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, EBS= 187 ± 2kJ mol‐1, a lifetime, τf= 6.3± 0.1ns at 298 K, and fluorescence quantum yields, φr= 0.07 ± 0.02 (298 K) and 0.20 ± 0.04 (77 K). The S1→ T intersystem crossing quantum yield was φisc= 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 EBT= 180 ± 20 kJ mol‐1. A unimolecular decay rate constant, k1= (2.3 ± 0.5)· 103 s‐1 at room temperature, and a molar absorption coefficient, εT443= 9500 ± 500 M‐1 cm‐1, were obtained for the triplet state. This species was quenched by O2 with ko2= (1.7 ±0.3)· 108M‐1 s‐1, and by benzoquinone with kq= (5.2 ± 0.3)‐109M‐1 s‐1. The latter value, as well as the high value determined for the triplet annihilation rate constant, k2= (2 ± 0.5)· 109M‐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.