Intramolecular Energy Transfer in Fullerene Pyrazine Dyads

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 red-shifted with respect to their strong UV ground-state absorption and formed with overall quantum yields (Φ) 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 × 10 9 M -1 s -1 ( 6 ) to 1.1 × 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 ( 1 O 2 ) with quenching rate constants for 1 − 3 of (1−1.5) × 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) × 10 7 M -1 s -1 .