Roles of Localized Electronic Structures Caused by π Degeneracy Due to Highly Symmetric Heavy Atom-Free Conjugated Molecular Crystals Leading to Efficient Persistent Room-Temperature Phosphorescence

Abstract

Conjugated molecular crystals with persistent room-temperature phosphorescence (RTP) are promising materials for sensing, security, and bioimaging applications. However, the electronic structures that lead to efficient persistent RTP are still unclear. Here, the electronic structures of tetraphenylmethane (C(C₆H₅)₄), tetraphenylsilane (Si(C₆H₅)₄), and tetraphenylgermane (Ge(C₆H₅)₄) showing blue-green persistent RTP under ambient conditions are investigated. The persistent RTP of the crystals originates from minimization of triplet exciton quenching at room temperature not suppression of molecular vibrations. Localization of the highest occupied molecular orbitals (HOMOs) of the steric and highly symmetric conjugated crystal structures decreases the overlap of intermolecular HOMOs, minimizing triplet exciton migration, which accelerates defect quenching of triplet excitons. The localization of the HOMOs over the highly symmetric conjugated structures also induces moderate charge-transfer characteristics between high-order singlet excited states (S _m ) and the ground state (S₀). The combination of the moderate charge-transfer characteristics of the S _m -S₀ transition and local-excited state characteristics between the lowest excited triplet state and S₀ accelerates the phosphorescence rate independent of the vibration-based nonradiative decay rate from the triplet state at room temperature. Thus, the decrease of triplet quenching and increase of phosphorescence rate caused by the HOMO localization contribute to the efficient persistent RTP of Ge(C₆H₅)₄ crystals.

Keywords: aggregation induced emission; persistent room‐temperature phosphorescence; spin–orbit coupling; transfer integral; triplet exciton diffusion.

Figure 1

Optical characteristics and crystalline structures of C(C₆H₅)₄, Si(C₆H₅)₄, and Ge(C₆H₅)₄. a) Chemical structures of C(C₆H₅)₄, Si(C₆H₅)₄, and Ge(C₆H₅)₄. b) Changes in the luminescence of the three crystals under excitation at 280 nm and after ceasing excitation. c) Absorption and d) fluorescence spectra in THF at RT. In (d), the peak at 280 nm is caused by scattering of excitation light and the emission intensity was normalized to 1. Emission spectra of the crystals e) under excitation at 280 nm at RT and f) after ceasing excitation. In (e), the rapid increase below 300 nm is caused by scattering of the excitation light from the crystals. In (f), emission intensity was normalized to 1. g) RT emission decay characteristics of the crystals at 490 nm after ceasing excitation. h) Crystalline structures of Ge(C₆H₅)₄ at RT.

Figure 2

Temperature dependence of the phosphorescence characteristics of C(C₆H₅)₄, Si(C₆H₅)₄, and Ge(C₆H₅)₄ crystals. a) τ_p(T). b) k _nr(T) + k _q(T). Dashed lines are fitted with a sum of two exponential functions.

Figure 3

Comparison of calculated physical parameters related to diffusion of triplet excitons and illustration of the difference of hole and electron transfer integrals using molecular orbitals. a) Summary of |H _h| and |H _e| of a representative dimer showing the largest H _h ² H _e ² and experimentally observed k _q(RT), τ_p(RT), and L _T(RT) in air of C(C₆H₅)₄, Si(C₆H₅)₄, and Ge(C₆H₅)₄. i) Conformations in crystalline structures determined by XRD at RT are used. GGA:PW91 and TZP were used as exchange‐correlation functionals and the Slater‐type all‐electron basis, respectively, to calculate |H _h| and |H _e|. Data of dimer 3 for C(C₆H₅)₄. Data of dimer 5 for Si(C₆H₅)₄ and Ge(C₆H₅)₄. ii) The value is determined using microscope in ref. 59. b) Structures of molecular orbitals causing the small |H _h| of dimer 5 in the Ge(C₆H₅)₄ crystalline lattice. c) Structures of molecular orbitals related to the large |H _e| of dimer 5 in the Ge(C₆H₅)₄ crystalline lattice.

Figure 4

Relationship between $P_{\mathrm{p}}\left(\mathrm{RT}\right){\left|\partial ⟨{\mathrm{\Psi }}_0^{1\left(0\right)}\left|\overline{H_{\mathrm{SO}}}\right|{\mathrm{\Psi }}_1^{3\left(0\right)}⟩\mathrm{/}\partial Q_{\mathrm{p}}\right|}^2$ and ω_p for a) Si(C₆H₅)₄ and b) Ge(C₆H₅)₄ monomers. Conformations including normal vibration modes were optimized at T₁ using density functional theory (Gaussian09/B3LYP/6‐31G(d)). SOC data were treated as perturbations based on the scalar relativistic orbitals. Hybrid‐B3LYP and TZP were used as exchange‐correlation functionals and the Slater‐type all‐electron basis set, respectively.

Figure 5

SOC between S_m and T_n (n = 1–8) and MOs involved in the SOC of dimer 2 of C(C₆H₅)₄, Si(C₆H₅)₄, and Ge(C₆H₅)₄ crystals. a) Relationship between ${\left|⟨{\mathrm{\Psi }}_n^1\left|\overline{H_{SO}}\right|{\mathrm{\Psi }}_n^3⟩\right|}^2$ and m for dimer 2 of C(C₆H₅)₄, Si(C₆H₅)₄, and Ge(C₆H₅)₄. b) MOs related to the S₉–S₀ and T₇–S₀ transitions of dimer 2 of Ge(C₆H₅)₄. c) MOs related to the S₉–S₀ and T₄–S₀ transitions of dimer 2 of Ge(C₆H₅)₄. d) MOs related to the S₉–S₀ and T₈–S₀ transitions of dimer 2 of Si(C₆H₅)₄.

Figure 6

Relationships between photophysical parameters related to k _p and m of dimer 2 in C(C₆H₅)₄, Si(C₆H₅)₄, and Ge(C₆H₅)₄ crystals. Relationships between a) m and ${\mu }_{{\mathrm{S}}_m\to {\mathrm{S}}_0}{}^2$, b) m and ${\left|⟨{\mathrm{\Psi }}_n^1\left|\overline{H_{SO}}\right|{\mathrm{\Psi }}_n^3⟩\right|}^2$ averaged for n = 1–8, c) m and λ_m ² averaged for n = 1–8, and d) m and ${\mu }_{S_m\to S_0}{\hspace{0.17em}}^2{\lambda }_m{\hspace{0.17em}}^2$ averaged for n = 1–8.