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Polymorph-dependent luminescence response to acid vapours and its application in safety protection of file information Huapeng Liu, Zhuoqun Lu, Kaiqi Ye, Zuolun Zhang, and Hongyu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b14474 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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ACS Applied Materials & Interfaces

Polymorph-dependent luminescence response to acid vapours and its application in safety protection of file information Huapeng Liu, Zhuoqun Lu, Kaiqi Ye, Zuolun Zhang,* and Hongyu Zhang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China. KEYWORDS: Polymorphs, Acid sensing, Information protection, Luminescence, Crystal ABSTRACT: A Schiff base, (E)-1-(phenylimino)methyl-2-hydroxylnaphthalene (PIHN), was found to form both non-luminescent and luminescent polymorphs. A unique luminescence ‘turn on’ behaviour was observed when the non-luminescent polymorph was fumed with the vapour of aliphatic acids with two or three carbons in the main chain, while the luminescent polymorphs almost did not change the emission colour under the same condition. As we know, this is the first report on polymorph-dependent acid response which discloses the influence of crystalline phase on acid-responsive behaviour. The formation of hydrogen bonds between PIHN and aliphatic acid is proposed to be the reason for the responsive behaviour of the non-emissive polymorph, and such a mechanism is different from the common protonation mechanism. A novel safety protection method of file information has been developed based on the polymorph-dependent luminescence response of PIHN. In addition, we disclose that a crystalline form could show multiple responsive behaviours towards different acids, which benefits the further design of novel acid sensors, such as the sensors that can qualitatively analyses the species of acid source in an acidic environment.

1. INTRODUCTION Organic solid materials have become a hot topic of material study due to their wide applications. 1–5 Among various solid materials, those responding to external stimuli, such as light, electricity, heat, pressure, acids/bases and solvent vapours, are promising active materials for sensors, logic gates, information storage, and electronic devices.6–14 Therefore, they have attracted increasing attention in recent years. Especially, the solids with luminescence response are of special interest because of the high sensitivity, rapidity and portability of luminescence detection. Various forms of organic solids, including powders, films and single crystals, have been applied in stimuliresponsive studies.15–20 The organic polymorphs, different forms of single crystals of an organic molecule, are of special research interest because their difference in molecular conformations and/or packing structures may lead to different responsive behaviours and thus facilitate the comparison and in-depth understating of responsive phenomena.21 From another perspective, polymorphs with different responsive behaviours allow the selection of desired responsive properties. Therefore, the construction of polymorphs represents a strategy for obtaining desired responsive materials, and such a strategy avoids the complicated chemical modification of material molecule. To date, organic polymorphs have been widely used in the study of pressure/mechanical force responsive materials.6,22–25 However, their responsive behaviours to other environmental stimuli are less explored.8,21,26

Especially, the acid-responsive behaviour of polymorphs has not been reported although such behaviour has been extensively studied with powders and films due to its importance for environment monitoring and ecological supervision.27–29 Therefore, the influence of crystalline form on acid-responsive behaviour has not been disclosed. In addition, the acid-responsive mechanism of organic solids is limited to the protonation of nitrogen atoms of material molecules.30,31 Understanding the relationship between crystalline form and acid-responsive behaviour, as well as the exploration of new acid-responsive mechanisms, is of importance for the development and application of novel acid-responsive materials. Here, we report the polymorph-dependent luminescence response of a Schiff base, PIHN (Figure 1a), to acid vapours and propose a new acid-responsive mechanism. Moreover, PIHN was applied in safety protection of file information by utilizing the polymorph-dependent response.

2. EXPERIMENTAL SECTION General information. (E)-1-(phenylimino)methyl-2hydroxylnaphthalene (PIHN) was synthesized according to a known procedure.32 NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer with tetramethylsilane as the internal standard. Emission spectra were recorded using a Maya2000 Pro CCD spectrometer. Absolute fluorescence quantum yields of solids were measured on an Edinburgh FLS920 spectrometer combined with a calibrated integrating sphere. FT-IR

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microspectroscopy was carried out with a Thermo Fisher Scientific Nicolet iN10 microscope spectrometer using a liquid-nitrogen-cooled detector.

Crystal growth. Saturated dichloromethane solution of PIHN was added in test tubes and then it was covered with hexane (twice the volume as the dichloromethane solution). After standing at room temperature for several days, block-shaped crystals appeared in most test tubes and needle-like crystals appeared in the rest of test tubes. Platelike crystals were prepared by slow evaporation of a PIHN solution (with the mixture of dichloromethane and hexane (1:6) as the solvent) in test tubes at room temperature. Single-crystal X-ray diffraction. Single crystal Xray diffraction data were collected on a Rigaku R-AXIS RAPID diffractometer using the ω-scan mode with graphite-monochromator Mo·Kα radiation. The structures were solved with direct methods using the Olex2 programs and refined with full-matrix least-squares on F2. Nonhydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated and refined isotropically. The structures were deposited at the CCDC under the following numbers: 1584702 (PIHN-B), 1584704 (PIHN-N) and 1584716 (PIHN-P).

Figure 1. Chemical structure of PIHN (a), photographs of the CH2Cl2 solution (b, c), block-shaped polymorph (d, g), needlelike polymorph (e, h) and plate-like polymorph (f, i) of PIHN taken under room light and UV light, and emission spectra of the needle-like and plate-like polymorphs under the excitation wavelength of 365 nm (j).

3. RESULTS AND DISCUSSION Luminescent properties. PIHN is a known compound (Figure 1a),32 but its photophysical properties have not been reported. We found that the compound is

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non-emissive in both polar and non-polar solvents (Figure 1c). Three polymorphs of PIHN were readily obtained by simple crystallizations (see ESI), and they can be easily distinguished by shape and luminescence (Figure 1d–i). The blocked-shaped polymorph (PIHN-B) is non-emissive, while the needle-like (PIHN-N) and plate-like (PIHN-P) polymorphs display bright green and yellow emissions, respectively. All the polymorphs show a clean NMR spectrum which fits the structure of PIHN (Figure S1). A broad singlet at 15.40 ppm and a sharp singlet at 9.40 ppm, which correspond to the hydrogens of the OH and N=CH groups, respectively, can be clearly seen. The FT-IR spectra of the three polymorphs are similar (Figure S2). There are obvious peaks at 3000−3075 cm-1, which correspond to the O−H and C(Ar)−H stretching vibrations. The formation of both emissive and non-emissive crystals of a single compound was rarely observed. The emission spectrum of PIHN-N shows the maximum (λem) at 530 nm and a shoulder peak at 496 nm, while that of PIHN-P exhibits the λem at 550 nm (Figure 1j). The photoluminescence quantum yield (ΦPL) of PIHN-N is 0.08, and that of PIHN-P is much higher, being 0.19.

Crystal structures. To disclose the influences of molecular conformations and packing modes on luminescent properties, single-crystal X-ray diffraction was carried out (Table S1). Solvent molecules are not included in these crystals. There is one molecule in the asymmetric unit of PIHN-B/PIHN-N, while two molecules exist in the asymmetric unit of PIHN-P. Due to the existence of intramolecular hydrogen bond between the OH and the CH=N groups (Figure 2 and Table S2), the iminomethyl-2hydroxylnaphthalene moieties in all the polymorphs show a nearly planar structure, with the torsion angles between the naphthalene and the CH=N groups ranging from 0–3°. The major difference in the molecular conformations of these polymorphs lies in the different torsion angles between the phenyl and the CH=N groups. In PIHN-B and PIHN-N, the angles are 36.1 and 25.9°, respectively, and corresponding angles in the two isomeric molecules of PIHN-P are 8.7 and 10.0° (Figure 2). Therefore, the molecules become more planar in the order of PIHN-B, PIHN-N and PIHN-P, which is closely related to the gradually improved ΦPL in the same order, as well as the red-shifted emission from PIHN-N to PIHN-P. These polymorphs possess obviously different molecular packing modes. In PIHN-B, molecules form molecular chains along the crystallographic b axis through intermolecular C−H···O hydrogen bonds (Figure 3a and Table S2). These chains pack together along the c axis through C−H···π hydrogen bonds formed between the phenyl groups of neighbouring chains (Figure 3b), and thus a layered structure can be observed in the crystal (Figure S3). There are no obvious interactions between the layered structures. In PIHN-N, the π-systems of adjacent molecules take a slipped face-to-face packing along the b axis to form a chain structure (Figure 3c). C−H···π interactions exist between neighbouring molecular chains, resulting in a herringbone structure in the crystal. In PIHN-P, two isomeric molecules with different conformations form, respectively, slipped face-to-face π-stacking similar to that observed in PIHN-N (Figure 3d). Different from PIHN-B,


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ACS Applied Materials & Interfaces polymorphs PIHN-N and PIHN-P possess intermolecular π···π interactions, which may restrict vibrations of molecules to a larger extent in the excited state and thus serve as another factor leading to improved ΦPL.

the ‘turn on’ of luminescence. After the crystal was taken away from the acetic acid vapour, the emission was gradually turned off (Figure 4b and S5). The luminescence on-off switching process can be repeated many times (Figure 4c), suggesting a reversible acid-responsive behaviour of PIHN-B. In contrast to PIHN-B, polymorphs PIHN-N and PIHN-P did not show obvious change in luminescence colour when treated with acetic acid vapour (Figure S6). The above results indicate that the luminescence response of PIHN to acetic acid vapour is polymorph-dependent, with the ‘turn on’ of emission for PIHN-B and almost no response for PIHN-N and PIHN-P. To the best of our knowledge, the polymorph-dependent response to acid vapours has not been reported before.

Figure 2. Top and side views of molecular structures observed in PIHN-B (a), PIHN-N (b) and PIHN-P (c, d). The dashed yellow lines represent the intramolecular hydrogen bonds.

Figure 4. The emission turn-on process of PIHN-B in acetic acid vapour (a), the emission turn-off progress caused by the removal of acetic acid vapour (b), and repeated on/off switching (c)

Figure 3. Molecular packing features of PIHN-B (a, b), PIHN-N (c) and PIHN-P (d). The hydrogen atoms in PIHN-P are omitted for clarity. The dashed yellow and green lines represent the intermolecular C−H···O and C−H···π hydrogen bonds, respectively, while the red arrows represent the π···π interactions.

Acid responses. When a piece of block-shaped nonemissive crystal placed in the centre of a petri dish was surrounded by cotton that absorbed acetic acid, green emission gradually emerged and, finally, the whole crystal became luminescent (Figure 4a). 1H-NMR spectrum of a fully converted crystal confirmed the existence of a large amount (ca. 0.1 equivalent) of acetic acid (Figure S4), suggesting that acetic acid went into the crystal and caused

In order to disclose the mechanism of the polymorphdependent response, many other vapours were tested (Table S3). It was found that the vapour of hydrochloric acid or formic acid led to a weakly red-emissive state for all three polymorphs (Figure S7). Besides acetic acid vapour, PIHNB also responded reversibly to the vapours of propionic acid, acrylic acid, isobutyric acid and 2,2-dimethylpropanoic acid by on-off switching of green emission. However, the vapour of n-butyric/n-pentanoic acid did not cause the emergence of luminescence. Therefore, it seems that the generation of green emission for PIHN-B depends on the species and size of acids. Only for the aliphatic acids with two or three carbons in the main chain, green emission can be turned on. PIHN-N and PIHN-P did not show obvious luminescence-colour change when fumed by the vapour of any aliphatic acid larger than formic acid (Table S3). In addition, alcohols, aldehydes and common organic solvents did not lead to luminescence responses for all the polymorphs (Table S3), suggesting the importance of an acidic environment in the responsive behaviour.


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Considering the strong acidity of hydrochloric acid and the similar responses of these polymorphs to the vapour of hydrochloric acid, hydrochloric acid vapour is believed to cause protonation of nitrogen-containing PIHN molecules in these polymorphs.33,34 The protonation leads to the formation of highly polar structures that emit low-energy red emission.35,36 The protonation by formic acid also seems to occur considering the similar responsive behaviours towards hydrochloric acid and formic acid vapours. The distinct responses of PIHN-B to hydrochloric acid and aliphatic acids with two/three carbons in the main chain suggest that protonation is not the reason for the response to these aliphatic acids. We note that the reversal of protonation-caused luminescence change of organic solids usually needs the treatment of base,37–39 while acetic acid fumed PIHN-B can recover without any chemical treatment. This result indicates an advantage of PIHN-B regarding sensing application and further supports that the response of PIHN-B to acetic acid vapour is not caused by protonation. When PIHN-B was fumed by the vapour of the mixture of hydrochloric and acetic acids, yellow emission which can be regarded as the combination of green and red emissions was observed. After the vapour was removed, the yellow emission was gradually changed to red emission due to the disappearance of green component caused by the escape of acetic acid molecules (Figure S8). This result further supports our inference that the green and red emissions are caused by different mechanisms. To understand the role of aliphatic acids (with two or three carbons in the main chain) in the responsive behaviour of PIHN-B, we tried to grow single crystals of PIHN in these acids. Unfortunately, the single crystals could not be obtained. It was also not possible to analyse the interaction mode between PIHN and acid through single-crystal X-ray diffraction of the acid-fumed crystal due to the poor diffraction data obtained. We propose that the aliphatic acid molecules entering PIHN-B interact with PIHN molecules through weak hydrogen bonds, which makes at least some of the initially twisted PIHN molecules become more planar and, meanwhile, further restricts molecular vibrations. Both factors are beneficial to the luminescence as indicated by the crystallographic study.40 Once the aliphatic acid molecules leave the crystal, the original molecular packing mode is restored and, correspondingly, the emission is turned off. The relatively stronger acidity of formic acid than other applied aliphatic acids is likely the reason for the protonation caused by formic acid vapour.41 Further analyses of the single-crystal structure of PIHN-B reveal that there are channels in the (100), (010) and (001) planes, which correspond to the surface of the crystal, for the entering of aliphatic acid molecules (Figure S9). However, the widths of the channels are less than 4.0 Å, which may lead to difficulty for the entering of large-size acids and thus cause the nonresponse to the acids with the main chain longer than three carbons. For PIHN-N and PIHN-P, the entering of smallsize aliphatic acids such as acetic and propionic acids is also possible. However, the formed hydrogen bonds may not change the molecular conformation significantly due to the originally more planar molecular conformation in these polymorphs. In addition, the polarity of the DPIN

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molecules should not be significantly affected by weak hydrogen bonds. Therefore, the emission colour and spectrum are not greatly affected. Notably, PIHN-B displays three kinds of behaviours towards the fuming of different acid vapours, i.e. generating red/green emission or no response. This result indicates that a polymorph could show multiple acid-responsive behaviours through different responsive mechanisms. Such a finding benefits the design of novel acid sensors, such as the sensors that can qualitatively analyse the species of acid source in an acidic environment.

Information protection. The interesting acid response of PIHN was found to be useful in safety protection of file information. As shown in Figure 5, after the letters ‘JLU’ written on a black paper with the dichloromethane solution of PIHN as the ink were dried in air, almost no trace was observed under both room light and UV light. Upon fuming the paper with acetic acid vapour, the letters could still not be seen under room light. However, they were clearly observed under UV light due to the emergence of green emission. Such emission ‘turn on’ behaviour caused by acetic acid vapour is similar to that observed for bulk crystal of PIHN-B, suggesting that PIHN molecules in the ink crystallize into a form same as or close to that of PIHN-B. Compared with some other informationprotection ways requiring one ingredient for detecting the secret information, such as the starch letters visualized by iodine, as well as the letters written with CoCl2 solution and visualized by heat, our method requires more ingredients to visualize the information, i.e. UV light and acid vapour. Therefore, the security of information can be better assured. The unique information protection method described above benefits from the emission ‘turn on’ behaviour of organic solids in the acid-responsive process. As we know, acid vapour induced emission ‘turn on’ of organic solids has not been reported. Therefore, similar ways for safety protection of file information have not been disclosed. The black paper is an optimal choice for the present method, because the paper with other colors makes the non-emissive letter detectable under UV light before fuming (Figure S10).

Figure 5. Photographs showing the utility of PIHN in safety protection of file information: (a) a black paper under room light with the letters ‘JLU’ written on it using the dichloromethane solution of PIHN as the ink; (b) the paper under UV light; (c) the paper under room light after the fuming of acetic acid vapour; (d) the fumed paper under UV light. For b and d, a black paper was used as the background to remove the influence of reflected UV light.


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4. CONCLUSIONS In summary, based on a solution-state non-luminescent Schiff base, PIHN, both non-luminescent and luminescent polymorphs have been obtained. The non-luminescent polymorph shows a unique response, i.e. ‘turn on’ of green emission, to the vapours of aliphatic acids with two or three carbons in the main chain, while the luminescent polymorphs almost do not respond to these acids. Thus, polymorph-dependent acid response has been achieved for the first time. Based on experimental results, a new mechanism different from the common protonation mechanism has been proposed for the responsive behaviour of the non-emissive polymorph, that is, the formation of hydrogen bonds between PIHN and aliphatic acid. By utilizing the special luminescence ‘turn on’ response, a unique method for protecting the safety of file information has been developed. The present study discloses the influence of crystalline phase on acid-responsive behaviour, which benefits the further design of novel acid sensors. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 1H NMR and 13C NMR (Figure S11) spectra; molecular packing structures; emission spectra; photographs; single-crystal XRD data; and parameters for the hydrogen bonds and π···π interactions. Crystallographic data of PIHN-B (CIF) Crystallographic data of PIHN-N (CIF) Crystallographic data of PIHN-P (CIF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Z.Z.) *E-mail: [email protected] (H.Z.)

ORCID Zuolun Zhang: 0000-0002-8924-4938 Hongyu Zhang: 0000-0002-0219-3948

Author Contributions H.Z. conceived the idea. H.L. prepared the materials and designed the experiments. H.L., Z.L., and K.Y. performed measurement. H.L., Z.Z., and H.Z. analyzed the data. H.L., Z.Z., and H.Z. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51622304, 51773077, 51603082 and 51773079).

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