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). Phosphorescence generally refers to the spin-forbidden radiative transition from triplet to singlet states. RTP is common in coordination complexes, which have a lifetime of μs-scale, owing to the increased radiative transition caused by transition metal (eg, Ir, Pt, etc.). (Yam et al., 2015). In contrast, pure organic compounds, in principle, have an ultralong phosphorescence lifetime of second-scale, however, their RTP phenomenon is almost unobservable due to the weak spin-orbit coupling (SOC) effect. (TurroRamamurthy and Scaiano, 2010). Namely, 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 incorporated into organic molecules to promote ISC process for achieving ultralong RTP, in combination with the suppression of the nonradiative quenching through rigid environment, (Wu et al., 2020; Zheng et al., 2020; Zhou et al., 2020; Chen et al., 2021; Xie et al., 2021), such as crystal engineering and host-guest system. Very recently, Bechtold et al. reported that the nonplanar aromatic hydrocarbon, named as 5,6,11,12,17,18-hexahydrobenzo [2,1-p]chrysene (HD), (Salla et al., 2019), can show an ultralong RTP, which was attributed to the pronounced SOC induced by non-planar configuration. However, the enhanced SOC also reduces the RTP lifetime. Therefore, it is urgent to probe the origin of the ultralong RTP in the nonplanar aromatic hydrocarbons.

The 5′-phenyl-1,1’:3′,1″-terphenyl (PT) has a faint RTP in crystal, while HD shows an efficient RTP with a lifetime of 380 m by introducing saturated ethylene bridges between the central benzene and outer benzene rings of PT. (Li et al., 2015; Salla et al., 2019). Therefore, PT and HD molecules are good prototype to expound the dependence of ultralong RTP on the nonplanar conformation. We thus take these two molecules as example to quantitatively evaluate the nature of the molecular excited states by combing quantum and molecular mechanics (QM/MM) approach and time-dependent density functional theory (TDDFT) coupled with the thermal vibration correlation function (TVCF) formalism and unravel the origin of the ultralong RTP in nonplanar aromatic hydrocarbons, especially the relationship between RTP and the nonplanar conformation.

The QM/MM models were built based on the crystal structures from X-ray single-crystal diffraction as shown in Figure 1B. The central one molecule was chosen as active QM part, while the remaining molecules were defined as rigid MM part, which was performed by using ChemShell 3.7 (Sherwood et al., 2003) packages interfacing ORCA (Neese, 2018) for QM and DL_POLY (Smith and Forester, 1996) with the GAFF (Wang et al., 2004) for the MM part. The geometry optimization and harmonic vibrational frequency of the ground state (S₀) and the lowest triplet (T₁) excited states were calculated at (TD) B3LYP/def2-SVP level. Based on the T₁-geometry, TD-B3LYP/def2-SVP method was used to calculate their electronic structure nature, including excitation energies and natural transition orbitals (NTOs) of the low-lying excited states. All the above calculations were implemented by Gaussian 09 software. (Frisch et al., 2009). At the same level, the SOC matrix elements (ξ) were evaluated by using Beijing Density Function package (BDF) (Liu et al., 1997; Liu et al., 2003; Hirao and Ishikawa, 2004; Li et al., 2013).

The phosphorescence lifetime τ _p = 1/(k _p + k _nr) is determined by radiative decay rate k _p and nonradiative decay rate k _nr. The former can be estimated by Einstein spontaneous emission relationship k p = f E 2 1.499  s ⋅ cm - 2 , where ƒ is the oscillator strength, E is the vertical excitation energy in wavenumber. While the latter is evaluated by the TVCF rate theory with the Lorentz broadening width of 100 cm⁻¹ implemented in MOMAP package. (Niu et al., 2018). It is noted that Duschinsky rotation effect is not considered.

The light-emitting behavior of organic phosphors is governed by the molecular excited-state process. We thus explored the excited-state electronic structure, including excitation energy, NTOs and SOC matrix elements, and excited state decay rates of T₁→S₀ to account for the origin the ultralong RTP in nonplanar aromatic hydrocarbons.

In summary, we explored the nature of low-lying excited states, excited-state decay rates and vibrationally resolved phosphorescence spectra by QM/MM models coupled with the TVCF rate formalism in the MOMAP program, to unravel the origin of the ultralong RTP in nonplanar aromatic hydrocarbon. Theoretical results demonstrated that, from PT to HD, the introduction of the ethylene bridges offer σ-electron to strengthen spin-orbit coupling (SOC) between singlet and triplet excited states, such a change not only largely accelerates the ISC process for efficient RTP, but also increases the radiative and nonradiative decay rates of T₁→S₀ process, hindering the ultralong phosphorescence lifetime of twisted HD molecule. Beyond the SOC effect, the ethylene bridges also reduce the electron-vibration coupling associated with the torsional vibration modes in low frequency regions (<300 cm⁻¹), strongly reducing the nonradiative decay rate of T₁→S₀, and thereby facilitating to the ultralong RTP. Therefore, the synergistic effect of SOC and vibronic coupling caused by the ethylene bridges make a slight enlargement of the nonradiative decay rate for nonplanar HD, thereby generating an efficient RTP in twisted hydrocarbon with ultralong lifetime.