Vibration Induced Emission (VIE): An Intrinsic Fluorescence Tuning

Chapter 3

Downloaded by CORNELL UNIV on October 27, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch003

Vibration Induced Emission (VIE): An Intrinsic Fluorescence Tuning Mechanism of N,N′Disubstituted-dihydribenzo[a,c]phenazines Wei Chen,1 Zhi Lin,2 Jianhua Su,1 and He Tian*,1 1Key

Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, P. R. China 2College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China *E-mail: [email protected]

In pursuit of design and modification of photophysical properties on molecular level, N,N′-disubstituteddihydrodibenzo[a,c]phenazines were demonstrated as a vivid example of elaborate manipulation via ‘vibration induced emission’ (VIE), which is coined for this particular change in configuration and planarity motion. Significant alternation was observed upon environmental change in polarity, viscosity and temperature, including dual fluorescence and large Stokes shift, etc. In-depth investigations were utilized such as temperature dependent steady-state spectroscopy, nanosecond time-resolved spectroscopy, femtosecond dynamics and computational simulation of the reaction energy surfaces, which manifest this phenomenon as a novel mechanism attributed to intrinsic switch-ability of these molecules. In light of this case, the concept of VIE can be introduced as a universal criteria for facile control of the photophysical pattern and hopefully be extended to versatile applications in photoelectric and biomedical disciplines.

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


Introduction Due to the prosperity of luminescent materials in field of photo-active devices and biosensors, fluorescence mechanisms have always been regarded as the utmost doctrines in development of applicable molecules (1). Anomalous luminescence phenomena of some specific molecules, including large Stokes shift and dual fluorescence (2), can be potentially exploited to unveil the theoretical excited-state dynamics of fluorophores, thus receiving considerable interest. Currently, several mechanisms that lead to large Stokes shift, have been proposed and acknowledged, such as excimer/exciplex formation, charge transfer (CT), excited-state intramolecular proton transfer (ESIPT) (3), and Förster/ fluorescence resonance energy transfer (FRET) (4), facilitating the rational design of opto-functional molecules. Herein, we envisioned a novel mechanism of photochemical process with a plain nomenclature of ‘Vibration Induced Emission’, which indicates an intrinsic response of particular molecules upon environmental variations (Scheme 1). Conventional photochemical or photophysical processes usually involve the transfer of electrons, protons or overall energy. Excimer/exciplex formation (5, 6) and charge transfer (CT) processes (7, 8), for instance, include the intermolecular and intramolecular transfer of electrons and as a result of intramolecular interactions, can produce intramolecular charge transfer (ICT) state. Park et al. reported a donor-acceptor cocrystal in a loosely packed manner, showing a strong red-shifted luminescence based on intermolecular charge transfer (6). With an analogous functionality, TICT (twisted intramolecular charge transfer) appears with perpendicularly organized electron donor and acceptor (9, 10), leading to electronic decoupling of the overall molecule. Tang et. al. designed BODIPY derivatives which has remarkable solvatochromism phenomena as the molecules changed from locally excited states to TICT states upon increase of solvent polarity (11). ESIPT, on the other hand, involves tautomerization in a unimolecular basis, which leads to strong redistribution of the electronic density (12–15). Chou et al. reported 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), of which a seven-member-ring hydrogen bond validates the intramolecular proton transfer and a large Stokes shift upon tautomerization (16). FRET occurs through nonradiative dipole-dipole coupling between two light-sensitive chromophores (17–20). In this sense, Würthner et. al. reported a system where polymerized vesicles loaded with water-soluble perylene diimide were designed for sensitive pH response and its fluorescence color changes covered the whole visible range (21). Nevertheless, there are still several chromophores with large Stokes shift that might not attribute to mechanisms mentioned above. One typical scaffold is the biphenyl derivatives (22, 23), where a geometrical planarization occurs upon excitation. Recently, Würthner et. al. observed a type of zwitterionic perylene bisimide-entered radical with precedented stability and a distorted structure was obtained via structural analysis (24, 25). Similarly, anomalously large Stokes shift was also observed with several polycyclic aromatic molecules in the bent-to-planar motion (26, 27). Yet these structural vibrations were rarely studied intensively. 22 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 1. Diagrammatic sketch of fluorescent mechanisms: (A) Twisted Intramolecular Charge Transfer (TICT); (B) Excited-state Intramolecular Proton Transfer (ESIPT); (C) our newly coined Vibration Induced Emission (VIE). (Original mechanism diagram.) Recently, we have observed an intriguing phenomenon from the N,N′-disubsitituted-dihydrophenazines derivatives (Figure 1) upon investigation on their hole-transporting properties (28) with the basic scaffold of N,N′-dimethylphenazine (DMP). The solutions of these molecules are colorless in appearance and yet emit pronounced red fluorescence. For N,N′-dimethyl-9,14-dihydrodibenzo[a,c]phenazine (DMAC), N,N′-diphenyl-dihydrodibenzo[a,c]phenazines (DPAC), and N′-phenyl-N′-fluorenyl-dihydrodibenzo[a,c]phenazines (FlPAC), the absorption maxima locate at ~ 350 nm, while the peak wavelengths of fluorescence locate at ~ 600 nm and hardly change with the solvent, giving an anomalously large Stokes shift calculated to be > 11000 cm-1 (Figure 2). These unique photophysical behaviors can be rationalized by excited-state configuration transformations induced by vibration, and we named this new mechanism as Vibration Induced Emission (VIE) (29–32). What needs to be emphasized is that VIE turned out to be one of the intrinsic fluorescence property of the molecules, which is irrelevant with non-radiative energy dissipations caused by rotational motion mechanisms. 23 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


It is not, to be precise, limited to enhanced fluorescence emission, but instead refers to dynamic changes in intrinsic photophysical properties upon vibrational alternation.

Figure 1. (A) Structures of DMP, DMAC, DPAC and FlPAC; (B) Photographic images of color (emission) and sol-gel phase transition of DPAC doped in thermosensitive organogel under visible and 365 nm UV light. (Modified with permission from ref. (36). Copyright © 2015 American Chemical Society.)

Vibration Induced Emission (VIE) The behavior of compound FlPAC appeared to be a perfect interpretation of this new mechanism. Careful examination of FlPAC showed a typical dual emission phenomenon: a dominant (anomalous) emission band maximized at ~610 nm, while a weak but non-negligible (normal) emission band located around the blue region, which was identical with the excitation spectra (370 nm). The blue emission band showed pronounced Stokes shift as a result of solvatochromism caused by charge transfer, which red shifted from cyclohexane to acetonitrile by 2730 cm-1. In contrast to the normal emission, the red emission was insensitive to the solvent polarity. Thus, the probability of TICT mechanism as an explanation of this anomalous emission could be entirely eliminated. However, as is reported in the literature (33), the absorption maximum of dihydrophenazines DMP located at ~ 343 nm, while the emission band maximized at ~ 470 nm. The large Stokes shift up to 8000 cm-1 can attribute to symmetrical inhibition of S0→S1 transition (34). Schuster et. al. subsequently reported compound DMAC and invoked a similar symmetry rule to explain the unique photophysical properties of DMAC (35). According to this theory, a compound like FlPAC whose C2 symmetry has been broken down should not show anomalous red emission. However, this inference was decidedly inconsistent with the experimental data. 24 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In addition, symmetry rule should not be dependent on temperature and viscosity, while the photophysical properties of DMAC were dramatically varied with the external environment based on our experiments. Therefore, the explanation of forbidden transition was not applicable to rationalize the anomalous Stokes shift of DMAC and FlPAC.

Figure 2. (A) Illustrative scheme of dihydrophenazine DPAC for VIE mechanism; (B) Typical emission spectrum of DPAC, indicating a remarkable Stokes shift from solid to dispersed solution. (Modified with permission from ref. (29). Copyright © 2015 Royal Society of Chemistry.) With further analysis of the single crystals of dihydrophenazines (36), we have found that all molecules were bent along the N1−N2 axis, which were more like saddle-shaped (V-shaped) structures rather than planar structures of phenazines (34). We then reasonably suspected that the vibration of two aryl rings along N1-N2 axis resulted in the excited-state configuration transformation from bent to planar state, inducing the red emission of dihydrophenazines. In solid state, the vibration was restricted due to the physical constraint, which blocked the planarization of the electronic structure in excited state, giving the result of a normal blue emission. In solution, however, the molecule structure was actively free and could vibrate to a co-plane in the excited state, inducing the anomalous red emission. Yamaguchi et al. reported a series of planarized 9-phenylanthracene derivatives, and the results showed that the planar and rigid structure definitely caused red-shifted and intense emission due to the effective π–conjuagation (37). To verify our conjecture, we then attempted to control the vibration of dihydrophenazines through changing temperature and viscosity. The results indicated that the optical properties of dihydrophenazines were 25 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


indeed temperature/viscosity dependent. With the decrease of temperature and the increase of viscosity, the blue emission intensity was gradually enhanced accompanied by the gradual reduction of red emission, which results from the hindrance of large-amplitude internal vibration. Moreover, the VIE mechanism can also be easily demonstrated by addition of a poor solvent (water) to a good solvent (THF). Results revealed that the dual emission of compound DPAC could be finely tuned by the ratio of water, caused by the restriction of structural vibration in excited state. In order to further validate the new mechanism, we performed a comprehensive study in cooperation with Prof. Chou and his coworkers, including nanosecond time-resolved spectroscopy, femtosecond dynamics and computational simulation of the reaction energy surfaces. With the in-depth research, a sequential, three-step kinetics (Figure 3) was established (36). The first stage was formation of initial charge transfer state (R*) with the solvent effect; The second stage is structural relaxation to local minimum state (I*) owing to the steric hindrance of N,N′-disubsituted side chain, which can be classified as an intermediate; The ultimate stage is structural vibration to the final planarization state (P*), with the elongation of the π-delocalization over the benzo[a,c]phenazines moiety. The anomalous photophysics of saddle-shaped dihydrophenazines were derived from the combination of the three excited states. Density functional Theory (DFT) calculations for FlPAC revealed that the bent angle of the two aryl rings along the N1−N2 axis underwent a transition from 136°(R*) to 133°(I) and ultimately vibrated to be 160°(P*), which is a thermodynamically stable structure (8, 38). In light of these confirmative phenomena and demonstrations, VIE mechanism proposed for vibration induced configurational change of dihydrophenazine is confirmed.

Figure 3. Three-step kinetic mechanism of compound FlPAC. (Reprinted with permission from ref. (36). Copyright © 2015 American Chemical Society.) 26 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

In merit of the tremendous manipulation potency of these dihydrophenazine molecules, a full-color display is plausible for further applications in luminescent materials. It is noteworthy that these anomalous photophysics properties including dual fluorescence, large Stokes shift and environmental responsive luminescence are not constrained to N,N′-disubstituted dihydrophenazines or simple C-C (C-N) single-bond rotation (37). Instead, any scaffold affording a controllable planar structure with electrons shared in the heterocyclic system has an intrinsic probability for VIE fluorescence mechanism, which can definitely be exploited as a design criteria for future functional fluorescent molecules.


Technological Applications In principle, the VIE mechanism can be utilized to develop versatile fluorescent materials through the hindrance or control of the vibration. Currently available molecules such as N,N′-disubstituted dihydrophenazine and its derivatives is applicable in various fields including white-light materials, environmental responsive indicators and photoelectronic devices. In reality, there are still tremendous possibilities of applications beyond our imagination. Herein, we demonstrated several aspects of utilities in optoelectronic and sensory systems based on N,N′-disubstituted dihydrophenazines.

Monomolecular White-Light-Emitting Materials High-efficiency white-light-emitting materials have received great attention for their prospective applications in display devices. The strategies followed so far to obtain white emission have relied most on the combination of three (blue, green and red) or two (blue and orange) complementary colors from different fluorophores (27, 28). In comparison with multimolecular white materials, monomolecular white light generators show obvious advantages, such as better stability, better color reproducibility and simpler fabrication procedures, which have become a current research hot spot (39–41). However, the field is hindered because it remains a challenge to excavate applicable monomolecular white light materials with standard white light illumination. As N,N′-disubstituted dihydrophenazines have the privilege of dual fluorescence properties, these small molecules thus qualify themselves as candidates for novel monomolecular white light emitting materials, which is further manifested by experimental data. As is mentioned above, N,N′-disubstituted dihydrophenazines showed two emission bands in solution, located at ~ 460 nm (blue) and ~ 600 nm (reddish orange) respectively and can be tuned for white light on basis of VIE mechanism. A series of novel VIE fluorophores based on dihydrophenazines have been designed and synthesized (21, 42). Experimentally, DPAC and the blue-emitting fluorophore which serves as an energy donor and acceptor, respectively, were ligated via a non-conjugated six-member ring to form M1 (Figure 4), which both performed dual fluorescence (29). The emissions were easily tuned from red to blue through the control of solvatochromism of the blue emission, which is proved as a result of normal ICT effect. Following the VIE mechanism as a 27 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


guiding principle, vibrational modes of the molecule can be adjusted for dual fluorescent manipulation. By addition of good solvent H2O to poor solvent THF, a near-white light emission can be achieved in an appropriate ratio. For compound M1, the solution gave a near white emission (CIE 0.28, 0.27) with the water fraction of 90 %. The color-tunable luminescence ascribed to the restriction of molecular vibration, generating from the aggregation of molecules in poor solvent. Moreover, a white light could also be obtained via control of the intramolecular energy transfer (IET) efficiency. With the increase of solvent polarity (from cyclohexane to acetonitrile), the IET efficiency was reduced due to the decrease of overlapping area between energy donor emission and energy acceptor absorption, resulting in the enhancement of blue emission accompanied with the weakening of red emission. The combination of IET and VIE effect leaded to a close white emission of M1 in acetonitrile (CIE 0.34, 0.36).

Figure 4. Structure of compound M1.

Environmental Sensors Environmental monitoring plays an irreplaceable role in the industrial production and exploration of the nature, where the most frequently measured physics parameters are viscosity and temperature (42). Although conventional measures do exist for viscosity and temperature monitoring, fluorescent sensors draw considerable attention due to the high sensitivity, simplicity, low-cost of implementation and the flexibility in signal readout. On the basis of the VIE mechanism, a change of the external environment is expected to influence the molecular vibration of N,N′-disubstituted dihydrophenazines, resulting in a viscosity/temperature-dependent phenomenon, the ratio of the dual emission intensity in particular. This expectation has been primarily demonstrated by using n-butanol with a relatively high viscosity (2.948 cP/ 293 K), as the media to finely tune the ratiometric emission. Upon temperature decrease, the viscosity increases gradually, causing the red emission to decrease and the blue emission to increase. Prominent luminescence response can thus validate its potential utilization of N,N′-disubstituted dihydrophenazines as viscometers or thermometers. 28 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In addition, Chen et. al. reported a new approach to monitor cryogenic temperature by use of compound FlPAC in MeTHF (2-methyl-tetrahydrofuran) solution as a soluble thermometer (43). On basis of VIE mechanism, the vibration mode of the excitation state can be inhibited when temperature is lowered down, which is unachievable through other mechanism. Luminescence of the solution varied from blue via magenta to orange, with the temperature increasing from 138 K to 343 K. It was noteworthy that the thermometer performed extremely high sensitivity at low temperature, which reached as high as 19.4 % per K at 138 K. The study not only provides a successful example of a ratiometric fluorescent thermometer, but also demonstrates a practical application of VIE mechanism.

Hole-Transporting Organic Light-Emitting Materials With its advantages in light weight, abundant variations and easy processibility for industrial promotion, organic photovoltalic materials have become an optimal candidate for the substitution of conventional inorganic materials. Hole-transporting materials, which serves as a decisive component in the OLED devices, can be modulated to increase the stability of the overall devices as well as manipulate the energy level of the light-emitting layer and the electrode. In fact, the in-depth investigation of the fluorescence originates in the process of performance perfection of optoelectronic materials. As the electron-donating groups in the hole-transporting materials appears to be electron abundant, sharing the electrons in a planar conjugated scaffold, an intrinsic fluorescence change can be achieved if the planar configuration could be disrupted. N,N′-disubstituted dihydrophenazines, in this case, possess satisfying hole-transport mobility as they usually include electron donors for stronger electropositivity. We therefore designed and synthesized a series of novel N,N′-disubstituted dihydrophenazine derivatives, with HOMO level between 2.83 eV and 5.08 eV, perfectly matching the HOMO level of the anode and light-emitting materials (29). Electronic devices based on compound b has a maximum luminance intensity up to 17437 cd/m2 as well as high current efficiency and density. It can thus be envisioned that analogous molecules can be applied to hole-transporting materials and most importantly, a bi-directional application, where more electron-abundant heterocyclic molecules utilized as hole-transporting materials can be further considered for vibrational motion manipulation, would definitely flourish the two fields, forming a circulated backflow that would propel the design and application of both vibration induced fluorescence molecules and optoelectronic materials.

Summary and Perspectives In conclusion, a series of N,N′-disubstituted dihydrophenazines have been intensively studied on their distinct photophysical properties, such as dual emissions, anomalously large Stokes shift and environmental sensitivity. With the full verification of steady-state spectroscopy, nanosecond time-resolved spectroscopy, femto-picosecond time-resolved spectroscopy and computational simulation, we proposed an elaborately coined mechanism Vibration Induced 29 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Emission (VIE) in order to emphasize the intrinsic phenomenon of our designed molecules. The most distinct feature of this particular mechanism is that it can address a dynamic emission change in color and a very large Stokes shift, which are unachievable through other mechanism. Also, this mechanism is experimentally distinguishable from others because this kind of phenomena can be demonstrated in dispersed system such as solutions. This new mechanism not only enriches the basic fluorescence theories in photoelectric materials, but also provides a new approach to obtain monomolecular emitters with color-tunable fluorescence and environmental-sensitive sponsors. As a primitive example of these photophysical phenomena, N,N′-disubstituted dihydrophenazine and its derivatives are proved be potentially applicable in various organic functional materials. Apart from monomolecular white emitters and thermometers reported, utilization in biomedicine, new energy resources and information materials could also be realized by molecular modification. For applications in biosensors and cell imaging, water solubility of dihydrophenazines need to be largely improved, which could be solved by introducing water soluble groups, such as crown ethers, amino acids and quaternary ammonium salts. It is also feasible to affect the structural vibration by molecular self-assembly, giving a great potential for N,N′-disubstituted dihydrophenazines to apply in molecular machines. It is worth noting that the highly-rich and redox-active properties of dihydrophenazines could add versatility of this kind of molecules and can therefore be utilized as good electron donors in dye-sensitized solar cells (DSSCs), as well as hole-transport materials in organic light emitting diodes. Finally, we envision that as a key supplement of luminescent mechanism, VIE would be a gold mine for researchers to dig over and be regarded as a brand-new concept of molecule design for optoelectronic materials.

Acknowledgments We thank those people with contributions to this research: Academician Benzhong Tang in Hong Kong University of Science and Technology, Prof. Pi-Tai Chou and his group in National Taiwan University, Prof. Hongbing Fu and his group in Institute of Chemistry Chinese Academy of Sciences, and Dr. Zhiyun Zhang, Dr. Wei Huang in our lab.

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Vibration Induced Emission (VIE): An Intrinsic Fluorescence Tuning

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