Effect of pyridyl and thienyl groups on the excited state properties of stilbene-like molecules

The relaxation properties of the lowest excited states of singlet and triplet multiplicity of sometrans-stilbene-like molecules with two-ring or three-ring polycyclic groups containing nitrogen and sulphur heteroatoms have been studied by stationary and pulsed fluorimetric and photolytic techniques. The kinetic parameters of the competitive relaxation processes, derived from the measured quantum yields and lifetimes, have been compared with the corresponding values of the hydrocarbon analogues to have information on the heteroatom effect on the photoisomerization mechanism. Then, π* states introduced by the nitrogen atom can play a deactivating role, mainly through vibronic coupling and consequent internal conversion to the ground state. The heavy atom introduced with the thiophene ring favours the spin-orbit coupling and then the rate of intersystem crossing to the triplet manifold. In any case, the position of the heteroatom is an important factor affecting the nature of the lowest excited states and then the radiative and reactive behaviour of these compounds. The presence of polycondensed rings leads to a decrease in the quantum yield of photoisomerization to thecis isomers, as reported for the corresponding hydrocarbons, owing to an increase in the torsional energy barrier. The latter is drastically reduced in the polar solvents.