with permission from John Wiley and Sons.
1.6.2 Polymerization‐induced Emission
Recently, Tang and coworkers have proposed polymerization‐induced emission (PIE), which is another conceptual innovation related to CTE [22]. It describes the process where the nonemissive monomers can be converted into luminescent polymers through polymerization. AIE process occurs mainly by physically manipulating the molecular motions, whereas PIE is achieved through the chemical ways accompanied by the CTE process. As versatile polymerization methods can be utilized to construct the PIE polymers, and these unusual luminescent polymers own good processability, the PIE acts as a promising solution in developing novel soft luminescence materials.
The working principle underneath is similar with CTE, nonconjugated subunits with rich electrons, such as phenyl, hydroxyl, and carbonyl groups, ether, and amide, can be connected into polymer chains by chain polymerization or step polymerization, and such polymer chains can be entangled and form multilevel structures through diverse intra‐ or interchain motions. Then, the electron‐rich moieties will aggregate into a cluster with electron overlapping in multiple microstructures and finally generate visible light. The emission intensity of the PIE polymers will increase with promoting the polymerization degree and the molecular weight. The intrinsic diverse structures endow the PIE polymers the potential to create diverse luminescence performance.
1.6.3 Excited‐state Through‐space Conjugation
From the abovementioned discussion, molecular motions are mainly detrimental to luminescence. Tang and coworkers have recently revealed a new role of molecular motions in the excited‐state deactivation process of the tetraphenylethane (s‐TPE) derivatives as presented in Figure 1.11a and b [13a].
The s‐TPE only contains four phenyl rings connected by the saturated single bond but can intensely emit visible light with the peak at 467 nm in the solid state. The dilute solution of s‐TPE shows ultraviolet emission that basically stems from the isolated phenyl rings, but with adding more than 70% water into the dilute solution, s‐TPE molecules can aggregate accompanied with a notable emission peak at 460 nm emerging, showing the typical AIE property. Why do the nonconjugated systems emit the visible light? Due to the highly twisted and flexible structure of s‐TPE, it shows no obvious intermolecular π–π interaction in the single crystal. The theoretical simulation of the exciton coupling also shows that there is no notable intermolecular coupling in the excited state, so it should be the intramolecular interaction that affects the emission.
Further optimization of the excited‐state structures shows that s‐TPE will encounter substantial conformational reorganization, and during the relaxation, the distance between the geminal phenyl rings can gradually decrease. When it decays to the S1,min, two geminal phenyl rings reach a close‐contacting conformation with large through‐space overlapping of π orbitals. Indeed, the transition energy gaps decrease along with the phenyl rings getting close, so the emission wavelengths can be redshifted with such intramolecular motions. Finally, the calculated emission at S1,min becomes 460 nm, which is well consistent with the experimental data. The EVC analysis shows that s‐TPE molecules own much higher excited‐state motion ability in the dilute solution, so the conformation with intramolecular TSC can be easily disturbed by the vigorous molecular motions. Once the s‐TPE molecules aggregate, the molecular motions will be restricted to a certain degree, as indicated by the suppressed structural reorganization, which, thus, stabilizes the through‐space conjugated conformation and reduces the nonradiative decay rates, and so the solid‐state s‐TPE can emit enhanced visible light.
Figure 1.11 Schematic illustration of the excited‐state (a) intramolecular through‐space conjugation of s‐TPE and (c) intermolecular through‐space complex of s‐DPE. Molecular orbitals involved in the transition between the excited state and the ground state of (b) s‐TPE and (d) s‐DPE.
Source: Adapted from Refs. [13a, b] with permission from American Chemical Society.
A certain degree of molecular motions in the solid state will facilitate the formation and stabilization of emissive states with TSC. The diphenylethane (s‐DPE) is another nonconventional AIEgen as presented in Figure 1.11c and d [13b]. It contains no long‐range through‐bond conjugated structure but two isolated phenyl rings. The absorption and photoluminescence spectra in the dilute THF solution are ascribed to the electronic transition of the phenyl rings, with peaks at 270 and 285 nm, respectively. However, with addition of water into its dilute solution, a new emission peak at 355 nm gradually emerges, showing the AIE phenomenon. The emission peak in the solid state of s‐DPE is also redshifted to 355 nm with a small fraction of the emission at 285 nm coming from the isolated state. Different from s‐TPE, the isolated s‐DPE molecule cannot form the intramolecular TSC, so the only way to redshift the light emission is to form the intermolecular TSC. The optimization of the dimer of s‐DPE based on two adjacent s‐DPE molecules in the single crystal shows that the two s‐DPE molecules gradually get closer and turn the orientation of two phenyl rings from face‐to‐edge at S0,min to face‐to‐face at S1,min and finally form an excited‐state through‐space complex (ESTSC) with significant orbit overlapping between the two respective phenyl rings of the two s‐DPE molecules. Meanwhile, the energy gap decreases from 5.32 to 4.33 eV along with the light‐driven solid‐state intermolecular motions, which is the luminescence origin of s‐DPE in the solid state.
As the characteristic ultrafast transient absorption spectra reveal the most probable conformations in the excited‐state timescale, the femtosecond transient absorption (fs‐TA) spectroscopy has been utilized to prove the existence of ESTSC [13b]. The steady‐state absorption spectra of s‐DPE in dilute solution and crystal film have been first measured, respectively. It has been found that the absorption spectra in these two conditions are matched with each other, indicating there are no species involving intermolecular electronic coupling in the ground‐state crystal film. Then, the fs‐TA spectra of s‐DPE in the dilute solution show that there are no notable absorption peaks in the long‐wavelength region in the excited state but only peaks located in the short‐wavelength region. However, when it comes to the crystal state, the fs‐TA spectra of s‐DPE crystal film show that the peaks from 340 to 450 nm almost disappear, but new peaks at 590 nm emerge as the dominant characteristic absorption peaks with the lifetime of 525 ps, which is similar to a characteristic transient absorption peak at 570 nm for the literature‐reported naphthalene excimer. According to the theoretical simulation, this long‐wavelength characteristic peaks can be assigned to the ESTSC. Since the ground‐state s‐DPE crystal will not form complexes, the ESTSC can only be formed in the excited state along with the intermolecular motions.
What is the driving force for the molecular motions that lead to the formation of ESTSC? The Mulliken charges have been mapped onto molecular structures of the isolated s‐DPE and the ESTSC [13b]. It shows that partial molecular charges of two carbon atoms of the isolated s‐DPE connecting two phenyl rings and the central ethane moiety are negative, whereas the other carbon atoms show positive partial charges. Hence, it is proposed that such opposite partial charge distribution will induce the transient dipole to attract the adjacent two molecules approaching to each other and finally
