Excited state properties of chlorophyll f in organic solvents at ambient and cryogenic temperatures

Chlorophyll f is a photosynthetic pigment that was discovered in 2010. In this study, we present investigations on spectral and dynamic characteristics of singlet-excited and triplet states of Chl f with the application of ultrafast time-resolved absorption and fluorescence spectroscopies. The pigment was studied at room temperature in two organic solvents: pyridine and diethyl ether that have different characters of coordination of the chlorophyll magnesium (Mg) atom (hexa- and penta-coordination, respectively). Cryogenic measurements (77 K) were performed in 2-methyltetrahydrofuran (hexa-coordination). The singlet-excited state lifetime was measured to be 5.6 ns at room temperature regardless of Mg coordination and 8.1 ns at 77 K. The fluorescence quantum yield of Chl f was also determined in pyridine to be 0.16. The triplet state lifetime was studied in detail in pyridine at room temperature, and the inherent lifetime was estimated to ~150 μs. Selective measurements at 77 K demonstrated that the metastability of the triplet state greatly enhances, and its lifetime increases by a factor of more than three.