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 meiotetra-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.

Spectroscopy, persistent hole burning, and holographic applications of naphtophthalocyanine/haloanthracene systems

“…In contrast to the results in [1], and in correspondence with the observations in [2], the long living IS is not likely the triplet state of H 2 -TBNP, since in general phthalocyanines have a much shorter triplet state lifetime of the order of 1 ms [8]. An unusual feature of the observed gated holes is their small width of about 400 MHz, which is close to that for nongated materials, and about 10-20 times less than typically observed in other gated PSHB materials [1][2][3][4]. The reason for such narrow holes is likely related to a relatively low concentration of acceptor molecules 1-ClA of about 10 À4 mol/l due to the lesser influence of the electric field induced by D-A pair formation [1].…”

“…An unusual feature of the observed gated holes is their small width of about 400 MHz, which is close to that for nongated materials, and about 10-20 times less than typically observed in other gated PSHB materials [1][2][3][4]. The reason for such narrow holes is likely related to a relatively low concentration of acceptor molecules 1-ClA of about 10 À4 mol/l due to the lesser influence of the electric field induced by D-A pair formation [1]. This conclusion requires verification by future experiments on materials with higher acceptor concentration, which are expected to have more efficient gated hole burning.…”

“…Based on the results of the study of gated hole burning for porpyrins in the presence of halogenated aromatic compounds [1][2][3], we propose the following two-photon gating mechanism in our case. It involves the selective excitation by the l 1 -photon of H 2 -TBNP (donor, D); electron transfer reaction in the excited (likely triplet) states of donor to 1-ACl (acceptor, A) through the intermediate state (IS); and the induced l 2 -photon formation of a stable photo-product SP,…”

“…Many of these materials exhibit photon-gated two-photon PSHB mechanisms [1][2][3][4], which permit nondestructive one-photon readout. In addition, some of these systems have high hole burning efficiency and allow fast single shot hole burning in the nanosecond [1] and even the picosecond [3] time scale. In phthalocyanines, due to the increase in size of their skeleton and p-electron system, the position of the S 0 -S 1 transition is shifted to the infrared (700-800 nm) as compare to 630-660 nm in porphyrin derivatives.…”

Photochemical hole burning of organic dye doped in inorganic semiconductor

“…The Ar ϩ laser can induce T-T transition of ZnTTBP as a gating light. 20,21 Traces ͑d͒ and ͑e͒ are spectra after the irradiation of 7.5 mW/cm 2 dye laser, and 7.5 mW/cm 2 dye laser together with 5 mW/cm 2 Ar ϩ laser, respectively, for 10 min in both cases. No enhancement of hole formation by the existence of the Ar ϩ laser is observed in traces ͑c͒ or ͑e͒.…”