Theoretical Insight Into the Ultralong Room-Temperature Phosphorescence of Nonplanar Aromatic Hydrocarbon.

Ke Qin,Jia Gao,Wenqi Gong,Wei Yao,Deping Hu,Huifang Shi,Huili Ma,Zhongfu An

doi:10.3389/fchem.2021.740018

Abstract

Purely aromatic hydrocarbon materials with ultralong room-temperature phosphorescence (RTP) were reported recently, but which is universally recognized as unobservable. To reveal the inherent luminescent mechanism, two compounds, i.e., PT with a faint RTP and HD with strong RTP featured by nonplanar geometry, were chosen as a prototype to study their excited-state electronic structures by using quantum mechanics/molecular mechanics (QM/MM) model. It is demonstrated that the nonplanar ethylene brides can offer σ-electron to strengthen spin-orbit coupling (SOC) between singlet and triplet excited states, which can not only promote intersystem crossing (ISC) of S1→Tn to increase the population of triplet excitons, but also accelerate the radiative decay rate of T1→S0, and thus improving RTP. Impressively, the nonradiative decay rate only has a small increase, owing to the synergistic effect between the increase of SOC and the reduction of reorganization energy of T1→S0 caused by the restricted torsional motions of aromatic rings. Therefore, a bright and long-lived RTP was obtained in aromatic hydrocarbon materials with twisted structure. This work provided a new insight into the ultralong RTP in pure organic materials.

Highlights

Ultralong room-temperature phosphorescence (RTP) in purely organic materials has been gaining more attention in encryption (An et al, 2015; Ma et al, 2021), display (Wang et al, 2019; Tan et al, 2021), bioimaging (Wang et al, 2020; Wang et al, 2021a) and so on (Yu et al, 2017; He et al, 2019; Zhao et al, 2020)
We explored the excited-state electronic structure, including excitation energy, natural transition orbitals (NTOs) and spin-orbit coupling (SOC) matrix elements, and excited state decay rates of T1→S0 to account for the origin the ultralong RTP in nonplanar aromatic hydrocarbons
It rationally speculated that the intersystem crossing (ISC) process of S1→Tn should be largely promoted, which is responsible for the bright RTP in HD

Summary

Introduction

Ultralong room-temperature phosphorescence (RTP) in purely organic materials has been gaining more attention in encryption (An et al, 2015; Ma et al, 2021), display (Wang et al, 2019; Tan et al, 2021), bioimaging (Wang et al, 2020; Wang et al, 2021a) and so on (Yu et al, 2017; He et al, 2019; Zhao et al, 2020). Pure organic compounds, in principle, have an ultralong phosphorescence lifetime of second-scale, their RTP phenomenon is almost unobservable due to the weak spin-orbit coupling (SOC) effect. The ultralong RTP in aromatic hydrocarbon materials is extremely rare, (Clapp, 1939; Bilen et al, 1978), because of the forbidden intersystem crossing (ISC) process between singlet and triplet excited states. To overcome this issue, the heavy atoms (eg., Br and I) (Cai et al, 2018; Wang et al, 2021b) and carbonyl groups (Zhao et al, 2016; Jia et al, 2020) were

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