Görner, Helmut, Griesbeck, Axel G., Herinrich, Thomas, Kramer, Wolfgang, and Oelgemoeller, Michael (2001) Time-resolved spectroscopy of sulfur- and carboxy-substituted N-alkylphthalimides. Chemistry: a European journal, 7 (7). pp. 1530-1538.

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Abstract

The photophysical and photochemical properties of N-phthaloylmethionine (1), S-methyl-N-phthaloylcysteine methyl ester (2) and N-phthaloyltranexamic acid (3) were studied by time-resolved UV/Vis spectroscopy, using laser pulses at 248 or 308 nm. The quantum yield of fluorescence is low (Φf<10⁻²) for 1–3 in fluid and glassy media, whereas that of phosphorescence is large (0.3–0.5) in ethanol at −196 °C. The triplet properties were examined in several solvents, at room temperature and below. The spectra and decay kinetics are similar, but the population of the π,π* triplet state, as measured by T–T absorption, is much lower for 1 and 2 than for 3 or N-methyltrimellitimide (5′) at ambient temperatures. The quantum yield (ΦΔ) of singlet molecular oxygen O₂(1Δg) formation is substantial for 3 and 5′ in several air- or oxygen-saturated solvents at room temperature, but small for 2 and 1. The quantum yield of decomposition is substantial (0.2–0.5) for 3 and small (<0.05) for 2 and 1. It is postulated that photoinduced charge separation in the spectroscopically undetectable ³n,π* state may account for the cyclization products of 1 and 2. In aqueous solution, this also applies for 3, whereas in organic solvents cyclization involves mainly the lower lying ³π,π* state. Triplet acetone, acetophenone and xanthone are quenched by 1–3 in acetonitrile; the rate constant is close to the diffusion-controlled limit, but smaller for benzophenone. While the energy transfer from the triplet ketone occurs for 3, a major contribution of electron transfer to the N-phthalimide derivative is suggested for 1 and 2, where the radical anion of benzophenone or 4-carboxybenzophenone is observed in alkaline aqueous solution.

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