Jahn-Teller Distortion Found to Govern Electron Transfer in Novel Copper Compounds
Researchers have demonstrated that the Jahn-Teller distortion (JTD), a structural change in response to electronic excitation, plays a critical role in controlling the efficiency of electron transfer within a newly studied class of copper compounds. The findings, published this week, could pave the way for the design of more effective copper-based photosensitizers for a range of applications.
The study focused on heteroleptic Cu(I) bisphenanthroline–naphthalene diimide (CuHETPHEN-NDI) donor-acceptor dyads. These compounds exhibit a JTD, a phenomenon where the geometry of a molecule changes to reduce its energy when it becomes electronically excited. Scientists have long known about the JTD in copper complexes, but its direct influence on photoinduced electron transfer (PET) has remained largely unexplored until now.
Using a combination of 20-femtosecond broadband transient absorption spectroscopy, coherent vibrational wavepacket (CVWP) analysis, and quantum chemical calculations, the research team directly observed the nuclear motions that guide the system from an initial metal-to-ligand charge-transfer (¹MLCT) state to a charge-separated (CS) state. The analysis revealed that the JTD governs charge separation through a vibronically controlled conical intersection.
“We directly track nuclear motions that steer the system from the metal-to-ligand charge-transfer state to the CS state,” explained the research team in their published work. The researchers identified two key vibrational signatures during this process. A low-frequency breathing mode, involving changes in the bond distance between copper and nitrogen atoms (Cu-N), modulates the absorption of the naphthalene diimide anion and acts as a crucial link in the process. A higher-frequency mode also evolves along the PET trajectory.
The study also showed that increasing the steric bulkiness of groups attached to the phenanthroline ligand slows down both the JTD and the subsequent charge separation. This suggests that the structural dynamics around the copper center are critical for efficient electron transfer. Specifically, the researchers found that steric hindrance suppresses vibronic coupling, leading to slower charge separation.
The Jahn-Teller effect is a well-established phenomenon in chemistry, notably influencing the structure of octahedral complexes. As noted in Wikipedia, the effect is responsible for the tetragonal distortion of the hexaaquacopper(II) complex ion, where axial copper-oxygen distances differ from equatorial distances. The current research extends this understanding to the realm of photoinduced electron transfer, demonstrating a direct link between the distortion and the efficiency of charge separation.
Related research, published last year, demonstrated that lattice-induced strain can also affect the excited state dynamics of Cu(I) based photocatalysts. That study, conducted on single crystals, showed faster intersystem crossing in crystalline structures compared to solutions, highlighting the importance of the surrounding environment on the photocatalytic process.
The findings suggest a pathway for designing improved Cu-based photosensitizers by carefully controlling the structural dynamics to optimize PET. Further research is planned to explore the impact of different ligand modifications on the JTD and its influence on electron transfer efficiency.
