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

be hindered and make the excitons stabilized in the higher excited state to generate enhanced anti‐Kasha emission. However, these results still remain controversial. Zhou et al. have recently employed more DFT functions to recheck the excited‐state properties of the BODIHY derivatives and have challenged the SOKR mechanism [11b]. They have found that the TDA‐PBE method used in Ref. [11a] may not describe the correct order of the excited states, and the energy gaps between S₃ and S₂ states obtained from this method are small enough to generate efficient internal conversion from S₃ and S₂ states. Hence, the emission of BODIHY derivatives in higher viscosity may not be induced by the SOKR. Instead, they proposed that the restriction of access to the CI caused by the flip‐flop motion is responsible for the AIE behaviors of BODIHY derivatives.

Guo et al. has recently reported another luminogen named DMF‐BP‐PXZ with highly efficient light emission from the S₂ state (Figure 1.9a, b) [11c]. According to the calculation, the S₁ state is a transition‐forbidden dark state, and the internal conversion dominates the decay process in the solution due to the severe molecular motions, so the rapid internal conversion from S₂ to S₀ state, through the intermediate S₁ state, quenches the light emission. However, its k_IC from S₂ to S₀ can be suppressed by four orders of magnitude, and the fluorescence radiation rate can be enhanced in the solid state, which leads to the efficient light emission from S₂ state to the ground state. In this regard, further experimental evidence for the stable population of higher excited states is still highly desired to solidify the claim on the anti‐Kasha emission, but the mechanism demonstrated earlier also belongs to the category of RIM, and it validates the wide reliability of the RIM mechanism.

      Exciton population on the higher excited states can also occur in the triplet states. Taking the ClBDBT as an example (Figure 1.9c, d), it exhibits white‐light emission under UV light and persistent yellow afterglow in the room temperature [11d]. According to the calculated energy levels, T₁ and T₂ states are all lower than the S₁ state in energy, which makes both T₁ and T₂ accessible for the exciton population coming from the S₁ state. Furthermore, the T₂ state mainly contains the (n,π*) transition character, which leads to a larger spin–orbit coupling (SOC) between T₂ and the S₀ and a higher radiative decay rate, whereas the T₁ state contains more (π,π*) transition character. At room temperature, the small energy gap between T₂ and T₁ can promote the thermal population from T₁ to T₂. According to Boltzmann distribution, T₂ has a smaller population than T₁, but the faster radiative decay from T₂ results in a balanced emission intensity from both T₂ and T₁ states. Thus, the combined anti‐Kasha blue light from T₂ and the yellow light from T₁ generate the efficient white‐light emission at room temperature.

      In fact, it is also the RIM process in the crystal state that stabilizes the specific electronic structures of T₂ and T₁ and restricts the nonradiative decay from triplet states to the ground state, and then the balanced dual emission can be restored.

Figure 1.9 Molecular structure, calculated energy levels, fluorescence, and internal conversion rate constants of DMF‐BP‐PXZ in the (a) solution and (b) solid state.

      Source: Adapted from Ref. [11c] with permission from John Wiley and Sons.

      (c) Molecular structure, calculated energy levels, and (d) emission spectra of ClBDBT in the solid state.

      Source: Adapted from Ref. [11d] with permission from Springer Nature.

1.6 Through Space Conjugation

      Most of the classical AIEgens are constructed by chromophores with through‐bond conjugation (TBC), and their emission can be enhanced through restricting the nonradiative decay driven by molecular motions, whereas the light emission can also be boosted by promoting the radiative rates through the through‐space conjugation (TSC) [13]. The TSC plays a key role in radiative decay processes of molecular systems with clusterization‐triggered emission (CTE) property [21]. Moreover, a certain degree of molecular motions in the solid state will facilitate the intra‐ or intermolecular excited‐state TSC for the nonconjugated molecules and stabilize the radiative channels and, thus, promote the emission intensity [13].

      1.6.1 Clusterization‐Triggered Emission

Traditional luminogens, generally, consist of aromatic groups or other conjugated building blocks and take the two‐dimensional conjugated structure connected by chemical bonds. In such molecular systems with TBC, the EVC between emissive states and the ground state, the access to CIs, or the dark states driven by molecular motions are usually the detrimental causes for emission quenching. However, researchers have discovered and developed a nonconventional type of luminogens without any long‐range conjugated structures but containing only isolated units such as heteroatoms with lone‐pair electrons, unsaturated C=C, C=O, and C≡N groups as presented in Figure 1.10. Although this kind of luminogens lacks luminescent centers intuitively, and they are almost nonemissive in the dilute solution, they can emit visible light efficiently in the aggregate or solid state, showing the typical AIE behaviors. Because of the lack of largely conjugated luminescent centers and much higher motion ability than the traditional molecules with long‐range conjugation, such nonconjugated molecules are nonemissive in the isolated state. However, it can be noticed that these heteroatom‐containing groups usually exist as amide, imide, ketone, anhydride, or ester subunits connected by the saturated bonds in the macromolecule backbones, and they are usually rich in electrons. Hence, once the aggregation or cross‐linking occurs, electron‐rich moieties can cluster together and form a relatively stable long‐range through‐space electron ocean and energy bands by the electron overlapping for the electronic transition, which is similar to the energy band structures in the inorganic semiconductors. Upon excitation, electrons can jump into the excited state based on the stabilized through‐space energy bands and light up the whole cluster system.

Such nonconventional phenomenon has been coined as CTE; it deciphers that the severe molecular motions and nonconjugated structures make the CTE luminogens nonemissive in the isolated state or the dilute solution, whereas diverse intra‐ and intermolecular interactions in the cluster or cross‐linking can hamper the molecular motions and stabilize the conformations with large electron overlapping and thus boost the light emission of CTE luminogens, in which the packing density and the cluster size can affect the emission efficiency [21]. Taking the polyacrylonitrile (PAN) [21b] and poly(N‐hydroxysuccinimidyl methacrylate) (PNHSMA) [21c] as the example, they contain no aromatic groups and are nonconjugated in the isolated polymer backbone (Figure 1.10c). Therefore, these two molecules are nearly nonemissive in the dilute solution. But they can emit strong blue light in the concentrated solution or solid powder. CTE phenomenon also exists in the natural products such as starch, cellulose, and protein; they can show notable blue light in the solid state under UV excitation [21d].