INFLUENCE OF CHAIN-LENGTH AND DERIVATIZATION ON THE LOWEST SINGLET AND TRIPLET-STATES AND INTERSYSTEM CROSSING IN OLIGOTHIOPHENES

On the basis of configuration interaction calculations, we first descr ibe the nature of the lowest singlet and triplet excited states in oli gothiophenes ranging in size from two to six rings. We calculate the v ertical excitation energies from the singlet ground state S-0 to the f irst one-photon allowed singlet excited state S-1 as well as the energ y difference between the ground state and the lowest triplet state T-1 . The computed transition energies are in very good agreement with the measured values and indicate a strong confinement of the lowest tripl et. We also uncover the nature of the higher-lying triplet excited sta te T-n that is coupled via a large oscillator strength to T-1. The evo lution with chain length of the T-1-T-n excitation energies compares w ell with the experimental evolution based on photoinduced absorption d ata. Next, we investigate the geometry relaxation phenomena occurring in the S-1 and T-1 states; more pronounced and localized bond-length d eformations are calculated in the triplet state than in the singlet, c onfirming the more localized character of T-1. We also analyze the inf luence on the lowest excited states of grafting electroactive end-grou ps on the conjugated path of terthiophene. Finally, the various mechan isms involved in the nonradiative decay of the singlet excitations are discussed, and results are presented as a guide toward the optimizati on of light emission efficiency in conjugated systems.