Ultrafast Photo-Induced Charge Transfer Unveiled by Two-Dimensional Electronic Spectroscopy

TL;DR

This study employs two-dimensional electronic spectroscopy and quantum chemistry to reveal ultrafast charge transfer processes in a lutetium bisphthalocyanine dimer, highlighting charge localization and dynamics within 100 femtoseconds.

Contribution

It demonstrates the capability of 2D-ES combined with quantum chemistry to directly observe and analyze ultrafast charge transfer dynamics in complex molecular systems.

Findings

Charge transfer occurs within 30 fs in the anionic form.

Charge localization on the macrocycle nearest to the counter-ion within 100 fs.

Charge transfer states influence electronic structure and reaction pathways.

Abstract

The interaction of exciton and charge transfer (CT) states plays a central role in photo-induced CT processes in chemistry, biology and physics. In this work, we use a combination of two-dimensional electronic spectroscopy (2D-ES), pump-probe measurements and quantum chemistry to investigate the ultrafast CT dynamics in a lutetium bisphthalocyanine dimer in different oxidation states. It is found that in the anionic form, the combination of strong CT-exciton interaction and electronic asymmetry induced by a counter-ion enables CT between the two macrocycles of the complex on a 30 fs timescale. Following optical excitation, a chain of electron and hole transfer steps gives rise to characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor how the excited state charge density ultimately localizes on the macrocycle closest to the counter-ion within 100 fs. A comparison with the dynamics in the radical species further elucidates how CT states modulate the electronic structure and tune fs-reaction dynamics. Our experiments demonstrate the unique capability of 2D-ES in combination with other methods to decipher ultrafast CT dynamics.