Packing Effect on Light Emission of Naphthyridine-Based Luminophor: Insights from Quantum Mechanics and Quantum Mechanics/Molecular Mechanics Calculations

Luminescent properties of the naphthyridine-based luminophor, 2,7-di(9,9-dimethylacridin-10(9H)-yl)-1,8-naphthyridine (DMAC-ND), have been explored by using quantum mechanics and quantum mechanics/molecular mechanics calculations. Based on different packing models for DMAC-ND monomer in tetrahydrofuran (THF) solution and its crystal and amorphous aggregated states, the morphology dependence of light absorption and emission has been explored. Calculations reveal that the intersystem crossing rates (k_isc) from S₁ to T₁ are comparable with their corresponding non-radiative decay rates (k_nr) from S₁ to S₀ in crystal and amorphous phases, while the k_isc value in THF solution is 6 orders of magnitude smaller than its corresponding k_nr, suggesting that effective intersystem crossing (ISC) may occur only in the aggregated configurations. The predicted reverse intersystem crossing rates (k_risc) are also comparable with their corresponding non-radiative decay rates from T₁ to S₀, and there would be an effective upconversion process in the aggregated state. The predicted k_risc values show notable morphology and temperature dependences, and the aggregation and the increase in temperature can facilitate the reverse intersystem crossing process. Based on the independent gradient model and energy decomposition analysis, combined with the estimation of the Huang-Rhys factors, such remarkable packing effects on the luminescent properties of DMAC-ND can be ascribed into the strong intermolecular interactions and the restriction of low-frequency vibrations in the crystal and amorphous phases.