THE PHOTOPHYSICS OF BONELLIN: A CHLORIN FOUND IN MARINE ANIMALS

— The photophysical properties of bonellin, a free‐base chlorin, were studied in ethanolic solution. For the singlet excited state the following data were determined: an energy level, E B S = 187 ± 2kJ mol ‐1 , a lifetime, τ f = 6.3± 0.1ns at 298 K, and fluorescence quantum yields, φ r = 0.07 ± 0.02 (298 K) and 0.20 ± 0.04 (77 K). The S 1 → T intersystem crossing quantum yield was φ isc = 0.85 ± 0.1. No phosphorescence was observed at 298 K and 77 K. Based on quenching experiments the triplet state energy level was determined to be E B T = 180 ± 20 kJ mol ‐1 . A unimolecular decay rate constant, k 1 = (2.3 ± 0.5)· 10 3 s ‐1 at room temperature, and a molar absorption coefficient, ε T 443 = 9500 ± 500 M ‐1 cm ‐1 , were obtained for the triplet state. This species was quenched by O 2 with ko 2 = (1.7 ±0.3)· 10 8 M ‐1 s ‐1 , and by benzoquinone with k q = (5.2 ± 0.3)‐10 9 M ‐1 s ‐1 . The latter value, as well as the high value determined for the triplet annihilation rate constant, k 2 = (2 ± 0.5)· 10 9 M ‐1 s ‐1 , might reflect an electron transfer mechanism. Copper bonellin had a shorter triplet lifetime (>20 ns), which offers a possible explanation for its lack of photodynamic action.