Wavelength-dependent photochemistry in Cr(CNPh)6: a study of photosubstitution and photoinduced electron transfer using time-resolved spectroscopy.

Abstract

The wavelength dependence of photosubstitution, photoinduced electron transfer, and the time-resolved spectra of Cr(CNPh)6, a compound having low-lying MLCT states, were investigated. Photosubstitution quantum yields increase with increasing excitation energy while photoinduced electron transfer quantum yields decrease with increasing excitation energy. At the lowest excitation energy used (532 nm, or 18,800 cm(-1)), the quantum yields for both electron transfer and photosubstitution reach the same maximum value, 0.29. Picosecond time-resolved absorption spectra at 355 and 532 nm excitation wavelengths show two features: a bleach signal centered at 400 nm and an excited state absorption (ESA) in the 600 nm region. The ESA signal is much weaker for 532 nm excitations than for 355 nm excitations. Following a 355 nm flash, the bleach and ESA decay exponentially with the same lifetime of 23 micros. This implies a simple ligand dissociation followed by recombination. Bleach recovery kinetics after a 532 nm flash are more complicated: two or three exponential components are required to fit the data. Cr(CNPh)6 exhibits two photochemical mechanisms: at high excitation energy, a simple charge neutral dissociation occurs; at low energy, it is proposed that a phenylisocyanide radical anion dissociates, forming a radical pair that is responsible for the observed substitution and electron transfer reactivity, and the complicated nanosecond kinetics. The primary processes for both reactions occur in less than 20 ps.