Conformational equilibria in EE-1,3-di-(3'-thienylethenyl)benzene: One-way adiabatic interconversion of rotamers in S1 and their excited state properties

A refined procedure of the kinetic fluorescence analysis (KFA) (accounting for the lack of isoemissive points in the fluorescence spectra) has been applied to study the rotamerism of the EE isomer of 1,3-di-(3'-thienylethenyl)benzene [(3TE)2B]. The effect of the excitation wavelength and temperature on the fluorescence spectra, quantum yields and decay profiles allowed the excited state properties (radiative and reactive) of the two most abundant conformers (rotamers), implying the simultaneous, restricted rotation around the quasi-single bonds of the ethenic bridges with both the central ring and the 3'-thienyl groups, to be obtained. At high temperature (above 320 K), another activated process of the lowest excited singlet state, implying the interconversion of the short-lived rotamer into the longer-lived one, is operative. The EE → EZ photoisomerization was also investigated as a function of temperature. The present results lead to a complete and quantitative description of the conformational equilibrium including the radiative and radiationless relaxation rate parameters of the lowest excited states of the two rotamers. A simultaneous singlet and triplet mechanism for the geometrical photoisomerization, with an involvement of the former above room temperature only, seems to reasonably account for the experimental results. The role played by the rotamers in the photoreactive behaviour of (3TE)2B is also discussed