Qingxia Xiong,Chao Xu,Nianming Jiao,Xiang M*,Yanqiang Zhang,Suojiang Zhang*

a Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100049, China

b School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

c Zhengzhou Institute of Emerging Industrial Technology, Zhengzhou 450000, China

d Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China

Keywords:N-Allylquinoline Room-temperature phosphorescence Anti-counterfeiting

A B S T R A C T Pure organic room-temperature phosphorescence(RTP)materials have been attracting much attention recently.Herein,we report a facile approach combining heavy atom effect and external solvent stimuli to realize RTP. [8]N-Allylquinolinium bromide under 365 nm UV exhibited intense green RTP emission with response upon adding chloroform.This interesting phenomenon endowed[9]N-allylquinolinium bromide great potential as an anti-counterfeiting material.

Materials with room-temperature phosphorescence (RTP) are normally inorganic compounds or organometallic complexes[1–5]. They have extensive applications in biological imaging,anti-counterfeiting, organic light-emitting diodes, chemical sensors and molecular switches [6–12]. Compared to inorganic and metal-containing materials,it is hard for the electrons to emit from triplet states in pure organic materials as the compounds have weak molecular spin-orbit coupling and high sensitivity to external quenchers[13–16]. To realize RTP in mental-free organic materials,some very practical strategies have been proposed,such as introducing heavy atoms or heteroatoms (like N, O, S, P),polymer matrix,crystallization and supramolecular assembly[17–22]. They are based on the principles to facilitate the intersystem crossing(ISC)and suppress the non-radiative dissipation[23–27].

Anti-counterfeiting of high-value items including banknotes,legal documents, currency, brands, medical products and luxury items,is a challenging task worldwide.Many techniques have been designed by using anti-counterfeiting materials,especially various security inks due to their convenience, reliability, accuracy and practicability. Related to luminescent inks, a lot of luminescent materials have been designed, like colloidal photonic crystals,polymers, nanobeads and nanorods, carbon dots, nanoclusters,rare earth-doped nanostructures and other hybrids [28–34].Herein, we report a facile approach that combine heavy atom effect and external solvent stimuli to realize RTP in pure organic materials and explored its applications in anti-counterfeiting as well.

A series of N-allylquinolinium-based salts were designed and synthesized. To our surprise, they displayed strong RTP upon the external solvent stimuli. These materials exhibited excellent optical features including matched colors and variations in phosphorescence lifetime.Their properties demonstrate a possible encoding method by combining an organic salt and a volatile solvent to encode images, which are difficult to be counterfeited and reversely engineered. These materials are convenient to get and could be efficient security inks in protecting special items as government documents, banknotes and high-value merchandise.

The N-allylquinolinium-based salts (1–3) were designed and synthesized according to the reference methods (Scheme 1). The salts were characterized by NMR, EA and ESI-mass [10]spectrometry(Figs.S1-S6 in Supporting information).Moreover,the faint yellow crystal(1),deep yellow crystal(2)and colorless crystal(3)suitable for single crystal X-ray diffraction analysis were obtained by slow evaporation of their CHCl3(or ethyl acetate) solutions at room temperature, respectively. The molecular structures of 1–3 are shown in Fig.1. The crystallographic data of the salts is listed in Table S1 in Supporting information.

Fig.1. Single-crystal XRD structures of salts 1 (a), 2 (b) and 3 (c).

When excited by UV at 365 nm, 1 showed very weak orange luminescence and 2 barely emitted light,while 3 emitted a strong blue light.The photographs of the salts are displayed in Fig.2.The UV–vis absorption spectra of the three salts are displayed in Figs.S7-S9 in Supporting information.As to their emission spectra,1 showed very weak signal around 553 nm with low quantum yield near to 0% (Fig. 3a), 2 with no emission signal, and 3 with strong luminescence around 383 nm (Fig. 3b), which is consistent with the photographs as observed. After the addition of CHCl3, the emission of 1 was surprisingly boosted so that a green light could be observed by naked eyes (Fig. 2a), 2 was still nonluminous(Fig.2b),and on the contrary the photoluminescence of 3 became weaker(Fig.2c).The emission intensity of 1 was greatly enhanced as[18]Femfrom almost zero to 2.76%with a major peak at 519 nm and a shoulder one at 489 nm and the lifetime was up to 2.73 ms(Fig.3a and Fig. S10 in Supporting information), while 3 displayed much weaker luminescence at 383 nm (Fig. 3b). Moreover, the original weak orange luminescence of 1 could be restored via the evaporation of CHCl3so that CHCl3could be a reversible photoluminescence on-off switch. For other solvents such as ethanol,acetonitrile, acetone, methanol, ethyl acetate, dimethyl sulfoxide,ethyl ether, tetrahydrofuran, deionized water, petroleum ether,dichloromethane and tetrachlormethane, there is no such phenomena, which demonstrated its specialty.

Fig.2. Chemical structures of salts 1–3,and their photographs recorded under UV light (lex=[2]365 nm) and the addition of CHCl3.

Fig.3. The[3]photoluminescent emission of 1 under the mode of phosphorescence(a)and [4]3 under the mode of fluorescence (b) excited by 370 nm UV.

Based on the above results,we also assumed the mechanism for the emission changes.With the external heavy atom effect,1 and 2 emitted weak phosphorescence in pure solid state. As to 2, it showed barely emission which is different from Zhang’s work[35].It is because Iis a stronger electron donor compared to Br- and allyl group is also a stronger electron accepter compared to methoxy group, leading to more charge transfer effect from Ito allyl group in 2.For the PF6counterpart,there was no heavy atom perturbation or significant charge transfer involved, and the fluorescence emission characteristic p-p* arising from the quinolinium ring can be mostly observed in 3. After the addition of CHCl3,the crystalline morphology of the compound might have some changes,increasing the mixing of S1 and T1 states,and thus to accelerate the ISC process from the singlet to triplet in the molecules, ultimately leading to the blue shift and inducing the intense green phosphorescence emission of 1. As to 2 and 3,the perturbation is not strong enough to turn on the luminescence of 2 and weaken the fluorescence of 3 so that 2 is still lightless and 3 showed weaker blue fluorescence. Furthermore, to gain an insight of the photoluminescence behaviors experimentally, the changes of molecular arrangements in different states were investigated [19]via powder X-ray diffraction (PXRD) for 1. As shown in Fig. 4, the PXRD patterns of the pure 1 exhibited sharp and intense peaks and agreed with the diffraction peaks obtained by single crystal structure simulation, which indicates that the original solid is crystalline state. After the addition of CHCl3, the diffraction peaks became weaker and some peaks even disappeared (2u=10-15) which might prove the existence of some perturbation in the molecules.

Based on the specific recognition to CHCl3,1 could be used as a special toner in the printer to code the document and use the solvent as the chromogenic reagent owing to the property of photoluminescence under soaking condition. [10]Figs. 5a and b displays an English acronym corresponding to‘Ionic Liquid Center’written on filter paper with the powder 1 under natural light and UV light, respectively. Compared to the image Figs. 5a and b,intense green color emerged clearly in the writing marks despite the original blue background (Fig. 5c) after the addition of CHCl3.This anti-counterfeiting approach may help the development of solvatochromism technique in potential applications such as document encryption and information storage.

Fig. 4. PXRD patterns of 1 at experimental and simulated conditions.

Fig.5. Photographs of‘ILC’written on filter paper by using 1 under natural light(a),365 nm UV (b) and the addition of CHCl3 (c).

In summary, we successfully presented a concise and very simple approach to achieve pure organic RTP materials based on heavy atom effect and external solvent stimuli. The three Nallylquinolinium-based salts (anion as Br, I, PF6, respectively)with the diverse luminescence behaviors.Different from 2 and 3,1 exhibited weak orange light in the pure solid state,while its green light were greatly boosted after the addition of CHCl3([18]Fem: from almost zero to 2.76%).The reversible photoluminescence behaviors endowed 1 great potential as a notable anti-counterfeiting material, which may stimulate new molecular engineering endeavors in the design of advanced anti-counterfeiting materials.

Acknowledgments

We gratefully acknowledge the financial support from the Instrument Developing Project of the Chinese Academy of Sciences(No.YJKYYQ20170009),the National Natural Science Foundation of China (NSFC Nos. 21722603 and 21871083) and the Innovation Program of Shanghai Municipal Education Commission.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.04.010.