Phosphorescence Quenching of Heavy-Atom-Free Dopant Chromophores Triggered by Thermally Activated Triplet Exciton Diffusion of a Conjugated Crystalline Host

Abstract

Persistent room-temperature phosphorescence (pRTP) from aggregated molecular crystalline materials is important for stable high-resolution afterglow imaging. However, an unclear understanding concerning the deactivation mechanism from the lowest triplet excited state (T1) precludes the development of highly efficient pRTP. Here, we report the reasons for the presence and absence of pRTP from a heavy-atom-free conjugated dopant in heavy-atom-free conjugated molecular crystals. N,N-di(9H-fluoren-2-yl)phenanthrene-3-amine (DFAP) was separately doped into fluorene crystals and (S)-2,2′-bis(diphenylphosphino)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthalene ((S)-H8-BINAP) crystals. DFAP doped in fluorene crystals with stronger intermolecular interactions did not generate RTP, while DFAP doped in (S)-H8-BINAP crystals with weaker intermolecular interactions showed strong pRTP. The pRTP of DFAP doped in (S)-H8-BINAP crystals is explained by the deep confinement of the T1 of DFAP in the (S)-H8-BINAP crystals with a large T1 energy. The absence of RTP from DFAP doped in fluorene crystals is explained by the temperature dependence of phosphorescence and quantum calculations, which suggest that deactivation is mainly caused by quenching after diffusion of triplet excitons of the host crystals. The temperature dependence of the triplet–triplet annihilation-based delayed fluorescence of the fluorene crystalline host confirms that the triplet diffusion of the fluorene crystalline host deactivates the triplet excitons of DFAP.