G. Bartocci

Fluorescence analysis of trans-2-styrylnaphthalene in solution

Abstract Trans -2-styrylnaphthalene (2-StN) in n -hexane solution at 20°C is a mixture of two species, whose absorption spectra and fluorescence quantum yields ( q ), lifetimes (τ) and spectra ( F ) are determined, and whose relative abundances are estimated. The major species (≈0.67) has τ = 27.6 ns, q = 0.66 and a structured F . The minor species (≈0.33) has τ = 5.2 ns, q = 0.73 and a more diffuse F . These results are obtained by a general method of determining the fluorescence and absorption properties of two species from those of the mixture.

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.

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.

Photoisomerization mechanism and conformational equilibria of styrylnaphthalenes. A study based on photophysical properties and molecular-orbital calculations

The photophysical properties of trans-1- and trans-2-styrylnaphthalene (1-StN and 2-StN) have been studied. The conformational isomers of 2-StN have been reinvestigated by measuring the solvent and temperature effects and by m.o. calculations (CNDO/S) on the nature of the lowest excited states. Fluorescence quantum yields and lifetimes and kinetic parameters of decay have been obtained for the two conformers in n-hexane and ethanol. The energy difference between the two conformers and the activation energy for their trans→cis photoisomerization have been obtained from the temperature effect in both solvents. Evidence has been obtained for two photoisomerization mechanisms, a triplet mechanism predominant below room temperature and a singlet mechanism favoured in ethanol and/or at higher temperatures.

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.

Fluorimetric and theoretical study of the rotamerism of trans-styrylanthracenes

Abstract The ground state conformational equilibrium of trans-n -styrylanthracenes ( n =1, 2, 9) has been studied by stationary and pulsed fluorimetric techniques in non-polar solvents. Parallel theoretical calculations of the potential energy curves for the internal rotation of the anthryl group have been carried out with a modified CS-INDO method. The behaviour of the mixture of almost isoenergetic rotamers of 2-styrylanthracene has been particularly investigated to add some complementary and revised information to that recently reported in the literature.

Effect of temperature on the photophysical properties of styrylpyridines

The effect of temperature on the photophysical parameters (fluorescence quantum yield and lifetime) of the three isomeric trans-styrylpyridines has been studied in 3-methylpentane and a mixture of ethyl ether, isopentane and ethanol in the proportions 5 : 5 : 2 (v/v). The Arrhenius parameters (A= 1012–1013 S–1, ΔE= 1–3 kcal mol–1) indicate that the activated twisting around the double bond, which is dominant at room temperature, takes place in the singlet manifold, as in stilbene. At low temperature (below 100–150 K, depending on nitrogen position) the S1 [graphic omitted] T1 process becomes important and, because of differences in the relative weight of intersystem crossing, different photophysical behaviour is exhibited by the three isomers. The observed dependence of the lifetimes on the excitation wavelength gives hints about the presence of conformational isomers.

Decay pathways of the first excited singlet state of cis-1-styrylpyrene

Abstract The cis isomer of 1-styrylpyrene displays an unusual fluorescence emission in fluid media at room temperature. The photophysical properties of the fluorescent state were studied in non-polar solvents in a large temperature range. The fluorimetric and photochemical results evidenced a substantial contribution of an adiabatic singlet mechanism to the cis→trans photoisomerization.

Fluorescence of conformational isomers of trans 2-styryl-naphthalene. Further evidence for different radiative decay parameters of the two rotamers

Abstract The fluorescence properties of the conformer mixture of trans 2-styrylnaphthalene in inert solvents have been reinvestigated. The temperature, excitation wavelength and oxygen quenching effects on fluorescence spectra, quantum yields and decay profiles have been carefully examined. The results obtained give further confirmation to our previous fluorescence analysis thus supporting the assignment of different radiative decay parameters to the two conformers.