A new twisted donor–acceptor–donor (D–A–D) multi-photofunctional organic molecule comprising phenoselenazine as the electron donor (Ds) and dibenzo[a,j]phenazine (DBPHZ) as the electron acceptor (A) has been developed. The developed selenium-incorporated D–A–D compound features multi-color polymorphism, distinct mechanochromic luminescence, chemically-stimulated luminochromism, thermally-activated delayed fluorescence, and room-temperature phosphorescence. The internal heavy atom effect on the photophysical properties of the D–A–D system has been investigated through a comparison with the physicochemical properties of a previously developed sulfur analogue and a tellurium analogue.

Exploring the design principle for switching between thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) is a fundamentally important research to develop triplet-mediated photofunctional organic materials. Herein systematic studies on the regioisomeric and substituent effects in a twisted donor–acceptor–donor (D–A–D) molecular scaffold (A = dibenzo[a,j]phenazine; D = dihydrophenazasiline) on the fate of its excited state have been performed. The study revealed that regioisomerism clearly affects the emission behavior of the D–A–D compounds. Moreover, distinct differences in TADF, dual TADF & RTP, and dual RTP were observed, depending on the host used. Furthermore, organic light-emitting diodes (OLEDs) fabricated with the developed emitters achieved high external quantum efficiencies (EQEs) of up to 7.4% for RTP-based OLEDs.

A new series of multifunctional compounds displaying thermally activated delayed fluorescence (TADF), room-temperature phosphorescence (RTP), aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE) have been developed, and their optical properties investigated. The materials are based on a twisted donor–acceptor (D–A) motif using acenaphtopyridopyrazine as the acceptor core, and various donors are used for tuning their photophysical behaviour. Time-resolved spectroscopy analysis using different matrices gives a detailed image of their photophysical properties. Solution-processed organic light-emitting diodes (OLEDs) using the new compounds as emitters gave high external quantum efficiency (EQE) of up to 15.3% in the CBP matrix. The triplet excited state of the new emitters poses an opportunity for the photochemical formation of singlet oxygen (¹O₂) species in solution. We demonstrate that the efficiency of ¹O₂ generation correlates with the ΔE_ST and triplet decay lifetime. The maximum ¹O₂ quantum efficiency obtained is ∼48% for NQPy-CBZ.