Abstract
The widespread application of Re(I) carbonyl complexes in photocatalysis, solar energy conversion, sensing, and imaging has driven efforts to understand their excited-state dynamics. Re(I) carbonyl complexes provide an ideal platform for illustrating excited-state mapping by spectroscopic and computational methods, as systematic ligand modification has produced a rich range of diimine and triimine systems with closely spaced electronic states and competing relaxation pathways. Recent advances in spectroscopic techniques have enabled mapping of the excited-state landscape across different time scales. The discussion is organized around the different stages of excited-state evolution. Resonance Raman (RR) spectroscopy is first used to probe the initially populated Franck–Condon states. Faster dynamics, analyzed by femtosecond transient methods, describe the evolution toward the thermally equilibrated excited state (THEXI), while slower dynamics and longer-lived states are examined using nanosecond transient methods. Vibrational spectroscopies, including transient infrared (TRIR) and transient Raman techniques, provide structural evidence for both short-lived intermediate states and the longer-lived THEXI state. As such, the techniques are discussed with emphasis on the information they provide and their limitations, and their application to Re(I) systems, where the electronic complexity increases from metal-to-ligand charge-transfer (MLCT)-dominated systems to systems with a complex interplay among MLCT, intraligand charge-transfer (ILCT), intraligand (IL), and mixed-character states. These examples highlight how solvent effects, charge-transfer character, and ligand-driven structural changes influence excited-state properties, switching, and lifetimes. In parallel, computational methods connect experimental spectra to electronic structure, excited-state energies, and orbital distributions. Recent developments in time-dependent Density functional theory (TDDFT)-based excited-state analysis, simulations of RR, TRIR, spectroelectrochemical, and transient absorption spectra, Marcus-type treatments of charge and energy transfer, and nonadiabatic wavepacket dynamics are also considered. Selected examples further extend these concepts to Re(I) probes in DNA and protein environments, illustrating how combined spectroscopic and computational approaches can reveal excited-state behavior in complex molecular and biological settings.