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.

Excited state mechanisms in crystalline carbazole: the role of aggregation and isomeric defects

“…Dopants can be used to facilitate ISC, for example, N,N-di(9H-fluoren-2-yl)phenanthrene-3amine (DFAP) has been successfully used to increase triplet-triplet annihilation delayed fluorescence (TTADF) in fluorene crystals. 59 The rate equation of the concentration of triplets [T ] is…”

Thermo-Reversible Persistent Phosphorescence Modulation Reveals the Large Contribution Made by Rigidity to the Suppression of Endothermic Intermolecular Triplet Quenching

“…Commercially available (S)-H 8 -BINAP crystals often generate green pRTP after ultraviolet (UV) excitation has ceased (Supplementary Figure S1A) because of the presence of (S)-BINAP as an impurity (Supplementary Figure S2). After careful repeated purification of the (S)-H 8 -BINAP using silica column chromatography, however, we noted that the purified crystals of (S)-H 8 -BINAP did not produce pRTP (Supplementary Figure S1B) (Hirata et al, 2020). First (S)-BINAP powder was dissolved in molten pure (S)-H 8 -BINAP at 220 °C.…”

Excited State Mechanisms in Crystalline Carbazole: The Role of Aggregation and Isomeric Defects

“…However, the generation of triplets is limited by the relatively low k ISC which is two orders of magnitude lower than k f (Table I). Dopants can be used to facilitate ISC, for example, N,N-di(9H-fluoren-2-yl)phenanthrene-3-amine (DFAP) has been successfully used to increase triplettriplet annihilation delayed fluorescence (TTADF) in fluorene crystals [56].…”