Mechanism of photon-gated persistent spectral hole burning in metal-tetrabenzoporphyrin/halomethane systems: Donor-acceptor electron transfer

Abstract

We have observed photon-gated persistent spectral hole burning for meso-tetra-p-tolyl(tetrabenzoporphyrinato)zinc or -magnesium molecules in the presence of several different halomethanes (chloroform, methylene chloride, and methylene bromide) in poly(methyl methacrylate) thin films at liquid helium temperatures. Depending upon the sample composition and porphyrin/halomethane concentration ratio, hole formation is up to 300 times more efficient when two photons are absorbed by the porphyrin as compared to one-photon excitation. After the first (site-selecting) photon excites the singlet-singlet origin absorption near 630 nm, the most efficient photon gating occurs when the second (gating) photon excites a strong triplet-triplet transition near 480 nm. The hole width at 1.4 K is approximately 2.5-GHz full width at half-maximum in an inhomogeneously broadened origin absorption 300 cm-1 in width. Analysis of the halomethane concentration dependence, the action spectrum of the second photon, the photoproduct spectrum, and the dependence of the hole depth on the time delay between the site-selecting and gating light pulses confirms that the mechanism is donor-acceptor electron transfer from an excited triplet state of the porphyrin to a nearby halomethane molecule. © 1987 American Chemical Society.