absorption measurements to investigate the thermodynamic properties and oxygen-dependence of triplet formation and decay. Using EPR spectroscopy it is only possible to observe the triplet state at cryogenic temperatures. Hence, initially it was important to provide a direct comparison of the excited-state dynamics of Pchlide at both cryogenic and room temperatures by using time-resolved absorption measurements after laser excitation at 450 nm (ps to µs timescale) as described in the Supporting Information. The time-resolved difference spectra from these measurements (Figures S1-S8) were fitted to a sequential kinetic model by global analysis to yield evolution associated difference spectra (EADS) (Figure 2).16 The time-resolved spectral changes at a range of temperatures between 10 K (Figure 2) and room temperature (Figure 2), and in the presence and absence of oxygen could all be fitted to four components with only minor differences in the lifetimes of the various excited state species (Table 1). The first three excited-state components have been described previously and correspond to the formation of an ICT state from the S1 excited state (~5 ps), solvation of the ICT state (~200 ps) and decay of the S1/SICT excited state into the long-lived triplet state on the nanosecond timescale (~4 ns).7–13 Hence, it appears that temperature and presence of oxygen only have a minimal effect on these three excited-state processes, which are directly related to the formation of the triplet state. Although there are only minor differences in the electronic spectra of the different Pchlide excited singlet states, these species have previously been confirmed by corresponding time-resolved fluorescence and measurements. In contrast, the triplet state has a distinctly blue-shifted bleach at ~629 nm and less intense excited state absorption features.16,17 The remaining long-lived component (τ4), which relates to the decay of the triplet state, occurs on longer timescales than the 80 µs time window of the pump-probe measurements.
absorption measurements to investigate the thermodynamic properties and oxygen-dependence of triplet formation and decay. Using EPR spectroscopy it is only possible to observe the triplet state at cryogenic temperatures. Hence, initially it was important to provide a direct comparison of the excited-state dynamics of Pchlide at both cryogenic and room temperatures by using time-resolved absorption measurements after laser excitation at 450 nm (ps to µs timescale) as described in the Supporting Information. The time-resolved difference spectra from these measurements (Figures S1-S8) were fitted to a sequential kinetic model by global analysis to yield evolution associated difference spectra (EADS) (Figure 2).16 The time-resolved spectral changes at a range of temperatures between 10 K (Figure 2) and room temperature (Figure 2), and in the presence and absence of oxygen could all be fitted to four components with only minor differences in the lifetimes of the various excited state species (Table 1). The first three excited-state components have been described previously and correspond to the formation of an ICT state from the S1 excited state (~5 ps), solvation of the ICT state (~200 ps) and decay of the S1/SICT excited state into the long-lived triplet state on the nanosecond timescale (~4 ns).7–13 Hence, it appears that temperature and presence of oxygen only have a minimal effect on these three excited-state processes, which are directly related to the formation of the triplet state. Although there are only minor differences in the electronic spectra of the different Pchlide excited singlet states, these species have previously been confirmed by corresponding time-resolved fluorescence and measurements. In contrast, the triplet state has a distinctly blue-shifted bleach at ~629 nm and less intense excited state absorption features.16,17 The remaining long-lived component (τ4), which relates to the decay of the triplet state, occurs on longer timescales than the 80 µs time window of the pump-probe measurements.
170227_ACS_Guillem_Final_e_superfinal.docx