Serena Ciorba

Photobehavior of the geometrical isomers of two 1,4-distyrylbenzene analogues with side groups of different electron donor/acceptor character.

The photobehavior of two 1,4-distyrylbenzene analogues where the central benzene ring is asymmetrically substituted with a pyrid-4-ylethenyl group at one side and thien-2-ylethenyl or a p-nitrostyryl group at the other side, has been studied in two solvents at room temperature. The four geometrical isomers (EE, ZE, EZ, and ZZ) of each compound were separated by the combined use of HPLC and spectrophotometric techniques. The radiative/reactive competition in their excited state relaxation was particularly examined: the diabatic/adiabatic contributions were estimated and a reasonable interpretation of the photoisomerization mechanism was proposed. The role of the conformational isomers was also investigated by measured and computed spectral data. Since the different electron donor/acceptor character of the side groups of these molecules can induce charge transfer phenomena that can affect the relaxation pathways of their excited states, the photobehavior was compared in inert and polar solvents to clarify the role of the intramolecular charge transfer. The latter was found to affect markedly the relaxation properties and to induce interesting fluorosolvatochromic effects, particularly in the p-nitro derivative. The participation of the triplet state in the reaction mechanism of the latter was also investigated by flash photolysis and sensitized experiments.

Fluorescence/photoisomerization competition in trans-aza-1,2-diarylethenes

The effect of the presence and position of the nitrogen heteroatom on the photobehaviour of a series of symmetric and asymmetric 1,2-diarylethenes, where one or both the one-, two- or three-membered aryl groups, linked to the ethene bridge, contain(s) nitrogen heteroatoms, has been investigated by fluorimetric and photochemical techniques. The yield of the radiative pathway is generally reduced with respect to the corresponding hydrocarbons, due to a smaller energy barrier for trans→cis (E→Z) photoisomerization in the singlet state, which often can be slightly favoured in the aza-compounds. All compounds investigated exist in solution as mixtures of two or more conformers, which can have different spectral and kinetic properties. Attempts to separate the intrinsic properties by selective photoexcitation, at a wavelength where the light is mainly absorbed by one conformer, allowed in some favourable cases the spectra and decay times of the largely prevalent (quasi-pure) conformer to be obtained. In two cases, the formation of intramolecular (hydrogen-bond-type) interactions between the nitrogen atom and the nearest hydrogen atom of the ethene bridge (proved by NMR measurements and confirmed by theoretical calculations) were found to stabilize a specific conformer thus affecting a selective relaxation of the excited molecule.

Adiabatic pathways in the conformational and geometrical photoisomerizations of the 1,2-distyrylbenzene isomers.

The photophysical behavior of the geometrical isomers of 1,2-distyrylbenzene (1,2-DSB) has been investigated as a function of temperature by stationary and pulsed fluorimetric techniques in a nonpolar solvent. The temperature effect on the spectral properties, fluorescence quantum yields, and decay profiles allowed the role played by the conformational isomers of these molecules in the relaxation processes of the lowest excited states to be clarified. In the case of the EE geometrical isomer, a conformer (or rotamer) re-equilibration, implying an adiabatic interconversion during the lifetime in the S(1) state, was found to be operative. The deactivation channels of the excited cis isomers (ZE and ZZ) markedly depend on the conformational geometry, which is responsible for the adiabatic (rotamer-specific) cis --> trans photoisomerization. The radiative and radiationless relaxation rate parameters of the isomers and conformers of 1,2-DSB, derived from the experimental and calculation results, allowed a complete and reasonably quantitative description of their behavior in the S(1) state.