Molecular Insight into the Water-Induced Enhancement of Room-Temperature Phosphorescence in Organic Aggregates

Organic room-temperature phosphorescence (RTP) from triplet excitons has shown great potential for biological imaging and sensing, but these applications in aqueous environments are often limited by the moisture-mediated phosphorescence quenching. Water-induced enhancement of RTP can overcome this limitation, but the underlying mechanism remains unclear. This study focuses on two control prototypes CT and CTW, composed of trimesic acid (TMA, guest) and cyanuric acid (CA, host), while CTW introduces 20 wt.% water to CT, leading to enhanced RTP. Theoretical calculations demonstrate that the molecular conformation of TMA manipulated by intermolecular interactions governs the RTP property of aggregates. From CT to CTW, the TMA tends to a more coplanar geometry due to the decreased values in the span of deviation from plane. This conformational change not only increases the spin-orbit coupling (SOC) of S₁→T_n, thereby accelerating the intersystem crossing process and radiative transition for promoting RTP efficiency, but also reduces the SOC of T₁→S₀, suppressing the non-radiative transition to prolong RTP lifetime. Exciton dynamics reproduce the prolonged RTP lifetime from CT to CTW in experiments, which is dominated by the SOC, rather than the electron-vibration coupling. The findings offer novel insights for developing water-doped materials with improved RTP performance.