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 \AA{}) than in acetonitrile (0.38 \AA{}), 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 ($6.85\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) than in acetonitrile ($1.1\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$), attributed due to enhanced spin-orbit coupling and reduced energy gaps. These factors collectively suppress nonradiative transitions and extend exciton lifetimes.