Excited state behaviour of some thio-analogues of 1,3-distyrylbenzene

Abstract

The excited state properties of 1, n-distyrylbenzene ( n = 2, 3 or 4) and hetero-analogues have being investigated in the authors’ laboratory by fluorimetry, nanosecond and femtosecond laser flash photolysis, and conventional photochemical techniques as well as by the help of theoretical calculations. The aim of the present article is to survey recent results of the study on the internal rotation around double bonds, i.e. cis– trans photoisomerization ( cis and trans are indicated in the text by Z and E, respectively) and single bonds (i.e. conformational equilibria) in some thio-analogues of E, E-1,3-distyrylbenzene where the central or side benzene rings were replaced by thiophene rings. Some peculiarities in the photobehaviour of these compounds will be thus evidenced. The main effect of the sulphur heteroatom in the side rings is a decrease in the radiative relaxation and an increase in the E, E → Z, E photoisomerization yield caused by decreased torsional barriers in S 1 and/or increased triplet yields, which opens the way to isomerization in the triplet manifold. On the other hand, when sulphur is in the central ring, the fluorescence decrease favours non-reactive and non-radiative deactivations. In the case of 2,5-distyrylthiophene with a central thiophene ring, the photobehaviour of the stereoisomers with one or both cis double bonds ( Z, E and Z, Z) showed diabatic/adiabatic formation of the E, E isomer in both lowest excited states of singlet and triplet multiplicities. In the case of Z, Z, the adiabatic process (directly from Z, Z * to E, E *) is characterized by a “one photon–two bonds” mechanism. The presence of conformational equilibria in solutions of distyrylbenzenes was found to play a role in their relaxation properties since different conformers can have different deactivation channels, specifically in the case of the 1,3 compound with side 3′-thienyl groups. The results obtained by selective irradiation, as a function of the excitation wavelength and temperature, and supported by theoretical calculations, evidenced uncommon rotamer interconversion during the short S 1 lifetime of this compound.