PUB - Publikationen an der Universität Bielefeld

Excited-state properties of three different pyrazine derivatives 4-6 were probed by emission and transient absorption spectroscopy. They display emission maxima at 464 (4), 417 (5), and 515 nm (6) that are redshifted with respect to their strong UV ground-state absorption and formed with overall quantum yields (Phi) of 0.156, 0.22, and 0.13, respectively. Once photoexcited, these triplet excited pyrazines undergo rapid intermolecular energy transfer to a monofunctionalized fullerene derivative (7) with bimolecular rate constants ranging from 3.64 x 10(9) M-1 s(-1) (6) to 1.1 x 10(10) M-1 s(-1) (4). The product of these bimolecular energy-transfer reactions is in all cases the fullerene triplet excited state. Functionalization of pristine C-60 With the investigated pyrazine derivatives promotes the UV-vis absorption characteristics and, in turn, improves the light-harvesting efficiency of the resulting dyads 1-3 relative to pristine C-60. Photoexcitation of the pyrazine moieties in dyads 1-3 leads to the formation of their singlet excited states. In contrast to the pyrazine models, photoexcitation of dyad 1-3 is followed by rapid intramolecular deactivation processes of the latter via energy transfer to the fullerene ground state with half-lives between 37 and 100 ps. In turn, energy transfer transforms the short-lived and moderately redox-active singlet excited states of pyrazine into the highly reactive fullerene triplet excited state. The latter is found to produce effectively singlet oxygen (O-1(2)) with quenching rate constants for 1-3 of (1-1.5) x 10(9) M-1 s(-1). Similarly, reductive quenching of the triplet excited states in dyads 1-3 via electron transfer with diazabicyclooctane (DABCO) occurs with rate constants of (5.2-9.4) x 10(7) M-1 s(-1).