New Mechanistic Insights into the AIE Phenomenon

Chapter 2

New Mechanistic Insights into the AIE Phenomenon Downloaded by UNIV OF ROCHESTER on October 2, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch002

Zikai He,1,2 Engui Zhao,1,2 Jacky W. Y. Lam,1,2 and Ben Zhong Tang*,1,2 1Department

of Chemistry, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China 2HKUST Shenzhen Research Institute, No. 9 Yuexing First RD, South Area, Hi-tech Park Nanshan, Shenzhen 518057, China *E-mail: [email protected]

Luminescent materials with characteristics of AIE have drawn extensive attention. AIE now becomes not only a phenomenon but also a synonym of a class of novel functional materials. Deciphering of the underlying mechanisms is of great importance to fundamental understanding, luminogens explorations, and advanced applications. In this chapter, we conduct an in-depth mechanistic discussion on this special photophysical process focusing on the recently proposed mechanism of the restriction of intramolecular motion. We derived that the structural rigidification of a flexible luminogen is the intrinsic cause of AIE effect, which may serve as a rationale design principle for novel AIE systems.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by UNIV OF ROCHESTER on October 2, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch002

Introduction Luminescent materials with characteristics of aggregation-induced emission (AIE) have drawn extensive attention since the debut of the AIE concept. In 2001, we reported an abnormal photophysical property of the silole systems (1). A series of silole derivatives (i.e. HPS) were then found to be nonluminescent in dilute solutions but became highly emissive when aggregated. Since the nonemissive silole molecules were induced to emit by aggregate formation, this novel phenomenon was termed as “aggregation-induced emission”. AIE now becomes not only a phenomenon but also a synonym of a class of functional materials (2–4). Deciphering of the underlying mechanisms of the AIE phenomenon is of great importance to fundamental understanding, luminogens explorations, and advanced practical applications (5). Theoretically, an excited luminogen molecule can decay through the photophysical and/or photochemical pathways (6). The photophysical one includes nonradiative and radiative processes. The photochemical one results a chemical reaction. In solution, the excited AIE luminogens (AIEgens) decay mainly through nonradiative photophysical or photochemical processes. In aggregated states, they decay mainly through radiative photophysical process. The collective effects give the unique AIE properties. Therefore, the investigations on the AIE mechanism should focus on finding out the detailed decay processes that account for these photoinduced behaviors. Numerous efforts have been continuously devoted to deciphering the AIE working principle. A number of possible mechanisms have been put forward, including conformational planarization, J-aggregate formation, E/Z isomerization, the restriction of twisted intramolecular charge transfer, as well as the excited-state intramolecular proton transfer, but none of them can be fully supported by the experimental data or perfectly applicable to all the AIE systems (7). With great and persistent efforts, the restriction of intramolecular rotation (RIR) process has been proposed to be the main mechanistic picture for the AIE effect by our group (8). However, some newly emerging AIE systems that are absent from multiple rotors, bring about some ambiguous issues to the RIR mechanism. As is well known, rotation and vibration are the two main modes of molecular motion accompanied by energy consumption of excited state. We proposed that the AIE effect of these rotor-absence luminogens maybe originate from the restriction of intramolecular vibrations (RIV). We integrated the RIR with the RIV as the restriction of intramolecular motion (RIM) as a more comprehensive AIE mechanism (Figure 1) (7). Derived from RIM, here we realized that the structural rigidification of a flexible luminogen is the intrinsic principle for AIE systems. Besides, we revisited the E/Z isomerization and photocyclization decay pathway, which may serve as considerable nonradiative photochemical processes during the excited states relaxation of AIEgens.

6 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 1. (Upper panel) Propeller-shaped AIEgens governed the restriction of intramolecular rotations. (Lower panel) Shell-like AIEgens working under the restriction of intramolecular vibrations. Adapted with permission from reference (7). Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

Restriction of Intramolecular Rotations (RIR) The RIR mechanism was proposed from sysmmetrical investigation of the prototypical AIEgens, namely hexaphenylsilole (1, HPS) and tetraphenylethylene (2, TPE). The external control experiments, internal structural modifications as well as theoretical calculations verified our RIR hypothesis. The RIR mechanism is the mostly widely used for explanation and exploration of novel AIE systems. After careful examination of the structure of HPS, we can find that the silole core is linked to six phenyl rings through single bonds, which makes the molecule conformational flexible (Figure 2A). As revealed by its single crystal structure, HPS molecule takes a propeller-like conformation with the large torsion angles between the peripheral phenyl rings and the central silole core (Figure 2B) (8). The flexible propeller-like structure will rationally explain its photoluminescence (PL) behavior.



Figure 2. (A, C) Molecular structures and (B, D) single crystal conformations of HPS and TPE. Figure 3 demonstrates the AIE behaviors of HPS. HPS is soluble in many common organic solvents, such as THF, chloroform, acetonitrile and acetone, less soluble in methanol, and insoluble in water. Thus, water is used as a nonsolvent to induce aggregation of HPS molecules in a water-miscible solvent system. As shown in Figure 3A, the dilute solution of HPS in acetone is nonemissive, with a negligible fluorescence quantum yield (ΦF ~ 0.1%). Upon increasing water fractions (fw), the ΦF shows insignificant change before fw reaches 50 vol %, but starts to rise swiftly afterwards. At fw = 90 vol %, the ΦF is boosted to 22%, which is 220-fold higher than that of the acetone solution (8). The RIR mechanism is therefore proposed to explain the AIE effect of HPS. In solution, its multiple phenyl rotors can dynamically rotate against the silole stator via the C-C single-bonds, which serves as a nonradiative decay pathway for the excited states. In aggregate, such rotations are suppressed due to the physical constraints from around molecules. The nonradiative pathway is thus blocked, leading that the radiative channel becomes the dominant decay pathway. As media with high viscosity can slow down the intramolecular rotations, AIEgens in higher viscous media should exhibit stronger emission. Considering that the viscosity of glycerol (934 cp at 25 °C) is three orders of magnitude higher than that of methanol (0.544 cp at 25 °C), the PL of HPS were then measured in glycerol/methanol mixtures. With gradually increasing the viscosity of the solvent 8 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


mixture, the PL peak intensity of HPS linearly increases on the semilog scale with glycerol fractions (fG) in the range of 0–50 vol % at room temperature (Figure 3B). The fluorescence enhancement in this fG region should be primarily attributed to the viscosity effect, because the HPS molecules are soluble in these mixtures. In the glycerol/methanol mixtures with fG larger than 50 vol %, the PL intensity increases sharply, which is due to the formation of HPS nanoaggregates in solvent mixture with low solvating power.

Figure 3. (A) Plots of fluorescence quantum yield of HPS vs. water fraction in acetone/water mixtures and (B) its PL peak intensity vs. glycerol fraction in glycerol/methanol mixtures; [HPS] = 10 μM. Reprinted with permission from reference (8). Copyright 2003 American Chemical Society.

The external control experiments strongly support the RIR mechanism and prove that the silole emission can be modulated through physical and engineering manipulation (9, 10). Similarly, the structure modification can also serve as internal control experiments to examine the RIR mechanism through steric (11) and conjugation (12) modulation. For example, bulky isopropyl (i-Pr) groups are attached to different peripheral phenyl rings of HPS yielding 3-5 (Figure 4). All these three siloles are fluorescent in solutions, with the increase in solution ΦF in the order of 5 > 4 > 3, which is consistent with the difference in their rotational barriers caused by the steric hindrance from adjacent substituents. Such high rotation barriers of 3-5 will lead to structural rigidification, which plays a decisive role in making them more emissive in solutions than HPS (11). Similar to the silole systems, a great deal of work has been done with TPE derivatives, aiming at proving the RIR mechanism. As shown in Figure 2, TPE have the four phenyl rings linked to ethylene through single bonds, enjoying a flexible configuration (Figure 2C). As revealed by its single crystal structure, TPE molecule also takes a propeller-like conformation (Figure 2D). A variety of experiments have been carefully designed for RIR mechanism verification, including host-guest inclusion (13), steric effect (14), conjugation effect (15), intermolecular coordination (16), covalent bonding (17), and metal-organic framework locking (18), etc. 9 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 4. (A) Chemical structures and fluorescence photographs and (B) PL spectra of solutions of siloles 3–5 in acetone (10 µM). Adapted with permission from reference (11). Copyright 2005 American Chemical Society. For example, TPE is tethered with α-, β- or γ-cyclodextrins (CDs) through ester bonds between CDs and monocarboxylic acid-substituted TPEs to afford CDs-substituted TPE adducts. Among the three derivatives, TPE-α-CD (6) shows highest fluorescence intensity in solution as compared to others because α-CD has the smallest cavity size. In the small α-CD cavity, the intramolecular rotations of the phenyl rings and diphenylmethylene units become more restricted (Figure 5A), which accounts for its intensified emission (13). Multiple methyl groups are attached to the o-positions of the phenyl rings in TPE, giving the sterically crowded TPE derivative, TPE-TM (7). In THF solution, 7 shows a bright emission with a ΦF of 64.0% (Figure 5B). The four o-methyl groups increase the bulkiness and rotational barriers of the phenyl groups, efficiently suppressing nonradiative decay due to the restricted rotational freedom (14). When two diphenyl groups are attached to the o-positions of the phenyl rings in TPE, the resulted folded luminogen (Z)-o-TPE-BBP (8) is also emissive in a dilute THF solution with a ΦF of 45.0%. As the rotations of aryl rings are restricted due to the intramolecular through-space π-interactions and steric effect, the nonradiative decay rate of the excited states is decreased (Figure 5C) (15). Tetrakis(bisurea)-decorated TPE (9) (16) and tetra(4-pyridylphenyl)ethylene (10) (17) are weakly emissive in solutions, but exhibit “turn-on” fluorescence after addition of sulfate anion and Hg2+ cation, respectively (Figure 5D and 5E). The intramolecular rotations of TPE should be curbed by formation of coordination complexes. When tetrakis(4-carboxyphenyl)ethylene (11), a TPE derivative decorated with four carboxylic acid groups, was used as the ligand to construct MOFs, researchers can generate various luminescent MOFs (18). Anchoring AIEgens by metal ions within a robust matrix is hence supposed to be an effective method to restrict the intramolecular motion (Figure 5F). Electromagnetic radiation at frequency of tetrahertz (10 (12) Hz, 4.1 meV) is low enough in energy to probe low-frequency intermolecular interactions and some low energy intramolecular motion. It is sensitive to the relaxation dynamics in condensed matter. Terahertz time-domain spectroscopy (THz-TDS) has been applied to provide directly experimental support to the RIR mechanism. The measurement verifies that the phenyl ring rotations of TPE occur at the THz 10 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


frequencies with the higher overall absorption at the high temperature than the low temperature (19). The phenyl ring rotations is found play the key role in deactivating the excited state of TPE at room temperature.

Figure 5. (A) Chemical structure of TPE-α-CD. Reprinted with permission from reference (13). Copyright 2013 Royal Society of Chemistry. (B) Plots of I/I0 of TPE and TPE-TM (7) vs. water fractions in THF/water mixtures (10 μM), where I0 and I are the PL intensities in THF solution and a THF/water mixture, respectively. Inset: fluorescence photographs of TPE and 7 in THF solutions. Reprinted with permission from reference (14). Copyright 2014 Royal Society of Chemistry. (C) PL spectra of (Z)-o-TPE-BBP (8) in THF/water mixtures. Inset: Photographs of (Z)-o-TPE-BBP in THF/water mixtures (fw = 0, 90%) taken 11 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


under the illumination of a UV lamp. Reprinted with permission from reference (15). Copyright 2013 Royal Society of Chemistry. (D, E) Schematic illustrations of coordination-induced restriction of intramolecular rotations based on luminogens 9 (reprinted with permission from reference (16). Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA) and 10 (adapted with permission from reference (17). Copyright 2012 Royal Society of Chemistry.). (F) Representative example of metal-organic frameworks constructed by using carboxylic acid TPE derivatives (11). Adapted with permission from reference (18). Copyright 2011 American Chemical Society.

E/Z Isomerization Different from HPS, the situation of TPE becomes complicated by the fact that its central ethylenic double bond can be broken by UV irradiation. TPE could undergo another rotation after being excited, the rotation of the opened double bond. As a result, there is an issue of concern of the AIE mechanism to TPE involving E/Z isomerization process. What is the relationship between isomerization and emission processes of a TPE-based AIEgen? Herein, we summarize the recent understandings on these issues and give the conclusive answer that E/Z isomerization is involved in deactivation process, but play a minor role. It is the subsequent result of the double bond rotation rather than the reason for AIE effect. To investigate the role of E/Z isomerization in the PL process of TPE based AIEgens, a TPE derivative with polar substituents was synthesized and separated into E and Z isomers (Figure 6A) (20). After elaborate experiments, the E/Z isomerization process was followed by tracing the photoirradiation-induced changes in the chemical shifts of the E and Z isomers under “normal” PL spectrum measurement conditions by proton NMR. The E/Z isomerization process did occur with a considerable amount (~15% in 5 min). To further discriminate effect of E/Z isomerization and RIR process, another luminogen (TPE-Fl) was synthesized by linking TPE and fluorescein (Fl) units together. Its emission behaviors were investigated under UV (330 nm) and visible (480 nm) light irradiations that were capable and incapable of breaking ethylenic double bond of the TPE unit, respectively. Two wavelength excitation experiments revealed that E/Z isomerization was involved, but played a minor role in the luminescence quenching process of TPE-Fl (Figure 6B). RIR was confirmed to play a predominant role. On the other hand, we found that the major step of the E/Z isomerization was also an intramolecular rotation of the diphenylmethylene units. The rotations occur around newly formed single bond after UV irradiation and accounts for one of the nonradiative decays of the TPE units. The results here offer a more comprehensive picture of emission behavior in TPE-based AIEgens, filling up the gap in the mechanistic study. Particularly, it settles the concern and extends the content of RIR mechanism.



Figure 6. (A) Chemical structures of (E/Z)-TPE-FM and TPE-Fl. (B) Changes in the Z ratio of TPE-FM with irradiation time (measured for twice). (C) Relative PL quantum yields of TPE-Fl in solution measured at different excitation wavelengths using Fl as reference. Adapted with permission from reference (20). Copyright 2016 Royal Society of Chemistry.

Restriction of Intramolecular Vibrations (RIV) Recently, some newly emerging AIE systems without multiple rotors, such as the nonplanar THBA, cannot be explained perfectly by the RIR mechanism. THBA has no rotators, as its phenyl rings are locked by ethane tethers. However, it exhibits typical AIE behavior: nonemissive in solution but highly luminescent as aggregates (Figure 7) (21). As is well known, rotation and vibration are the two main modes of molecular motion which can consume excited state energy. We proposed that the AIE effect of THBA may be mainly originated from the restriction of intramolecular vibrations (RIV). The phenyl rings could be viewed as vibration parts which are connected by a flexible heptagon bridge. Upon aggregation, the substantial intramolecular vibrations are restricted to block nonradiative decay pathway. Theoretical investigations verify that the intramolecular vibrations of the fixed phenyl rings are the key energy consumption modes of the excited states. As shown in Figure 7C, the isolated THBA molecules have six normal modes that consume significant amounts of excited state energy. In comparison, the clustered THBA molecules have only three normal modes consuming less amounts of excited state energy. In the cluster, a decrease in the number of vibrational normal modes and a loss of ~30% in the energy consumption of excited-state lead confirm that RIV leads THBA to radiatively decay after forming cluster. 13 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 7. (A) PL spectra of THBA (15) in THF/water mixtures with different water fractions (fw) and (B) change in PL intensity of 15 with water fraction ([15] = 20 μM). Plots of reorganization energy vs. normal mode wavenumbers for excited states of (C) molecular and (D) clustered species of 15. Adapted with permission from reference (21). Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 8. Chemical structures of COT containing AIEgens 16 and 17.



The RIV hypothesis was also proved in other AIE systems, such as AIEgens bearing dibenzocyclooctatetraene (COT) moieties, 16 (22) and 17 (23), which are reported by Iyoda et al. and Yamaguchi et al., respectively (Figure 8). They are nonemissive in solutions but become fluorescent in crystals. In the solution, the flexible COT cores can undergo vibrational inversion among the various molecular conformations. Such intramolecular motion can dissipate the excited state energy through nonradiative decay pathways. In the crystals, these conformational motion are restricted by the intermolecular interactions. The excited states mainly undergo the radiative decay, resulting in emissive aggregates governed by the RIV mechanism.

Restriction of Intramolecular Motion (RIM) Now we know both the intramolecular rotations and vibrations can dissipate the excited state energy to make solution states nonemissive. Restriction of these intramolecular motion can block the nonradiative decay pathway, leading to AIE effect. Therefore, we would like to combine the RIR and RIV as restriction of the intramolecular motion (RIM). A series of AIE-active luminogens from 18 to 21 containing both vibratable cores and rotatable peripheries are shown in Figure 9. Since both RIR and RIV are involved in such systems, the luminogens are AIE active as expected, which can be explained using the principle of RIM. On the one hand, the nonplanar butterfly-like AIEgens contain bendable cores, such as phenothiazine in 18 (24), 11,11,12,12tetracyano-9,10-anthraquinodimethane in 19 (25), pentacenequinodi-methane in 20 (26) and 21 (27), respectively. On the other hand, these bendable cores are decorated with various rotatable groups. As a result, there are two nonradiative channels dissipating the excited state energy: (i) the intramolecular vibrational motion and (ii) the intramolecular rotational motion. With addition of poor solvent into their solution, the molecules must form aggregates. The rotations of the aryl groups and the vibrations of the bendable cores are restrained by a variety of intramolecular interactions and constrained surroundings in solid states. Thereby, RIM turns on the emission of these molecules. The RIM should be a general AIE mechanism. The principle of RIM will greatly extend the scope of AIE research, because it not only provides new insights into the photophysical fundamentals but also opens up new avenues to the explorations of new AIEgen systems.



Figure 9. Examples of luminogens whose AIE activities are ascribed to the process of restriction of intramolecular motion (RIM). The structures are optimized by Chem 3D.

Restriction of Intramolecular Photocyclization As an alternative mechanism, the deactivation of TPE from its first excited states through the intramolecular photocyclization pathway is rarely considered. It is quite intriguing, considering its existence as the intermediate in the photo-oxidative reaction of stilbene and TPE (28). Recently, theoretical chemists from Switzerland reported that 75% of the trajectories (45/60) proceed through photocyclization (Figure 10). In comparison, only 5% of the trajectories (3/60) proceed through deactivation channel of ethylenic twist during their theoretical simulation (29). The remaining trajectories (12/60) persist in the excited states without change through the time length of the simulation. In detail, the TPE system, after excitation to the S1 state, evolves adiabatically on the same potential energy surface. Potential energies of S0/S1/S2/S3 are shown in magenta/red/blue/green curves, respectively, while the actual electronic state is indicated in black curve. All the energies are relative to the initial (0 fs) S0 energy. The phenyl ring torsions can bring the S1 and S0 close together, leading the system 16 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


to the conical intersection (Figure 10A). The nonradiative relaxation channel to the ground state results in various photoproducts, depending upon the precise conical intersection topology encountered by the trajectory.

Figure 10. (A) Relevant geometrical parameters (upper panel) and electronic state potential energies (lower panel) as a function of time for the photocyclization process. The time evolution of photocyclization is described by the relevant C-C distance. (B) Time evolution of the Θ twist angle for 60 trajectories. The trajectories are computed at the PBE0/def2-SVP level. Adapted with permission from reference (29). Copyright 2016 Royal Society of Chemistry. Although the photodynamical cycle of TPE is rather complicated, there is little doubt that photocyclization plays a key role. Figure 10B shows a time evolution of the twist angle Θ for the ensemble of trajectories. Red/blue/green lines represent molecules in S1/S2/S3 state, respectively, whereas S1/S0 crossing points are indicated by black dots. The cyclization dynamics can be easily distinguished from the ethylenic twist. The phenyl rings are initially close to one another and cyclization dominates. So restricting the torsional motion would greatly block the nonradiative decay and promote the radiative pathways. These findings will be of considerable value for the interpretation of the TPE-based AIE systems and extending the scope of RIM mechanism.

Conclusion The RIM mechanism now can be the unification of the RIR, RIV and RIP mechanisms. The new RIM mechanism with broader contents provides the simple, fundamental and comprehensive AIE mechanisms to work together for explanation and creation of AIE family. Intrinsically, the intramolecular motion described here boosts the nonradiative decay rates, arising from the flexible isolated molecular structures. Upon aggregation, such intramolecular motion is restricted, contributing to the enhanced structural rigidification and dramatically decreased nonradiative decay rates. Thus, the radiatve relaxation channels become favorable. Generally, in an AIE system, the flexible structures 17 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

weaken molecular rigidity and promote intramolecular motion to accelerate the nonradiative decay. The solutions become poorly emissive. The aggregation induces the structure rigidification and blocks the nonradiative decay channels, making intense fluorescence. Therefore, molecular rigidity of a flexible structure is the key factor of an AIE system.

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