Unveiling the regulation of triplet excitons by intermolecular interactions in 4-phenylmorpholine: Insights from transient absorption spectroscopy and theoretical modeling.

Heteroatom-containing derivatives of 4-phenylmorpholine are cost-effective phosphorescent materials. In this study, femtosecond transient absorption (TA) spectroscopy of 4-phenylmorpholine reveals that the excited-state absorption signal at 625 nm decays as the triplet-triplet absorption (TTA) at 475 nm increases, thereby confirming that intersystem crossing (ISC) occurs within 2.27 ns. Subsequent nanosecond-TA measurements show that the TTA signal decays within 363.9 ns. To further understand how intermolecular interactions influence triplet excitons in crystalline environments, quantum-mechanics/molecular-mechanics simulations were performed. The simulations indicate that configurational changes are significantly smaller in crystals (0.15 Å) than in acetonitrile (0.38 Å), due to stronger intermolecular interactions that suppress molecular vibrations and reduce high-frequency recombination energy. Moreover, the ISC rate in crystals is 6 times higher (6.85×10^{7}s^{-1}) than in acetonitrile (1.1×10^{7}s^{-1}), attributed due to enhanced spin-orbit coupling and reduced energy gaps. These factors collectively suppress nonradiative transitions and extend exciton lifetimes.