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

The wavelength dependence of photosubstitution, photoinduced electron trans fer, and the time-resolved spectra of Cr(CNPh)(6), a compound having low-ly ing MLCT states, were investigated. Photosubstitution quantum yields increa se with increasing excitation energy while photoinduced electron transfer q uantum yields decrease with increasing excitation energy. At the lowest exc itation energy used (532 nm, or is 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 ex cited 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 mu s. This implies a simple ligand dissociation followed by recombinatio n. 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) e xhibits two photochemical mechanisms: at high excitation energy, a simple c harge neutral dissociation occurs; at low energy, it is proposed that a phe nylisocyanide radical anion dissociates, forming a radical pair that is res ponsible for the observed substitution and electron transfer reactivity, an d the complicated nanosecond kinetics. The primary processes for both react ions occur in less than 20 ps.