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Pressure Tuning Dual Fluorescence of 4-(N, NDimethylamino) Benzonitrile Yu-Xiang Dai, Shitong Zhang, Haichao Liu, Kai Wang, Fangfei Li, Bo Han, Bing Yang, and Bo Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00709 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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The Journal of Physical Chemistry

Pressure Tuning Dual Fluorescence of 4-(N, N-Dimethylamino) Benzonitrile

Yuxiang Dai,† Shitong Zhang,‡ Haichao Liu,‡ Kai Wang,*,† Fangfei Li,† Bo Han,† Bing Yang,*, ‡ and Bo Zou,*,†

†

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

‡

State Key Laboratory of Supramolecular Structure and Materials College of Chemistry, Jilin University, Changchun 130012, China.

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Abstract The intramolecular charge-transfer (ICT) emission band in dual fluorescence of 4-(N,Ndimethylamino) benzonitrile（ DMABN ） molecular crystal exhibits increase in response to compression up to 10 GPa. On the basis of Raman and angle-dispersive X-ray diffraction (ADXRD) experiments combining with computational studies, the mechanism of this phenomenon could be assigned to the change of the intramolecular geometrical conformation, especially for the decrease of the dihedral angle between the dimethylamino (NMe2) and phenyl moiety. Meanwhile the reduction of excited-state energies and the HOMO-LUMO band gap leads to the redshifts of photoluminescence (PL) spectra and the absorption edge, respectively. Competing with the aggregation caused quenching (ACQ) effect, the planarity of molecular conformation and the slight rotation of the NMe2 group under high pressure both could enhance the ICT process, which will contribute to the revelation of the ICT mechanism and designs of new piezochromic luminescent materials.

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Introduction Donor-acceptor material system has been attracting wide attention in recent years for its versatile applications such as organic light emitting diode (OLED), organic photovoltaic (OPV), organic sensor and so on.1-2 In early researches of the CT materials, dialkylaminobenzonitrile derivates reported by Lippert in 19623 were most researched, due to their intriguing photophysical properties, especially for the dual-fluorescence phenomenon of the most traditional model compound DMABN (Figure 1), since those studies on the dual fluorescence phenomenon of the CT materials could help to better cognition and further application on donoracceptor materials.2 By now, many researches had been carried out on DMABN to understand its energy and electronic structural features, and many competing hypotheses were proposed to explain the dual fluorescence in DMABN4, and two of the mechanisms are most accepted. One is the intramolecular structural change by a 90° twist of the NMe2 group or the twisted intramolecular charge-transfer (TICT)5-9 the other is the planarization of the molecule in the CT state (PICT)10-12. Recently, based on the change in the excited state from a locally excited (LE) state to an ICT state, a few piezochromic crystal materials with dramatic switching in the luminescence color and intensity have been reported.13-14 Thus, applying pressure could be an effective method to tune and study dual fluorescence in DMABN crystal. DMABN crystallizes in monoclinic P21/c symmetry at the temperatures ranging from 173 K to 301 K.15-17 It is interesting to note that the NMe2 group is not coplanar with the phenyl ring. The pyramidal geometry of amino group has been considered as the feature of its molecular structure as well. According to the reported literature17, the maximum of the emission band of DMABN is at about 370 nm and a shoulder peak exists in range from 450 to 500 nm. When the temperature drops to 173 K, the intermolecular space is slightly squeezed and unit-cell volume decreases by

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2.1%. These are derived from a slippage of alternating molecules and a significant reorganization of molecular packing. The dihedral angle between the NMe2 group and the phenyl ring is nearly 10° at 301 K and 7° at 173 K respectively.17 It is also worthy to notice that DMABN molecules in both room-temperature phase and low-temperature phase (173 K) are arranged in a herringbone style with weak π-π interactions between staggered antiparallel molecules. While dipoledipole interactions can explain the antiparallel intermolecular orientation

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, the weak close

contacts and attractive orbital interactions of the π-systems between vicinal DMABN molecules indicate that the molecules are relatively isolated in an electronic sense. So far, luminescence of DMABN under ambient pressure has been studied in several literatures.19-22 As is known, dual fluorescent properties in DMABN crystal are related to molecular configurations and arrangements,17 which can be regulated by the external pressures effectively.13-14,

23-31

To gain deep insight into the intriguing dual fluorescence of DMABN

molecule, we put DMABN crystal into conditions of extreme pressure up to about 15 GPa and investigated the structural, conformational, optical and photophysical properties by applying in situ high-pressure PL spectroscopy, UV-visible absorption spectra, Raman spectroscopy and ADXRD techniques, combining with the theoretical calculations and simulations. Now that piezochromic luminescent crystal materials have appealing application prospect in optical storage, pressure sensors, and writing tools.32-45 Thus, this study will not only offer an intriguing example for understanding the relationship between dual fluorescence and structural property of DMABN molecular crystals, but also contribute to developing promising solid-state luminescent materials.

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Experimental Section A piece of T301 steel with a predrilled 130 µm compartment was placed between the parallel diamonds. Commercially purchased DMABN crystal (>98%) was placed in a symmetric diamond anvil cell to conduct high-pressure PL, UV-visible absorption, Raman and ADXRD experiments. Ruby balls were used to measure the pressure through the ruby fluorescence method.46 Nitrogen was used as the transmitting medium of pressure for PL, UV and Raman experiments while silicon oil was used for ADXRD experiments, respectively. The temperature was maintained at approximately 298 K. The excitation spectrum of the DMABN crystal was carried out with a RF-5301PC, and the excitation spectrum (Figure S1, Supporting Information) revealed the good excitation wavelength λex ranging from 320 to 335 nm. Piezochromic PL spectra were measured with a micro-Raman spectrometer (Horiba-JY LabRam HR Evolution) using a 325 nm laser as the excitation source. High-pressure UV-visible absorption spectra were measured with an optical fiber spectrometer (Ocean Optics, QE65000) using a Deuterium-Halogen light source. Raman spectra were measured with a micro-Raman system assembled around a spectrometer (iHR 550, Horiba Jobin Yvon) with a thermoelectrically cooled charge-coupled device (CCD, Syncerity, Horiba Jobin Yvon). In order to avoid fluorescent background, the Raman measurements were separated from the PL measurements and a 785 nm wavelength laser (0.5 mW) was used in the Raman experiments. Although it is difficult to control the pressures at exactly the same values in different experiments, we tried our best to compare the PL and Raman spectra at similar pressures. In situ high-pressure ADXRD experiments were performed by using the 4W2 beamline at the High-Pressure Station of Beijing Synchrotron Radiation Facility. The wavelength of 20 × 30 µm2-size monochromatic beam was kept as the 0.6199 Å. The geometric parameters

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were calibrated by the CeO2 standard. The ADXRD patterns were collected by the Mar345 detector and integrated by the Fit2D software.47 Materials Studio 5.0 was used to analyze the crystal structure of DMABN under high pressure on the basis of the ADXRD patterns. Theoretical calculations were carried out with a Gaussian 09 Version D.01 Package. PL spectra under variable temperatures were carried out with an Edinburgh FLS-920.

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Results and Discussion The PL spectra of the DMABN crystal under high pressure are shown in Figure 2. According to the reported literature, the broad emission band from 330 to 450 nm is LE emission band and the other broad emission band at region after 450 nm is assigned as ICT emission band.17 The PL lifetime data (Figure S2, Supporting Information) collected at ambient conditions can support the assignment of these two emission bands. The emission intensity of the LE band in Figure 2a was continuously reduced during the process of compression, which is corresponding to aggregation caused quenching (ACQ)48 in solid-state or aggregated structures. While it is worth noting that the intensity of the ICT band was enhanced during the compression process below 10 GPa (Figure 2a). Above 10 GPa, the ICT band quenched gradually with the increase of pressure in Figure 2b. Meanwhile both the LE and ICT emission bands showed continuous redshifts in PL spectra under increasing external pressure (Figure 2), which can be identified as being derived from the pressure-induced conformational planarization based on general views.14, 49-52 The PL spectra of DMABN crystal recorded in the decompression process indicate these phenomena are reversible (Figure S3, Supporting Information). In Figure 3, the redshifted absorption spectra during compression were observed, which are correlated with the redshifts of PL spectra. The geometry of DMABN molecule was altered to adapt to the close-packed crystal structure and the HOMO-LUMO band gap was predicted to decrease gradually under increasing pressure.53 Based on this theoretical analysis, the frontier orbital’s contribution of molecular conformations in DMABN crystal under different pressures will be transformed. It is intriguing to note that the small absorption peak attributed to ICT transition in range from 440 nm to 450 nm was enhanced with the increase of the applied pressure, which is consistent with the enhancement of ICT emission band in Figure 2a.

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To investigate the molecular conformation under high pressure, we measured the Raman spectra of DMABN crystals at various pressures up to 14.92 GPa (Figure 4), and the results were assigned based on reported studies (Table S1, Supporting Information).54-57 The external modes at 45−200 cm-1 region in Figure 4a are translational and librational vibrations at several pressures. With continuous compression, the blueshifts of external modes indicate that the intermolecular interactions were enhanced.58-59 The internal modes can reflect molecular variations and the chemical environment around specific groups. The blueshifts of the ring breathing mode at 790 cm-1, the υs(NC2) mode at 946 cm-1, the ρsMe mode at 1173 cm-1,the υs(ph−CN) mode at 1232 cm-1, the δsMe mode at 1450 cm-1 and the C≡N stretching mode at 2216 cm-1 also demonstrated the enhancement of intermolecular interactions.60-61 Importantly, the ν(ph−N) frequency is a complex mode with additional contributions from the ring CH in-plane bending mode. Both the ν(ph−N) mode and the ring CH in-plane bending mode are useful in evaluating the bonding conformation of the amino group and the electron distribution in aromatic amine compounds.57 The frequencies of the υs(ph−N) mode at 1376 cm-1 and the CH in-plane bending mode at 1185 cm-1 were both increased under high pressure, which indicated the decrease of the ph-N bond length, the decrease of the amino inversion angle, and the increase in the extent of nπ conjugation. The intensity of the ρ sMe mode at about 1135 cm-1 also increased indicating the constraints of methyl is reduced, which may be related to the planarity of the pyramidal NMe2 group. The Raman results of the released samples in Figure 4 also indicate that the observed phenomena are completely reversible. In situ high-pressure ADXRD experiments were subsequently performed to further analyze the crystal structure of DMABN. Selected ADXRD patterns of DMABN vs. pressure are shown in Figure 5. With the increase of pressure, it is evident that all the diffraction peaks shifted to higher

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angles, indicating lattice spacing was decreased and the unit cell was squeezed. Above 10 GPa, all the sharp diffraction peaks started to broaden and lose their intensity and finally became one new broad peak up to 14.98 GPa, which indicates the original crystal structure of DMABN was broken and the arrangement of DMABN molecules tuned to be disordered.62 The diminishment of X-ray diffraction patterns is coincident with the quenching of ICT emission band in Figure 2b. In addition, the diffraction pattern of the recovered samples returns to that at ambient conditions, indicating that all the transitions are reversible, which is consistent with the PL and Raman results. The Pawley refinement results indicate that the compression of DMABN crystal is anisotropic, and the a-axis is the most compressible axis (Figure 6a). It is worth noting that the unit-cell contraction over 1.97 GPa is about 4.6% (Figure 6b), which is bigger than that at 0.1 MPa/173 K (2.1%)17. Under low temperature, the ICT emission intensity was also enhanced, while the LE emission intensity was reduced (Figure S4, Supporting Information). So the PL results during compression up to 10 GPa are similar to the low-temperature PL results. Thus it allows us to conclude that the high-pressure molecular conformation is similar with that at 0.1 MPa/173 K, and the dihedral angles between NMe2 group and the phenyl ring of DMABN molecules are smaller than those at ambient conditions. According to the reported literature, this dihedral angle is associated with the ICT emission band.17 To clarify the origin of the transitions in PL spectra and the excited state properties in the crystal of DMABN, quantum chemical calculations of these two reported molecular conformations with different dihedral angles between NMe2 group and the phenyl ring were further carried out. In Figure 7a, the molecular conformation with ~10° dihedral angle between NMe2 group and the phenyl ring is of higher excitation energy than the more planar conformation (dihedral angle

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is ~7°), reﬂecting that the planarity of molecular conformation could be responsible for the redshifted emission under high pressure. Furthermore, natural transitional orbital (NTO) analysis also demonstrates the difference of S1/S2 excited states between these two conformations in Figure 7b. Because there is larger orbital overlap between “hole” and “particle” in the S2 state than that in the S1 state, it is evident that the S1 state dominates ICT emission and the S2 state dominates LE emission. The distinct oscillator strengths of these two different conformations can further illustrate the difference of PL intensity under high pressure. The S1 oscillator strength of the conformation with ~10° dihedral angles (0.0289) is smaller than that with ~7° dihedral angles (0.0316), indicating that radiative transition rate of the ICT state is higher in a more planar conformation, which corresponds to the enhancement of ICT emission band under high pressure. Meanwhile, the smaller S2 oscillator strength of the conformation with ~7° dihedral angles (0.5243) in comparison to that with ~10° dihedral angles (0.5387) also coincides with the pressure-induced fluorescence quenching of LE emission. The frontier orbital’s contributions of DMABN in different molecular conformations are seen in Figure S5 (Supporting Information), which depicts the calculated energy levels of the HOMO-1, HOMO, LUMO, and LUMO+1 of different molecular conformations. Geometrical transitions and the reduction of the HOMO– LUMO band gap correlate with the redshifts of the absorption edge with increasing pressure in Figure 3, which is of similar magnitude of the pressure-induced redshifts in the PL spectra. The decrease of the dihedral angle between NMe2 group and the phenyl ring facilitates the delocalization of the nitrogen lone pair to the π system of the cyanophenyl group, leading to greater overlap of the frontier orbitals and achieving high intensity of ICT emission, theoretically. In addition, we have also built a series of models to investigate the relationship between the ICT emission and the torsion of NMe2 group. Figure 8 shows the calculated excitation energies

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and oscillator strengths of molecular conformations with decreasing C8-N2-C4-C3 torsion angles, which can also match the high-pressure PL results. It shows that rotating NMe2 group in the direction to decrease C8-N2-C4-C3 torsion angles can also increase the oscillator strength of ICT band and enhance ICT emission. Meanwhile the decrease of S1/S2 excitation energies also corresponds with the pressure-induced redshifts of LE and ICT bands. So planarity of molecular conformations and the slight rotation of charge-donor group in an appropriate way can act as the mechanism to enhance ICT emission intensity. Meanwhile the rehybridization of amino N atom from sp3 to sp2 in a more planar molecular conformation can also cause the enhancement of ICT emission band.51 The increased overlap of the frontier orbitals and the enhanced conjugation of the nitrogen lone pair and ring π cloud also lead to efficient charge transfer.13, 57 Besides the well-known ACQ effects in the condensed phase, the energy transfer from S1 to S2 is another reason leading to the quenching of LE fluorescence. However, the ACQ phenomena of LE and ICT emission band are inevitable under higher pressures and the disordered crystal structure over 10 GPa also correlates with the quenching of ICT emission.

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Conclusion In summary, we have investigated dual fluorescence of DMABN crystal under high pressure. The quenching of LE emission band and the enhancement of ICT emission band under the external pressure stimuli were observed. These changes are related to the molecular conformation with the decrease of the dihedral angle between the NMe2 group and phenyl ring. The increased oscillator strength of ICT transition in this more planar molecular conformation is responsible for the increased radiative transition rate of the ICT emission band. The reduction of the HOMO−LUMO band gap is also consistent with the redshifted absorption edge of DMABN. Meanwhile the slight rotation of NMe2 can also affect the ICT emission band and ACQ phenomena of LE and ICT emission band are inevitable under higher pressures. We anticipate that the conformational planarity and the rotation of donor groups can tune the excited state, which will inspire the development of new solid-state piezochromic luminescent materials.

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Figure 1. (a) Chemical formula of DMABN; (b) The scheme of the intermolecular charge transition process.

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Figure 2. (a) PL spectra of DMABN single crystal recorded in the compression from 0.73 GPa to 10.36 GPa, (b) from 10.36 GPa to 15.22 GPa.

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Figure 3. UV-visible absorption spectra of DMABN single crystal recorded in the compression from 0.73 GPa to 15.22 GPa.

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Figure 4. Raman spectra of DMABN powder crystals at selected pressures: (a) 45 cm−1 to 1330 cm−1, (b) 1366 cm−1 to 2300 cm−1.

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Figure 5. Representative synchrotron XRD patterns of DMABN powder crystals at high pressures.

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Figure 6. (a) Variations in lattice parameters; (b) Unit cell volume as a function of pressure.

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Figure 7. (a) Excitation energies and (b) NTO analysis of different DMABN molecular conformations with different dihedral angles between the NMe2 group and the phenyl ring. The percentages on the arrow are the proportions of transitions. The calculations are carried out using td/M06-2X/6-31g (d, p) method, and f is the oscillator strength.

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Figure 8. (a) The atomic label of the DMABN molecule, (b) Excitation energies of different DMABN molecular conformations with different C8-N2-C4-C3 torsion angles, f is the oscillator strength.

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Supporting Information The excitation spectrum under ambient conditions, room-temperature lifetimes, PL spectra recorded in the decompression, PL spectra under selected temperatures. the assignment of the major Raman bands and partial calculated results are shown in the Supporting Information. Corresponding Author *E−mail: [email protected] *E−mail: [email protected] *E−mail: [email protected] Notes The authors declare no competing financial interests. Acknowledgments This work is supported by National Natural Science Foundation of China (Nos. 21673100, 91227202), Changbai Mountain Scholars Program (No. 2013007), and Program for Innovative Research Team (in Science and Technology) in University of Jilin Province. 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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Table of Contents Graph

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Figure 1. (a) Chemical formula of DMABN; (b) The scheme of the intermolecular charge transition process. 76x50mm (300 x 300 DPI)


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Figure 2. (a) PL spectra of DMABN single crystal recorded in the compression from 0.73 GPa to 10.36 GPa, (b) from 10.36 GPa to 15.22 GPa. 152x75mm (300 x 300 DPI)



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Figure 3. UV-visible absorption spectra of DMABN single crystal recorded in the compression from 0.73 GPa to 15.22 GPa. 76x76mm (300 x 300 DPI)


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Figure 4. Raman spectra of DMABN powder crystals at selected pressures: (a) 45 cm−1 to 1330 cm−1, (b) 1366 cm−1 to 2300 cm−1. 152x80mm (300 x 300 DPI)



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Figure 5. Representative synchrotron XRD patterns of DMABN powder crystals at high pressures. 76x86mm (300 x 300 DPI)


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Figure 6. (a) Variations in lattice parameters; (b) Unit cell volume as a function of pressure. 76x39mm (300 x 300 DPI)



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Figure 7. (a) Excitation energies and (b) NTO analysis of different DMABN molecular conformations with different dihedral angles between the NMe2 group and the phenyl ring. The percentages on the arrow are the proportions of transitions. The calculations are carried out using td/M06-2X/6-31g (d, p) method, and f is the oscillator strength. 76x48mm (300 x 300 DPI)


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Figure 8. (a) The atomic label of the DMABN molecule, (b) Excitation energies of different DMABN molecular conformations with different C8-N2-C4-C3 torsion angles, f is the oscillator strength. 76x48mm (300 x 300 DPI)



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Table of Contents Graph 41x44mm (300 x 300 DPI)


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