Ultrafast Dynamics in Light-Harvesting Molecules with Optical and X-ray Spectroscopies

Abstract

Light-harvesting materials, such as the N719 dye, have been introduced as one of the next-generation energy sources. Photophysical and electrochemical studies on such systems have been carried out to understand the light-harvesting phenomena with a particular interest in noble-metal free photosensitizers and photocatalysts. The realization of ultrashort optical and X-ray pulses together with their spectroscopic applications enabled to finely trace the corresponding ultrafast changes in the photosensitizers and photocatalysts, both globally and locally.

This study investigates the ultrafast dynamics of energy conversion in model complexes by means of ultrafast optical and X-ray methods to obtain more insights into their steady state performances. Differences in the optical absorption spectrum with respect to the arrival time changes between two pulses revealed a global interaction in a bimetallic photocatalyst such as Ir(III)-Co(III) triads, Ru(II)-Mo(VI) dyads, and Fe(II)-Co(III) dyads. Complementary to this, time-resolved X-ray absorption and emission spectroscopy visualized the electronic and geometric structure changes of the target element that inherently trigger the relevant chemical events in photoactive molecules like Ru(II), Cu(I), and Fe(II) photosensitizers. This study finds that the 3d-chelates undergo a femtosecond symmetry breaking after the charge separation that often recombines non-radiatively, whereas the 4d- and 5d-chelates with their chemical potential in the charge separation state can perturb the nearby electron densities such that either a chemical energy or an electron gets transferred on the picosecond timescale.