Anna Spalletti

Intramolecular Charge Transfer of Push–Pull Pyridinium Salts in the Singlet Manifold

The solvent effect on the photophysical and photochemical properties of the iodides of three trans (E) isomers of 2-D-vinyl,1-methylpyridinium, where D is a donor group (4-dimethylaminophenyl, 3,4,5-trimethoxyphenyl and 1-pyrenyl), was studied by stationary and transient absorption techniques. The results obtained allowed the negative solvatochromism and relaxation pathways of the excited states in the singlet manifold to be reasonably interpreted. Resorting to ultrafast absorption techniques and DFT calculations allowed information on the excited state dynamics and the role of the solvent-controlled intramolecular charge transfer (ICT) processes to be obtained. The structure-dependent excited state dynamics in nonpolar solvents, where the ICT is slower than solvent rearrangement, and in polar solvents, where an opposite situation is operative, was thus explained. The push-pull character of the three compounds, particularly the anilino-derivative, suggests their potential application in optoelectronics.

Role of internal conversion on the excited state properties of trans-styrylpyridines

Abstract A theoretical and photophysical study of the three isomeric trans- n -styrylpyridines ( n = 2,3,4) has been carried out to evaluate the role of internal conversion (IC) on their deactivation pathways from the singlet manifold. Calculations by the QCFF/PI and CNDO/S methods and application of current theories of radiationless transitions allowed the rate constants for IC to be evaluated. The fluorescence lifetimes measured in the picosecond region and the fluorescence and trans → cis photoisomerization quantum yields from previous works allowed the rate constants for the non-radiative and non-reactive deactivation processes to be calculated. The experimental results were in a reasonable agreement with the theoretical predictions on the role of IC (markedly more important for the 2 and 4 derivatives) and confirmed that intersystem crossing is a low efficiency process for these azastilbenes. On the basis of the temperature and the solvent viscosity effects on the electronic spectra, an interpretation of the excitation energy effect on the quantum yields of the 3 isomer was attempted.

Ultrafast photoinduced intramolecular charge transfer in push–pull distyryl furan and benzofuran: solvent and molecular structure effect

The excited state deactivation pathways of push-pull distyryl furan and benzofuran derivatives in several organic solvents were investigated in detail by using time-resolved transient absorption and fluorescence spectroscopies, with nano- and femto-second time resolution. Solvent polarity was found to play a key role in determining the efficiencies of fluorescence, intersystem crossing and internal conversion. The triplet yield gradually decreased, while the internal conversion increased upon increasing the solvent dielectric constant. However the fluorescence showed a different solvent polarity effect in the low and high solvent polarity region, with a reversal of the trend of fluorescence properties (quantum yield and lifetime). This fact points to an emitting state of a different nature (smaller and larger dipole moments) in the two cases, as also suggested by the huge fluorosolvatochromism. In fact the ultrafast spectroscopic investigation evidenced the presence of two transients characterized by peculiar spectral shapes assigned to a locally excited (LE) and a charge transfer (CT) state. In the more polar solvents the CT state was the longer lived, fluorescent one and an intramolecular charge transfer process was found to be operative and to become faster (up to ∼200-250 fs) in the higher polarity media. On the contrary, distyrylfuran, which exhibits the same molecular skeleton without the push-pull character showed a similar excited state dynamics in solvents of different polarities.

Effect of the nature of the aromatic groups on the lowest excited states of trans-1,2-diarylethenes

The photophysical and photochemical behaviour of a series of diarylethenes has been investigated experimentally. For the same compounds the potential-energy curves of trans→cis photoisomerisations and the ethenic bond orders in the S1 and T1 states have been calculated. The purpose of the study is to assess which properties of the aryls govern the photophysical behaviour of the diarylethenes. It is found that the relative energies of the La and Lb states of the larger aryl with respect to the ethene Bu state determine to a large extent the photophysical and photochemical properties of the diarylethenes.

Excited state behaviour of some trans-stilbene analogues bearing thiophene rings

Abstract The effect of replacing the phenyl group(s) of trans -stilbene with thienyl groups, or polycyclic groups containing a thienyl moiety, on the relaxation properties of the lowest excited states was studied by fluorometric, photochemical and laser flash photolysis techniques, as well as by theoretical calculations, for four trans compounds in non-polar and polar solvents. In some cases, a larger contribution of intersystem crossing and, consequently, a triplet mechanism to trans → cis photoisomerization, with respect to the parent hydrocarbons, was found. Although the compound with a single thienyl group, 2-styrylthiophene, shows reactive relaxation in the singlet manifold as in the case of stilbene, the presence of two heteroaromatic rings in di(2-thienyl)ethene enhances the spin-orbit coupling, thus leading to a mixed singlettriplet mechanism in non-polar solvents. The presence of polycondensed rings in dibenzothienylethene and thienyl-naphthothienylethene reduces the isomerization yield due to an increase in the torsional barrier for twisting in the singlet manifold. Therefore these compounds deactivate mainly through fluorescence emission and intersystem crossing, which leads to a predominant triplet mechanism for trans → cis photoisomerization. Polar solvents reduce the activation barrier to twisting, thus favouring isomerization in the singlet manifold.

Comprehensive Photophysical Behaviour of Ethynyl Fluorenes and Ethynyl Anthracenes Investigated by Fast and Ultrafast Time-Resolved Spectroscopy

Detailed investigations by time-resolved transient absorption and fluorescence spectroscopies with nano- and femtosecond time resolutions are carried out with the aim of characterising the lowest excited singlet and triplet states of three ethynyl fluorenes (1-3) and three ethynyl anthracenes (4-6) in solvents of different polarity. The solvent is found to modify the deactivation pathways of the lowest excited singlet state of compounds 1-4, thus changing their fluorescence, intersystem crossing and internal conversion efficiencies. The fluorescence and triplet yields gradually decrease, while the internal conversion quantum yield increases upon increasing the solvent dielectric constant. These experimental results, coupled with the marked fluorosolvatochromic effect, point to the involvement of an emitting state with a charge-transfer (CT) character, strongly stabilised by polar solvents. This is proved by ultrafast spectroscopic studies in which two transients, distinguished by characteristic spectral shapes assigned to locally excited (LE) and CT states, are detected, the CT state being the longer lived and fluorescent one in highly polar solvents. The intramolecular LE→CT process, operative in highly polar media, becomes particularly fast (up to ≈300 fs) in the case of the NO(2) derivative 1. No push-pull character is found for 5 and 6, which exhibit different photophysical behaviour; indeed, the solvent polarity does not modify significantly the dynamics of the lowest excited singlet states. Quantum mechanical calculations at the TDDFT level are also used to determine the state order and nature of the lowest excited singlet and triplet states and to rationalise the different photophysical behaviour of fluorine and anthracene derivatives, particularly concerning the intersystem crossing process.

Photophysics and photochemistry of 2,6-distyrylpyridine and some heteroanalogues

The photophysical and photochemical properties of some symmetric (E,E)-2,6-di(arylvinyl) derivatives of pyridine, where the side aryl groups were phenyl, pyridyl, furanyl, thienyl and thiazolyl were investigated. The conformational equilibria between the two more abundant rotamers and their role in the kinetic competition between the radiative and reactive (E,E→E,Z) relaxation channels of their lowest excited singlet state are described and compared with those of the parent hydrocarbon, 1,3-distyrylbenzene. The effect of intramolecular hydrogen bonds between the nitrogen atom and the ethenic hydrogens on the nature and properties of the S1 state is discussed.

A photophysical and theoretical study of styrylanthracenes

The photophysical properties of the three isomeric trans-n-styrylanthracenes (n-StAs, with n= 1, 2, 9), whose lowest excited states are expected to display a prevalent aromatic character, have been studied in different solvents. Fluorescence quantum yields and lifetimes have been measured as a function of temperature in a wide temperature range. All trans compounds, including the two rotamers of 2-StA, are relatively highly fluorescent and do not photoisomerize to the cis compounds. Semiempirical MO + Cl Calculations have been performed to assess the shape of the potential-energy surfaces in the lowest excited states of the compounds investigated. These calculations indicate that S1 and T1 in all the n-StAs have a sizeable barrier on the torsional coordinate from trans to cis. The barrier for the cis- to trans- process is expected to be lower. The combined results of the experimental and theoretical studies offer a clear description of the excited-state behaviour of trans StAs.

Evidence of adiabatic channels in the singlet photoisomerization of cis-1,2-diarylethenes: a fluorimetric study

Abstract The deactivation of the first excited singlet state (S1) of some cis isomers of 1,2-diarylethenes was studied by fluorimetric and photochemical measurements. When at least one of the aryl groups is a large polycyclic group of low S1 energy (as in the case of pyrenyl and anthryl derivatives) the minimum at 90° (typical of stilbene) in the potential energy curve as a function of the twisting coordinate may disappear. This leads to an unusual fluorescence emission at room temperature and favours the adiabatic 1cis* → 1trans* isomerization mechanism instead of the well-known diabatic pathway, which implies internal conversion to the ground state from the 1perp* configuration.

Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

Three (donor-π-acceptor)(+) systems with a methyl pyridinium or quinolinium as the electron-deficient group, a dimethyl amino as the electron-donor group, and an ethylene or butadiene group as the spacer have been investigated in a joint spectroscopic and TD-DFT computational study. A negative solvatochromism has been revealed in the absorption spectra, which implies a solution color change, and interpreted by considering the variation in the permanent dipole moment modulus and orientation upon photoexcitation. The fluorescence efficiency decreases upon increasing solvent polarity, in agreement with the excited-state optimized geometries (planar in low-polarity media and twisted in high-polarity media). Femtosecond transient absorption has revealed the occurrence of a fast photoinduced intramolecular charge transfer (ICT) and the molecular factors that determine an efficient ICT. Considering the crucial role of the ICT in tuning the nonlinear optical (NLO) properties, these compounds can be considered promising NLO materials.