Vibrational dynamics of a perylene-perylenediimide donor-acceptor dyad probed with femtosecond stimulated Raman spectroscopy

The ultrafast vibrational dynamics of the photoinduced charge-transfer reaction between perylene (Per) and perylene-3,4:9,10-bis(dicarboximide) (PDI) were investigated using femtosecond stimulated Raman spectroscopy (FSRS). Specifically probing the structural dynamics of PDI following its selective photoexcitation in a covalently linked dyad reveals vibrational modes uniquely characteristic to the PDI lowest excited singlet state and radical anion between 1000 and 1700 cm ^-1. A comparison of these vibrations to those of the ground state reveals the appearance of new ^1*PDI and PDI ^-• stretching modes in the dyad at 1593 and 1588 cm ^-1, respectively. DFT calculations reveal that these vibrations are parallel to the long axis of PDI and thus then may be integral to the charge separation reaction. The ability to differentiate excited state from radical anion vibrational modes allows the evaluation of the influence of specific modes on the charge transfer dynamics in donor-bridge-acceptor systems based on PDI molecular constructs.