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2023 - Influence of chirality on the aggregation of a new A-π-D-π-A small molecule with a benzodithiophene core: Spectroscopic and morphological investigations [Articolo su rivista]
Parenti, Francesca; Capelli, Raffaella; Mucci, Adele; Mortalò, Cecilia; Paolicelli, Guido; Pigani, Laura; Vanossi, Davide; Caselli, Monica

2021 - Spin Multiplicity and Solid-State Electrochemical Behavior in Charge-Transfer Co-crystals of DBTTF/F4TCNQ [Articolo su rivista]
Solano, F.; Inaudi, P.; Chiesa, M.; Kociok-Kohn, G.; Salvadori, E.; Da Como, E.; Vanossi, D.; Malandrino, M.; Carmieli, R.; Giacomino, A.; Fontanesi, C.

Charge-transfer crystals exhibit unique electronic and magnetic properties with interesting applications. The charge-transfer single crystal formed by dibenzotetrathiafulvalene (DBTTF) together with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) presents a long-range ordered supramolecular structure of segregated stacks, with a unitary degree of charge transfer. Thus, the crystal structure is composed of dimerized radical molecules with unpaired electrons. The energy levels and the spin degrees of freedom of this material were investigated by solid-state electrochemistry and electron paramagnetic resonance (EPR) spectroscopy. The electrochemical data, supported by density functional theory calculations, show how this organic Mott insulator has an electronic gap in the range of hundreds of meV. EPR experiments show the presence of a ground-state S = 1 triplet spin state along with localized S = 1/2 spins. The calculations also predict a ground-state triplet configuration, with the singlet configuration at 170 meV higher energy. DBTTF/F4TCNQ seems to be a candidate material for organic electronic and spintronic applications.

2021 - The Fundamental and Underrated Role of the Base Electrolyte in the Polymerization Mechanism. The Resorcinol Case Study [Articolo su rivista]
Bonechi, M.; Innocenti, M.; Vanossi, D.; Fontanesi, C.

The Kane-Maguire polymerization mechanism is disassembled at a molecular level by using DFT-based quantum mechanical calculations. Resorcinol electropolymerization is selected as a case study. Stationary points (transition states and intermediate species) leading to the formation of the dimer are found on the potential energy surface (PES), and elementary reactions involved in the dimer formation are characterized. The latter allow to further propagate the polymerization chain reaction, when applied recursively. In this paper, the fundamental role of the sulfate anion (a typical base electrolyte) is addressed. Investigation of the PES in terms of both stationary-state properties and of ab initio molecular dynamics results (dynamic reaction coordinate) allows the appreciation in detail of the critical role of the base electrolyte anion in making the proton dissociation from the initial radical ion, a feasible (downhill in energy) process.

2021 - Wavy graphene sheets from electrochemical sewing of corannulene [Articolo su rivista]
Bruno, Carlo; Ussano, Eleonora; Barucca, Gianni; Vanossi, Davide; Valenti, Giovanni; Jackson, Edward A.; Goldoni, Andrea; Litti, Lucio; Fermani, Simona; Pasquali, Luca; Meneghetti, Moreno; Fontanesi, Claudio; Scott, Lawrence T.; Paolucci, Francesco; Marcaccio, Massimo

The presence of non-hexagonal rings in the honeycomb carbon arrangement of graphene produces rippled graphene layers with valuable chemical and physical properties. In principle, a bottom-up approach to introducing distortion from planarity of a graphene sheet can be achieved by careful insertion of curved polyaromatic hydrocarbons during the growth of the lattice. Corannulene, the archetype of such non-planar polyaromatic hydrocarbons, can act as an ideal wrinkling motif in 2D carbon nanostructures. Herein we report an electrochemical bottom-up method to obtain egg-box shaped nanographene structures through a polycondensation of corannulene that produces a new conducting layered material. Characterization of this new polymeric material by electrochemistry, spectroscopy, electron microscopy (SEM and TEM), scanning probe microscopy, and laser desorption-ionization time of flight mass spectrometry provides strong evidence that the anodic polymerization of corannulene, combined with electrochemically induced oxidative cyclodehydrogenations (Scholl reactions), leads to polycorannulene with a wavy graphene-like structure.

2019 - Excited-state intramolecular proton transfer in a bioactive flavonoid provides fluorescence observables for recognizing its engagement with target proteins [Articolo su rivista]
Vanossi, D.; Caselli, M.; Pavesi, G.; Borsari, C.; Linciano, P.; Costi, M. P.; Ponterini, G.

A benzothiophene-substituted chromenone with promising activity against Leishmania and Trypanosoma species exhibits peculiar fluorescence properties useful for identifying its complexes with target proteins in the microorganism proteomes. The emission spectra, anisotropy and time profiles of this flavonoid strongly change when moving from the free to the protein-bound forms. The same two types of emission are observed in organic solvents and their mixtures with water, with the relative band intensities depending on the solvent ability to establish hydrogen bonds with the solute. The regular emission prevails in protic solvents, while in aprotic solvents the anomalously red-shifted emission occurs from a zwitterionic tautomeric form, produced in the excited state by proton transfer within the intramolecularly H-bonded form. This interpretation finds support from an experimental and theoretical investigation of the conformational preferences of this compound in the ground and lowest excited state, with a focus on the relative twisting about the chromenone-benzothiophene interconnecting bond. An analysis of the absorption and emission spectra and of the photophysical properties of the two emitting tautomers highlights the relevance of the local microenvironment, particularly of the intra- and intermolecular hydrogen bonds in which this bioactive compound is involved, in determining both its steady-state and time-resolved fluorescence behaviour.

2019 - Optoelectronic Properties of A‐π‐D‐π‐A Thiophene‐Based Materials with a Dithienosilole Core: An Experimental and Theoretical Study [Articolo su rivista]
Parenti, Francesca; Caselli, Monica; Vanossi, Davide; Buffagni, Mirko; Imperato, Manuel; Pigani, Laura; Mucci, Adele

Two A‐π‐D‐π‐A thiophene based small molecules, with a central dithienosilole core and dicyanovinyl end groups were synthesized. These compounds differ only for the presence of alkyl and alkylsulfanyl chains, respectively, on thiophene beta positions. The computational data together with the spectroscopic and electrochemical findings (obtained by means of absorption, steady‐state/time‐resolved emission techniques and cyclic voltammetry) revealed that both molecules possess low electronic and optical band gaps, broad absorption spectra and a good stability both in p and n‐doping states, properties that make them suitable for optoelectronic applications. In either compounds the HOMO‐LUMO transition involves an intramolecular charge transfer from the electron‐donor dithienosilole unit to the two terminal electron‐acceptor DCV groups. A marked positive emission solvatochromism was observed for both molecules and was interpreted on the basis of the symmetry breaking in the S1 excited state. The two synthesized compounds were also compared to their shorter precursors and to similar oligothiophenes to understand how the nature of the building block influences the characteristics of the final materials. Furthermore, it was possible to better understand the contribution of the sulfur atom in modulating the optical properties of the small molecules studied.

2019 - Redox-Active Ferrocene grafted on H-Terminated Si(111): Electrochemical Characterization of the Charge Transport Mechanism and Dynamics [Articolo su rivista]
Fontanesi, C.; Como, E. D.; Vanossi, D.; Montecchi, M.; Cannio, M.; Mondal, P. C.; Giurlani, W.; Innocenti, M.; Pasquali, L.

Electroactive self-assembled monolayers (SAMs) bearing a ferrocene (Fc) redox couple were chemically assembled on H-terminated semiconducting degenerate-doped n-type Si(111) substrate. This allows to create a Si(111)|organic-spacer|Fc hybrid interface, where the ferrocene moiety is covalently immobilized on the silicon, via two alkyl molecular spacers of different length. Organic monolayer formation was probed by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and X-ray photoelectron spectroscopy (XPS) measurements, which were also used to estimate thickness and surface assembled monolayer (SAM) surface coverage. Atomic force microscopy (AFM) measurements allowed to ascertain surface morphology and roughness. The single electron transfer process, between the ferrocene redox probe and the Si electrode surface, was probed by cyclic voltammetry (CV) measurements. CVs recorded at different scan rates, in the 10 to 500 mV s−1 range, allowed to determine peak-to-peak separation, half-wave potential, and charge-transfer rate constant (KET). The experimental findings suggest that the electron transfer is a one electron quasi-reversible process. The present demonstration of surface engineering of functional redox-active organometallic molecule can be efficient in the field of molecular electronics, surface-base redox chemistry, opto-electronic applications.

2018 - Thiophene based A-D-A small molecules with a dithienosilole core: synthesis, theoretical calculations and optoelectronic properties [Abstract in Atti di Convegno]
Parenti, Francesca; Buffagni, Mirko; Caselli, Monica; Mucci, Adele; Pigani, Laura; Vanossi, Davide; Zambon, Alfonso

Polythiophenes are conjugated materials finding application in many optoelectronic devices.1 Recently, much attention has been devoted to oligothiophenes, the shorter analogous compounds of these polymers. These semiconductor materials are structurally well-defined, monodisperse, free of defects and often possess higher polarizability and charge mobility with respect to the corresponding polymers.2 Thiophene-based small molecules3 are well studied as photoactive materials in organic solar cells. In particular, molecules with A-π-D-π-A architecture, formed by a central electron donating building block (D) and two terminal electron accepting groups (A) linked by π-conjugated bridges, show a low energy absorption band making them suitable as active layers in organic solar cells.4,5 Little modifications of the backbone, i.e. by changing the length of the alkyl substituents or the type of the functional groups, will often result in remarkable modification of the band gap and other optical properties. Here, we present the theoretical calculations, the synthesis and the characterization of two A-π-D-π-A small molecules E1 and E2, where the central unit is a dithienosilole, the terminal units are methyldicyanovinyl functionalized thiophene rings and the π-bridges are alkyl or alkylsulfanyl functionalized thiophene rings, respectively. The aim of this research is to study the structural, electronic and optical properties of these molecules to better understand the role of the sulfur atom of the alkylsulfanyl chains and to gain insight on the properties of these materials in view of their application in optoelectronic and photovoltaic devices

2017 - An Integrated Theoretical/Experimental Study of Quinolinic-Isoquinolinic Derivatives Acting as Reversible Electrochromes [Articolo su rivista]
Sassi, Mauro; Salamone, Matteo M; Beverina, Luca; Longoni, Gianluca; Fontanesi, Claudio; Vanossi, Davide; Cigarini, Luigi; Ruffo, Riccardo

A series of compounds, featuring an ethenylic bridge and quinoline and isoquinoline end capping units possessing systematically varied substitution patterns, were prepared as molecular materials for electrochromic applications. The different structures were optimized in order to maximize the electrochromic contrast in the visible region, mostly by achieving a completely UV-absorbing oxidized state. Density functional theory (DFT) calculations are exploited in order to rationalize the correlation between the molecular structure, the functional groups' electronic properties, and the electrochemical behavior. It is shown that the molecular planarity (i.e. ring/ring pi conjugation) plays a major role in defining the mechanism of the electrochemical charge transfer reaction, while the substituent's nature has an influence on the LUMO energy. Among the compounds here studied, the (E)-10-methyl-9-(2-(2-methylisoquinolinium1- yl)-vinyl)-1,2,3,4-tetrahydroacri-dinium trifluoromethanesulfonate derivative shows the most interesting properties as an electrochromophore.

2017 - Ferrocene Molecular Architectures Grafted on Si(111): A Theoretical Calculation of the Standard Oxidation Potentials and Electron Transfer Rate Constant [Articolo su rivista]
Fontanesi, Claudio; Innocenti, Massimo; Vanossi, Davide; Da Como, Enrico

The standard oxidation potential and the electron transfer (ET) rate constants of two silicon-based hybrid interfaces, Si(111)/organic-spacer/Ferrocene, are theoretically calculated and assessed. The dynamics of the electrochemical driven ET process is modeled in terms of the classical donor/acceptor scheme within the framework of Marcus theory. The ET rate constants, k(ET), are determined following calculation of the electron transfer matrix element, V-RP, together with the knowledge of the energy of the neutral and charge separated systems. The recently introduced Constrained Density Functional Theory (CDFT) method is exploited to optimize the structure and determine the energy of the charge separated species. Calculated ET rate constants are k(ET) = 77.8s(-1) and k(ET) = 1.3 x 10(-9) s(-1), in the case of the short and long organic-spacer, respectively.

2017 - Novel oligothiophenes with reduced HOMO-LUMO band gap for Optoelectonics [Abstract in Atti di Convegno]
Parenti, Francesca; Buffagni, Mirko; Zambon, Alfonso; Caselli, Monica; Vanossi, Davide; Mucci, Adele

Here we present the synthesis of A-π-D-π-A thiophene-based oligomers (Figure 1); in the design of these materials we incorporated both electron-donor and electron-acceptor groups by having a central donor dithienosilole, two terminal methyldicyanovinyl acceptor groups and two bithienyl units, functionalised with alkyl or alkylsulfanyl chains, as π-bridges.

2016 - An Integrated Experimental/Theoretical Study of Structurally Related Poly-Thiophenes Used in Photovoltaic Systems [Articolo su rivista]
Vanossi, Davide; Cigarini, Luigi; Giaccherini, Andrea; da Como, Enrico; Fontanesi, Claudio

In this work, a series of eight thiophene-based polymers (exploited as “donors” in bulk heterojunction photovoltaics cells), whose structures were designed to be suitably tuned with the electronic characteristics of the [6,6]-Phenyl C61 butyric acid methyl ester (PCBM), is considered,. The electronic properties of the mono-, di-, trimeric oligomers are reckoned (at the Hartree-Fock and DFT level of the theory) and compared to experimental spectroscopic and electrochemical results. Indeed, electrochemical and spectroscopic results show a systematic difference whose physical nature is assessed and related to the exciton (electron-hole) binding energy (Je,h). The critical comparison of the experimental and theoretical band gaps, i.e., the HOMO-LUMO energy difference, suggests that electrochemical and DFT values are the most suited to being used in the design of a polythiophene-based p-n junction for photovoltaics.

2016 - Optical and photophysical properties of anisole and cyanobenzene-substituted perylene diimides [Articolo su rivista]
Pagoaga, Bernard; Mongin, Olivier; Caselli, Monica; Vanossi, Davide; Momicchioli, Fabio; BLANCHARD DESCE, Mireille; Lemercier, Gilles; Hoffmann, Norbert; Ponterini, Glauco

One- and two-photon absorption cross-sections and spectra and the photophysical properties of eight perylenetetracarboxy-3,4:9,10-diimide (PDI) derivatives are reported and analyzed. The investigated compounds are characterized by direct binding of the phenyl rings of the substituents to the bay positions of the perylene core. They have been designed to test the effects of differences in the electronic nature – electron donating (anisole) or accepting (cyanobenzene) – and binding topology (cis or trans, meta or para disubstitution or tetrasubstitution) of the bay substituents on the above optical and photophysical observables. (TD)DFT and Hu¨ckel MO calculations have provided theoretical information on the ground-state geometries, the MOs and the electronic spectra of several model compounds. For tetrasubstituted and cis disubstituted derivatives, strong steric interactions in the bay area determined the preferred conformations, with perylene cores distorted near the substituted bay(s) and a 42–441 twisting of the substituent rings relative to the core, quite irrespective of the electronic nature of the substituents. On the other hand, in trans-disubstituted PDI steric hindrance in the bay areas was much weaker and similar in the cyanobenzene and the anisole derivatives. So, the large differences found in the conformational preferences were completely attributable to electronic effects. With electron-accepting cyanobenzene, the substituent rings were found normal to the central planar perylene core, thus enabling the assignment of the moderate spectroscopic effects to inductive interactions. The DFT analysis of the PDI trans-disubstituted with electron-donating anisoles gave quite strongly distorted perylene-core geometries and less twisted (591) substituent rings. The corresponding increased substituent/core conjugative interactions resulted in new CT allowed electronic transitions and an extremely pronounced solvent-polarity dependence of the emission spectra and intensities. All anisole substituted PDI feature a very fast radiationless decay path in polar solvents, likely related to a relaxation to a charge-separated configuration in the lowest excited-state.

2016 - Structure, Stoichiometry, and Charge Transfer in Cocrystals of Perylene with TCNQ-Fx [Articolo su rivista]
Salzillo, Tommaso; Masino, Matteo; Kociok Köhn, Gabriele; Di Nuzzo, Daniele; Venuti, Elisabetta; Della Valle, Raffaele Guido; Vanossi, Davide; Fontanesi, Claudio; Girlando, Alberto; Brillante, Aldo; Da Como, Enrico

Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics.

2015 - A novel synthetic strategy for magnetite-type compounds. A combined experimental and DFT-computational study [Articolo su rivista]
Cigarini, Luigi; Vanossi, Davide; Bondioli, Federica; Fontanesi, Claudio

The dynamics of the early stage reaction between benzyl alcohol and Fe(acetylacetonate)3 is studied by exploiting the Dynamic Reaction Coordinate (DRC) approach, at the PBE0/6-31G* level of theory. Analysis of the DRC trajectory provides a detailed molecular insight into the catalytic effect observed in the acidic reaction environment, compared to the neutral one. The presence of an additional proton in the reaction system, meant to simulate an acidic reaction environment, dramatically affects the reaction path: both by decreasing the activation energy of the complex dissociation and leading to the formation of acetone.

Fontanesi, Claudio; Vanossi, Davide; L., Cigarini; Mucci, Adele; E., Da Como

The electric properties of a large number of different complex composed systems (with particular focus on interfacial systems) are rationalized within a “Lego-like sum approach” of the electronic properties of the single components. Such a general modelistic approach span extremely different worlds: from Mott-Schottky barrier to Tafel plots in electrochemical systems [1,2]. In particular, in the field of organic semiconductors exploited to assemble photovoltaic devices, the open circuit potential is rationalized on the basis of the reciprocal HOMO LUMO energy differences between the donor and the acceptor partners [3]. Although such an approach seems by far too much crude in its strategy, the straight comparison of HOMO LUMO MOs energy levels, of the donor and acceptor building blocks, is yet the most widespread tools exploited in the modelization of photovoltaic organic based system [4]. In this work a series of seven thiophene based polymers (donors) are considered, whose structure was designed (both by introduction of ring structures of various chemical nature in the polymeric backbone structure – heteropolymers – and also by various lateral alkyl chains) so to suitably tune the electronic properties of the [6,6]-Phenyl C61 butyric acid methyl ester, often addressed as PCBM, (acceptor) and obtained a satisfactory solubility. Examples of the polymers structure are shown in Chart 1. The electronic properties of the mono-, di-, tri-meric oligomer species are reckoned and compared to experimental spectroscopic and electrochemical results.

2014 - Linear and nonlinear optical properties of V-shaped D–p–A–p–D chromophores: effects of the incorporation of aromatic rings in the polyenic p-bridges of open-chain ketocyanines [Articolo su rivista]
Momicchioli, Fabio; Ponterini, Glauco; Vanossi, Davide

Following previous studies on a and b polarizabilities of ketocyanines, a subgroup of D–p–A–p–D quadrupolar chromophores with moderately V-shaped structure, the present work analyses the effects of modifying the p-bridges connecting the D (NMe2) and A (CO) groups. This aim is pursued through a detailed comparison between the previously studied ketocyanines (KC2, KC3) and a Michler’s ketone analogue (KM1) bearing styrenic (in the place of polyenic) p-bridges. First, we report a spectroscopic study, including absorption and fluorescence anisotropy spectra, aimed to probe the electronic peculiarities of KM1 as well as to derive consistent three-state model (TSM) parameters for the three compounds. The paper goes on with an extensive theoretical study, carried out in the framework of the density functional theory (DFT), encompassing the structure, the electronic spectrum, a and b polarizabilities and two-photon absorption (TPA) cross-sections (sTP). Calculations performed according to the sum-over-states (SOS) approach are discussed with reference to the performances of few-state descriptions, it is shown that such descriptions (including TSM), which have been proved to be quite reliable in the case of KC2 and KC3, lose their effectiveness with KM1 because of the electronic characteristics related to the styrenic p-bridges. As to the TPA cross-sections, the results of TSM and SOS approaches concerning the TSM g - c and g - e transitions are supplemented by those obtained using the quadratic response theory. A common qualitative conclusion, traceable to the degree of bending of the V-shaped structure, is that in the case of KM1 the allowed (g - e) and the ‘‘forbidden’’ (g - c) transitions both should be observable in the TPA spectrum, as confirmed by experiment.

2013 - Electropolymerization of ortho-phenylenediamine. Structural characterisation of the resulting polymer film and its interfacial capacitive behaviour [Articolo su rivista]
Vanossi, Davide; Pigani, Laura; Seeber, Renato; Paolo, Ferrarini; Baraldi, Pietro; Fontanesi, Claudio

The physico-chemical characteristics of thin poly-(orth o-phenylenediamine) (PPD) films, obtained by electrochemical oxidation of the relevant monomer, are investigated using electrochemical, morpholog-ical and spectroscopic techniques. In particular, cyclic voltammetry and electroche mical impedance spec-troscopy (EIS) techniques are used to collect information concerning the redox, conductivity and double layer capacitance properties of the PPD film. AFM imaging and Raman spectroscopy results are exploited to characterize the film structure. In this respect, Raman spectra of two possible PPD oligomers are cal-culated at the B3LYP/6-311G level of the theory.

2013 - Regiochemistry in the electrochemical assisted grafting of glassy carbon. With focus on sterical hindrance of lateral chains in the electroreduction process of multi-functionalized bithiophene [Articolo su rivista]
Tassinari, Francesco; Vanossi, Davide; Mucci, Adele; Parenti, Francesca; Fontanesi, Claudio

The electrochemically assisted grafting process of glassy carbon (GC) surfaces is pursued by using the 7-[(5,50-diiodo-2,20-bithiophen-4-yl)sulfanyl]heptanenitrile (DIBHN). Which is an ad hoc synthesized bithiophene derivative: the DIBHN features two non-equivalent iodine substituents in the 5 (which is under the sterical hindrance of the sulfanylheptanenitrile group) and 50 positions. CV measurements show the occurrence of two reduction processes, the second, more cathodic one, induces the grafting. This result is in qualitative agreement with theoretical calculations (B3LYP/6-311G level of the theory) concerning the comparison of the optimized structures of the DIBHN neutral and radical anion species (first electroreduction process): the C–I bond length of the iodine in the 5 position varies from 2.098 (neutral) to 2.835 Å (radical anion), while the C–I bond length of the iodine in the 50 position remains virtually unchanged when comparing the geometries of the neutral and radical anion structures. Raman spectra are recorded for various polymer film thicknesses; the comparison with theoretical calculated Raman spectra further support the effective grafting of the glassy carbon surface.

2012 - Chemical asymmetry and alpha and beta polarizabilities of D-A-D' chromophores: a three-state-model and TDDFT-SOS analysis of apenta-heptamethine ketocyanine [Articolo su rivista]

The essential-state model, here amounting to a three-state model, has been employed to account for the effects of chemical asymmetry on the electronic alpha and beta polarizabilities of a pentaheptamethine ketocyanine (KC2,3), a prototypic D-A-D' chromophore. A suitable model, based on the idea of a ‘chromophoric site’, has been set up in terms of the three-state model features previously derived for the parent symmetric pentamethine and heptamethine ketocyanines, KC2 and KC3. This approach has been found to reproduce very well the experimental transition energies and dipoles. From the resulting properties of the ground and two relevant lowest excitedstates, average alpha and beta vec have been evaluated according to the SOS approach. The performances of themodel have been tested by comparison with the results of TDDFT SOS (hyper) polarizability calculations considering up to twenty excited states. A detailed analysis of the results for the threeketocyanines has shown a rapid convergence of the SOS expansion that supports the reliability of descriptions based on a few low lying excited states (here corresponding to p - p* excitations). However, while only two excited states were necessary for the symmetric compounds, for KC2,3 avalue of beta vec comparable with the converged value, as well as with that predicted by the experimentally-based three-state model, has been obtained including at least three excited states.Both the TDDFT SOS and the three-state model descriptions have emphasized the important role played by the three-level term contributions in the determination of beta vec. Moreover, bothdescriptions agree in predicting that KC2,3 features average alpha and beta vec values in between those of KC2 and KC3

2012 - Functionalization of glassy carbon surface by means of aliphatic and aromatic amino acids. An experimental and theoretical integrated approach [Articolo su rivista]
Vanossi, Davide; Benassi, Rois; Parenti, Francesca; Tassinari, Francesco; Giovanardi, Roberto; Florini, Nicola; DE RENZI, Valentina; Arnaud, Gaelle Francoise; Fontanesi, Claudio

Glassy Carbon (GC) electrode surfaces are functionalized through electrochemical assisted grafting, in oxidation regime, of six amino acids (AA): -Alanine (-Ala), L-Aspartic acid (Asp), 11-aminoundecanoic acid (UA), 4-Aminobenzoic acid (PABA), 4-(4-Amino-phenyl)-butyric acid (PFB), 3-(4-Amino-phenyl)-propionic acid (PFP). Thus, a GC/AA interface is produced featuring carboxylic groups facing the solution. Electrochemical (cyclic voltammetry and electrochemical impedance spectroscopy) and XPS techniques are used to experimentally characterize the grafting process and the surface state. The theoretical results are compared with the experimental evidence to determine, at a molecular level, the overall grafting mechanism. Ionization Potentials, Standard Oxidation Potentials, HOMO and electron spin distributions are calculated at the CCD/6-31G* level of the theory. The comparison of experimental and theoretical data suggests that the main electroactive species is the “zwitterionic” form for the three aliphatic amino acids, while the amino acids featuring the amino group bound to the phenyl aromatic moiety show a different behaviour. The comparison between experimental and theoretical results suggests that both the neutral and zwitterionic forms are present in the acetonitrile solution in the case of 4-(4-Amino-phenyl)-butyric acid (PFB) and 3-(4-Amino-phenyl)-propionic acid.

2011 - Dissociation Dynamics of Asymmetric Alkynyl(Aryl)Iodonium Radicals: An ab Initio DRC Approach to Predict the Surface Functionalization Selectivity [Articolo su rivista]
Fontanesi, Claudio; Bortolotti, Carlo Augusto; Vanossi, Davide; M., Marcaccio

The dissociation process of neutral open-shell [4-F—(C6H4)—I—CtC—(CH2)4—Cl] and [4-NO2—(C6H4)—I—CtC—(CH2)4—Cl] asymmetric iodonium radicals was studiedtheoretically. Vertical electron affinities and DRC (dynamic reactioncoordinate) results were obtained and compared with experimentalevidence. In particular, the fluorine and nitro substituent groups wereselected because of (i) their opposite electron-withdrawing/electrondonatingeffects and (ii) experimental evidence that the grafting ability,in terms of alkynyl/aryl grafting ratio, increases with decreasingelectron-withdrawing nature of the para-position substituent on the phenyl ring. DRC results show that the dissociation dynamicsof the iodinealkynyl carbon bond, for the nitro-substituted iodonium, occurs on a longer time scale than that of the fluorinesubstitutediodonium. This finding is in agreement with the overall experimental results.

2011 - Electronic spectra and (hyper)polarizabilities of non-centrosymmetric D–A–D chromophores. An experimentally based three-state model and a theoretical TDDFT study of ketocyanines [Articolo su rivista]
Ponterini, Glauco; Vanossi, Davide; Krasnaya, Z. h. A.; A. S., Tatikolovc; Momicchioli, Fabio

The electronic structure, spectra and linear and second-order polarizabilities of two symmetricketocyanines, which are prototypic examples of D–A–D chromophores, have been investigatedwith two different toolsets: (i) the so-called ‘essential-state model’, here comprising three states,the ground and two lowest excited 1pp* states, has been adapted for these non-centrosymmetric,yet symmetric compounds to determine their permanent electric dipole moments, polarizabilitiesand first hyperpolarizabilities making use of experimental transition energies and moments; (ii)extensive TDDFT calculations have provided ground-state conformational results consistent withNMR-derived structural information, energies and dipole moments of up to 20 lowest-lyingelectronic states as well as, within the sum-over-states (SOS) scheme, the most relevantcomponents of the polarizabilities and first hyperpolarizabilities. The two levels of descriptionform consistent pictures of the ketocyanine excited states that provide the most relevantcontributions to hyperpolarizabilities: extension of the SOS set beyond the three states of thebasic model left unchanged (within B10%) the calculated vector component of the second-orderpolarizability tensor along the direction of the ground-state dipole moment (by). Both approachesindicate that these D–A–D compounds, in spite of their quasi-linear structure, reminiscent of thatof centrosymmetric quadrupolar chromophores, feature significant second-order molecularpolarizabilities. These rapidly increase with the length of the polyenic bridges in thechromophores. About half of the total value of by is predicted to come from the three-level-termpart, by,3, most of which derives from the contribution involving the three electronic states of theessential-state model.

2010 - Experimental and Theoretical Study of the p- and n-Doped States of Alkylsulfanyl Octithiophenes [Articolo su rivista]
C., Bruno; F., Paolucci; M., Marcaccio; Benassi, Rois; Fontanesi, Claudio; Mucci, Adele; Parenti, Francesca; L., Preti; Schenetti, Luisa; Vanossi, Davide

The charge-transfer and spectral properties of two octithiophenes, namely 4′,4′′′,3′v,3v′-tetra(octylsulfanyl)-2,2′:5′,2′′:5′′,2′′′:5′′′,2′v:5′v,2v:5v,2v′:5v′,2v′′-octithiophene and 4,3′′,4v,4v′′-tetrabromo-4′,4′′′,3′v,3v′-tetra[(R)-2-methylbutylsulfanyl]-2,2′:5′,2′′:5′′,2′′′:5′′′,2′v:5′v,2v:5v,2v′:5v′,2v′′-octithiophene, OT1 and OT2, respectively,are characterized by cyclic voltammetry and spectroelectrochemistry under ultradry conditions. The analysisof the voltammetric results shows the formation of up to the dication for both OT1 and OT2 and up to thetetraanion (OT1) and trianion (OT2) anions. The optical properties of the OT1 (2+, 1+, neutral, 1-, 2-)species were probed by in situ UV-vis-NIR spectroelectrochemistry. The calculated standard potentials atthe B3LYP/cc-pVTZ level of the theory allowed the rationalization of the experimental electrochemical results.The UV-vis-NIR spectra were successfully compared with the theoretical electronic transitions and oscillatorstrength data obtained by time-dependent B3LYP/6-31G* calculations. Theoretical redox potentials and opticaltransitions properties are calculated including “the solvent effect” within the CPCM model. The consistencyobtained between experimental and theoretical results indicates the existence of the hypothesized high-spin/high-charge p- and n-doped electronic states for the OT1 and OT2 octithiophenes here studied.

2010 - On the Electroreduction Mechanism of Iodonium Salts on Glassy Carbon Electrodes" [Abstract in Atti di Convegno]
K., Daasbjerg; S. U., Pedersen; Florini, Nicola; Parenti, Francesca; Bortolotti, Carlo Augusto; Vanossi, Davide; Fontanesi, Claudio

Glassy carbon (GC) surfaces can be functionalized exploiting the electrochemical reduction of iodonium salts of general formula [RIR’]+ [1]. The overall mechanism could be roughly sketched as: IR’ bond cleavage:[RIR’]+ + e + GC  R’GC + IR Route (1) IR bond cleavage:[RIR’]+ + e + GC  RGC + IR’ Route (2) Upon electroreduction the I-R or the I-R’ bond dissociate, leading to a neutral closed shell organic iodide and an open shell radical, the latter reacts with the GC electrode (grafting). Several factors can influence the R/R’ ratio grafted on the GC surface. In fact, different amounts of the R and R’ radicals can be formed depending on the electronic structure of the neutral open shell [R-I-R’] • radical. Moreover, the different radicals can exhibit different reactivity toward the carbon surface, as well as different electrochemical stabilities (the radical itself could be reduced to a negative closed shell form). To clarify the interplay of the various factors affecting the final surface functionalization, a number of iodoniums has been considered and experimental evidences (electrochemical and XPS) are compared with theoretical results calculated at the DFT level of the theory (electron affinities, potential energy surfaces of competitive reaction pathways).

2010 - On the electroreduction mechanism of iodonium salts on glassy carbon electrodes [Abstract in Atti di Convegno]
K., Daasbjerg; S. U., Pedersen; DE RENZI, Valentina; Florini, Nicola; Parenti, Francesca; Bortolotti, Carlo Augusto; Vanossi, Davide; Fontanesi, Claudio

Glassy carbon (GC) surfaces can be functionalized exploiting the electrochemical reduction of iodonium salts of general formula [RIR’]+ [1]. The overall mechanism could be roughly sketched as: IR’ bond cleavage: [RIR’]+ + e + GC  R’GC + IR Route (1) IR bond cleavage: [RIR’]+ + e + GC  RGC + IR’ Route (2) Upon electroreduction the I-R or the I-R’ bond dissociate, leading to a neutral closed shell organic iodide and an open shell radical, the latter reacts with the GC electrode (grafting). Several factors can influence the R/R’ ratio grafted on the GC surface. In fact, different amounts of the R and R’ radicals can be formed depending on the electronic structure of the neutral open shell [R-I-R’] • radical. Moreover, the different radicals can exhibit different reactivity toward the carbon surface, as well as different electrochemical stabilities (the radical itself could be reduced to a negative closed shell form). To clarify the interplay of the various factors affecting the final surface functionalization, a number of iodoniums has been considered and experimental evidences (electrochemical and XPS) are compared with theoretical results calculated at the DFT level of the theory (electron affinities, potential energy surfaces of competitive reaction pathways).

2008 - First- and Second-Order Polarizabilities of Simple Merocyanines. An Experimental and Theoretical Reassessment of the Two-Level Model [Articolo su rivista]
Momicchioli, Fabio; Ponterini, Glauco; Vanossi, Davide

Taking four merocyanines [(CH3)2N-(CHdCH)n-C(CH3)O; n ) 1-4] (Mc1-4) as test D-A systems, weperformed a close experimental and theoretical examination of the two level model with reference to itsability to provide correct predictions of both absolute values and dependence on the conjugation path lengthof first- and second-order molecular polarizabilities. By 1H NMR spectroscopy merocyanines Mc1-4 werefound to be ∼1:1 mixtures of two planar conformers with cis and trans arrangements of the sC(CH3)Oelectron-acceptor group and all trans structure of the polyene like fragment. The degree of bond lengthalternancy (BLA) in the -(CHdCH)n- fragment, was quantified by extensive full geometry optimizations atboth semiempirical and ab initio level. DFT (6-31G**/B3LYP) optimized geometries were considered to bemost reliable and were used for calculations of the excited-state properties. The applicability of the two levelmodel, reducing the general sum-over-states (SOS) expansion to only one term involving the ground state (g)and the lowest-lying 1(ππ*) CT state (e), was checked by analysis of fluorescence and near UV absorptionspectra. Measurements of the basic two-level model quantities (Ege, μge and Δμeg), by which the dominantcomponents of r and tensors are expressed (RXX, XXX, X ≡ long molecular axis), were designed to giveapproximate free-molecule values. It is proposed, in particular, an adjustment of the solvatochromic methodfor the determination of Δμeg, based on accurate measurements of absorption spectral shifts in n-hexane/diethyl ether mixtures with small diethyl ether volume fractions. Such an approach led to Mc1-4 XXX’smatching well in both value and n-dependence with EFISH data reported in the literature for similarmerocyanines. For the fluorescent Mc4, the results were qualitatively well reproduced by an approach, whichcombines absorption and fluorescence solvent shifts. All the measured quantities were calculated for bothtrans and cis Mc1-4 by three semiempirical INDO-based approaches aiming at evaluating the performancesof different integral parametrizations and CI extensions: ZINDO/S, CS INDO SCI, CS INDO SDCI. In allcases, RXX and XXX were found to rise proportionally to about n1.3 and n2, respectively, in qualitatively goodagreement with the experimental values. As to the absolute values, however, experimental RXX’s and XXX’swere best reproduced by CS INDO SDCI combining Ohno-Klopman parametrization and CI including bothsingle and double excitations. The validity of the two-level model was checked by comparison with convergedSOS calculations for the longest chain merocyanine (Mc4) and finite field calculations of linear polarizabilitiesfor all of the four dyes (Mc1-4).

2007 - Electronic spectra and fluorescence properties of multichromophoric sulfonylureas [Articolo su rivista]
Baraldi, Ivan; Caselli, Monica; Ponterini, Glauco; Vanossi, Davide

The multichromophoric character of two sulfonylurea herbicides, SMT and BNS, has been investigated in its manifestations in the electronic absorption spectra and in some fluorescence properties through a combined experimental and theoretical approach. After a theoretical analysis of the most stable structures of these flexible systems, the UV absorption spectra of the two multichromophoric compounds have been analysed both experimentally and theoretically, and most transitions have been assigned to individual chromophores, also by comparison with four suitable reference compounds (5-8). Finally, some experimental information concerning the fluorescence spectra and quantum yields have been analysed with reference to the contributions from single fluorophores and the role of low-lying n -> pi* states. (c) 2006 Elsevier B.V. All rights reserved.

2007 - Solvent-dependent host-guest complexation of two homologous merocyanines by a water-soluble calyx[8]arene. Spectroscopic analysis and structural calculations [Articolo su rivista]
A., Lodi; Caselli, Monica; A., Casnati; Momicchioli, Fabio; F., Sansone; Vanossi, Davide; Ponterini, Glauco

The sulfonated calixarene I8C12 acts as a host for homologous merocyanines Mc1 and Mc2 in organic solvents, exhibiting neither selectivity towards the guest dyes nor solvent dependence of the complexation equilibria. In water, on the contrary, only the lower homologue, Mc1, is solubilized in the presence of the calixarene. A combination of UV–visible and fluorescence spectroscopic and photophysical analysis and MD structural simulation of the calixarene-dye complexes was employed to account for the observations, and suggests that a radical change in the complexation mode occurs upon moving from an organic to an aqueous environment.

2006 - Excitation energy transfer in ion pairs of polymethine cyanine dyes: Efficiency and dynamics [Articolo su rivista]
PONTERINI, Glauco; M., Fiorini; VANOSSI, Davide; A. S., Tatikolov; MOMICCHIOLI, Fabio

The present work deals with singlet excitation energy transfer (EET) occurring in contact ion pairs (CIPs) of several anionic oxonol analogues (acting as EE donors) and cationic cyanines (acting as acceptors) characterized by off resonance individual transitions. Combining conductometric and spectroscopic measurements with decreasing solvent polarity, we were able to observe a progressive ion pairing leading first to solvent-separated ion pairs (SSIPs) and then to CIPs. Analysis of the absorption spectra of three selected salts (A2, C1, A2, C2, and A1, C4) in chloroform-toluene mixtures showed that the transformation of SSIP into CIP involves the appearance of a certain exciton coupling, the extent of which decreases regularly with increasing gap between the local excitation energies. Fluorescence excitation spectra showed that EET occurs in CIP, and EET efficiencies were evaluated with a procedure expressly devised for weakly emitting donors. These were between 0.2 and 0.65 for the examined ion pairs involving anions A1 and A2. The spectroscopic study was complemented by a theoretical investigation aimed at establishing the dynamic regime of the observed EET. From classical MD simulations and local full geometry optimizations, A2, C1 and A2, C2 were found to form rather stable sandwich-type CIP structures with interchromophore distances (R) of about 0.45-0.50 nm. The donor-acceptor electronic coupling was calculated in terms of Coulombic interactions between atomic transition charges. For CIP, the electronic coupling was decidedly beyond the limit of the weak coupling required for an incoherent Forster-type mechanism. Thus, we tried to arrange the EET dynamics within the theory developed by Kimura, Kakitani, and Yamato (J. Phys. Chem. B 2000, 104, 9276) for the intermediate coupling case, which provides analytical expressions of time- dependent occupation probability, EET rate, and coherency in terms of two basic quantities: the electronic coupling and a correlation time related to the Franck-Condon factor. The latter was shown to be primarily modulated by Forster's spectral overlap integral (related in turn to the excitation energy gap). Calculations were carried out for the three sample systems using three values of the electronic coupling roughly corresponding to CIP, 1.0, and 2.0 nm interchromophore distances. At the CIP distance, EET in both A2, C1 and A2, C2 was predicted to occur with a partial exciton mechanism, very short transfer times (about 10 fs), and high degree of coherence. In A1, C4 (having the largest energy gap), EET was found to occur with a hot-transfer mechanism. More or less hot-transfer dynamics appeared to be retained by all three systems at R=1.0 nm. Fully incoherent EET appeared to become operative only at distances larger than 2.0 nm.

2004 - Electronic structure and photochemistry of squaraine dyes: basic theoretical analysis and direct detection of the photoisomer of a symmetrical squarylium cyanine [Articolo su rivista]
Momicchioli, Fabio; A. S., Tatikolov; Vanossi, Davide; Ponterini, Glauco

The photoisomerization kinetics of a squaraine dye has been the object both of experimental investigation and of interpretation in the framework of a qualitative theoretical model formulated by the aid of simple HMO calculations and orbital symmetry considerations. Such a model has first confirmed that the electronic structure and the spectroscopic properties of symmetrical squaraines are related to those of the parent cyanines, with ketocyanines as intermediate systems. Extension of the approach to structures twisted by 90degrees about a polymethine bond has then provided insight into the electronic aspects and the mechanism of the photoisomerization of the squaraine under study. The reaction, previously indirectly investigated by. uorescence analysis, has been directly monitored by laser. ash photolysis. These experiments indicate that, while photoisomerization is likely the main radiationless decay route from the spectroscopic minimum of the lowest excited singlet state (S-1), the cis photoisomer is produced with only a 1% yield, likely because of an unfavourable cis/trans branching ratio from the perpendicular minimum of the S-1-state potential energy surface. In contrast with what found for symmetrical cyanines, an increase in the solvent polarity was found to accelerate both the direct, excited-state reaction and, to a much larger extent, the ground-state back-isomerization. Such observations are consistent with predictions of the theoretical model and provide a clue for the identification of the isomerization coordinate.

2003 - Photoisomerization of simple merocyanines: a theoretical and experimental comparison with polyenes and symmetric cyanines [Articolo su rivista]
Baraldi, Ivan; Momicchioli, Fabio; Ponterini, Glauco; A. S., Tatikolov; Vanossi, Davide

The structure and electronic spectra of merocyanines are intermediate between those of acyclic polyenes and cationic symmetric cyanines of comparable sizes, shifting from one to the other as a result of a solvent polarity change. In this paper we address, both theoretically and experimentally, the question of how the photochemical behaviour of simple merocyanines compares with that of polyenes and symmetric cyanines. After a brief theoretical re-appraisal of the absorption maxima dependence on the chain length in the three classes of chromogens, we analyse the calculated potential-energy curves for two photoisomerization coordinates of a polyene, a symmetric cyanine and a merocyanine of comparable sizes. The results concerning both the energy curves and the nature of the relevant electronic states, particularly the TICT (twisted intramolecular charge transfer) character of the S-1 state at the perpendicular geometry, reveal the existence of a near relationship between merocyanines and symmetric ionic polymethines, traceable back to their being odd-alternant systems. The presence of a low-lying (1)(npi*) state at the trans geometry and the associated solvent-modulated vibronic coupling with the (1)(pi(H)pi(L)*) state dominate the experimentally accessible photochemical behaviour of two simple merocyanines and make it peculiar with respect to those of both polyenes and symmetric ionic cyanines.

2003 - Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study [Articolo su rivista]
Baraldi, Ivan; A., Spalletti; Vanossi, Davide

The experimental results on the rotameric equilibrium and electronic spectra of aza-derivatives of trans-stilbene and 1,4-diphenylbutadiene, have been rationalized by a theoretical study which combines simple ab initio calculations of molecular energies for the ground state with a theoretical analysis of the splitting of the conjugation band developed at CS INDO Cl level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the H-1-NMR experiments, the ab initio geometry of ZE-2-pyridylphenylbutadiene is consistent with the presence of the N.H intramolecular hydrogen bond. As regards the Franck-Condon excited states of aza-derivatives, our theoretical results show that the first singlet excited state has (pi(H), pi(L)(*)) character in all compounds except for E-4,4'-dipyridylethene, where S-1 has (n, pi*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the (1)(n, pi(L)(*)) and (1)(pi(H), pi(L)(*)) states which occurs in solvents of high polarity. The inclusion of the most important. doubly- and triply-excited configurations in the CI calculations shows that the (1)A(g)(-) excited state is above the spectral region analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.

2003 - Solvent influence on absorption and fluorescence spectra of merocyanine dyes: a theoretical and experimental study [Articolo su rivista]
Baraldi, Ivan; G., Brancolini; Momicchioli, Fabio; Ponterini, Glauco; Vanossi, Davide

The solvaton-CS INDO model, previously successfully used to describe the solvatochromic properties of merocyanines, has been extended to the study of the solvent influence on the fluorescence spectra (fluorosolvatochromism) of these dyes. A ketocyanine (M1) and a stilbazolium betaine (M2) were chosen as representatives of positively and negatively solvatochromic behaviours, respectively. The gap of experimental knowledge concerning the emission properties of M2 was filled by a spectrofluorometric analysis in a set of solvents covering a large range of the E-T(30) scale. Solvato- and fluorosolvatochromism were described by calculating the S-0 (eq.) --> S-1 (Franck-Condon) and S-1 (eq.) --> S-0 (Franck-Condon) transition energies as a function of a polarity factor related to the static dielectric constant of the solvent, and ranging from 0 to 1. The absorbing S-0 (eq.) and emitting S-1 (eq.) units (solute molecule + solvent cage) were approximated using the S-0 and S-1 geometries of the unsolvated molecule and the respective charge distributions fitted to the current value of k(epsilon). The calculation results fully confirm that S-0 and S-1 states of merocyanines can be viewed as a mixture of a neutral and a zwitterionic structure whose composition is controlled by the solvent polarity. The plots of the calculated spectral data (absorption and emission maxima and corresponding Stokes shifts) vs k(epsilon) are in fairly good agreement with those of the experimental data over almost the entire range of the normalized E-T(N) values, thus showing that specific solvent interactions are at least partly simulated within the solvaton-CS INDO scheme. The methodological prerequisites for a correct prediction of solvatochromic shifts are recalled with reference to previous conflicting theoretical interpretations.

2002 - Dimerization of green sensitizing cyanines in solution. A spectroscopic and theoretical study of the bonding nature [Articolo su rivista]
Baraldi, Ivan; Caselli, Monica; Momicchioli, Fabio; Ponterini, Glauco; Vanossi, Davide

The bonding nature in cyanine-dye aggregates has been investigated by studying dimerization in solution of 3, 3'-disulfopropyl-4,5,4',5'-dibenzo-9-ethyloxacarbocyanine (D) and three other oxacarbocyanine analogues (E, F, G) used as photographic sensitizers in the green spectral region. Quantitative information on the monomer-dimer equilibrium of dye D in different solvents and of its analogues (E, F, G) in water was obtained by measurement of the absorption spectrum as a function of dye concentration and of temperature. Dimerization was found to be generally driven by enthalpic factors traceable to strong attractive van der Waals interactions between the two large and highly polarizable dye molecules. Entropic contributions to DeltaG(0) usually favour dissociation but are smaller than the enthalpic ones. The visible absorption spectrum of the dimer consists in a classic two-branched exciton band with a marked splitting (1600 cm(-1) in water). The experimental observations were the subject of a theoretical study including classical molecular dynamics (MD) and Monte Carlo (MC) calculations of the dimer structure and comparative analysis of monomer and dimer spectra by the CS INDO CI method. Computer simulations led to three similar H-type structures, the most stable of which is characterized by a distance of 4.7 Angstrom between the planes of the chromophores and an endoendo configuration of the sulfopropyl substituents. The calculated dimer spectrum was clearly interpreted in terms of exciton model but a quantitative agreement with the two-maximum exciton band could be obtained only by assuming substantial deviation of the long molecular axes from parallelism. On the basis of normal coordinate calculations it is suggested that such configurations may occur with a high probability in virtue of twisting vibrational motions of extremely low frequency. (C) 2002 Elsevier Science B.V. All rights reserved.

2001 - Exciton-like and charge-transfer states in cyanine-oxonol ion pairs. An experimental and theoretical study [Articolo su rivista]
Baraldi, Ivan; Momicchioli, Fabio; Ponterini, Glauco; A. S., Tatikolov; Vanossi, Davide

Absorption and fluorescence emission properties of cyanine-oxonol mixed dyes, i.e., salts formed by a cationic cyanine with an anionic oxonol as counterion, were investigated both theoretically and experimentally in order to probe the effects of ion pairing occurring in low-polarity solvents. We analyzed, in particular, three model systems (S1, S2, and S3) built combining thiacarbo- and thiadicarbocyanine (C1, C2) with two vinylogous oxonol chromophores (A1/A1F, A2). in systems S1 (C1-A1) and S2 (C2-A2), where the visible absorption bands of the individual ions are almost superimposed, the formation of ion pairs gives rise to marked spectral alterations traceable to interchromophore resonance interactions. On the contrary, in system 53 (C2-A1F), whose components absorb widely apart, the spectrum of the contact ion pair and that of the dissociated form differ only for the relative band intensities. In both cases, however, contact ion pairing results in complete quenching of the emission of the chromophoric units. Such behaviors, emphasized by absorption and fluorescence emission and excitation spectra of both the mixed dyes and their components in solvents of different polarities, were the subject of a theoretical study based in particular on the calculation of structures and electronic spectra of the contact ion pairs. Molecular dynamics (MD) simulations and local full geometry optimizations led to two types of structures characterized by almost parallel and orthogonal arrangements of the long molecular axes. CS INDO SCI calculations using both arrangements emphasized the role of the exciton coupling between the local HOMO-LUMO excitations of the two chromophoric units. The most striking spectral characteristics in low-polarity solvent turned out to be explainable in terms of parallel type arrangements, even if an appreciable contribution of the orthogonal type structure was to be invoked for a complete interpretation of the S1 spectral properties. In all contact ion pairs, independently of the structure, the lowest excited singlet is a forbidden anion --> cation charge transfer (CT) state explaining why no fluorescence emission was observed in such systems.

2000 - A recent development of the CSINDO model. Treatment of solvent effects on structures and optical properties of organic dyes [Articolo su rivista]
Baraldi, Ivan; Momicchioli, Fabio; Ponterini, Glauco; Vanossi, Davide

Treatment of solvent effects on structures and optical properties of organic dyes

2000 - Exciton effects in the dimer and higher aggregates of a simple merocyanine dye. A CSINDO CI based theoretical study [Articolo su rivista]
P., Millie; Momicchioli, Fabio; Vanossi, Davide

A simple streptomerocyanine with five methine carbons (Mc) has been used as a suitable model system to investigate theoretically the optical properties in organized assemblies of merocyanine dyes. The subject was treated by a methodology making use of a semiempirical quantum chemistry description of the isolated chromophore within the Frenkel exciton theory. Both an extended CS INDO CIPSI and easier CS INDO SCI (with different parametrizations of electron repulsion integrals) schemes were applied in order to establish the role played by the quality of the wave functions. The exciton states were built on the (1)(pi pi*) (HOMO-LUMO) excited-state responsible for the color band of Me. Exciton effects were first analyzed for two stable dimer structures, having H- and J-type character, located by use of a semiempirical intermolecular potential and the simulated annealing method. Then, attention was shifted from dimers to higher-order model aggregates built in such a way as to obtain typical H (columnar) and J (staircase) arrangements. Exciton-state properties (energies and transition dipole moments) were studied as a function of the aggregation number N (up to N = 50). The results are discussed with reference to the approximations used to calculate the excitonic matrix as well as to the effects of changing from CIPSI to simple SCI schemes.

2000 - Regarding enumeration of molecular isomers [Articolo su rivista]
Baraldi, Ivan; Vanossi, Davide

In this paper, a simple theoretical approach to counting of substitution isomers is described. It is based on Polya's theorem and on point groups as recently described by us [Baraldi, I.; Fiori, C.; Vanossi, D. J. Math. Chem. 1999, 25, 23-30] and extended in this paper. Several applications are developed that range from molecules without symmetry to molecules with icosahedral symmetry (Ih) The problem of the appearance of stereoisomers is also analyzed.

1999 - On the cycle index of point groups [Articolo su rivista]
Baraldi, Ivan; Fiori, Carla; Vanossi, Davide

In this brief work we express the cycle index of the molecular point groups as a function of a limited number of initial geometrical parameters. Such parameters are the number m of elements composing the domain D of sites of substitutions in the molecule belonging to the point group G, and the numbers (n(C-n),n(sigma),...) of sites of D lying on the symmetry elements (C-n,sigma,...) for G.

1998 - Solvent effects within the CS INDO method. Geometrical distortion and solvatochromism of merocyanine dyes [Articolo su rivista]
Baraldi, Ivan; Momicchioli, Fabio; Ponterini, Glauco; Vanossi, Davide

This work represents the first step of a theoretical study aiming at explicitly including solvation in the CS INDO CI calculations of ground and excited state properties of organic dyes. The reported theoretical treatment falls within the electrostatic continuum theories and introduces solute-solvent interaction in the SCF calculation according to Klopman's solvaton model. The procedure was tested on two merocyanines characterized by opposite solvatochromic behaviours. It was found that solvaton systems most suitable for pi-conjugated donor-acceptor dyes, like merocyanines, can be built from the net pi-electron charges. The results show that the combined solvaton/CS INDO scheme is able to describe rather well the modifications of both geometries and absorption spectra of merocyanines as a function of the medium polarity. The advantages of the proposed approach with respect to other semi-empirical procedures are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.

1997 - On the character tables of finite point groups [Articolo su rivista]
Baraldi, Ivan; Vanossi, Davide

A new method for the "complete" construction of the character tables of all the finite point groups is presented. In an explicit way, only the character tables of the five nonisomorphic rotational groups (Cn, Dn, T, O, I) have been derived, because those of the remaining groups can be obtained through isomorphisms and direct products. The characters of the 1-dimensional irreducible representations are calculated using the generators and the defining relations of the groups, while for the multi-dimensional ones explicit formulas are given.

1997 - The stable and photochemical isomers of some merocyanines: A H-1 NMR and theoretical CS INDO study of the structures and electronic spectra [Articolo su rivista]
Baraldi, Ivan; S., Ghelli; Z. A., Krasnaya; Momicchioli, Fabio; A. S., Tatikolov; Vanossi, Davide; Ponterini, Glauco

The conformations and electron distributions of four merocyanines were investigated by H-1 NMR spectroscopy in low-polarity solvents and CS INDO calculations on the isolated molecules. The two approaches gave consistent results. Some information about the torsional dynamics of these dyes in the ground state was obtained from a rough H-1 NMR line shape analysis, and H-1 NMR experiments performed on photolysed samples made it possible to identify the conformations of the longest-lived photoisomers of three compounds and to provide an estimate of their lifetimes. The effects of an increase in the solvent polarity on the structures and absorption spectra of the four dyes were investigated by, respectively, measuring their H-1 NMR spectra in three more solvents with increasing dielectric constants, and CS INDO CI calculations of the electronic spectra, including solvent shift effects, evaluated within the classical solvaton model. All the results related to the medium polarity effects could be interpreted according to the generally accepted description of the merocyanine chromophore as the resonance hybrid of a neutral and a charge-separated form.

1995 - Electronic spectrum of porphyrins. CS INDO CI study [Articolo su rivista]
Baraldi, Ivan; A., Carnevali; Ponterini, Glauco; Vanossi, Davide

The electronic spectra of porphin, chlorin and their magnesium derivatives have been studied quantum-mechanicallywith the CS INDO CI method. Both the SOPS, and T,-T, one-photon absorption spectra have been analyzed togetherwith the correlation effects. The main result regards the T-T spectrum of porphin. To obtain a correct description of thecomplete spectrum it is necessary to take account of the correlation effects. Gouterman’s model of four orbitals is toosimple to describe the region of the Soret band.