Charge-Transfer Complexes: Halogen-Doped Anthracene as a Case of Study.

Charge transfer (CT) crystals exhibit unique electronic and magnetic properties with interesting applications. We present a rational and easy guide which allows to foresee the effective charge transfer co-crystal production and that is based on the comparison of the frontier molecular orbital (MO) energies of a donor and acceptor couple. For the sake of comparison, theoretical calculations have been carried out by using the cheap and fast PM6 semiempirical Hamiltonian and pure HF/cc-pVTZ level of the theory. The results are then compared with experimental results obtained both by chemical (bromine and iodine were used as the acceptor) and electrochemical doping (exploiting an original experimental set-up by this laboratory: the electrochemical transistor). Infra-red vibrational experimental results and theoretically calculated spectra are compared to assess both the effective donor-acceptor (D/A) charge-transfer and transport mechanism (giant IRAV polaron signature). XPS spectra have been collected (carbon (1 s) and iodine (3d5/2)) signals, yielding further evidence of the effective formation of the CT anthracene:iodine complex.

Growth Dynamics of Ultrathin Films of Benzo[1,2-b:4,5-b']dithiophene Derivatives on Au(111): A Photoelectron Spectroscopy Investigation.

Ultrathin films of a stereoisomeric mixture of benzo[1,2-b:4,5-b']dithiophene derivatives were grown by thermal evaporation in vacuum on Au(111), and they were studied in situ by photoelectron spectroscopy. X-ray photons from a non-monochromatic Mg Kα conventional X-ray source and UV photons from a He I discharge lamp equipped with a linear polarizer were used. He I photoemission results were compared with density functional theory (DFT) calculations: density of states (DOS) and 3D molecular orbital density distribution. Au 4f, C 1s, O 1s, and S 2p core-level components suggest a surface rearrangement as a function of film nominal thickness, with the variation of the molecular orientation, from flat-laying at the initial deposition to tilted toward the surface normal at coverages exceeding 2 nm. Eventually, the DFT results were exploited in assigning of the valence band experimental structures. Moreover, polarization-dependent photoemission confirmed the tilted arrangement of the molecules, starting at 2 nm. A variation of the work function of 1.4 eV with respect to the clean substrate was measured, together with a valence band offset of 1.3 eV between the organic layer and gold.

The Fundamental and Underrated Role of the Base Electrolyte in the Polymerization Mechanism. The Resorcinol Case Study.

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.

Excited-state intramolecular proton transfer in a bioactive flavonoid provides fluorescence observables for recognizing its engagement with target proteins.

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.

Optical and photophysical properties of anisole- and cyanobenzene-substituted perylene diimides.

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 Hückel 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 42-44° 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 (59°) 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.

An Integrated Experimental/Theoretical Study of Structurally Related Poly-Thiophenes Used in Photovoltaic Systems.

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 ( J e , 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.

A novel synthetic strategy for magnetite-type compounds. A combined experimental and DFT-computational study.

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.

Linear and nonlinear optical properties of V-shaped D-π-A-π-D chromophores: effects of the incorporation of aromatic rings in the polyenic π-bridges of open-chain ketocyanines.

Following previous studies on α and ß polarizabilities of ketocyanines, a subgroup of D-π-A-π-D quadrupolar chromophores with moderately V-shaped structure, the present work analyses the effects of modifying the π-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) π-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, α and ß polarizabilities and two-photon absorption (TPA) cross-sections (σTP). 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 π-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.

Chemical asymmetry and α and ß polarizabilities of D-A-D' chromophores: a three-state-model and TDDFT-SOS analysis of a penta-heptamethine ketocyanine.

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 α and ß polarizabilities of a penta-heptamethine 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 excited states, <α> and ß(vec) have been evaluated according to the SOS approach. The performances of the model 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 three ketocyanines 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 π→π* excitations). However, while only two excited states were necessary for the symmetric compounds, for KC2,3 a value of ß(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 ß(vec). Moreover, both descriptions agree in predicting that KC2,3 features <α> and ß(vec) values in between those of KC2 and KC3.

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.

The electronic structure, spectra and linear and second-order polarizabilities of two symmetric ketocyanines, which are prototypic examples of D-A-D chromophores, have been investigated with two different toolsets: (i) the so-called 'essential-state model', here comprising three states, the ground and two lowest excited (1)ππ* states, has been adapted for these non-centrosymmetric, yet symmetric compounds to determine their permanent electric dipole moments, polarizabilities and first hyperpolarizabilities making use of experimental transition energies and moments; (ii) extensive TDDFT calculations have provided ground-state conformational results consistent with NMR-derived structural information, energies and dipole moments of up to 20 lowest-lying electronic states as well as, within the sum-over-states (SOS) scheme, the most relevant components of the polarizabilities and first hyperpolarizabilities. The two levels of description form consistent pictures of the ketocyanine excited states that provide the most relevant contributions to hyperpolarizabilities: extension of the SOS set beyond the three states of the basic model left unchanged (within ∼10%) the calculated vector component of the second-order polarizability tensor along the direction of the ground-state dipole moment (ß(y)). Both approaches indicate that these D-A-D compounds, in spite of their quasi-linear structure, reminiscent of that of centrosymmetric quadrupolar chromophores, feature significant second-order molecular polarizabilities. These rapidly increase with the length of the polyenic bridges in the chromophores. About half of the total value of ß(y) is predicted to come from the three-level-term part, ß(y,3), most of which derives from the contribution involving the three electronic states of the essential-state model.

Dissociation dynamics of asymmetric alkynyl(aryl)iodonium radicals: an ab initio DRC approach to predict the surface functionalization selectivity.

The dissociation process of neutral open-shell [4-F-(C(6)H(4))-I-C≡C-(CH(2))(4)-Cl] and [4-NO(2)-(C(6)H(4))-I-C≡C-(CH(2))(4)-Cl] asymmetric iodonium radicals was studied theoretically. Vertical electron affinities and DRC (dynamic reaction coordinate) results were obtained and compared with experimental evidence. In particular, the fluorine and nitro substituent groups were selected because of (i) their opposite electron-withdrawing/electron-donating effects and (ii) experimental evidence that the grafting ability, in terms of alkynyl/aryl grafting ratio, increases with decreasing electron-withdrawing nature of the para-position substituent on the phenyl ring. DRC results show that the dissociation dynamics of the iodine-alkynyl carbon bond, for the nitro-substituted iodonium, occurs on a longer time scale than that of the fluorine-substituted iodonium. This finding is in agreement with the overall experimental results.

Wavy graphene sheets from electrochemical sewing of corannulene.

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.

Optoelectronic Properties of A-π-D-π-A Thiophene-Based Materials with a Dithienosilole Core: An Experimental and Theoretical Study.

Two A-π-D-π-A thiophene-based small molecules with a central dithienosilole core and dicyanovinyl (DCV) end groups were synthesized. These compounds differ only by the presence of alkyl and alkylsulfanyl chains, respectively, on the thiophene beta positions. 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 good stability both in p and n-doping states, which make them suitable for optoelectronic applications. In both 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.

Redox-Active Ferrocene grafted on H-Terminated Si(111): Electrochemical Characterization of the Charge Transport Mechanism and Dynamics.

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.

First- and second-order polarizabilities of simple merocyanines. An experimental and theoretical reassessment of the two-level model.

Taking four merocyanines [(CH3)2N-(CHCH)n-C(CH 3)O; n = 1-4] (Mc1-4) as test D-A systems, we performed a close experimental and theoretical examination of the two level model with reference to its ability to provide correct predictions of both absolute values and dependence on the conjugation path length of first- and second-order molecular polarizabilities. By (1)H NMR spectroscopy merocyanines Mc1-4 were found to be approximately 1:1 mixtures of two planar conformers with cis and trans arrangements of the C(CH 3)O electron-acceptor group and all trans structure of the polyene like fragment. The degree of bond length alternancy (BLA) in the -(CHCH)n- fragment, was quantified by extensive full geometry optimizations at both semiempirical and ab initio level. DFT (6-31G**/B3LYP) optimized geometries were considered to be most reliable and were used for calculations of the excited-state properties. The applicability of the two level model, reducing the general sum-over-states (SOS) expansion to only one term involving the ground state (g) and the lowest-lying (1)(pipi*) CT state (e), was checked by analysis of fluorescence and near UV absorption spectra. Measurements of the basic two-level model quantities ( Ege, microge and Deltamicro(eg)), by which the dominant components of alpha and beta tensors are expressed (alpha XX , beta XXX , X identical with long molecular axis), were designed to give approximate free-molecule values. It is proposed, in particular, an adjustment of the solvatochromic method for the determination of Deltamicro(eg), 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 beta XXX 's matching well in both value and n-dependence with EFISH data reported in the literature for similar merocyanines. For the fluorescent Mc4, the results were qualitatively well reproduced by an approach, which combines absorption and fluorescence solvent shifts. All the measured quantities were calculated for both trans and cis Mc1-4 by three semiempirical INDO-based approaches aiming at evaluating the performances of different integral parametrizations and CI extensions: ZINDO/S, CS INDO SCI, CS INDO SDCI. In all cases, alpha XX and beta XXX were found to rise proportionally to about n (1.3) and n (2), respectively, in qualitatively good agreement with the experimental values. As to the absolute values, however, experimental alpha XX 's and beta XXX 's were best reproduced by CS INDO SDCI combining Ohno-Klopman parametrization and CI including both single and double excitations. The validity of the two-level model was checked by comparison with converged SOS calculations for the longest chain merocyanine (Mc4) and finite field calculations of linear polarizabilities for all of the four dyes (Mc1-4).

Ferrocene Molecular Architectures Grafted on Si(111): A Theoretical Calculation of the Standard Oxidation Potentials and Electron Transfer Rate Constant.

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 E T , are determined following calculation of the electron transfer matrix element, V R P , 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 E T = 77.8 s - 1 and k E T = 1.3 × 10 - 9 s - 1 , in the case of the short and long organic-spacer, respectively.

An Integrated Theoretical/Experimental Study of Quinolinic-Isoquinolinic Derivatives Acting as Reversible Electrochromes.

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 π 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-methylisoquinolinium-1-yl)-vinyl)-1,2,3,4-tetrahydroacri-dinium trifluoromethanesulfonate derivative shows the most interesting properties as an electrochromophore.

Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study.

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 CI level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the 1H-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 (piH, piL*) character in all compounds except for E-4,4'-dipyridylethene, where S1 has (n, pi*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the 1(n, piL*) and 1(piH, piL*) 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 1A(g)- excited state is above the spectral region analyzed.

Experimental and theoretical study of the p- and n-doped states of alkylsulfanyl octithiophenes.

The charge-transfer and spectral properties of two octithiophenes, namely 4',4''',3'(v),3(v)'-tetra(octylsulfanyl)-2,2':5',2'':5'',2''':5''',2'(v):5'(v),2(v):5(v),2(v)':5(v)',2(v)''-octithiophene and 4,3'',4(v),4(v)''-tetrabromo-4',4''',3'(v),3(v)'-tetra[(R)-2-methylbutylsulfanyl]-2,2':5',2'':5'',2''':5''',2'(v):5'(v),2(v):5(v),2(v)':5(v)',2(v)''-octithiophene, OT1 and OT2, respectively, are characterized by cyclic voltammetry and spectroelectrochemistry under ultradry conditions. The analysis of the voltammetric results shows the formation of up to the dication for both OT1 and OT2 and up to the tetraanion (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 at the 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 oscillator strength data obtained by time-dependent B3LYP/6-31G* calculations. Theoretical redox potentials and optical transitions properties are calculated including "the solvent effect" within the CPCM model. The consistency obtained 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.

Electronic structure and photochemistry of squaraine dyes: basic theoretical analysis and direct detection of the photoisomer of a symmetrical squarylium cyanine.

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 90[degree] 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 fluorescence analysis, has been directly monitored by laser flash 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.