Pressure-Induced Emission Enhancement of Carbazole: The

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Letter

Pressure-Induced Emission Enhancement of Carbazole: the Restriction of Intramolecular Vibration Yarong Gu, Kai Wang, Yu-Xiang Dai, Guanjun Xiao, Yuguo Ma, Yuancun Qiao, and Bo Zou J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01796 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017

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The Journal of Physical Chemistry Letters

Pressure-Induced Carbazole:

the

Emission Restriction

Enhancement of

of

Intramolecular

Vibration Yarong Gu,a Kai Wang,a,* Yuxiang Dai,a Guanjun Xiao,a Yuguo Ma,b Yuancun Qiao,c Bo Zou a,* a

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

b

Beijing National Laboratory for Molecular Sciences, Centre for the Soft Matter Science and

Engineering and the Key Lab of Polymer Chemistry & Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China c

North China Institute of Aerospace Engineering, Langfang 065000, China

Contributions K.W. and B.Z. designed and performed experiments, and analyzed data. Y.D., G.X. and Y.Q. assisted in performing experiments. B.Z., Y. M. and K.W. provided intellectual input. Y.G., K.W., Y.D., G.X. and B.Z. wrote the manuscript. Corresponding Author *Kai Wang, E-mail: [email protected]. *Bo Zou, E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: Pressure-induced emission enhancement (PIEE), a novel phenomenon on the enhancement of the solid-state emission efficiency of fluorophores, has been arousing wide attention in recent years. However, researches on PIEE are still in the early stage. To further pursue more enhanced efficiency, discovering and designing more PIEE systems would be urgently desirable and of great importance. In this letter, we found carbazole presented a conspicuous emission enhancement under high pressure up to 1.0 GPa. In situ high-pressure infrared spectroscopy, angle-dispersive X-ray diffraction analysis combined with Hirshfeld surface theory calculation indicated that the PIEE of carbazole was attributed to the decrease of non-radiation vibration process. This phenomenon mainly resulted from the restriction of N–H stretching vibration by increased N−H···π interactions under high pressure. Our study puts forward a mechanism of PIEE related to the restriction of intramolecular vibration, which provided a deep insight into the essential role of intermolecular interaction in fluorescence emission properties.

TOC GRAPHICS

Organic fluorophores have attracted widespread attention because of their tremendous utilities in the fields of organic light-emitting diodes, light emitting organic field effect transistors, and


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bioprobes, etc.1-3 In practical application, these fluorophores are generally fabricated into solid film. On account of the strong electronic interactions between their aromatic rings, they tend to aggregate. The formation of aggregate often quenches emission, which has been a thorny issue for developing effective devices.4-6 Tang’s group observed a phenomenon of aggregationinduced emission (AIE): a group of non-emissive molecules emitted intensely in aggregate state.7-9 The AIE can be explained by restriction of intramolecular rotation (RIR) and restriction of intramolecular vibrations (RIV). These two distinct mechanisms deem that the restriction of molecular motion would result in the decrease of non-radiative vibration process, thus enhancing the photoluminescence (PL).10-12 Pressure is a powerful tool to tune optical properties such as absorption and PL emission.13-16 The application of external stress could also achieve an emission enhancement. For example, our group reported that the propeller-shaped tetraphenylethene crystal showed the PIEE phenomenon between 1.5-5.3 GPa using a diamond anvil cell (DAC) technique.17 Tang et al. also observed PIEE phenomenon in the propeller-shaped hexaphenylsilole film.18 The RIR mechanism was responsible for the above-mentioned PIEE phenomenon in propeller-shaped molecules. Yang and Zou et al. reported a PIEE in 4-(2-(4’-(diphenylamino)-[1,1’-biphenyl]-4-yl)-1Hphenanthro[9,10-d]-imidazol-1-yl)benzonitrile, which was ascribed to the rehybridization of nitrogen atom in TPA driven by weak hydrogen bonding interactions.19 The PIEE would greatly facilitate applications in optical pressure-sensing devices. Therefore, discovering and designing more PIEE systems would be urgently desirable and of great importance.


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Figure 1. Packing arrangement of carbazole crystal in part of one layer at ambient conditions. Red dashed lines represent C−H···π hydrogen bonds. Blue dashed lines represent N−H···π hydrogen bonds. Carbazole is a typical blue-emitting organic fluorophore. The crystal structure is formed by alternating layers along the c-axis. Within each layer, molecules are linked by intermolecular N−H···π hydrogen bonds and C−H···π hydrogen bonds, exhibiting herringbone motifs. (Figure 1).20 External compression can affect hydrogen bonds and as a consequence a changing of the vibration frequency of molecular groups might be occur as well. The changes of the nonradiative vibration process have effects on the photoluminescence. We applied in situ fluorescence spectroscopy, infrared (IR) spectroscopy, and angle-dispersive X-ray diffraction (ADXRD) measurements combining with Hirshfeld surface theory calculation to systematically investigate the pressure response of carbazole and hence the effect of the pressure on the emission properties.


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Figure 2. (a) PL spectra of carbazole crystal at range of pressure from 0.0 GPa to 10.3 GPa excited by 355-nm laser. The red arrows represent the changes of PL intensity. (b). Corresponding PL photographs under high pressure. Figure 2a showed PL spectra of carbazole crystal ranging from 0.0 GPa to 10.3 GPa. We chose 355-nm laser as the excitation source in high-pressure PL experiments. At ambient conditions, solid-state carbazole showed a typical crystal fluorescence spectrum with several vibronic bands. Upon compression up to 1.0 GPa, we found that the intensity of the main bands located in the range of 385-489 nm was enhanced. Figure S1f (Supporting Information, (SI)) indicated that the best excitation wavelength λex was 367 nm. In order to eliminate the effect of pressure-induce shift of excited wavelength on the maximum PL intensity, a series of fluorescence spectra were measured by changing the wavelength of Xenon lamp (Figure 1a-e, SI). It demonstrated that pressure-triggered fluorescence enhancement was observed for every excited wavelength under 1.0 GPa. The intensity gradually decreased when external pressure was beyond 1.0 GPa. The change of intensity could also be observed by naked eyes as shown in Figure 2b. As well as these


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changes, the fluorescence spectrum was red-shifted 40 nm up to 10.3 GPa. The quenched PL spectra of carbazole crystal recorded in the decompression process indicated the phenomena were reversible (Figure S2, SI). In Figure S3 (SI), the absorption bands showed red shift with increasing pressure. The color of sample changed gradually from colorless to yellowish. The redshift phenomenon in herringbone pattern could be ascribed to the pressure-induced decrease of intermolecular distances and the revolving of the molecular planes. The decrease of intermolecular distances and the revolving of the molecular planes increased the intermolecular interaction between an excited molecule and unexcited adjacent molecules. The strong intermolecular coupling would cause a resonance dimer from which a red-shifted band was emitted.21-23

Figure 3. High-pressure IR spectra of carbazole up to 10.5 GPa at room temperature in the region of ν(C−H) and ν(N−H) vibration modes. We further performed the high-pressure IR spectra measurements in order to study the changes of intermolecular interactions. In Figure 3, the wide IR absorption band around 3050 cm−1 was assigned to the C−H stretching vibration ν(C−H).24 The corresponding blue-shift of ν(C−H) with increasing pressure indicated that C−H bonds were shortened. The interatomic distances were shortened by compression, from which the vibration showed blue-shift. The blue-shift of C-H


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stretching vibration meant that the frequency of stretching vibration increased. That would increase the non-radiation vibration process, resulting in the decrease of emission. At the same time, the shape of ν(C−H) band obviously changed. That resulted from the change of complex C−H···π interactions. When compressed, the molecules would spontaneously adjust their position in order to adapt the smaller volume. The closer motion of molecules could ultimately lead to the strengthening of intermolecular interactions. From the perspective of crystallography, the molecules in herringbone motif became closer and the intermolecular interactions became stronger, which meant the molecules revolved to more parallel. That promoted effective intermolecular π-π stacking interactions which were responsible for the emission quenching. The peak at 3420 cm-1 can be definitely assigned to N−H stretching vibration ν(N−H).25-26 It is intriguing that the N−H stretching vibration ν(N−H) showed a red-shift (8 cm-1) from 3420 cm-1 to 3412 cm-1 up to 2.0 GPa, followed by a blue shift beyond 2.0 GPa. This indicated that N−H bonds were firstly elongated, and then shortened with increasing pressure. Below 1.0 GPa, N−H bonds were elongated, demonstrating that the N−H···π interactions got stronger with increasing pressure.27-28 The N−H···π interactions restricted N−H stretching vibration, so inhibiting the nonradiative vibration process. As a result, the increase of radiative energy enhanced the emission below 1.0 GPa. However, beyond 1.0 GPa, the effective π-π stacking interactions could quench the emission.


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Figure 4. (a) Selected ADXRD patterns of carbazole at different pressures. (b) Compression of unit cell volume of carbazole with respect to pressure. The inset shows compression rate of lattice constants under different pressure. (c) The evolution of molecular arrangement with increasing pressure. Viewing down c-axis, the two sets of ‘herringbone’ pattern are at different viewing depths, black is closer to viewer. To verify the effect of the revolving of molecules on emission properties, we performed in situ high-pressure ADXRD study. As the ADXRD measurements could display the information of


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crystal structure, which had a direct relationship with its intrinsic properties, such as optical, mechanical properties.29-32 Figure 4a showed representative ADXRD data of carbazole under different pressure. All the diffraction peaks shifted to higher angles. Moreover, upon the release of pressure, the diffraction pattern returns back to its original state, indicating that the compression process was reversible. In order to get more detailed information about changes of structure, we performed Pawley refinements for ADXRD patterns. Figure 4b showed the variations of unit cell volume with pressure. The pressure-volume data were fitted using the third-order Birch-Murnaghan equation of state. The continuous volume collapse indicated that no phase transition occurred, and the structure remained stable up to 11.0 GPa. The different lattice axis were compressed in different rates under high pressure, indicating that the anisotropic compression of carbazole under pressure (inset, Figure 4b). The continuous compression of caxis indicated that the distance between layers was decreased, which meant the interlayer interaction was gradually strengthened. Note that the b-axis was more compressible than a-axis, which meant the b-axis was more sensitive than a-axis to pressure. This fact led to the molecules more nearly parallel to the ac plane and packing closer (Figure 4c). The changes implied that the overlap of π electron was increased, which meant the increase of π-π interactions.33 This would verify the discussion made in IR analysis. The π-π interaction would ultimately quench the emission.12, 29 Therefore, below 1.0 GPa, the promoted N−H···π interactions suppressed the N−H stretching vibration that was ascribed as responsible cause for the emission enhancement. Beyond 1.0 GPa, the reduced distance between the intermolecular plane in the crystal promotes effective π-π stacking interactions that are responsible for the emission quenching.


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Table 1. Hirshfeld surface for calculated structure at 0.0 GPa, 0.7 GPa, 1.0 GPa, 2.0 GPa mapped with dnorm distance. 0.0 GPa

0.7 GPa

1.0 GPa

2.0 GPa

Front

Back

Front

Back Footnotes: ① represents N−H···π hydrogen bonds; ② represents C−H···π hydrogen bonds. In order to verify the evolution of intermolecular interactions under high pressure, we carried out Hirshfeld surface theory calculations. As shown in Table 1, Hirshfeld surface under elevated pressure visually displays the evolution of intermolecular interactions. Decomposition of the Hirshfeld surface at ambient pressure revealed that the red areas which represented strong interactions were produced by N–H···π interactions (marked ① ) and C−H···π interactions (marked ② ). They were the shortest interactions within the structure, as expected for a herringbone pattern.30 At the beginning of compression, the red regions were enlarged, which meant that initial interactions became strong. Nevertheless, upon further compression, there appeared more new red regions. This indicated that the distance between C atoms and H atoms of adjacent molecules became closer, accompanied by the more intense interactions between molecules in herringbone pattern. The position and percent of C···C interactions contributing to


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Hirshfeld surface were summarized in Table S1 (SI) and Figure S4 (SI). The C···C interactions increased with pressure. At low pressure, the C···C interactions changed slightly, which indicates that π-π interactions are relatively weak. However, upon further compression, π-π interactions increased drastically, which implies that the molecules in herringbone patterns become more parallel. These analyses were in good accordance with ADXRD data. In summary, we found that carbazole crystal presented obvious PIEE under 1.0 GPa. Analysis by ADXRD, IR combined with theory calculated results demonstrated that below 1.0 GPa, PIEE was due to the restriction of N−H stretching vibration by N−H···π hydrogen bonds. Beyond 1.0 GPa, π-π interactions increased markedly, which eventually quenched the PL emission. Furthermore, the revolving of molecules and the decreased distance between molecules also explained the red-shift phenomenon of PL spectra. The study about PIEE in carbazole crystal not only provided a direct experimental evidence for RIV mechanism, but also facilitated the design of other promising PIEE materials. Experimental Methods Sample Preparation and high pressure Generation. Carbazole was purchased from Alfa Aesar and used as received. The high-pressure experiments were performed using DAC technique. The sample was loaded in the holes (diameter: 150 µm) of a T301 steel gasket, which was preindented to a thickness of 40 µm. Small ruby balls were inserted into the sample compartment for in situ pressure calibration according to the R1 ruby fluorescence method. In the high pressure PL and XRD experiments, the pressure-transition medium (PTM) was silicone oil (Aldrich). KBr was used as the PTM in IR experiment. Optical Measurements. In situ PL and absorption photographs of the samples were obtained using a camera (Canon Eos 5D mark II) equipped on a microscope (Ecilipse TI-U, Nikon).


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Absorption spectra were measured in the exciton absorption band region using a DeuteriumHalogen light source, and the excitation source, a 355 nm line of a UV DPSS laser was used for PL measurements. The optical fiber spectrometer is an Ocean Optics QE65000 spectrometer. The excitation spectrum was carried out with a Shimadzu RF-5301PC. IR micro spectroscopy of Carbazole was carried out with a Nicolet iN10 microscope spectrometer (Thermo Fisher Scientific, USA) using a liquid nitrogen cooled detector. ADXRD Measurements. In situ high-pressure powder angle-dispersive X-ray diffraction (ADXRD) experiments with a wavelength of 0.6199 Å beam were performed at the 4W2 High Pressure Station in Beijing Synchrotron Radiation Facility. CeO2 was used as the standard sample to do the calibration. The ADXRD patterns were collected for 300 s at each pressure and then were integrated with FIT2D program. The program of Materials Studio was used for further analyzing the ADXRD data to carry out lattice parameters. Computation details. Geometry optimization was performed for the carbazole at different pressures, based on the first-principles plane-wave pseudopotential density functional theory36 as implemented in the CASTEP package37. The starting structure CRBZOL0338 was obtained from the Cambridge Structure Database. The GGA functional of Perdew, Burke, and Ernzerhof39 was used in the calculation. To correct for the van der Waals interactions common in molecular crystal, the nonempirical scheme of TS40 was used. The convergence level for total energy, max force, max stress, max displacement, and SCF iterations were fine. In order to visualize and analyze the intermolecular interactions in the crystal structures of carbazole at different pressure, the Crystal Explorer 3.1 program41 was used. It has enabled us to construct the 3-dimensional Hirshfeld surfaces of molecules in crystals.42 And we also can get information about relative contributions of intermolecular contacts to the Hirshfeld surface area from it.43


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ASSOCIATED CONTENT Supporting Information. PL spectra excited by Xenon lamp under high pressure, excitation spectra under ambient conditions, UV-Vis absorption spectra under high pressure, PL spectra recorded in the decompression, C···C interactions on Hirshfeld surface at selected pressure, contribution rate of C···C interactions to the Hirshfeld surface area versus pressure. AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed. E-mail: [email protected] E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Authors gratefully acknowledge funding supports from the National Natural Science Foundation of China (Nos. 21673100, and 91227202), Chang Jiang Scholars Program (No. T2016051), Changbai Mountain Scholars Program (No. 2013007), and Jilin Provincial Science & Technology Development Program (No. 20150520087JH). ADXRD experiments were performed at Beijing Synchrotron Radiation Facility (4W2 beamline), which is supported by Chinese Academy of Sciences (No. KJCX2-SW-N20, KJCX2-SW-N03).


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(31) Sagara, Y.; Yamane, S.; Mitani, M.; Weder, C.; Kato, T. Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials. Adv. Mater. 2016, 28, 1073-1095. (32) Dong, Y. Q.; Lam, J. W.; Tang, B. Z. Mechanochromic Luminescence of AggregationInduced Emission Luminogens. J. Phys. Chem. Lett. 2015, 6, 3429-3436. (33) Heimel, G.; Hummer, K.; Ambrosch-Draxl, C.; Chunwachirasiri, W.; Winokur, M. J.; Hanfland, M.; Oehzelt, M.; Aichholzer, A.; Resel, R. Phase Transition and Electronic Properties of Fluorene: A Joint Experimental and Theoretical High-Pressure Study. Phys. Rev. B 2006, 73, 024109. (34) Wang, Q.; Li, S.; He, L.; Qian, Y.; Li, X.; Sun, W.; Liu, M.; Li, J.; Li, Y.; Yang, G. Pressure-Induced Emission Enhancement of A Series of Dicyanovinyl-Substituted Aromatics: Pressure Tuning of the Molecular Population with Different Conformations. Chemphyschem 2008, 9, 1146-1152. (35) Desiraju, G. R.; Gavezzotti, A. Crystal Structures of Polynuclear Aromatic Hydrocarbons. Classification, Rationalization and Prediction from Molecular Structure. ActaCrystallogr. Sect. B: Struct. Sci. 1989, 45, 473-482. (36) Kohn, W.; Hohenberg, P. Inhomogenous Electron Gas. Phys. Rev.1964, 136, B864-B871. (37) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. I. J.; Refson, K.; Payne, M. C. First Principles Methods Using CASTEP. Z. Kristallogr. - Cryst. Mater. 2005, 220, 567-570. (38)

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Figure 1. Packing arrangement of carbazole crystal in part of one layer at ambient conditions. Red dashed lines represent C−H•••π hydrogen bonds. Blue dashed lines represent N−H•••π hydrogen bonds. 76x45mm (300 x 300 DPI)



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Figure 2. (a) PL spectra of carbazole crystal at range of pressure from 0.0 GPa to 10.3 GPa excited by 355nm laser. The red arrows represent the changes of PL intensity. (b). Corresponding PL photographs under high pressure. 76x88mm (300 x 300 DPI)


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Figure 3. High-pressure IR spectra of carbazole up to 10.5 GPa at room temperature in the region of ν(C−H) and ν(N−H) vibration modes. 76x57mm (300 x 300 DPI)



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Figure 4. (a) Selected ADXRD patterns of carbazole at different pressures. (b) Compression of unit cell volume of carbazole with respect to pressure. The inset shows compression rate of lattice constants under different pressure. (c) The evolution of molecular arrangement with increasing pressure. Viewing down caxis, the two sets of ‘herringbone’ pattern are at different viewing depths, black is closer to viewer. 152x149mm (300 x 300 DPI)


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TOC 50x50mm (300 x 300 DPI)


Pressure-Induced Emission Enhancement of Carbazole: The

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