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

excited‐state nonradiative decay processes in open‐ and closed‐DPDBF and showed that the former exhibits an AIE property in contrast to the normal ACQ effect of the latter. The trajectory for open‐DPDBF showed that, after the initial excitation, the double bond length of open‐DPDBF increased quickly from its initial value of 1.37 to 1.55 Å after 10 fs, and the double bond rotation began correspondently (see Figure 3.39). There is a nonradiative transition point at 1206 fs; the energy for the S₀ and S₁ states approached each other with a gap of less than 0.5 eV. At this point, the two phenyl rings are nearly coplanar and the DBF ring is approximately perpendicular to the two phenyl rings. In contrast to open‐DPDBF, the C═C bond of closed‐DPDBF was restricted and the energy gap was relatively large at ~2 eV. Therefore, the energies of S₁ could not release to S₀ through such a point and emission was observed in solution.

Figure 3.39 (a) The chemical structures for open‐ and closed‐DPDBF. The temporal evolution of the energy gap (S₁–S₀) (red) and the average values of coordinates (b) C₂₁=C₃₃ (green) and (c) C₂₂–C₃₃=C₂₁–C₄ (green).

      Source: Reproduced with permission from Ref. [65]. Copyright 2012, Royal Society of Chemistry.

The studies reported above indicated that the single‐molecular nonradiative decay process of DPDBF mainly resulted from the rotation of the C═C bond, but further supplemental theoretical research of the AIE effect of DPDBF in the solid phase is needed. Blancafort et al. [66] combined solution and crystal computational simulation of DPDBF. In solution, the rotation of the C═C bond could reduce the energy of S₁ and eventually decayed further to the ground state through a (S₁–S₀) CI seam. But in crystal, the rotation was hindered by the surrounding molecules, which caused the CI structure to show higher energy than S₁. The CI seam is disfavored for solid DPDBF, and fluorescent intensity is significantly enhanced (see Figure 3.40). In 2015, they further investigated the MECI of DPDBF in the crystal state [67]. A cluster of 12 molecules (528 atoms) surrounding each other was relaxed during the MECI optimization, with one molecule being treated at the QM level. The results confirmed that the AIE effect of DPDBF was due to the packing of the molecules. Even when the molecules surrounding the excited molecule were allowed to relax, the rotation of the C═C bond was still hindered and the CI responsible for nonradiative decay in solution is not accessible energetically.

Figure 3.40 Calculated mechanisms for the photophysics of DPDBF in acetonitrile (a) and in the solid phase (b).

      Source: Reproduced with permission from Ref. [66]. Copyright 2013, Royal Society of Chemistry.

      In addition to these common AIE compounds discussed above, there are more examples to illustrate the importance of restricting the double bond rotation for certain AIEgens to render strong fluorescence. Liu et al. [68] report a computational study on the fluorescence quenching in methanol solution and fluorescence enhancement in crystal for 4‐diethylamino‐2 benzylidene malonic acid dimethyl ester (BIM).

The push−pull substitution of BIM could lead to a charge‐transfer (CT) structure and result in the fluorescence quenching of solution. The optimized results of the BIM molecule demonstrated that the double bond of the S₁ state was greatly stretched and its torsion was more serious than S₀, but the twisting of single bonds in the vicinity of a double bond was reduced. An S₁ minimum (referred to as S₁‐EM; see Figure 3.41) rendered weak emission and was energetically more stable than FC because of the torsion of a double bond. In addition, the rotation of both double bond and adjacent single bond could lead to the S₁ state geometry relaxing to the CT structure without barriers. But the energetic decrease from the S₁ state was much steeper for the former, suggesting that the former was the dominant S₁ decay channel. Due to the excited molecules decaying to a CT state, the fluorescence of solution was quenched through the S₁/S₀ CI near the CT intermediate.

      In the crystal state, the simulation works revealed that the energetic difference between FC and S₁‐EM state was much slighter than that of BIM in solution, suggesting that the surrounding molecules restricted the rotation of both double bond and single bond and blocked the energetic relaxation from the intramolecular motions. Moreover, the energy of the CT state was higher than that of the FC state, and the energy barrier made it impossible for BIM nonradiative decay through forming CT intermediate. Consequently, high emission channel was accessible for BIM molecules in crystal states.

Tang et al. [69] prepared a series of benzylidene methyloxazolone (BMO) derivatives with AIE activities. EZI process was observed in one BMO derivative BMO‐PH by ¹H NMR spectra (see Figure 3.42). When the Z‐isomer in CDCl₃ was irradiated by a UV light at 365 nm, the fraction of E‐isomer increased quickly in the first 35 minutes. To investigate the relationship between the rotation of a double bond and solution fluorescence quenching, theoretical calculations were carried out. The theoretical calculations of BMO‐PH via DFT/TD‐DFT showed that in the ground state, the energy barrier of a double bond rotation was at least 1.0 eV higher than the single bond rotation. But in the S₁ state, the barrier for the former was dramatically reduced and even lower than that for the latter. When torsion angles of the double bond were in the range of 70–120°, the formation of CI of S₁/S₀ was mainly responsible for the nonradiative decay of BMO‐PH in the solution. In the crystal state, no EZI product was detected through ¹H NMR, and high emission was observed.

Figure 3.41 Schematic representation of the conical intersection (left) and AIE mechanisms in BIM.

      Source: Reproduced with permission from Ref. [68]. Copyright 2016, American Chemical Society.

Figure 3.42 EZI process of BMO‐PH that was monitored by ¹H NMR spectra. No irradiation (upper spectra) and irradiation (lower spectra) by a 365‐nm UV lamp for 35 minutes in CDCl₃ (40 mM).

      Source: Reproduced with permission from Ref. [69]. Copyright 2013, Royal Society of Chemistry.

3.3 Conclusions

Most of the AIE molecules, especially the most extensively studied TPE and its derivatives, possess a critical carbon–carbon double bond. Therefore, whether the RDBR process is involved and plays a key role in the AIE mechanism is a very concerned question even from the moment when the AIE phenomenon is discovered. From no effect, minor effect, to key effect and major effect on the