Handbook of Aggregation-Induced Emission, Volume 1. Группа авторов
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conjugation, resulting in the increase of the electric transition dipole moment (μ) from 5.20 to 5.85 Debye (Table 2.1). Hence, upon aggregation, the calculated kr increases 6 times, while kic decreases about 4 orders of magnitude (Table 2.1). The nonradiative decay channels are generated by the coupling between the electrons and thermal vibrations that can be quantified by the molecular reorganization energy
Figure 2.2 (a) PES scheme of AIEgens in solution and solid states. (b–g) Overview of vibrational modes involved in the excited‐state nonradiative decay channels, which contribute to AIE.
A similar AIE mechanism is followed by the other siloles, with the degree of π‐conjugation of the 2,5‐substituents increasing from TPS [47], BrTPS [48], HPS [45], BTPES [48] to BFTPS [49] (Figure 2.3). As shown in Table 2.1, as the conjugation increases, ∆Eg in both gas phase and solid state decreases regularly. Simultaneously, the μ value increases drastically due to the electron delocalization. The balance of excitation energy and μ makes kr to first increase and then levels off as the extension of conjugation from TPS to BFTPS. After aggregation, λtotal for all AIEgens decreases obviously and kic decreases by more than 2 orders of magnitude from 9.30 × 105 s−1 for TPS to 1.22 × 108 s−1 for BFTPS. Overall, with the degree of π‐conjugation of the 2,5‐substituent increases, ΦF in the solid state first increases sharply, then levels off, and finally starts to decrease slightly, as the competition between kr and kic. Similar to the above discussed HPS, the dramatic decrease of kic upon aggregation turns the fluorescence on, which is mainly caused by the decoupling of the electron and low‐frequency rotational normal modes (Figure 2.4a) [38].
Figure 2.3 Overview of molecular structures discussed in Sections 2.3 and 2.4.
2.3.2 Stretching Vibrations of Bonds
Terephthalic acid (TPA) is a representative AIEgen, which could emit both fluorescence and phosphorescence simultaneously [41]. Through a combined QM/MM approach, the photophysical properties of TPA in both gas phase and crystal were investigated to unravel the effect of crystallization on the nature of the molecular excited states. It is found that the nature of S1 changes from (n,π*) in the gas phase to (π,π*) in crystal because the excitation energy of (n,π*) is increased up to 5.05 eV from 4.81 eV, whereas the (π,π*) state is reduced to 4.76 eV from 4.99 eV due to the strong electrostatic interaction upon aggregation. Accordingly, the oscillator strength of S1 is largely enhanced to 3.49 × 10−2 in the crystalline phase from 3.33 × 10−5 in the gas phase, which recovers the radiative decay from S1 to S0. And the resultant kr is largely increased by 3 orders of magnitude from 3.34 × 104 s−1 in the gas phase to 3.43×107 s−1 in the solid phase.
Table 2.1 Calculated HOMO–LUMO energy gap (∆Eg), electric transition dipole moment (μ), total reorganization energy (λtotal), kr, kic, and ΦF in gas phase and aggregate phase at room temperature for HPS, TPS, BrTPS, BTPES, and BFTPS, respectively.
| ∆Eg (eV) | μ (Debye) | λ total (meV) | k r (s−1) | k ic (s−1) | Φ F (%) | |
|---|---|---|---|---|---|---|
| In gas phase | ||||||
| HPS [37] | 3.59 | 5.20 | 492 | 1.05 × 107 | 3.76 × 1011 | 0.003 |
| TPS [38] | 3.21 | 0.58 | 1120 | 9.30 × 105 | 1.62 × 1010 | 0.01 |
| BrTPS [38] | 2.90 | 1.71 | 1161 | 5.55 × 106 | 5.72 × 109 | 0.09 |
| HPS [38] | 2.62 | 5.26 | 891 | 4.98 × 107 | 2.53 × 1010 | 0.20 |
| BTPES [38] | 2.48 | 6.03 | 667 | 6.76 × 107 | 2.66 × 1010 | 0.25 |
|
BFTPS [38]
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