The first molecule discussed here belongs to the typical propeller-shaped AIE system. Unlike crystals with periodic molecular packing, amorphous aggregates are structurally heterogeneous; it is a great challenge to investigate the aggregation effects on photophysical properties of AIEgens. Taking the emblematic hexaphenylsilole (HPS) as an example (see Scheme 1), the aggregation effect of HPS was systematically investigated by simulating four different sizes of amorphous aggregates (20, 30, 40, and 60) by combining MD and QM/MM calculations (Zheng et al., 2016). The embedded QM/MM model and exposed QM/MM model (insets in Figure 2A) were setup, respectively, to differentiate the different environments of HPS embedded inside the amorphous aggregate and exposed on the surface. In addition, for each aggregate size, five embedded molecules with different conformations were selected, and their photophysical properties were calculated accordingly to include the impact of the molecular packing difference. Compared to HPS crystal, the fluorescent emission in amorphous aggregate is red-shifted, giving a direct interpretation for the crystallization-enhanced emission phenomenon in the experiment (Dong et al., 2015; Zhao et al., 2020). The fluorescence quantum efficiency (FQE) was calculated by η _F≈k _r/(k _r + k _ic), where k _r is the radiative decay rate constant, and k _ic is the non-radiative decay rate constant, respectively. The FQE of both the embedded (>92.7%) and exposed HPS molecules (<7%) are size-independent, and the FQE of embedded HPS is 1–2 orders of magnitude larger than those of the exposed ones (Figure 2A). This is because the environment-insensitive k _r is hardly changed, but the k _ic of embedded HPS molecules are significantly smaller than those of exposed ones. Analyzing the key parameter of reorganization energy (λ) determining k _ic, it indicates that the λ of dihedral angles (λ _dihedral) mainly contributes to the different k _ic between the embedded and the exposed HPS molecules in amorphous aggregates (Figure 2B), implying that the densely packed amorphous aggregate significantly blocks the non-radiative decay channel of the excited state energy by retarding the electron–vibration coupling of the low-frequency rotational modes of phenyl rings in HPS. We conclude that the FQE of the nano-sized aggregate is size-independent, and the embedded molecules dominate their luminescent intensity; therefore we predict there is a linear relationship between the fluorescent intensity and aggregate size, which was also successfully confirmed in the experiment (Jiang et al., 2017; Figure 2C).

Cyclooctatetrathiophene (COTh, Scheme 1) is a non-aromatic annulene-based eight-member ring (Zhao J. et al., 2019); it demonstrates aromaticity reversal property upon excitation, following the Baird’s rule that the aromatic (anti-aromatic) molecule at the ground state (S₀) reverses to the anti-aromatic (aromatic) property at the lowest excited triplet state (T₁) (Baird, 1972). The aromaticity reversal can serve as a driving force inducing the significant conformational change to quench the emission. Thus, suppressing the excited-state aromaticity reversal of COTh turns the emission on (Zhao Z. et al., 2019). Theoretical calculations for COTh in both isolated and crystalline states were carried out to unravel the AIE mechanism at the atomic level. The isolated COTh was setup at the gas phase, and the crystalline state was simulated by the QM/MM model (Figure 3A) to consider the influence of molecular packing on its photophysical properties. Upon excitation, the dihedral angles of neighboring thiophene rings of COTh are dramatically changed; the eight-member ring became more planar (Figure 3B), while the corresponding bond lengths and bond angles are still similar. The change of dihedral angles upon irradiation in solution is more significant than those of the crystalline state, that is to say, the conformation of COTh in crystal is more constrained, supported by its smaller λ than that in solution. The larger λ in solution is mainly contributed by the change of dihedral angles (Figure 3C). In addition, the femtosecond transient-absorption spectra analysis also indicated that COTh has a very rapid molecular deformation in dilute solution, while the change is suppressed in the solid state.

The nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) analysis were performed based on the S₀ and T₁ transition-state structures. As shown in Figure 3D, the highly positive NICS_zz(1) at S₀ and negative NICS_zz(1) at T₁ indicate COTh is anti-aromatic at S₀ and aromatic at T₁; therefore the aromaticity reversal occurred upon excitation in the COTh system. It is also supported by the different ring-current of the eight-member ring at S₀ and T₁ states (Figure 3D). In general, the COTh molecule adopts a tub-like conformation at S₀ due to its non-aromatic feature (Liu et al., 2007; Nishinaga et al., 2010); it can suffer a quick conformational change to approach the planar/quasi-planar aromatic state (Kotani et al., 2020). Therefore, as illustrated in Figure 3E, there are two pathways for the isolated COTh molecule to stabilize the anti-aromaticity state at S₁, go-up and go-down to reach the minimum energy structures (MES) corresponding to the non-radiative decay of exciton, resulting in the quenched emission. However, crystal densely packing effectively restricts the molecular deformation process and essentially enhances its emission (Figure 3F).

Replacement of the C=C unit with its isoelectronic B–N unit can effectively alter the optoelectronic performances of polycyclic aromatic hydrocarbons (PAHs). NBN-5 and NBN-6 (Scheme 1) are two representative NBN-doped PAHs; NBN-6 is AIE-active, and NBN-5 could emit fluorescence in both solution and solid states (Wan et al., 2018). The photophysical properties of both compounds at different environments (including the dilute solution, amorphous aggregate, and crystal) were systematically explored by combining MD simulations and the thermal vibration correlation function–coupled QM/MM models (Zeng et al., 2021). The embedded and exposed QM/MM models were setup respectively (as discussed above) to consider the different molecular packing in amorphous aggregates. It is found that, upon excitation, the dihedral angle D₁ between rings A and B of NBN-6 exhibits significant changes in both solution (27.0°) and exposed (16.0°) states, which are much larger than those of the embedded (about 5.1°) and crystalline (3.9°) state; in the meanwhile, the conjugation and planarity of NBN-6 at S₀ are obviously improved after aggregation. Therefore, the rotation of ring A in NBN-6 can be effectively restricted in the aggregated state, leading to fluorescence enhancement. By contrast, the structural modifications of NBN-5 are pretty slight, with the largest structural change of D₁ 5.7° in solution (much smaller than that of NBN-6). Furthermore, the configurations of NBN-5 are insensitive to environments, keeping rigid and planar structures in all cases. It is clear to see that NBN-6 has intramolecular charge transfer (ICT) property; the highest occupied molecular orbital (HOMO) is located on ring B and naphthalene moiety, while the lowest unoccupied molecular orbital (LUMO) is distributed on rings A and B (Figure 4A). In contrast, both the HOMO and LUMO of NBN-5 are delocalized on the whole backbone (Figure 4B). The ICT property of NBN-6 leads to their relatively lower energy gap and redder fluorescence emission at all environments than those of NBN-5. AIEgens with ICT properties are quite sensitive to the environments (Zhang et al., 2020); the FQE of NBN-6 is highly environment-dependent, with k _ic decreasing by 2–4 orders of magnitude after aggregation; thus NBN-6 is AIE-active (Figure 4C). Meanwhile, the FQE of NBN-5 is environment-independent, showing bright emission in both solution and solid states (Figure 4D). Normal mode analysis of NBN-6 at different environments indicates that the suppression of the out-of-plane rotation and distortion of ring A after aggregation is the primary reason for the AIE effect.

Experimentally, a supramolecular host with a specific cavity can encapsulate proper-size AIEgens and form host–guest complexes, emitting fluorescence in the dispersed monomer (Liang et al., 2014; Song et al., 2014; Liang et al., 2016; Liow et al., 2017). However, the detailed structure–property relationship that determines the host–guest interaction–induced emission enhancement phenomenon remains elusive. A typical host molecule CD and a TPE derivative (G-3, Scheme 1) were taken as examples to study the influence of host–guest interactions on the photophysical properties of AIEgens by multiscale modeling protocol (Yang et al., 2021). MD simulations confirm that the host–guest inclusion complex 2CD/G-3(D) was formed by several cooperatively interplayed non-covalent interactions. On the one hand, the interior hydrophobic cavity of CD hosts one phenyl ring of the TPE moiety and partial PEG chain of the guest by the hydrophobic interaction. On the other hand, the exterior hydrophilic surfaces of CD fasten the PEG chain and adjacent phenyl rings of the TPE moiety of the guest by the intermolecular hydrogen bond and O-H^…π interactions, respectively. Importantly, three representative aggregates: G-3 aggregate, G-3 aggregate with 2CD, and 2CD/G-3(D) aggregate were also simulated by MD simulations to consider the aggregation effect. The QM/MM models for all three kinds of aggregates were setup accordingly to further obtain the photophysical properties (Figures 5A–C). The k _r and λ are calculated and summarized in Figure 5D, where λ measures the extent of intramolecular electron–vibration coupling; the decrease in λ implies the sharp reduction of k _ic. Introducing host–guest interaction, the fluorescent emission of the single inclusion complex 2CD/G-3(D) obviously increases relative to G-3 because of the slightly increased k _r and the sharply decreased λ. It is suggested that the host–guest interactions are responsible for hindering the non-radiative decay channel of the excited state energy of G-3. Upon aggregation, k _r of the G-3 aggregate and G-3 aggregate with 2CD are sharply boosted; at the same time, the corresponding λ is decreased, which is beneficial to the enhanced emission of aggregates. However, further increasing the concentration of CD is not conducive to luminescence because the high concentration of CD causes the disassembling of the 2CD/G-3(D) aggregate and the decrease of packing density; thus, the non-radiative decay channel is unblocked again. Therefore, the aggregation effect coupled with host–guest interactions in the G-3 aggregate with the 2CD system could significantly restrict the low-frequency rotational motions of phenyl rings and the C=C double bond twisting of the TPE moiety effectively; therefore the non-radiative decay channels of excited state energy in amphiphilic AIEgens are effectively blocked and finally enhances the fluorescent intensity.

The luminescent properties of organic molecules are highly morphology-dependent (Ma et al., 2016; Taylor and Wood, 2019; Fu et al., 2021). 4,4′-bis(9H-carbazol-9-yl)-methanone (Cz2BP, Scheme 1) was observed to emit room-temperature phosphorescence in a cocrystal consisting of chloroform but not in the amorphous nor the crystal phase (Li et al., 2015). The impact of the intermolecular hydrogen bond interactions on luminescent properties of Cz2BP was quantitatively investigated by the thermal vibration correlation function–coupled QM/MM calculations (Ma et al., 2019). It is found that compared with amorphous aggregate and crystal, the strong intermolecular hydrogen bond (C=O^…H−C) between Cz2BP and chloroform in cocrystal makes the densest molecular packing and effectively decouple the vibronic effect. For the T₁ state, responsible for phosphorescence, its relative compositions of (n, π*) and (π, π*) and the spin-orbital coupling coefficients (ξ) strongly depend on the aggregation. From amorphous to crystal to cocrystal, the ξ(T₁→S₀) decreases from 17.22, 9.57 to 5.52 cm⁻¹, while the corresponding (π, π*) components of the T₁ state are 59.8, 88.6, and 94.6%, respectively (Figure 6A). The electron–vibration coupling analysis (Figure 6B) indicates that the λ is dominated by high-frequency modes, including the stretching vibration of the C=O bond and the breathing vibration of benzene and carbazole units. In particular, the C=O stretching vibration (ω) is drastically reduced from 1,888.46 to 717.24 to 186.67 cm⁻¹ from amorphous to crystal to cocrystal, and the corresponding normal-mode displacement ΔQ is also regularly shortened, consecutively (Figure 6C). The quantum efficiency (Ф _P) of RTP is quantitatively analyzed by the equation Ф _P ≈ k _p/(k _p + k _nr); the results indicate that the calculated k _p of T₁→S₀ decreases about one order of magnitude in cocrystal, more importantly, k _nr of T₁→S₀ is largely reduced by 3−6 orders of magnitude from 1.87 × 10⁶ to 5.51 × 10³ s⁻¹ to 6.03 s⁻¹, leading to an efficient Ф _P (20.76%) in cocrystal relative to the extremely low Ф _P in the amorphous and crystal, and finally inducing a bright and long-lived RTP (Figure 6D). Therefore, both the decreased ξ(T₁, S₀) and the decreased λ of the C=O stretching vibration contribute to the sharply decreased k _nr, but the decoupling of the electron vibration from C=O plays the primary role in decreasing of k _nr.

Although a variety of doped organic systems with room temperature phosphorescence have been reported (Chen et al., 2021; Ning et al., 2021; Yan et al., 2021), the specific configuration and molecular packing of the guest molecules in the host matrix are still unknown. For example, we recently found that pure TPA crystalline powder only exhibits weak fluorescence, while the doping TPA matrix with no more than 0.1% guest molecule (MADBA, Scheme 1) simultaneously shows strong fluorescence, thermally activated delayed fluorescence, and efficient room temperature phosphorescence (Lei et al., 2021). We further setup a MADBA/TPA doping model with the molar ratio of 1: 190 by replacing two TPA molecules with a single large-sized MADBA molecule (Figure 7A). Then, MD simulations were performed to simulate the doped configurations and spatial molecular packing of the guest molecule in the host TPA matrix. To the best of our knowledge, this is the first use of MD simulations to study doped materials. The QM/MM model was further setup based on the equilibrated MD conformation of the doped system to study the photophysical properties. Since the larger volume of MADBA than that of the TPA molecule, the intermolecular distances between MADBA and surrounding TPA molecules are reduced with the intermolecular C–H^…π interactions enhanced, indicating a more rigid environment of MADBA in the doping system. After doping, the structural changes of MADBA from S₀ to T₁ in the doped state become smaller than those in the solution state. In addition, it is found that the energy gap (ΔE_ST) between S₁ and T₁ of MADBA is 0.98 eV, which is not facilitating the intersystem crossing (ISC) process (Figure 7B). The host TPA with a similar molecular structure shows higher T₁ energy (2.80 eV) than MADBA; therefore, the T₁ of TPA could act as a bridge to narrow the ΔE_ST from 0.98 to 0.31 eV to facilitate the ISC process of MADBA, namely, the intermediate T₁ of TPA is beneficial for the energy transfer from the T₁ of the host TPA to guest MADBA. Compared to pure MADBA crystal, the smaller ΔE_ST and larger ξ between S₁ and low-lying triplet states also support the easier ISC process in the doped MADBA/TPA system than that in the pure MADBA crystal (Figure 7C). Therefore, matching the energy levels between host and guest molecules could effectively bridge the energy transfer process of the low-lying triplet states and facilitate the ISC process, which will make room temperature phosphorescence more efficient in the doping system (Figure 7D). Therefore, doping trace amounts of MADBA is beneficial to promote ISC of excitons, thereby leading to phosphorescence emission in the host matrix.