Molecular Aggregates Associated THz Coherent Vibrations Contribute to Enhancements of Emission Efficiencies

Abstract

The control of excited-state vibrational and electronic energy flows in molecular solids has a considerable impact on the performance of optoelectronic devices. In this study, we applied a novel ultrafast pump-probe system with 3.2 fs resolution to demonstrate that the aggregated Pt(II) complex 4H, an efficient near-infrared emitter, exhibits prominent single-mode vibrational coherence (VC) with a frequency of 32 cm−1 (~ 0.96 THz) in the excited state. This single-mode VC is associated with the collective out-of-plane motions induced by intermolecular metal-metal-to-ligand charge transfer transitions, which occur through ultrafast intersystem crossings with lifetimes of 150 fs. Similar single-mode VC characteristics were observed in analogues of 4H and other Pt(II) complexes with intense NIR emission. The conservation of single-mode VC enables excited-state deactivation to proceed along low-frequency coordinates, which contributes to the suppression of nonradiative decay rates and causes highly intense near-infrared emission in aggregated Pt(II) complexes. These novel results highlight the importance of VC in understanding nonradiative processes, elucidating the foundations of VC in molecular solid, which serve as a benchmark for evolving the device performance.