Direct Observation of the Symmetrical and Asymmetrical Protonation

Jun 16, 2017 - ABSTRACT: The symmetrical and asymmetrical protonation states are realized via the formation of intermolecular hydrogen bonds inside 9 ...
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Direct Observation of the Symmetrical and Asymmetrical Protonation States in Molecular Crystals Suqian Ma, Jibo Zhang, Yingjie Liu, Jingyu Qian, Bin Xu,* and Wenjing Tian* State Key Laboratory of Supramolecular Structure and Materials, Jilin Unversity, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: The symmetrical and asymmetrical protonation states are realized via the formation of intermolecular hydrogen bonds inside 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA) molecular crystals. With the protonation of H2SO4, BP4VA molecules are protonated symmetrically, while the molecules are asymmetrically protonated by introducing HCl. The different protonation states of BP4VA crystals result in various supramolecular interactions, aggregation states, and even tunable optical properties. It provides a fundamental principle to understand the effect of protonation in organic conjugated molecules and an approach to expanding the scope of organic functional materials.

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in crystal.16 We have recently presented a detailed investigation on the protonation of pyridyl contained molecular crystal which performed remarkable red-shift emission compared with the neutral crystal.17 Here, we present a direct observation of symmetrical and asymmetrical protonation states via the formation of H-bonds in the molecular crystals based on a pyridyl contained organic molecule 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA). Confirmed by X-ray diffraction, the crystals selfassembled from BP4VA+H2SO4 solution (BP4VA-2H) are symmetrically protonated with two-side pyridyl moieties, while the crystals from BP4VA+HCl solution (BP4VA-1H) are protonated asymmetrically, in which a protonated pyridyl interacts with an unprotonated pyridyl to form an intermolecular H-bond as well as the compact head-to-tail supramolecular framework. The different protonation states not only contribute to the distinct molecular stacking, but also tune the optical properties. Detailed investigations indicate that both the protonated crystals exhibit red-shift fluorescence compared to the neutral BP4VA crystal, while BP4VA-1H crystal shows further bathochromic emission owing to the enhanced dipole moments and the compact stacking. These observations provide a deep understanding of the effect upon protonation and the structure−property relationships in organic conjugated molecules, and could provide further insights into the rational design of novel functional organic materials. Figure 1 shows the fluorescence images, configurations, and stacking modes of BP4VA, BP4VA-1H, and BP4VA-2H crystals. The neutral BP4VA crystals obtained from the solvent

ecently, luminescence behaviors of organic functional materials in the aggregate states have attracted wide interest due to their good performance in sensing, light emitting, and bioimaging.1−3 Noncovalent interactions construct the backbone of the aggregates, including hydrogen bonds (H-bonds), electrostatic interactions, metal ion coordination and so on.4 Among these interactions, H-bond plays a crucial role in the structures, functions and dynamics of chemical and biological systems. One important way to form H-bonds is represented as D−H···A, where D is donor and A is acceptor, in which the acceptor often contains electronegative atoms. Organic functional groups containing nitrogen atoms such as pyridyl and cyano are well-known electronegative agents capable of hydrogen bonding, in which a protonated group may interact with an unprotonated group to form an intermolecular H-bond.5 The multiple intermolecular hydrogen bonds induced by protonation can also control the aggregation structures and even tune the optical properties in organic functional materials.6,7 However, the task of understanding the structure−property relationship has been met with limited success because of the complex aggregation structures in solid state. Fortunately, organic molecular crystals can provide an efficient approach to investigating the relationship between the aggregation structures and the optical properties due to the well-ordered arrangement of molecules in crystals. 8−14 However, due to the complicated effects of the protonation on the aggregation structure and the related properties, only a few relevant reports demonstrated the crystalline protonation.15−17 For example, Xu and co-workers reported an AIEactive luminophore with benzothiazole moiety that showed different emission in acid-stimuli crystals.15 Wang and coworkers reported a functional chromophore containing 4pyrone that exhibited “turn-on” fluorescence upon protonation © XXXX American Chemical Society

Received: June 8, 2017 Accepted: June 16, 2017 Published: June 16, 2017 3068

DOI: 10.1021/acs.jpclett.7b01454 J. Phys. Chem. Lett. 2017, 8, 3068−3072

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The Journal of Physical Chemistry Letters

Figure 1. (a) Fluorescence images of BP4VA, BP4VA-1H, and BP4VA-2H crystals. (b) The configurations of one molecule in BP4VA-2H crystal and BP4VA-1H crystal. (c) Stacking modes of corresponding crystals.

Figure 2. Molecular stacking and intermolecular interactions in (a) BP4VA-2H crystal and (b) BP4VA-1H crystal. Bottom: the distance as well as the displacement between two molecules in a dimer.

molecules. The distance between the labeled N and H is 0.948 Å, which is larger than that of BP4VA-2H. Still, it is shorter than the reported N−H covalent bond distance, demonstrating that covalent bonds between N and H are formed. To further understand the different protonation states by H2SO4 and HCl, detailed analysis of molecular stacking should be taken in to account. Initially, the molecular stacking of the neutral BP4VA crystal is herringbone H-type, as shown in the middle of Figure 1c and Figure S1. Between two columns of the molecular cluster, C−H···N intermolecular hydrogen bonds act as the connectors to build up the stacking. In the same column, the adjacent anthracene planes overlap half with a perpendicular distance of 3.6 Å. The molecular packing of BP4VA-2H crystal shown in Figure 1c is a slide stacking H-aggregate in which the nitrogen atoms of the pyridyl in both sides have bonded to hydrogen atoms. As shown in Figure 2a, the molecules in BP4VA-2H crystal arrange with plenty of intermolecular H-bonds as the driving force. In each molecular column along the b axis, weak C−H···π interactions (1) formed between adjacent molecules. Among the molecular columns, each sulfate radical interacts with two neighboring protonated BP4VA molecules with weak N−H···O

vapor method are green emissive, as shown in the middle of Figure 1a.18 When a droplet of HCl and H2SO4 was added in the BP4VA solution separately, two kinds of protonated crystals BP4VA-1H and BP4VA-2H can be obtained by slowly evaporating the solvent, where BP4VA-1H shows red fluorescence (right in Figure 1a) and BP4VA-2H exhibits orange emission (left in Figure 1a). The analysis of X-ray diffraction data indicates that the molecular configuration of the orange emissive BP4VA-2H crystal is symmetrically protonated by H2SO4, as shown in Figure 1b. Two nitrogen atoms in the pyridyl groups are both trapping protons and the remaining sulfate radicals are dispersed between BP4VA molecules and are 2.8 Å away from the nearest proton bonded to nitrogen. The distance between the labeled nitrogen and hydrogen is 0.903 Å, which is shorter than the reported N−H covalent distance around 1 Å, indicating the formation of a N−H covalent bond.19 Surprisingly, the protonated crystal BP4VA-1H by HCl shows that only one side of the pyridyl captures a proton while the other side does not (right in Figure 1b), as confirmed by the Xray diffraction analysis. This kind of asymmetrical-protonated state has rarely been reported so far in organic conjugated 3069

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The Journal of Physical Chemistry Letters interactions (2) and C−H···O interactions (3). The distances and the angles of the intermolecular interactions are summarized in Table S1. As displayed in the bottom of Figure 2a, the planar anthracenes of two adjacent molecules overlap partially with a long axis displacement of about 2.2 Å and a short axis displacement of 1.6 Å. The perpendicular distance between the neighboring molecules is 3.6 Å, and a typical Haggregate is formed with the pitch angle θ1 of 63°. By contrast, the BP4VA-1H molecules form asymmetric conformations and arrange into a compact head-to-tail construction both along and perpendicular to the long molecular axis, as exhibited in Figure 1c and Figure 2b. It is obvious that between two adjacent molecules, who arrange along the long axis of the molecules, strong intermolecular Hbond N−H···N interactions (I) formed to connect the molecules in lines. The distance between the H and the N that form the H-bond is 1.81 Å, and the angle of N−H−N is 164.0°. Additionally, perpendicular to the long molecular axis, two protonated BP4VA molecules stack face-to-face as H-type dimers with a pitch angle θ2 of 82° and vertical distance of 3.5 Å. And looking perpendicularly to the anthracene plane, the anthracene planes overlap mostly with a long axis displacement of 1.6 Å and a short axis displacement of 0.5 Å, which indicates that strong π−π interactions exist in the molecular clusters. Although the protonation of the electronegative groups in organic molecular crystals have been studied in detail before, the unusual asymmetrical protonation state and the distinctive head-to-tail arrangement in BP4VA-1H crystal are scarcely reported. The question of why do BP4VA-1H molecules form such unique head-to-tail stacking in crystal should be given serious consideration. Comparing the three crystal structures, the greatest difference is the protonation states of BP4VA molecules: no protonation for neutral BP4VA, symmetrical protonation for BP4VA-2H, and asymmetrical protonation for BP4VA-1H. Thus, dipole moments of these complexes were calculated to investigate other intermolecular interactions than H-bonds. As shown in Figure S2, the dipole moments of BP4VA and BP4VA-2H complexes are quite small, 0.00 and 0.77 D respectively, probably due to the symmetrical geometries, while a dipole moment as strong as 10.18 D is obtained from BP4VA-1H complex, owing to the extremely unbalanced protonation states between vinyl anthracene. As a result, the strong dipole−dipole interactions between BP4VA1H molecules make it easier to form the compact head-to-tail conformation, which is similar to a number of freely moved magnets that have north poles and south poles arranging spontaneously into a well-organized array.20 The intrinsic molecular energy of an individual BP4VA-1H molecule is rather high since the structure is not symmetrical. However, the headto-tail construction dramatically decreases the disturbance of the active molecules as a whole. On the other hand, since the volume of sulfate radical is much larger than chloridion, as shown in Figure 2, the steric effect of sulfate radicals makes BP4VA-2H molecules stack with larger slide. Therefore, both pyridyl moieties on the BP4VA molecule have plenty of space to catch protons to form a two-side protonated crystal. The significant variety of the protonation states and the stacking structures could contribute to the great difference in optical properties. Figure 3 and Figure S3 show the photophysical properties of these BP4VA molecular crystals. Initially, the neutral BP4VA crystal emits bright green photons with a fluorescence peak at 531 nm and high photoluminescence quantum yield (PLQY) of 0.65, while due to

Figure 3. Normalized (a) PL spectra and (b) absorption spectra of BP4VA, BP4VA-1H, and BP4VA-2H crystals.

the protonation of H2SO4, the emission from BP4VA-2H crystals shows red shift to 605 nm and fluorescence quenching with low PLQY of 0.13. Furthermore, the HCl-treated crystals (BP4VA-1H) exhibit further bathochromic-shift emission to 640 nm and even lower PLQY, which is only 0.09. Combining PLQY with fluorescence lifetime results, radiative decay rates can be calculated (Table S2) and illustrate that BP4VA-1H crystal has the lowest rate of radiative decay. In dilute tetrahydrofuran (THF) solution, with addition of HCl or H2SO4, the emissions show gradual red shift, while, after being fumed in saturated HCl atmosphere, the amorphous film shows large red shift in emission, as exhibited in Figure S4 and Figure S5. Additionally, as for the solid state absorption of these crystals, the neutral BP4VA crystal has the narrowest spectra and centers its peak at 436 nm. Similar to the trend of fluorescence spectra, the absorption spectra of both BP4VA-2H and BP4VA-1H crystals show red shift compared to BP4VA crystal, while the absorption cutoff wavelength of BP4VA-1H crystal is longer than that of BP4VA-2H crystal. The theoretical study shown in Figure 4 indicates that, unlike the highest occupied molecular orbital (HOMO), which is mainly located on the vinyl anthracene plane, the lowest unoccupied molecular orbital (LUMO) of BP4VA is delocalized a little bit due to the electron-withdrawing of the terminal pyridyl groups. The neutral BP4VA has the largest band gap of 3.39 eV, which is in good accordance with the optical band gap estimated from the absorption spectrum. Since the protonation of nitrogen atoms can greatly enhance the electron-withdrawing ability of pyridyl, the LUMO of BP4VA2H and BP4VA-1H molecules is more delocalized, and the electron cloud density is significantly increased on the protonated pyridyl groups. The delocalization of the electron cloud density can help to stabilize the molecules in the excited states and give rise to the decrease of the band gap.21 Thus, both BP4VA-1H and BP4VA-2H molecules have small band gap less than 2 eV, which is in accordance with the shifts of the 3070

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Figure 4. Diagram of the molecular protonations and the theoretical calculated frontier orbital contribution based on B3LYP-6-31G** of BP4VA (green), BP4VA-2H (orange), and BP4VA-1H (red).

absorption and fluorescence spectra to longer wavelength. Another key factor that can affect the optical properties is the molecular stacking modes. In BP4VA-1H crystal, the compact head-to-tail molecular stacking and the increased π−π overlap could result in the enhancement of excitonic coupling between the transition dipoles of adjacent molecules. As a result, further red shifts occur in absorption and fluorescence of BP4VA-1H crystals compared to BP4VA-2H crystals.22 In summary, a direct observation of symmetrical and asymmetrical protonation states was realized from protonated BP4VA crystals by different acids. When triggered by H2SO4, X-ray crystal diffraction results indicate that the molecules in crystal BP4VA-2H are symmetrically protonated with both pyridyls capturing protons, while the molecules in crystal BP4VA-1H, which is treated by HCl, are uniquely asymmetrically protonated with only one pyridyl bonding to hydrogen. Both protonated crystals showed compact H-type molecular stacking and red-shift emission due to the protonation and various intermolecular interactions. Meanwhile, the molecules in BP4VA-1H crystals adopt compact head-to-tail stacking due to the asymmetrical protonation states and enhanced molecular dipole moments. As a result, the emission and absorption of BP4VA-1H crystals shift to even longer wavelength compared with BP4VA-2H crystals. The detailed study on the crystal structures and the optical properties of the protonated crystals provides us a valuable chance to understand the structure− property relationships upon protonation. Specially, the symmetrically protonated and the asymmetrically protonated states could provide further insights into the rational design of novel functional organic materials.

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CIF of BP4VA-1H crystals (CIF) CIF of BP4VA-2H crystals (CIF) Checkcif of BP4VA crystals (PDF) Checkcif of BP4VA-1H crystals (PDF) Checkcif of BP4VA-2H crystals (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.X.). *E-mail: [email protected] (W.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (2013CB834701), the Natural Science Foundation of China (No. 51373063, 51573068, 21221063), Program for Chang Jiang Scholars, Innovative Research Team in University (IRT101713018), and the Program for Changbaishan Scholars of Jilin Province.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01454. Experimental details, molecular stacking, dipole moments, fluorescence decays, photophysical properties, and crystal data (PDF) CIF of BP4VA crystals (CIF) 3071

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