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3.2.5 Other AIEgens Involving RBDR Process

In addition to TPE, there are many other AIEgens with a double bond, in which the RDBR process is also involved in their luminescence emission.

Figure 3.32 Energy variations of the S₁ and S₀ states along the steepest‐descent (SD) pathway in the S₁ state of TPE‐2OMe.

Figure 3.33 Molecular structures of dinitriles DCNT and DCNP.

Kobayashi et al. [59] prepared dinitriles DCNT and DCNP (see Figure 3.33) that exhibited AIE and isomerization properties. When the solution of their E‐ or Z‐isomer was exposed to a UV lamp, the central ethylenic bond substituted by cyano groups could rotate and result in photoisomerization and fluorescence quenching. In the packed state, no isomerization of them was observed on the same experimental conditions due to the locked conformation of the compounds, providing a bright emission.

The (E)‐CN‐MBE is a typical AIEgen having great photophysical and self‐assembling characteristics, whose Φ_f is dramatically enhanced almost 700‐fold from solution to aggregation [8]. But (Z)‐CN‐MBE was the opposite, which emitted no fluorescence in both solution and aggregated states. Park et al. reported that the solid (Z)‐CN‐MBE became intense emissive when it was exposed to a UV lamp under ambient temperatures due to the EZI process [60]. It was thought that the bent‐shape structure of (Z)‐CN‐MBE led to loose packing, which was unable to effectively restrict the rotation of double bond even in the solid state. Therefore, nonradiative photoisomerization occurred. In contrary, the planar molecular structure of (E)‐CN‐MBE was easier to form tight packing, effectively blocking the double bond rotation (see Figure 3.34).

This inference was confirmed by Yamamoto’s calculation results [61]. Electronic structural calculations were employed to analyze the mechanisms of AIE and photo/thermal E/Z isomerization of CN‐MBE. In addition to study the single‐point PE changing based on ethylenic bond rotation (φ) of isolated CN‐MBE, free energy (FE) including thermodynamic influence from the environment was also considered.

In the PE profile of CN‐MBE, it was revealed that isomerization from E‐ (φ = 180°) or Z‐form (φ = 0°) in the S₀ state is difficult for CN‐MBE because of the large energy barrier (34 kcal/mol). However, in the S₁ state, the torsional motion of the double bond reduced the energy from the vertically excited FC points of the E‐ or Z‐form to the minimum‐energy point (φ = 90°) having no barrier. And the geometry corresponding to the minimum‐energy point of the conical intersection (MECI) between the S₀ and S₁ states of CN‐MBE demonstrated that the C═C bond had a significant twisting (φ = 75°), indicating that the rotation around the ethylenic C=C bond of CN‐MBE was an important coordinate that led to the S₀/S₁ CIs.

In the FE profile of CN‐MBE, both the solution state and crystal phase were calculated. In the THF solution (see Figure 3.35), no energy barrier existed in the FE profile of the S₁ state from the vertically excited FC geometries of the E‐ and Z‐forms (φ = 180 and 0°) to the twisted geometry (φ = 90 °). The S₀/S₁ CIs could be reached efficiently and facilitate fluorescence quenching after CN‐MBE photoexcitation; the molecule would show no emission when dispersed in dilute solutions.

In the crystal state, due to packing mode being different for two isomers, the simulation crystal structure of two forms of CN‐MBE showed that fractional free volumes of the E‐ and Z‐forms of CN‐MBE were found to be 22.1 and 24.2%, respectively, which indicated that the E‐forms were more densely packed than the Z‐forms in the aggregated phase.

Figure 3.34 Photos of (E)‐ (above) and (Z)‐CN‐MBE (below) under room light and UV light.

Figure 3.35 (a) Free‐energy profile of the changes in the torsional angle (φ) of the ethylenic C═C bond site of CN‐MBE in THF solution. (b) CN‐MBE in THF obtained from MD simulations.

Due to steric hindrance from the close stacking, it was energetically demanding for the rotation around the ethylenic bond in order to reach the S₀/S₁ CIs when the (E)‐CN‐MBE crystal was excited. Therefore, high emission is permitted in the solid phase. However, because of the loosely packed structure that allowed for the rotation around the ethylenic bond to reach the S₀/S₁ CIs, (Z)‐CNMBE did not exhibit fluorescence. Obviously, the restriction of the rotation of a double bond of (E)‐CN‐MBE is crucial for its emission in aggregates.

Kimizuka et al. [62] demonstrated aggregation‐induced photon upconversion (iPUC) based on control of the triplet energy landscape. Using AIEgen (2Z,2′Z)‐2,2′‐(1,4‐phenylene) bis(3‐phenylacrylonitrile) (PPAN) (see Figure 3.36) as an acceptor and Pt^II octaethylporphyrin (PtOEP) as a donor, when triplet states of acceptor were populated by a triplet sensitizer in solution, the TTA‐UC emission was not observed. In contrast, crystalline powder samples displayed a clear UC emission. For explaining such phenomena, the structure on the ground state (S₀) and the excited triplet state (T₁) both in solution and in the crystal was simulated. It was revealed that in solution, the double bond was dramatically twisted and stretched from S_0min to T_1min, resulting in the barrier‐free T₁‐to‐S₀ intersystem crossing (ISC). And eventually, no fluorescence was observed. However, in the rigid crystalline state, this conformational‐twisting‐driven transition was effectively prohibited.

Gierschner et al. [63] prepared a series of dicyano‐distyrilbenzene (referred to DCS) derivatives with two different CN substitution patterns (α and β in Figure 3.37). The α‐series compounds were AIE‐active, but the β‐series were radiative in both solution and crystal states except for β‐DCS. Evidently, this phenomenon contradicted to the principal of RIR; hence, computational studies were carried out to inspect the main reason for the difference between α‐ and β‐series.

The TDDFT and CASSCF calculation results showed that there was a CI between S₁ and S₀ for each compound in CHCl₃ when the double bond twisted 90° (φ_DB = 90°; see Figure 3.38). The energy of CI was identical for both α‐ and β‐series (2.78 eV), but the barrier was different for them to reach CI. In α‐series, due to the energies of initially excited FC (E_FC), the structure range from 2.9 to 3.2 eV was higher than that of CI, making the CI available for nonradiative decay. In β‐series, the energies of E_FC are lower than CI, making it difficult to access CI and eventually showing high emission in solution. This difference mainly comes from the distinct electronic nature of two series; the negative charge of the cyano‐group dramatically stabilizes the FC structure of β‐series due to their bigger resonance structures, but in α‐series, this effect is relatively weaker. In the crystal state, every compound has a bright fluorescence because the significant intermolecular interaction prevented the rotation of a double bond.

Figure 3.36 Potential energy curves of S₀ at its optimized structures.

Figure 3.37 Molecular structure of α‐ (left) and β‐series (right) and their fluorescence quantum yields in CHCl₃ and crystal [63].

Figure 3.38 Left: TD‐DFT rigid torsional scans of one double bond φ_DB for α‐DBDCS and β‐DBDCS using the optimized S₀ state in CHCl₃. Top: CASSCF calculated HOMO‐ and LUMO‐like orbitals. Right: TD‐DFT‐calculated FC energies and DFT‐calculated ground‐state energies.

Diphenyl dibenzofulvene (DPDBF) is an AIEgen similar to TPE that was first reported by Tang et al. in 2007 [64]. Probably, the rotation of a double bond in DPDBF is responsible for fluorescence quenching of its solution similar to that of TPE. Shuai et al. [65] carried out a nonadiabatic dynamics simulation for the 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).

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).

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.

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).

Handbook of Aggregation-Induced Emission, Volume 1

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