Photophysical properties of β-alkylated quater-, octa-, dodeca- and hexadecathiophenes

The order and deactivation of excited electronic states of beta-alkylated oligothiophenes consisting of n = 4, 8, 12 and 16 thiophene rings are investigated by absorption spectroscopy as well as by steady state and time resolved fluorescence spectroscopy. Transition energies in oligothiophenes are linearly dependent on reciprocal chain-length only for n less than or equal to 8. For n > 8, chain-length dependence is correctly described by calculations which rake alternating bond-lengths into account. beta-Alkylation leads to enhanced nonplanarity in the ground state and thus to blue-shifted absorption maxima. Due to the quinoid structure of the excited state, which leads to planarization, fluorescence maxima are not blue-shifted versus unsubstituted oligothiophenes. In solution, up to the longest chain under investigation, n = 16, the 1B(u) state is energetically below the 2A(g) state, as inferred from comparison of radiative rate constants of fluorescence and absorption according to the Strickler-Berg relationship. 1B(u) is also below 2A(g) in the solid state, as concluded from the coincidence of the O-O-transitions of absorption and fluorescence. (C) 1998 Elsevier Science B.V.