Handbook of Aggregation-Induced Emission, Volume 1. Группа авторов

821 1.22 × 10⁸ 1.66 × 10⁹ 6.86 In solid phase HPS [37] 3.48 5.85 403 6.56 × 10⁷ 2.06 × 10⁷ 76.0 TPS [38] 3.26 0.60 932 1.15 × 10⁶ 3.32 × 10⁶ 25.8 BrTPS [38] 2.91 1.80 890 8.12 × 10⁶ 1.50 × 10⁶ 82.3 HPS [38] 2.70 5.90 753 7.43 × 10⁷ 1.57 × 10⁶ 97.9 BTPES [38] 2.49 5.75 592 6.57 × 10⁷ 1.93 × 10⁶ 97.1 BFTPS [38] 2.30 9.10 607 1.14 × 10⁸ 1.07 × 10⁷ 91.4

      The reorganization energy analysis from S₁ → S₀ process for each normal mode in the gas phase reflects that the C=O stretching vibration contributes the most (∼1974.37 cm⁻¹), consistent with the corresponding large structural modification (0.08 Å between S₀ and S₁). However, in the solid phase, this value is reduced to 212.96 cm⁻¹ and the corresponding structural change is only 0.01 Å. Such a remarkable reduction is due to the alternation to π → π* electronic transition, which is decoupled with the C=O stretching vibration. As a result, from the gas to solid phases, k_ic decreases about 1 order of magnitude from 4.97 × 10⁷ to 5.15 × 10⁶ s⁻¹. That is, the nonradiative decay process is blocked by the decoupling between the high‐frequency C=O stretching vibration and transition electrons (Figures 2.2c and 2.4b). Overall, the largely accelerated radiative decay rate and slowed nonradiative decay rate induce the observed strong fluorescence in the solid phase [41].

      2.3.3 Bending Vibration of Bonds

(CAAC^Ad)CuCl (see Figure 2.3) is a two‐coordinate Cu(I) complex, which exhibits highly efficient fluorescence in aggregate [50]. The excited‐state decay dynamics for (CAAC^Ad)CuCl in both solution and solid states is studied by the hybrid QM/MM approach, coupled with the TVCF rate formalism [42]. Analyzing the geometrical changes between S₀ and S₁ upon excitation, it is found that the complex is more flexible in solution. The largest changes appear in the coordination bond angles between copper and two ligands with ∠C₁−Cu−Cl, ∠Cu−C₁−N, and ∠Cu−C₁−C₄ decreasing from 9.67°, 12.82°, and 10.73° in solution to 3.66°, 4.46°, and 6.75° in the solid phase, respectively. And the transition character changes from metal‐to‐ligand charge transfer (MLCT) to hybrid MLCT and halogen‐to‐ligand charge transfer (XLCT) upon aggregation. The k_r are close in both phases, while k_ic decreases by about 3 orders of magnitude from 8.38 × 10⁷ to 1.47 × 10⁴ s⁻¹ (see Table 2.2). The λ_total decreases a lot from solution (7121 cm⁻¹) to solid state (2274 cm⁻¹). It is found that the largest contributions to λ_total come from several low‐frequency bending modes and one high‐frequency stretching mode in solution. These low‐frequency modes are assigned to be the bending vibrations associated with coordination bonds C₁−Cu and Cu−Cl, and the high‐frequency mode belongs to the stretching vibration of the C−N bond in carbene ligand. Upon aggregation, the vibrations of angles C₁−Cu−Cl and Cu–C1–N are restricted largely, while the stretching vibrations of the C₁−N bond are insensitive to the environment (Figure 2.4c). Overall, the strong solid‐state fluorescence of (CAAC^Ad)CuCl is induced by removing the nonradiative decay channels owing to the decoupling between transition electron and the bending vibrations of coordination bonds (see Figure 2.2d).

Figure 2.4 Calculated reorganization energies versus normal mode of (a) HPS, (b) TPA, (c) (CAAC^Ad)CuCl, and (d) COTh in both gas/solution and solid phases, respectively.

      2.3.4 Flipping Vibrations of Molecular Skeletons

Cyclooctotetraene (COT) is a prototype molecule with nonaromatic annulenes [51–53]. The COT derivative cyclooctatetrathiophene (COTh) was found to be AIE active and its AIE mechanism was further investigated theoretically and experimentally [43]. The theoretically optimized geometries in the S₀ and S₁ states show that the isolated COTh is much more flexible than that in a cluster. For example, from the S₀ to S₁ state, the modifications of the dihedral angles Θ_I–II and Θ_II–III (I, II, and III denote the aromatic rings marked in Figure 2.3) are 22.82° and 24.22° in the gas phase, much larger than those in the solid state of 12.78° and 12.96°. Upon excitation, the dihedral angles between the neighboring thienyl rings are decreased from S₀ to S₁. As shown in Table 2.3, for example, Θ_I–II decreases from 44.93° to 22.11° in the gas phase, while it reduces from 43.84° to 31.06° in the solid phase, indicating a better planarity of the central large eight‐membrane ring. Therefore, the conjugation of the central eight‐membrane improves and the oscillator strength of S₁ enhances accordingly. In addition, the excitation energy at the S₁ geometry increases from 2.42 eV in the gas phase to 2.69 eV in cluster. Therefore, k_r increases from 9.80 × 10⁴ to 6.05 × 10⁵ s⁻¹, owing to the enhancement of oscillator strength and excitation energy; see Table 2.2. The k_ic is closely related to the reorganization energy. For COTh, the λ_total decreases to 636 meV in crystal from 860 meV in the gas phase and then the k_ic decreases by 2 orders of magnitude. Thus, the Φ_F in the crystal is 3 orders of magnitude