Mechanisms of White-Light Emission and Pressure-Modulated Dual Emission in Weak Donor–Acceptor Systems: A Theoretical Perspective

Combining fluorescence and phosphorescence in a single organic molecule offers a promising approach to white-light emission. However, balancing singlet and triplet excited-state dynamics remains challenging. Recently, a series of weak donor-acceptor molecules incorporating sulfur atoms as heavy atoms have been synthesized. Among them, DBTDBFCZ exhibits single-component white-light emission in the solid phase and stimulus-responsive behavior. To elucidate the underlying photophysical mechanisms, we employ quantum mechanics/molecular mechanics calculations to investigate how the heavy-atom effect and the substitution position of sulfur atoms modulate excited-state character. The results show that the heavy-atom effect and substitution position synergistically tune the hybridized local and charge-transfer character. This governs intersystem crossing rates and radiative/nonradiative decay. In solution, DBFDBTCZ achieves an optimal balance between efficient triplet population and suppressed nonradiative decay, showing superior phosphorescence performance. In the solid phase, the same mechanism, combined with solid-phase induced redistribution of locally excited and charge-transfer components, enables DBTDBFCZ to produce white light through coexisting blue-violet fluorescence and yellow-green phosphorescence. Taking advantage of its stimulus-responsive nature, we further examine the effect of hydrostatic pressure on its dual emission. A moderate pressure of approximately 2.0 GPa is identified as the optimal condition. At this pressure, the singlet-triplet energy gap is reduced and spin-orbit coupling is enhanced. Consequently, both fluorescence and phosphorescence are simultaneously enhanced, promoting balanced dual emission. These findings provide a mechanistic understanding of white-light generation and pressure-controlled dual emission, offering a theoretical basis for designing stimulus-responsive organic dual-emission materials.