Charge transfer in covalently-linked porphyrin-donor complexes from picosecond transient absorption spectroscopy

Abstract

Octaalkyl free base porphyrins have been synthesized with covalently attached electron donors at a single meso position. Singlet-state lifetimes were determined from fluorescence quenching in three solvents and were found to be [le]1.5 ps for the N,N,N[prime],N[prime]-tetramethyl-1,4-phenylenediamine (TMPD) appended complex, 1.3 ns for the phenyl-TMPD-appended complex, and [ge]5 ns for the phenyl-appended, N,N-dimethylaniline (DMA) appended, and DMA-TMPD-appended porphyrin complexes in o-difluorobenzene. Single photon counting measurements and fluorescence quantum yield quenching studies of the phenyl-appended control and phenyl-TMPD charge-transfer complex gave 14 [+-] 3 and 2 [+-] 0.3 ns excited-state lifetimes, respectively. Fluorescence quantum yields of the TMPD and phenyl-TMPD porphyrins were higher in solvents with lower dielectric constants. Picosecond absorption spectroscopy was performed on molecules exhibiting fluorescence quenching to characterize the charge-transfer state and to determine the recombination kinetics. Conclusive evidence of charge transfer was obtained by observing the donor cation absorption at 606 nm for the TMPD and phenyl-TMPD porphyrins. 49 refs., 10 figs., 2 tabs.