Jahn-Teller Distortion Controls Electron Transfer in Photoexcited Cu(I) Donor–Acceptor Systems

Abstract

The Jahn–Teller distortion (JTD) is a defining structural response to electronic excitation in Cu(I)-based transition metal complexes, yet its role in photoinduced electron transfer (PET) remains largely unexplored. Here, we demonstrate that the JTD governs charge separation (CS) through a vibronically controlled conical intersection in heteroleptic Cu(I) bisphenanthroline–naphthalene diimide (CuHETPHEN-NDI) donor–acceptor dyads. Using 20-fs broadband transient absorption spectroscopy combined with coherent vibrational wavepacket (CVWP) analysis and quantum chemical calculations, we directly track nuclear motions that steer the system from the metal-to-ligand charge-transfer (¹MLCT) state to the CS state. Steric bulkiness of the pendant groups at the 2,9-positions of the phenanthroline ligand systematically slows both JTD and CS. Short-time Fourier transformation and Fourier filtering analyses identify two key vibrational signatures: a low-frequency breathing mode (~100 cm-1) via a bond distance change between Cu and ligated Ns (Cu-N) that modulates the NDI anion absorption and acts as a vibronic coupling coordinate, and a higher-frequency mode (~313 cm-1) that evolves along the PET trajectory. Normal mode analysis and potential energy surface calculations show that the JTD brings the 1MLCT and CS states into degeneracy, while the Cu-N breathing motion dynamically modulates donor–acceptor electronic coupling to enable ultrafast nonadiabatic electron transfer. Steric hindrance exerted by the groups at the 2, 9 positions of the phenanthroline ligands suppresses this vibronic coupling, leading to faster CVWP decoherence for the 313 cm-1 mode and slower CS. These findings unravel JTD-controlled vibronic coupling at conical intersection as a governing factor for CS and provide insight into designing Cu-based photosensitizers by harnessing structural dynamics to control PET.