Читать книгу Handbook of Aggregation-Induced Emission, Volume 1 - Группа авторов - Страница 37

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 larger than that in the isolated state. As shown in Figure 2.4d, the major differences in the reorganization energies between the gas phase and the solid state are related to the low‐frequency eight‐member annulene flipping vibrational mode. Thus, the fluorescence of COTh upon aggregation is induced by the suppression of the nonradiative decay channel contributed by the electron–vibration coupling between the transition electron and low‐frequency flipping vibration (see Figure 2.2e).

Table 2.2 Calculated k_r, k_ic, and Φ_F in both solution and aggregate phases at room temperature for TPA, (CAAC^Ad)CuCl, and COTh, respectively.