Excitation energies of terthiophene and its dioxide derivative:: a first-principles study

The excitation spectra of terthiophene and terthiophene-S,S-dioxide are investigated by time-dependent density-functional theory (TD-DFT) calculated data are compared with recent experimental results. The second singlet-triplet excitation energy (T-2) is calculated above the first singlet-singlet (S-1) one. In terthiophene-S,S-dioxide the formation of a bonding interaction in the lowest unoccupied molecular orbital decreases its kinetic energy and explains the red-shift of the excitation spectrum. The inter-system crossing (ISC) rate in terthiophene-S, S-dioxide is expected to be lower than in terthiophene due to the increased T2-S1 energy gap, which is also found to not decrease with inter-ring torsion as in terthiophene. (C) 2001 Elsevier Science B.V. All rights reserved.