When Does MMLCT Emerge? The Role of Crystal Packing in the Photophysics of Pt(II) Polymorphs
Year: 2026
Type: preprint
Abstract: Aggregation-induced metal-metal-to-ligand charge transfer (MMLCT) is widely used to rationalize the red-shifted and broadened emission observed in Pt(II) complexes. However, the underlying mechanism of MMLCT formation and its exact relationship with photoluminescence remain poorly understood. Polymorphism provides an ideal platform to isolate aggregation effects from molecular structural changes, because the same molecule can pack into different packing arrangements leading to strikingly different photophysical properties. By combining QM/MM calculations, excited-state electronic structure analysis, vibronic spectral simulations, and radiative rate constant calculations, we investigate two polymorphs exhibiting distinct photophysical behaviors. For the investigated compounds, the formation of the excimeric MMLCT species is only possible for one polymorph and that the trimer is the minimum aggregate needed for the MMLCT formation. The MMLCT species possess distinct properties from those of the rest of aggregates and the monomer, including vibronically unresolved redshifted absorption and emission spectra and larger photoluminescence quantum yields (PLQY). An in-depth analysis of the computed results evidences that the redistribution of the electronic density in the MMLCT states leads to a different electron-vibrational coupling scenario, where the contributions of high-frequency ligand vibrations to the total reorganization energy are reduced and low-frequency Pt···Pt···Pt stretching modes become more prominent. The MMLCT species also enhances the radiative decay through enhanced spin-orbit coupling and transition dipole moments. These findings establish a direct relationship between aggregation structure, electronic structure, and photophysical properties and provide a modelling framework for understanding the formation of excimers in Pt(II) complexes in their solid states.
Source: ChemRxiv
Authors: Meihui Liu, Daniel Escudero
Institution: KU Leuven
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7222082429