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

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.