Letter pubs.acs.org/JPCL
Proton-Triggered Hypsochromic Luminescence in 1,1′-(2,5-Distyryl1,4-phenylene) Dipiperidine Jinlong Chen, Suqian Ma, Jibo Zhang, Lijuan Wang, Ling Ye, Bao Li, Bin Xu,* and Wenjing Tian* State Key Laboratory of Supramolecular Structure and Materials, Jilin Unversity, Qianjin Street No. 2699, Changchun 130012, People’s Republic of China S Supporting Information *
ABSTRACT: A proton-triggered hypsochromic luminescent chromophore 1,1′-(2,5distyryl-1,4-phenylene) dipiperidine (DPD) was designed and synthesized. Upon treatment by hydrochloric acid (HCl), the emission of DPD showed a large hypsochromic shift in both THF solution and microcrystals. Theoretical calculations and powder X-ray diffraction experiments reveal that the switchable emission of DPD originated from the change of the distribution and the spatial arrangement of the frontier molecular orbitals, and the different stacking modes of DPD in microcrystals also contribute to the switchable emission of DPD in aggregates.
SECTION: Spectroscopy, Photochemistry, and Excited States
F
lamino units with more than 150 nm blue-shift emission upon the addition of TFA to the solutions.11 Bunz and co-workers reported that cruciform functional chromophores containing dibutylamino units showed blue-shift emission upon proton stimuli,9 and the proton-stimuli hypsochromic emission of the functional chromophores containing dibutylamino groups originated from the stabilized low HOMO and unchanged LUMO of the fluorophores by the protonation.9,11 Here, we present the proton-triggered hypsochromic luminescence of a functional chromophore, 1,1′-(2,5-distyryl1,4-phenylene) dipiperidine (DPD) containing piperidyl groups and investigate the related origin of the luminescent changes by combining experimental analyses with theoretical calculations. Upon titration by hydrochloric acid (HCl) in THF solution, the large blue shift in the emission spectra of DPD is observed, and the fluorescence of DPD microcrystals can be switched between green and blue by fumigation of acid/base vapor. The theoretical calculations and powder X-ray diffraction (PXRD) experiment reveal that the switched fluorescence either in solution or in microcrystals originates from the change of the distribution and the spatial arrangement of the frontier molecular orbitals (FMOs). Besides, different stacking modes in DPD microcrystals also contribute to the switchable fluorescence in aggregates. DPD (Figure 1a) was synthesized by the Witting−Horner reaction and Buchwald−Hartwig amination reaction,16−18 and
unctional chromophores containing embedded recognition elements showing shifts in both absorption and emission upon acid/base stimuli are of immense interest for their innately potential applications in sensors or sensor arrays for acid/base.1−6 Nitrogen is one of the good recognition elements for developing functional chromophores with acid/base stimuliresponsive character because the lone pair electrons can easily bond with protons. Great effort has been devoted to the molecular design and modification,1,7−10 as well as the related origin of the spectral shift upon acid/base stimuli of functional chromophores containing nitrogen atoms.5,11−15 Most investigations on the spectral shift upon acid/base stimuli showed that the functional chromophores possess bathochromic shift emission under the proton trigger.1,7−14 For example, Tang and coworks reported a heteroatom-containing luminogen (CP3E) with switched emission between blue and yellow by repeated protonation and deprotonation processes.8 Bunz and coworkers found that cruciform functional chromophores containing pyridine units showed nearly 100 nm red shift emissions upon the addition of excess trifluoroacetic acid (TFA) in solutions.7 We recently presented a detailed investigation on the structure−property relationship of functional chromophores containing pyridine groups BDP2VB,10 BP3VA,12 and BP4VA13 with remarkable bathochromic shift emission under the protonation stimuli. Consequently, the origin of the bathochromic shift luminescence was proposed to be the enhanced electron delocalization and electron-withdrawing ability induced by the protonation. However, chromophores with proton-triggered hypsochromic luminescence were rarely reported until now. Haley and co-workers reported nine functionalized chromophores containing dibuty© 2014 American Chemical Society
Received: July 2, 2014 Accepted: July 29, 2014 Published: July 29, 2014 2781
dx.doi.org/10.1021/jz501383d | J. Phys. Chem. Lett. 2014, 5, 2781−2784
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spectrum of DPD (Figure 1b), suggesting that the emission of DPD can be switched by the protonation stimuli. To further investigate the protonation process, the quantitative titration was performed on the DPD solution by exploiting the fluorescent spectroscopy with the addition of HCl. As shown in Figure 1c, the emission color changed gradually from yellow to blue with the increase of H+ content (−log[HCl]). Figure 1d shows the PL spectra of DPD in THF upon titration. Without HCl, the emission band originally peaked at 538 nm. Upon the addition of HCl, the emission became weak and underwent a blue shift. Meanwhile, a new emission band peaking at 430 nm appeared and became stronger and stronger with the increase of H+ content. When the concentration of H+ increased to 0.12 mol/L (−log[HCl] = 0.925), the original emission band at 538 nm disappeared. Moreover, the fluorescent quantum efficiency ΦF and the fluorescent lifetime τF decreased from 0.70 to 0.48 and from 6.48 to 3.76 ns (Figure S3, Supporting Information) when increasing the concentration of H+ from 0 (no HCl) to 0.27 mol/L (−log[HCl] = 0.563) in THF solution. Meanwhile, the nonradiative rate knr (knr = (1 − ΦF)/τF)19 increased from 0.04 to 0.13 ns−1, suggesting that the protonation enhanced the nonradiative process of the DPD molecule. The PL spectra of DPD crystals are shown in Figure 1b. It exhibits a strong green emission peaking at 536 nm with almost no shift compared to that in CHCl3 solution. It is observed from the crystal structure shown in Figure S4d (Supporting Information) that DPD crystals belong to the triclinic system, with space group P-1 possessing an inversion center at the centroid of the central benzene ring. Additionally, the molecules arrange into an absolute uniaxial orientation.20 Each unit cell consists of one crystallographically independent DPD molecule (Figure S4c, Supporting Information). The torsional angle between the vinyl and the peripheral phenyl ring θ(1,2,3,4) is 0.66°, while the peripheral piperidyl groups are more twisted out of the central benzene plane, noting that the torsional angle θ(5,6,7,8) is 76.56° (Figure S4b, Supporting Information). As a result of the twisted conformations of the peripheral piperidyl, the central benzene planes of the two adjacent DPD molecules in the crystal are far away from each other (6.202 Å), thus indicating that there are almost no intermolecular vibronic interactions between the adjacent molecules. Besides, there are slide π−π interactions between neighboring DPD molecules in the crystals (Figure S4c, Supporting Information). The interplane distance is 3.634 Å, and there is little overlap between adjacent π conjugations, indicating that the intermolecular vibrations merely affect the electron transition21−23. Hence, the PL of the DPD crystal, which is similar to that of the dilute solution, mainly originates from the single molecule, and the intermolecular interactions have little effect on the emission of the crystal. Meanwhile, the large distance between the center benzene rings of adjacent DPD molecules also avoids the formation of face-to-face π−π interactions, resulting in the fluorescent quantum efficiency of the crystal being as high as ΦF = 0.51. The protonation/deprotonation processes of DPD microcrystals were also investigated. As shown in Figure 2a, DPD microcrystals exhibit a strong green emission peaking at 540 nm, while the emission turned blue and shifted to 456 nm under 365 nm UV illumination after being fumed by HCl vapor. When treated by triethylamine (TEA) vapor, the fumed microcrystal recovered to its green emission (λem = 537 nm). The switch between green and blue emission can be repeated without fatigue by alternately fuming with HCl and TEA vapor
Figure 1. (a) Molecular structure of DPD and DPD−HCl. (b) Normalized UV−vis absorption and PL spectra of DPD in chloroform solution with excess HCl and PL spectra of the crystal. (c) Images of DPD in THF solution with different HCl concentration under 365 nm UV light. (d) PL spectra of DPD in THF upon titration with HCl.
all of the synthetic procedures and characterization data are summarized in the Supporting Information. The absorption and fluorescent spectra of DPD in chloroform solutions are shown in Figure 1b. It shows that DPD possesses a main absorption band peaking at 334 nm that can be attributed to the HOMO−1 → LUMO transition, while a shoulder peak around 394 nm represents the HOMO → LUMO transition (Table S1, Supporting Information). The maximum emission appears at 538 nm with a broad and featureless structure and a large Stokes shift (144 nm). DPD is a cruciform molecule, in which two benzenes and two piperidine arms are placed in the 1,4- and 2,5- positions of the central benzene ring. The piperidine ring embedded nitrogen atom can serve as the electron-donating unit relative to distyrylbenzene moieties, forming the donor−acceptor (D− A) structure. This structure can reduce the orbital overlap in the excited state to ensure a charge-transfer (CT) state. The emission spectrum of DPD at low temperature was added as Figure S1 (Supporting Information). Blue-shifted emission of DPD is observed at 77 K compared to that of 298 K, suggesting the absence of CT states, which results from the difference between the ground state and the excited state. The fluorescent spectra of DPD in solvents with different polarity are shown in Figure S2 and Table S2 (Supporting Information). As the solvent polarity enhancement, the emission showed a large red shift (λem = 508 nm in n-hexane; 522 nm in diethyl ether; 538 nm in chloroform; 536 nm in THF; 546 nm in acetonitrile). The large solvatochromism of the emission mainly originates from the cruciform structure of DPD. In the excited state, the cruciform structure can reduce the orbital overlap between the donor (piperidine) and the acceptor (distyrylbenzene) moieties to ensure a CT state that possesses an increased dipole moment, which indicates that the molecule should be more stable in a polar medium than that in a nonpolar medium. As a consequence, the excited molecules are stabilized by surrounding polar solvent molecules, leading to red-shifted emissions.18 When excess HCl was added into the chloroform solution, the shoulder absorption band of DPD at 394 nm disappeared, and instead, a dramatic hypsochromic shift (from 538 to 435 nm) with fine vibronic structure was observed in the emission 2782
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Figure 3. Theoretically calculated frontier orbitals of DPD and protonated DPD.
involved in the π-conjugation. The energies of the HOMO and LUMO of DPD−HCl are −5.76 and −2.22 eV, and both are lower than those of DPD. The reason for the HOMO change is that the nitrogen atoms in DPD molecules would be protonated upon the acid stimuli and the lone pair electrons of the nitrogen atoms would bond with the hydrogen proton (H+). This weakened the electron-donating ability of nitrogen atoms and made the nitrogen atoms no longer involved in the π-conjugation, resulting in the decrease in the energy of the HOMO. This result can also be proved by comparing the absorption and emission spectra with those of the analogue DSB,24,25 as shown in Figure S6 (Supporting Information). Similar absorption and emission spectra indicated a similar πconjugate structure of protonated DPD as that of DSB. As a consequence, the band gap broadened from DPD (3.29 eV) to DPD−HCl (3.54 eV), which led to the blue shift in both the absorption and emission of DPD−HCl (Figure 1b). From the above discussion, we conclude that the nitrogen atoms are no longer involved in the π-conjugation induced due to the protonation, which leads to the low energy of the HOMO resulting in an increased band gap contributing to the blue emission of DPD−HCl compared to the yellow/green emission of DPD in either solution or the microcrystal. In conclusion, we have presented the proton-triggered hypsochromic luminescence of functional chromophore,1,1′(2,5-distyryl-1,4-phenylene) dipiperidine (DPD) and detailed investigations of the related origin. After being treated by HCl, DPD gave a hypsochromic fluorescent emission in either the solution or microcrystal. The theoretical calculations and PXRD of the DPD microcrystal indicated that the hypsochromic emission originated from the change of the distribution and the spatial arrangement of the FMOs because the nitrogen atoms in DPD are no longer involved in the π-conjugation under the proton trigger, leading to the enhanced band gap. Besides, the different stacking modes of the DPD microcrystal also contributed to the switchable emission between green and blue in the aggregates. This study on DPD provides a detailed insight into the origin of the hypsochromic shift and is beneficial to the design and synthesis of novel functional chromophores containing embedded nitrogen atoms for such stimuli-response luminescent applications. Also, the remarkable color-changing properties of DPD in solution and the microcrystals suggest that it may be a potential candidate for fluorescent sensors in both solutions and solids. Moreover, the high emission efficiency and the uniaxially oriented packing in
Figure 2. (a) PL spectra of the DPD microcrystal fumed with HCl and TEA. (Inset) Images of the initial, HCl fumed, and TEA fumed under 365 nm UV light. (b) PXRD patterns of DPD microcrystals treated by HCl vapor and then TEA vapor, as well as the simulated patterns from the data of single crystals.
as the process is nondestructive in nature (Figures 2a and S5, Supporting Information). The blue shift of ∼81 nm in the fluorescence of DPD when being fumed with HCl vapor and the recovery toward the initial green emission (λem = 537 nm) demonstrated the significant fluorescent switching properties of the DPD microcrystal under protonation/deprotonation stimuli. In order to explain the switched emission between blue and green, X-ray diffraction (PXRD) of the DPD microcrystal was performed, and the patterns are shown in Figure 2b. The PXRD pattern of the initial microcrystal accorded well with the simulated pattern from the crystal data of DPD, suggesting that the initial microcrystal possesses the same stacking modes as the single crystals. After being fumed by HCl/TEA, the weak and altered diffraction pattern indicated the significant decrease of crystallinity and the different molecular aggregation structures. In other words, the protonation/deprotonation process results in the decrease of the crystalline degree and different stacking modes of the DPD microcrystal, which may contribute to the hypsochromic shift emission.12 For more insights into the origin of hypsochromic shift in solution and microcrystals, the FMO plots of DPD and DPDHCl calculated by the Gaussian 03 suite of programs at the B3LYP/6-31G** level of DFT are shown in Figure 3. The HOMO is primarily delocalized over the distyrylbenzene framework and nitrogen atoms, while the LUMO is located on the distyrylbenzene backbone. After the protonation of nitrogen atoms in the piperidine moieties, the HOMO changed significantly, and a hypsochromic shift was observed. Just as shown in Figure 3, unlike the HOMO of DPD, the HOMO of DPD−HCl mainly locates on the distyrylbenzene backbone, and there is mere electron cloud distributed in the nitrogen atoms, suggesting that the protonated nitrogen atoms are not 2783
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the crystal suggest that DPD may be applied in the organic optoelectronic fields.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed synthesis and characterization of 1,1′-(2,5-distyryl-1,4phenylene) dipiperidine, together with its photophysical properties and single-crystal structure. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail
[email protected] (W.T.). *E-mail
[email protected] (B.X.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by 2013CB834702, the Natural Science Foundation of China (No. 51373063, 21204027, 21221063), Project 2014013 supported by the Graduate Innovation Fund of Jilin University, and the Research Fund for the Doctoral Program of Higher Education of China (20120061120016).
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REFERENCES
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