Nonlinear Optical Pulse Suppression via Ultrafast Photoinduced Electron Transfer in an Aggregated Perylene Diimide/Oligothiophene Molecular Triad

A donor acceptor donor triad material in which two quinquethiophene moieties are attached via nonconjugated, flexible bridges to the 1,7-positions (80% isomer) and 1,6-positions (20% isomer) of a perylene diimide (PDI-5T) has been synthesized, and its nonlinear suppression of nanosecond laser pulses in the 680-750 nm range has been studied. The kinetics of the photoinduced charge separation processes have been characterized using femtosecond transient pump probe spectroscopy. Excitation of either the quinquethiophene donor or perylene diimide acceptor leads to ultrafast (<700 fs) photoinduced charge separation, yielding quinquethiophene and perylene diimide radical ions that are strongly absorbing in the red-near-IR region. Despite the short lifetime (52 ps) of the charge-separated state, reasonably strong nonlinear suppression of nanosecond pulses, with figures-of-merit up to 14, has been realized with 4 mM solutions of PDI-5T. Although the radical ion absorption (RIA) is much stronger at 750 nm than that at 680 or 700 nm, the best optical suppression figures-of-merit were observed at 680 and 700 nm. Comparison of the optical parameters at these wavelengths suggests that the stronger ground-state absorption, due to aggregates of PDI-ST, is responsible for the enhanced figure-of-merit at the shorter wavelength.