Ultrafast Tuning of Various Photochemical Pathways in Perylene-TCNQ Charge-Transfer Crystals

Excited-state dynamics in a strong charge-transfer (perylene)(3)-7,7,8,8-tetracyanoquinodimethane (P3T1) single-crystal system was studied by use of femtosecond transient absorption and upconversion time-resolved fluorescence techniques. Charge separation within 0.6-1.2 ps and subsequent recombination within 6.6-12 ps was demonstrated. Moreover, the excited-state dynamics in the P3T1 crystal was found to depend on excitation energies. Excitation of upper excited electronic states (4.96 eV) leads to efficient direct population of perylene triplet states via singlet fission. At lower excitation photon energies, enhancement of triplet state formation via intersystem crossing as a result of efficient spin-orbit charge transfer was observed. By varying the excitation wavelength, the performance of optoelectronic devices can be tuned, i.e., photon absorption, free carrier transfer, mobility, and recombination.