Excited-State Modulation For Controlled Room-Temperature Phosphorescence In Organic Materials

Organic RTP materials are widely used in organic optoelectronic devices,biological imaging,anti-counterfeiting and sensors due to their long lifetimes,large Stokes shift and higher signal-to-noise ratio.In recent years,organic RTP materials have developed rapidly with a large number of organic RTP materials prepared by the methods of crystallization induction,host-guest systems,halogen/hydrogen-bonding enhancement.It is found that the molecular conformations and molecular packing modes of organic molecules have an important influence on the photophysical behavior of excited states.In this paper,the excited states are modulated by changing the molecular conformations and molecular packing modes for controlling RTP emission.The main research contents are as follows:（1）A strategy to achieve controlled emissions of fluorescence and RTP by adjusting the molecular packing modes of 1,7-phenanthroline（PR）and1,4-diiodotetrafluorobenzene（DITFB）in the cocrystals was reported.By changing the chemical ratio of PR and DITFB to adjust the molecular packing modes in the cocrystals,the luminescence of the cocrystals changes from blue fluorescence（1:0）（P0D1）,to white fluorescence（2:1）（P2D1）,and finally to yellow pure phosphorescence（1:1）（P1D1）.The experimental and theoretical results reveal that more high-lying T_n states introduced in cocrystals decreases the S₁–T_n energy gap(ΔE_S1-Tn).Meanwhile,with the increase of halogen bonding between PR and DITFB,the intersystem crossing rate can be comparable with the fluorescence rate,and finally even two orders of magnitude larger than the fluorescence rate,resulting in P0D1shows pure fluorescence,P2D1 shows dual fluorescence and phosphorescence for white-light emission,P1D1 shows pure phosphorescence,thus realizing the effective control of fluorescence and phosphorescence emission channels.（2）At room temperature,solution-phase RTP is rarely realized,because T₁→S₀phosphorescence is too slow to compete with nonradiative decay and oxygen-quenching effect.We report that suppression of Kasha’s rule is a strategy to achieve solution-phase RTP from the high-lying T₂ state by spatially separating T₂ and T₁ on different parts of the molecule（Cz Cb DBT）composed of carbonyl（Cb）,dibenzothiophene（DBT）,and carbazole moiety（Cz）.On one hand,intersystem crossing（ISC）is much faster from S₁ to T₂ than that to T₁,owing to the small energy-gapΔE_S1-T2 and large spin-orbital couplingξ_S1-T2.On the other hand,T₂→T₁internal conversion is inhibited owing to spatial separation of T₂ and T₁.Also,combination of very fast radiative decay from T₂to S₀ owing to largeξ_T2-S0,the efficient solution-phase RTP emission from the T₂ state was finally achieved.（3）Tuning TADF and RTP emissions in an organic molecule remains a challenge.We put forth a strategy by modulating T₂ state to tune TADF and RTP emission in an organic D-A molecule.The photophysical properties of three different crystals of Cz Cb DBT were studied.It was found that these three crystals exhibit TADF,T₂ state phosphorescence and persistent T₁ state phosphorescence,respectively.Further crystal data analysis shows that the molecular stacking modes of the three crystals are similar,but the differences between their molecular conformations are very large;at the same time,through theoretical calculation to explore the emission mechanism,it is found that different molecular conformations of Cz Cb DBT lead to different excited state characteristics of T₂ state,and the different photophysical behaviors of T₂ state lead to the three crystals showing TADF,T₂ state RTP and persistent T₁ state RTP,respectively.