Molecular mechanism of aggregation-induced emission

Quantitative description of the blockage of NR-VR

As seen from the comparison between the radiative and nonradiative decay rate constants summarized in Tables 1 and 2 for AIEgens in the gas phase/solution and solid phase, it is easily found that the radiative decay rate constant is almost independent from the environment, while the nonradiative decay rate constants greatly decrease from solution to aggregate, which leads to the occurrence of strong fluorescence in aggregates. Taking HPS an example, we disclose the origin of the change of the nonradiative decay rate by analyzing the nonadiabatic coupling, electron–vibration coupling and vibration–vibration mode mixing (Duschinsky rotation effect, DRE) during the excited state decay process in Figure 9.^[66] The nonadiabatic coupling provide the driving force of internal conversion from S₁ to S₀, whose coupling matrix elements are calculated by $⟨{\mathrm{\Phi }}_{\mathrm{f}}|\frac{\partial }{\partial Q_{\mathrm{f}l}}|{\mathrm{\Phi }}_{\mathrm{i}}⟩\approx -\frac{⟨{\mathrm{\Phi }}_{\mathrm{f}}^0|\partial \widehat{V}/\partial Q_{\mathrm{f}l}|{\mathrm{\Phi }}_{\mathrm{i}}^0⟩}{E({\mathrm{\Phi }}_{\mathrm{i}}^0)-E({\mathrm{\Phi }}_{\mathrm{f}}^0)}$ based on the first-order perturbation theory; the electron–vibration coupling characterizes the ability of molecular vibrations to accept the electronic excited-state energy during the internal conversion, which is computed via ${\lambda }_i=\frac{1}{2}{\omega }_i^2{D}_i^2$ for the ith normal mode; the mode mixing corresponds to multichannel internal conversion of a vibration mode of S₁ to many modes of S₀, in which Duschinsky rotation matrix (DRM) is obtained via $\mathrm{DRM}=L_{\mathrm{i}}^T{L}_{\mathrm{f}}$ (L_i(f) is the normal mode with mass weighted in the initial (final) electronic states. From Figure 9, it can be seen that the nonadiabatic coupling is insensitive to the environment. While the electron–vibration coupling and the degree of mode mixing are very susceptible to aggregation: (i) the low-frequency vibration modes are significantly blue-shifted with the number of modes with energy less than 100 cm⁻¹ decreased from 15 to 6; (ii) the electron–vibration couplings are notably weakened, especially those of the low-frequency modes; and (iii) many modes are segregated from each other with many vanishing off-diagonal terms from Figure 8C to 8D (when the off-diagonal element of DRM is not zero, the corresponding two modes are mixing each other) when going from solution to aggregate. These changes slow down the nonradiative decay from S₁ to S₀, recovering strong light-emitting in aggregate.

Based on the above analysis, we propose the micromechanism of these investigated AIEgens and plot the change feature of the potential energy surface from solution to aggregate in Figure 10. In light of the important contribution of low-frequency modes to the AIE of HPS, we present the PESs of S₀ and S₁ of two low-frequency modes as a representative. In solution, the PESs of S₀ and S₁ are very flat and far apart from each other. Moreover, each mode of S₁ is mixing with many modes of S₀ (taking two modes an example). At room temperature, many vibrational modes with high quantum numbers are activated to dissipate the excited-state owing to strong electron–vibration coupling, and a lot of nonradiative channels are opened due to effective mode-mixings. Both of these two processes highly speed up the nonradiative decay rate. When going to aggregate, the PESs become very steep because the intramolecular vibrational motions are restricted by the intermolecular interactions in compact molecular-packing aggregate. Accordingly, the number of activated vibration states and the nonradiative decay channels are decreased because of the weak electron–vibration couplings and subtle mixing among modes. Consequently, the nonradiative decays rate is sharply decreased. Theoretical investigations of a tremendous amount of systems demonstrate that aggregation can effectively attenuate the couplings between electron and a great variety of vibration modes, including rotating,^{[15, 58-60, 66]} twisting,^{[15, 68]} stretching,^[21] bending,^[69] and flipping^[70] vibrations by virtue of their special intermolecular interactions (see Figures 10B–G), which all significantly restrict the molecular geometrical relaxation and slow down the nonradiative decay, thus inducing strong fluorescence upon aggregation. These findings provide sound theoretical evidences for the RIR, RIV and RIM mechanism proposed by Tang's group from the perspective of molecular conformations.^{[8, 9, 14, 68]} So far, theoretical calculation have drawn a clear picture to demonstrate the change of electron–electron, electron–vibration, vibration–vibration coupling, and the effect of them on the nonradiaitive decay rate constants from solution to aggregate, which unravels deep insights of various experimental phenomena.

Experimental verifications of the theoretical predictions

The AIE mechanism revealed above focused on the change of electron–vibration coupling and vibration–vibration mixing upon aggregation. In order to visualize the theoretical parameters in experiments to confirm their contributions to AIE, we bridge the theoretically calculated parameters and experimentally measurable physical parameters.

Resonance Raman spectroscopy (RRS) is a spectroscopic technique to characterize molecular excited-state dynamics, including but not limited to molecular vibrational redistribution and electron–vibration couplings.^{[71, 72]} Because the intensity $\sigma ({\omega }_j)$ of the RRS signal from the jth normal mode is proportional to the product of electron–vibration coupling ${\lambda }_j$ and frequency ${\omega }_j$ ($\propto {\omega }_j{\lambda }_j$) under the FC approximation and the resonance condition,^[72] we proposed that the RRS can be used as an effective tool to detect the aggregation effect on the λ, validating the AIE mechanism revealed above.^[73]

Taking two AIEgens HPDMCb and TPA as examples, their calculated RRS intensity (σ) and electron–vibration coupling (λ) are plotted in Figure 11 in isolated and solid phases. Comparing σ and λ of HPDMCb and TPA, it is clearly found that their distribution features among vibration modes are almost identical in both phases. For HPDMCb, the low-frequency peaks have remarkable blue-shift with sharply decreased RRS intensity while the high-frequency peaks are almost unresponsive in both position and intensity. Different from HPDMCb, the low-frequency peaks of TPA are insensitive to the environment while the high-frequency peaks have changed significantly in intensity. The intensity of the peak corresponding to C = O stretching vibration is largely decreased while that of the peak from C = C stretching vibration is remarkably increased when TPA going from solution to aggregate owing to the appearance of hydrogen bonding interaction. All these findings indicate that the RRS can rationally reflect the change of the electron–vibration coupling in different environments, which fully confirm the contribution of electron–vibration coupling to the AIE process. The AIE mechanism are thoroughly validated as the fact that the decoupling between the transition electrons and low-frequency vibrations in HPDMCb and high-frequency vibrations in TPA significantly slows down the nonradiative decay rate, making strong fluorescence appear upon aggregation.

Isotope effect (IE) has been widely applied to probe the nonradaitive decay process.^{[74, 75]} Herein, deuteration is always used an effective mean to decrease the nonradiative decay rate so as to greatly improve the luminescence efficiency for conventional luminogens. Conventional luminogens are mostly rigid π-conjugated systems, which obey the Fermi-golden rule under displaced harmonic oscillator model for the nonradiative decay rate.^[76] In this case, the nonradiative decay rate constant is proportional to$\frac{1}{\overline{\omega }}{e}^{-S}\frac{S^n}{n!}$, in which S can be determined by $S=\lambda /\hslash \omega$. $\lambda$ is equal to the difference of the excitation energies at the equilibrium S₀-geomtery and S₁-geometry, which is unchanged due to the same geometrical and electronic structures after deuteration. Thus, S is increased owing to the decrease of frequency, which consequently decreases the nonradiative decay rate based on the exponentially proportional relation between the rate constant and S as given above.

Differently, the flexible AIEgens in solution are beyond the description of the displaced harmonic oscillator model because the mixing among vibrational modes or DRE is severe in the excited-state decay process. Especially, in the AIEgens with large reorganization energy introduced from low-frequency modes, the DRE becomes more serious for much more activated low-frequency vibration states after deuteration, which sharply accelerate the nonradiative decay rate. Hence, the deuteration brings up two contradictory effects on the nonradiative decay rate, the negative effect through increased S and the positive effect via strengthened DRE. When the latter outstrip the former, there would be abnormal positive isotope effect, and vice versa. We define the isotope effect as $\mathrm{IE}=(k_{\mathrm{nr}}^{\mathrm{D}}-k_{\mathrm{nr}}^{\mathrm{H}})/k_{\mathrm{nr}}^{\mathrm{H}}$ and calculate the isotope effect of a series of AIE-active systems and non-AIE ones by combining TVCF rate formulism in MOMAP program and (TD) DFT with PCM for solution and ONIOM approach in Gaussian 09 program for solid as seen in Figure 1.^[60] It is expectedly seen that AIEgens in solution exhibit abnormal positive isotope effect owing to strong vibration–vibration coupling, while the AIEgens in solid phase recover normal negative isotope effect because of the decoupling of the vibration–vibration upon aggregation, as analyzed above. The isotope effects of non-AIEgens are independent from the environment owing to marginally vibration–vibration mixing. Overall, for AIEgens, a more remarkable IE exists in solid phase than in solution, while for non-AIEgens, high IE happens in both solid and solution phases. So far, these fully prove the important contribution of DRE or vibration–vibration coupling on the AIE. At the same time, the isotope effect can be used as a mean to probe the AIE property at the molecular level.

Restricted access to a MECP and experimental visualization

As analyzed above, the nonradiative decay via MECP is sometimes a dominant factor to quench emission in solution, and its removal leads to strong fluorescence in aggregate. Li and Blancafort first proposed restricted access to a conical intersection (RACI) to rationalize the AIE phenomenon of diphenyl dibenzofulvene (DPDBF).^[20] Since then, RACI has been used for the explanation of the AIE phenomenon for many AIEgens.^[77-82] We here illustrate the RACI mechanism via an example of 4-diethylamino-2-benzylidene malonic acid dimethyl ester (BIM) with AIE activity, whose fluorescence quenching in solution and fluorescence enhancement in crystal are investigated by Liu group via QM (TDDFT and CASSCF) and ONIOM (QM:MM) calculations.^[80] The calculated S₁ potential energy surface along the torsion coordinate of the styrene double bond in solution shows that upon excitation, BIM first initiates a FC-like local minimum (S₁-EM), then experiences a barrierless relaxation to a low-energy intermediate with charge-transfer character (S₁-CT), then reaches a S₁/S₀ conical intersection (S₁/S₀-CIb), and finally nonradiatively comes back to S₀ via the CIb, as depicted in Figure 13. The barrier to access S₁/S₀-CIb is very small from S₁-EM, which indicate that S₁/S₀-CIb is responsible for the fluorescence quenching of RIM in methanol. In the crystalline phase, the rigid environment precludes the torsion of the styrene double bond, which restricts the access to a conical intersection with much higher energy. As a result, RIM is trapped in the S₁-EM and the S₁-EM emits strong fluorescence.

How to prove the occurrence of nonradiative decay via VR ($k_{\mathrm{nr}}^{\mathrm{HR}}$), or CI or MECP ($k_{\mathrm{nr}}^{\mathrm{BHR}}$), or both them? We here call the temperature dependence of the nonradiative decay rate to mind. Because for organic fluorophores, the nonradiative decay via MECP always needs to cross over an energy barrier, either to a transition state (TS) or to a high-energy MECP, $k_{\mathrm{nr}}^{\mathrm{BHR}}$ is likely to be very sensitive to temperature and have a sudden growth when energy reaching the mutation point at certain temperature. On the contrary, the nonradiative decay via VR is a decay process from high-energy excited state to low-energy ground state without an energy barrier, $k_{\mathrm{nr}}^{\mathrm{HR}}$ is not as sensitive to temperature as $k_{\mathrm{nr}}^{\mathrm{BHR}}$ and behaves a monotonically slight increase with the increase of temperature. Consequently, the excited-state lifetime and luminescence quantum efficiency combining the two nonradiative processes will exhibit nonmonotonic behaviors with a “knee point” as temperature increases. Keeping this in mind, we exemplify fac-Ir(F₂ppy)(ppz)₂ (ppy = 2-phenylpyridine and ppz = phenylpyrazole) with comprehensive photophysical data from experiments^[83] to quantitatively investigate the $k_{\mathrm{r}}$, $k_{\mathrm{nr}}^{\mathrm{HR}}$, and $k_{\mathrm{nr}}^{\mathrm{BHR}}$ and excited-state lifetime τ at different temperatures in THF solution.^[84] The emissive state of fac-Ir(F₂ppy)(ppz)₂ is a triplet excited state. We first construct the PES of T₁ and S₀ by locating the minimum point of S₀ (S₀-geometry) and the local minimum point of T₁ with metal–ligand charge transfer transition (³MLCT) in the harmonic region, and TS and local minimum with metal–center transition (MC) and MECP between T₁ and S₀ (MECP) beyond harmonic region, as seen in Figure 14A. Herein, the optimizations except for the MECP are performed at the level of B3LYP with LANL2DZ for Iridium atom and 6–31G (d,p) basis set for the others in Gaussian 09 package. The MECP is optimized using Harvey's algorithm at the B3LYP/ECP-60-mwb/def2-SVP level and its activation energy is corrected by PWPB95-D3/ECP-60-mwb/def2-SVP in the ORCA package.^[85] From Figure 14A, it is easily seen that the emissive ³MLCT can directly radiatively decay to S₀ as light ($k_{\mathrm{r}}$), directly nonradiative decay to S₀ via vibration relaxation ($k_{\mathrm{nr}}^{\mathrm{HR}}$), and indirectly nonradiative decay to S₀ by a two-step reaction of ${}^3\mathrm{MLCT}\stackrel{\mathrm{TS}}{\rightleftarrows }{}^3\mathrm{MC}\stackrel{\mathrm{MECP}}{\to }{\mathrm{S}}_0$. In the two-step reaction, the barrier to populate the TS plays a decisive role because the largest geometrical variation takes place from the ³MLCT (pseudo-octahedral) to the ³MC (trigonal bipyramid) point (³MLCT → ³MC) via the broken of one Ir-N bond and the rotation of relative ligand.^[86]

Based on the obtained geometrical and electronic structure information, k_r is calculated with the Einstein relationship by considering Boltzmann population of three substates of triplet state; $k_{\mathrm{nr}}^{\mathrm{HR}}$ is computed with the TVCF approach in MOMAP program; and $k_{\mathrm{nr}}^{\mathrm{BHR}}$ is obtained by adapting the kinetic model in reference 86, in which ³MLCT → ³MC is the rate limiting step and the equilibration between ³MLCT and ³MC is considered and the rate constant can be simply calculated as $k_{nr}^{BHR}=A_{0}A\exp (-E_a/k_{B}T)$ under steady state approximation. Here, A₀ is a temperature-dependent prefactor and$A_0={[1+\exp ((E_b-E_c)/k_{B}T)]}^{-1}$; A stands for the pre-exponential factor obtained by canonical variational transition state theory (CVT) implemented in POLYRATE program^[87]; and E_a, E_b, E_c are the activation energies marked in Figure 14A. Figures 14C and 14D show the experimentally measured and theoretically calculated plot of the excited state lifetime of$\tau =1/(k_{\mathrm{r}}+k_{\mathrm{nr}}^{\mathrm{HR}}+k_{\mathrm{nr}}^{\mathrm{BHR}}$) versus temperature, as well as the calculated$\tau =1/(k_{\mathrm{r}}+k_{\mathrm{nr}}^{\mathrm{BHR}}$) for comparison, and the detailed data are collected in Table 3. The sigmoid-like dependency is well reproduced by the calculated results. Three distinct regimes are depicted in Figure 14D, that is, a slight lifetime decrease at 77–220 K and a second more significant drop at 220–320 K, and a third almost unchanged tendency at temperature higher than 320 K. The two key inflexion points are at ca. 220 and 320 K, which are very close to experimentally observed values, that is, 250 and 340 K. It is easily understood that k_r slightly grows up owing to the increasing population of the substates with the largest k_r value among the three substates from ³MLCT observed at 77–130 K, and then it is almost unchanged when T > 130 K because the thermally activated vibrations always largely broaden and redshift the spectrum lineshape (instead of the integral area). $k_{\mathrm{nr}}^{\mathrm{BHR}}$ is expected to slightly increase because of small relaxation energy that exclusively comes from high-frequency stretching and in-plane deformation vibrations.^[84] At the low-temperature regime, only k_r and $k_{\mathrm{nr}}^{\mathrm{HR}}$ components contribute to the lifetime because the temperature cannot provide enough energy to reach the barrier for proceeding significant $k_{\mathrm{nr}}^{\mathrm{BHR}}$. Therefore, the temperature effect is not obvious at low temperature. However, when temperature reaches to a certain point, the molecule has enough energy to cross over the barrier to produce the transform ³MLCT → TS → ³MC. $k_{\mathrm{nr}}^{\mathrm{BHR}}$ becomes significant only when temperature going above ca. 230 K and hugely influences the quantum efficiency and lifetime by directly competing with radiative decay at room temperature, and then evolves into the most prominent deactivation channel that significantly quenches the luminescence at high temperatures. The relevance of $k_{\mathrm{nr}}^{\mathrm{BHR}}$ is further illustrated in Figure 14D by switching off its contribution to lifetime of $\tau =1/(k_{\mathrm{r}}+k_{\mathrm{nr}}^{\mathrm{BHR}}$) (dashed line in Figure 14D). The sigmoid-like temperature-dependence is fully lost without taking $k_{\mathrm{nr}}^{\mathrm{BHR}}$ into consideration, which is not consistent with the experimental observation. At the same time, this work provides a criterion for judgment whether the MECP occur in the excited-state decay processes both in solution and aggregates.

Aggregation-dispelled isomerization and experimental confirmation

Except for the MECP, photoexcited isomerization is another nonradiative decay channel to quench luminescence beyond the harmonic region, termed as NR-ISO. Generally, most AIEgens have twist/rotor structures while aggregation-caused quenching (ACQ) molecules possess planar conjugated structures. However, Huang's group found six planar molecules with analogous structure that show completely differen emission behaviors in experiment. 1,2-di(2-selenophenyl)-ethene (SVS), 1,2-Di(2-thienyl)-ethene (TVT), and (E)-1,2-bis(thieno[3,2-b]thiophen-2-yl)ethane (TTVTT) molecules exhibit typical ACQ phenomena, while SVS(OEt), TVT(OEt), and TTVTT(OEt) show interesting AIE behavior.^[62] Therefore, it is very meaningful to investigate the mechanism of their completely different emission behaviors from their similar planar structures.

We choose TTVTT and TTVTT(OEt) as representatives and first construct the PESs of S₀ and S₁ by a relaxed scan at the level of (TD)B3LYP/6-31G(d, p) in Gaussian 09 package, as seen in Figures 15A and B. It is obvious that the PESs of the two compounds both have double wells, corresponding to cis- and trans-geometries, respectively. The trans-geometry is more stable owing to its lower energy than that of cis-geometry, which is in line with the single-crystal X-ray structure. The isomerization barriers from trans to cis are very high for TTVTT(OEt) (1.26 eV) and TTVTT (1.23 eV) in the S₀ state. However, the barriers sharply reduce in the S₁ state up to 0.45 eV of TTVTT(OEt) and 0.67 eV of TTVTT. Thus, the introduction of -OCH₃ destabilizes the planar conformation and facilitates the occurrence of isomerization from trans- to cis-TTVTT(OEt) in the S₁ state.^[88]

We further compare the luminescence property of trans- and cis-TTVTT(OEt) by computing their $k_{\mathrm{r}}$ and $k_{\mathrm{nr}}^{\mathrm{HR}}$ in the harmonic range of locally optical transition using the TVCF approach in MOMAP program. It is found that $k_{\mathrm{r}}$ of trans-TTVTT(OEt) (1.1 × 10⁸ s⁻¹) is two orders of magnitude larger than that of cis-TTVTT(OEt) (6.2 × 10⁶ s⁻¹), which mainly stems from larger transition dipole moment of trans-TTVTT(OEt) (18.56 Debye) with a more planar conjugated structure than that of cis-TTVTT(OEt) (6.42 Debye) with twisted conformation. On the contrary, $k_{\mathrm{nr}}^{\mathrm{HR}}$ of trans-TTVTT(OEt) (5.9 × 10⁶ s⁻¹) is five orders of magnitude smaller than that of cis-TTVTT(OEt) (1.4 × 10¹¹ s⁻¹), which is caused by the rigidity of trans-TTVTT(OEt) and flexibility of cis-TTVTT(OEt). Consequently, trans-TTVTT(OEt) emits very bright fluorescence with $k_{\mathrm{r}}$> >$k_{\mathrm{nr}}^{\mathrm{HR}}$ whereas cis-TTVTT(OEt) shows dark with $k_{\mathrm{r}}$< <$k_{\mathrm{nr}}^{\mathrm{HR}}$ in the gas phase. Overall, the whole photophyscial process in solution is described as the following: after absorbing a phonon the stable trans-TTVTT(OEt) first jumps from S₀ to S₁, then transforms to cis-TTVTT(OEt) along the S₁-PES, and finally nonradiaitively goes back to S₀ of cis-TTVTT(OEt) without generating visible fluorescence. The lifetime of cis- TTVTT(OEt) is calculated to be 7.1 ps, being consistent with the experimental one (<30 ps). When going in aggregate, the isomerization from trans-TTVTT(OEt) to cis-TTVTT(OEt) are inhibited, resulting in a strong fluorescence from trans-TTVTT(OEt). Therefore, the aggregation-dispelled photoisomerization from bright state (trans-conformation) to dark state (cis-conformation) is responsible for the unusual AIE activity of these highly planar molecules.

To confirm the photoisomerization mechanism proposed above, a series of measurements are performed, including UV−Vis and 1H NMR before and after irradiated by an UV lamp for TTVTT(OEt) in THF, and the results are shown in Figures 15D–F. As expected, after a period of irradiation, a new set of absorption bands start to appear and the intensity is gradually increased, while the intensity of original bands is contrarily decreased (Figure 15D), which indicates the generation of a new compound. A new set of peaks (7.66, 7.41, 7.37 ppm) in 1H NMR also arise after 3-hour irradiation, which underlines that the new compound is cis-TTVTT(OEt). By this time, the mixture of cis- and trans-TTVTT(OEt) exhibit nonemissive characteristics in solution, which verify the dark S₁ state of cis-TTVTT(OEt) with $k_{\mathrm{r}}$< <$k_{\mathrm{nr}}^{\mathrm{HR}}$. However, the UV–vis spectra of TTVTT (Figure 15E) are almost identical before and after irradiation for 10 hours, suggesting photoisomerization barely happened in TTVTT solution. The solid-phase TTVTT(OEt) upon UV light irradiations also displays unchanged UV–vis spectrum before and after irradiation, indicating the hindrance of photoisomerization. These perfectly confirm the mechanism of aggregation-dispelled isomerization in TTVTT derivatives, which can be extended to explain a series of systems.

Thus far, the microscope mechanism of molecular luminescence has been clearly illustrated by one example after another. To sum up, the bright emissions in aggregates are induced by the emergence of dipole-allowed radiation (EDAR) owing to the change of the composition, symmetry, and spatial distribution of electronic states, or the blockage of the nonradiation via vibration relaxation (BNR-VR) in harmonic region, or the removal of the nonradiation via an isomerization/a minimum energy crossing point (RNR-ISO/RNR-MECP) beyond harmonic region, as seen in Chart 1. In aggregates, all of the investigated AIEgens have weak electron–vibration coupling λ and small changes in geometry structures ΔQ during the excited-state decay processes. However, the AIEgens with different AIE mechanisms exhibit distinct features during excited-state decay processes in solution. The ones with EDAR always have relatively small λ and ΔQ, and their spectra and decay rates are merely affected by temperature. The ones with RNR-MECP always behave the largest λ and ΔQ, and the NR rates are abruptly increased as temperature rises to a key point. The emission properties of the ones with RNR-ISO strongly depend on the relative competition between the original compound and isomer. And the ones with BNR-VR exhibit moderate λ and ΔQ, and their spectra and rates also greatly change with the increase of temperature. More importantly, it is keenly realized that the microscope mechanism is system-dependent due to great individual characteristics in organic molecules. However, looking from the other side, it is conducive to the expansion of AIEgens, and AIE is experiencing the development from exotic phenomena in individual systems to universal phenomenon in a variety of systems. In addition, the relationship among AIE compound, AIE mechanism predicted by theoretical calculations, and the computational method are summarized in Table 4, which is helpful to design the AIE molecules from first principles.