Low Internal Reorganization Energy of the Metal–Metal-to-Ligand Charge Transfer Emission in Dimeric Pt(II) Complexes

To achieve near-infrared (NIR) emitters with high photoluminescence quantum yield, reduction of nonradiative decay is necessary. There, the energy gap law dictates that as the emission wavelength shifts to the red, the nonradiative decay of the excited state becomes more rapid, unless the vibronic couplings can be simultaneously suppressed. In this study, we apply density functional theory to investigate vibronic coupling effects in a series of high-efficiency square-planar Pt(II) emitters in both the monomeric and dimeric forms. We reveal that the magnitude of internal reorganization energy, which is a measure of the extent of intracomplex vibronic coupling between two electronic states, can be significantly reduced in the dimers by the formation of metal–metal-to-ligand-charge-transfer excimeric states on the lowest triplet excited-state potential energy surface, despite that in the monomer complexes the triplet state exhibits a mixed intraligand-charge-transfer/metal-to-ligand-charge-transfer character that leads to large reorganization energies. Furthermore, we demonstrate that the relative magnitudes of the reorganization energies and the ligand substituent effects can be fully rationalized using the transition density between the excited state and the ground state as a theoretical tool that represents the spatial distribution of excitations. Finally, we expand our calculations to 45 Pt(II) NIR emitters to show that internal reorganization energy is a key factor affecting the photoluminescence quantum yield, in accordance with the energy gap law. Our findings clearly elucidate the vibronic factors affecting nonradiative decays in molecular excited states and point to novel design principles for improved NIR emitters by controlling vibronic couplings through metallophilic interactions in molecular aggregates.