Excited-State THz Vibrations in Aggregates of Pt II Complexes Contribute to the Enhancement of Near-Infrared Emission Efficiencies.

The exploration of deactivation mechanisms for near-infrared(NIR)-emissive organic molecules has been a key issue in chemistry, materials science and molecular biology. In this study, based on transient absorption spectroscopy and transient grating photoluminescence spectroscopy, we demonstrate that the aggregated Pt II complex 4H (efficient NIR emitter) exhibits collective out-of-plane motions with a frequency of 32 cm -1 (0.96 THz) in the excited states. Importantly, similar THz characteristics were also observed in analogous Pt II complexes with prominent NIR emission efficiency. The conservation of THz motions enables excited-state deactivation to proceed along low-frequency vibrational coordinates, contributing to the suppression of nonradiative decay and remarkable NIR emission. These novel results highlight the significance of excited-state vibrations in nonradiative processes, which serve as a benchmark for improving device performance.