Spin-orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in Anthracene-Perylenebisimide Compact Electron Donor/Acceptor Dyads and Triads and Photochemical Dianion Formation.

Abstract

Perylenebisimide (PBI)-anthracene (AN) donor-acceptor dyads/triads are prepared to investigate spin orbit charge-recombination induced intersystem crossing (SOCT-ISC). Molecular conformation is controlled by connecting of PBI unit to either 2- and 9- position of anthryl moiety. Steady state, time-resolved transient absorption and emission spectroscopies reveal that chromophore orientation, electronic coupling and dihedral angle between donor and acceptor exert significant effect on photophysical property. The dyad PBI-9-AN with orthogonal geometry shows weak ground state coupling and efficient ISC (Φ∆ = 86%) as compared to PBI-2-AN (Φ∆ = 57%), the dyad showing more coplanar geometry. With nanosecond transient absorption spectroscopy, long-lived PBI localized triplet state was observed (τT = 139 µs). Time-resolved electron paramagnetic resonance spectroscopy demonstrate that electron spin polarization (ESP) pattern of triplet state is sensitive to geometry and number of AN units attached to PBI. Reversible and stepwise generation of near IR-absorbing PBI radical anion (PBI-·) and dianion (PBI2-) was observed upon photoexcitation in presence of triethanolamine, and it was confirmed that selective photoexcitation at the near IR absorption bands of PBI-· is unable to produce PBI2-.