Balanced Energy Gaps as a Key Design Rule for Solution‐Phase Organic Room Temperature Phosphorescence.

Metal‐free organic emitters that display solution‐phase room temperature phosphorescence (sRTP) remain exceedingly rare. Here, we investigate the structural and photophysical properties that support sRTP by comparing a recently reported sRTP compound (BTaz−Th−PXZ) to two novel analogous materials, replacing the donor group by either acridine or phenothiazine. The emissive triplet excited state remains fixed in all three cases, while the emissive charge‐transfer singlet states (and the calculated paired charge‐transfer T2 state) vary with the donor unit. While all three materials show dominant RTP in film, in solution different singlet‐triplet and triplet‐triplet energy gaps give rise to triplet‐triplet annihilation followed by weak sRTP for the new compounds, compared to dominant sRTP throughout for the original PXZ material. Engineering both the sRTP state and higher charge‐transfer states therefore emerges as a crucial element in designing emitters capable of sRTP. [ABSTRACT FROM AUTHOR]