Marcus inverted region in organic long-persistent luminescence host–guest systems designed from thermally activated delayed fluorescence molecules: a mechanistic study†

Abstract

Organic long-persistent luminescence (OLPL) systems have long been experimentally investigated. First reported by Ifor D. W. Samuel et al., OLPL can be observed upon doping thermally activated delayed fluorescence (TADF) molecules in host materials of PPT, TPBi and PMMA and is proposed to proceed via a two-photon mechanism. In this work, OLPL that occurs upon doping CzPhAP in PPT/TPBi systems with charge-separation features was theoretically investigated to understand the essence of transformation from TADF to OLPL and to provide insights for further research. Theoretical results of this study revealed that OLPL emission from CzPhAP:PPT/TPBi systems proceeded via a double-luminescence mechanism. The main S₃ state emission peak (observed at 608 nm) was a mixture of TADF and OLPL. Moreover, large reorganization energy associated with converting S₁ into a low-lying charge-separation S₃ state enabled S₃ fluorescence with a larger emission rate (7.92 × 10⁷ s⁻¹) compared to that of S₁ fluorescence (3.94 × 10⁷ s⁻¹, shoulder peak observed at 584 nm). Thermodynamic equilibria between S₁ and low-lying charge-separation states of S₃ and T₃ were constructed, which stimulated fast conversion among the S₁, S₃ and T₃ states. Furthermore, our investigations indicated that when the reorganization energy of the ISC process is smaller than that of rISC, a larger ΔE_ST value is required to obtain k_rISC > k_ISC. Although a larger ΔE_ST value will make the ISC process deeply rooted in the Marcus inverted region, the ISC process will be strongly hindered and TADF emission could not be observed. Meanwhile, OLPL emission was enhanced with a larger ΔE_ST owing to frustrated charge recombination to the neutral T₁ state and electron–hole dissociation could occur. More importantly, our study indicated that the essence of conversion of TADF into OLPL in CzPhAP:PPT/TPBi systems is due to a low-lying neutral T₁ state.