Photophysics of trans-stilbene analogues: Indolo[3,2-b]indole and its heterosubstituted sulfur and selenium derivatives

The rates of radiative and nonradiative depopulation channels of the lowest excited singlet and triplet states have been determined for indolo[3,2-b]indole and its derivatives in which one or two nitrogen atoms have been replaced by sulfur or selenium, Introduction of the heavy atom leads to activation of both S-0<--S-1 internal conversion and T-1<--S-1 intersystem crossing, The nonradiative rates of T-1 depopulation are affected much more strongly than the radiative ones, The results are interpreted in terms of a trans-stilbene-like character of excited indoloindoles containing heavy heteroatoms, The evidence for the geometry change upon excitation is also provided by the results of luminescence anisotropy measurements.