Observation of thermally activated intersystem crossing in room-temperature phosphorescence of weakly donor–acceptor–donor π-ternary molecules

The direct observation of thermally activated intersystem crossing (TA-ISC) processes in room-temperature phosphorescence (RTP) has not been well investigated. Here, we reported four weakly donor-acceptor-donor π-ternary molecules featuring isomerism-dependent phosphorescence behaviors. The isomerism was modulated by changing the substitution position, offering tunable molecular planarity and π-conjugation. As a result, regulated energy levels and orbital configurations of the excited states facilitate additional TA-ISC channels as revealed by theoretical investigations. The temperature-dependent photoluminescence and fs- and ns-transient absorption spectra undoubtedly demonstrate their existence and effectiveness in promoting efficient and persistent RTP. The ISC rate constant achieves a breakthrough value of up to 1.93 × 10⁹ s^-1 within a poly(vinyl alcohol) matrix film, surpassing that in many existing afterglow systems containing heavy or hetero atoms. Abundant triplet excitons are populated and enable remarkable room-temperature phosphorescence performance, facilitating potential applications in constructing thermosensitive glasses and anti-counterfeiting patterns. This study offers a practical mechanism for developing novel RTP materials.