Ultrafast absorption and photothermal studies of decarboxytrichochrome C in solution

Abstract

The trichrochromes are natural constituents of pheomelanins. Herein, time-resolved spectroscopic techniques are used to quantify the energetics and dynamics of the primary photoprocesses of trichochromes following excitation into the lowest excited singlet state of the molecule. The absorption spectrum of decarboxytrichochrome C (dTC) reveals multiple electronic states are accessible upon visible and UV excitation. Emission is not observed upon excitation into the lowest energy absorption band. Photothermal measurements reveal 91% of the photon energy is promptly released as heat. Femtosecond time-resolved absorption studies reveal a ground-state recovery time of ∼2.4 ps. Complete recovery of the ground state is not observed; 15% of the initially excited molecules do not recover on the ∼150 ps timescale. The combination of ultrafast absorption and photoacoustic data suggest photoexcitation produces a long-lived intermediate and the energy of this species is at least 133 kJ mol⁻¹ above the ground state of the dTC molecule.