A neutral dinuclear Ir(III) complex for anti-counterfeiting and data encryption

a. Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Department of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, Jilin Province 130024, P.R. China. E-mail: zhudx047@nenu.edu.cn; zmsu@nenu.edu.cn b. Key laboratory of supramolecular structure and materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China. c. Department of Chemistry, Durham University, Durham, DH1 3LE, UK.E-mail: m.r.bryce@durham.ac.uk

systems are presently facing various problems such as poor stability and high reaction temperature (the calcination temperatures usually reach 800 o C). 5 Therefore, there is an urgent requirement for new advanced materials with high thermal stability and facile preparation to combat counterfeiting and information leakage.
Phosphorescent transition-metal complexes, such as Ir(III) systems, have a great potential in this area due to their high luminescence quantum yields, easy handling, structural versatility and high photostability. 6However, traditional phosphorescent materials are usually regarded as first-level data encryption, with the disadvantage of security information being exposed immediately under ultraviolet (UV) light illumination.This means that encoded data is not secure and can be easily substituted by compounds with a similar emission colour.Therefore, the design of novel luminescent materials with more covert and reliable anticounterfeit features is an appealing challenge.Piezochromic luminescent (PCL) compounds are a class of ''smart'' materials whose fluorescent properties change in response to external pressure or mechanical grinding. 7It has been shown that this process can be reversed and the original emission colour can be restored by altering the molecular packing mode in the solid state in response to external stimuli, such as heating or recrystallization.

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Taking advantage of this reversible property, PCL materials are widely used in sensors, memory chips, and security inks.9 However, PCL compounds generally require sophisticated synthesis (>10 steps) to introduce different components into one molecule. 10Moreover, similar to most conventional dyes, these luminophores suffer from aggregation caused quenching (ACQ), which results in low phosphorescence quantum yields in the solid state. 11All the aforementioned drawbacks significantly limit the real-world applications of PCL materials.In contrast, aggregationinduced emission (AIE), as coined by Tang et al. in 2001, 12 is a property of compounds which emit weakly when dispersed in dilute solution but show strong emission when aggregated due to restricted intramolecular motions.This abnormal phenomenon has attracted considerable interest in the field of electroluminescent devices and chemical sensing. 13The motivation for the present work is to develop phosphorescent transition-metal complexes with combined AIE and PCL properties and to exploit them as candidates for applications in anti-counterfeit and data encryption.
Herein, we designed and synthesized a new dinuclear Ir(III) complex (ppy) 2 Ir-(bsbd)-Ir(ppy) 2 (PIBIP) with a Schiff base bridging ligand (bsbdH 2 ) and phenylpyridine (ppy) cyclometalating ligands (Scheme S1, ESI †).The photophysical properties demonstrate that PIBIP is AIE active and simultaneously shows PCL properties.Moreover, the emission colour of PIBIP can be converted to the original colour upon simple treatment by organic solvent.These results lead to the demonstration of a second-level anti-counterfeit trademark and data encryption device using PIBIP as security ink.PIBIP is the first neutral dinuclear Ir(III) complex which exhibits both PCL and AIE characteristics, to the best of our knowledge.These combined properties are important for device applications. 7The flexible nature of the bsbd spacer allows the ligand to adopt optimal coordination geometries at the metal centres so as to facilitate the coexistence of PCL and AIE.The structure of PIBIP was established by its 1 H NMR and mass spectra, and single-crystal X-ray structure (ESI †).The pristine solid sample of PIBIP (hereafter abbreviated as P) shows yellow phosphorescence (λ max =587 nm), as shown in Fig. 1.When the powder P was thoroughly ground in an agate mortar for 5 min, a significant bathochromic shift (λ max = 613 nm) was observed in the orange-emitting ground sample G (Fig. 1).The emission intensity of G shows a significant weakening that is clearly visible to naked eyes.The photoluminescence quantum yield (PLQY) is decreased dramatically in the grinding process from 20% (P) to 7% (G) (Table S1, ESI †).To investigate the reversibility of this PCL behavior, G was wetted with dropwise addition of dichloromethane (DCM) solvent (sample D) and the emission colour reverted to the initial emission colour of P within a few seconds (Fig. 1).Notably, the emission can perfectly revert to G when D was further ground.This PCL behaviour of PIBIP was shown to be highly reversible for several grinding-wetting cycles (Fig. S4, ESI †).
In order to establish the origin and mechanism of this reversible piezochromic behaviour of PIBIP，the 1 H NMR spectra of both P and G were obtained.The results show Powder X-ray diffraction (PXRD) was carried out to investigate the aggregation states of P and G.The sharp peaks in the XRD pattern (Fig. 2a) unambiguously show that the P sample is a wellordered crystalline structure.In sharp contrast, the ground sample G exhibits weak and broad diffraction signals, which imply a crystalline to amorphous phase transition during the grinding process.The clear reflection peaks of pristine solid P reappeared after the sample G was heated (sample H) or wetted with dropwise DCM solvent (sample D).In addition, upon heating G to 350 o C, the DSC traces exhibited a clear broad exothermic recrystallization peak at ca. 198 o C.This peak is at a similar temperature at which thermal recrystallization begins to take place (Fig. 2b).Therefore, when G was heated at 198 o C for 1 min, the crystalline state was recovered and consequently the emission colour revered to the original one (Fig. 1).PIBIP emitted only weakly in amorphous G, but became strongly emissive in crystalline P.This is typical crystallizationinduced emission enhancement (CIEE) behaviour. 14These results provide a rational explanation for the variation in PL intensity.
The single crystal X-ray structure analysis of PIBIP demonstrates that no intermolecular interactions exist in the molecular packing (Fig. S8, ESI †).By contrast, obvious intramolecular π-π interactions between the bridging phenyl ring and the adjacent phenyl rings of ppy are observed (Fig. 3).This structure might be easily modified when mechanical pressure is applied, resulting in a red-shift of the PL spectra. 15The excited-state lifetimes (τ) for PIBIP significantly decreased from P (0.97 μs) to G (0.49 μs) (Table S1, ESI †).Thus, it can be concluded that the changed τ in the PCL process is associated with altering the solidstate molecular packing and/or the intramolecular interactions.The similar τ of both the P and D states is consistent with them having the same molecular arrangement.Encouraged by the excellent PCL performance of PIBIP, anticounterfeit trademark and data encryption devices were constructed (Fig. 4).MASK is a fluorescent emitter selected from the literature, 17 (for the structure see Fig. S2 in the ESI †) which shows a similar emission peak to G at around 610 nm (Fig. S3, ESI †).Thus, it is difficult to distinguish G from MASK due to their very similar emission colour and emission intensity.Furthermore, no colour change is observed when mechanical grinding is applied to MASK, which is an exclusive method to differentiate G and MASK (Fig. S3, ESI †).As shown in Fig. 4a and b, the anti-counterfeit trademark adopted the shape of a 'flower' comprising a central 'stamen' and a 'petal'.The 'stamen' is the as-prepared powder of MASK which emits orange fluorescence upon excitation with a 365 nm UV lamp and the 'petal' is the sample of G (detailed method is given in supporting information).This device could provide a second-level anti-counterfeit function.As shown in Fig. 4a, orange light was rapidly observed under the illumination of a standard UV lamp (at 254 and 365 nm), presenting a first-level anti-counterfeit system.When this 'flower' was sprayed with dichloromethane (DCM) solvent, the emission of the 'petal' changed immediately from orange (G) to yellow (D) (Fig. 4b).A second-level anticounterfeit system was then successfully obtained as follows.Upon further grinding the 'petal', the original emission colour was regenerated and the first-level anti-counterfeit trademark reappeared (Fig. S5, ESI  †).Furthermore, a simple, convenient and efficient technology for data encryption and decryption was designed (Fig. 4c).G was used as a 'cryptographic ink' while MASK was used as a control reagent.In the encryption stage, the characters 'NENU' were written on a filter paper by using MASK, then the powder G was carefully spread on it as the letters 'AIPE'.So G was hidden by MASK even under UV light, because G and MASK emitted unitary orange light.In the decryption stage, the yellow security letters 'AIPE' appeared clearly when DCM solvent was sprayed onto the as-prepared letters 'NENU'.In contrast to the orange-emitting background, the letters 'AIPE' showed intense yellow emission which is in agreement with the change in emission colour from G to D. Moreover, 'AIPE' can be easily hidden again by grinding.This simple process demonstrates an excellent encryption and decryption reversibility for several cycles.These data suggest that complex PIBIP has the potential to be employed in practical applications as an anti-counterfeit and security protection ink with simple optical authentication.
Since PIBIP exhibits strong luminescence in the solid state (Table S2, ESI †), the AIE property was probed by using different ratios of THF-water mixtures.As shown in Fig. 5, PIBIP exhibits very weak Please do not adjust margins Please do not adjust margins weak emission in pure THF solution, where it was well dissolved.Nevertheless, the PL intensity was dramatically enhanced when the water fraction reached 50%, increasing by a maximum of up to about 100-fold in comparison with pure THF solution.The PL intensity then decreased with increasing water content >50% water fraction, but the PL intensity at 90% water fraction is still stronger than in pure THF solution.There are two possible reasons for this behaviour.First, after aggregation, the molecules covered within the surface of the nanoparticles did not emit light, leading to a decrease in phosphorescent intensity.Second, crystalline particles and amorphous particles simultaneously form when the water fraction is increased.The former particles would enhance emission intensity but the latter do not. 8Thus, the measured overall PL intensity depends on the combined actions of the two kinds of nanoparticles.Furthermore, the UV-visible absorption profile showed a Mie scattering effect for the mixtures of PIBIP with high water content (Fig. S6, ESI †). 18Transmission electron microscopy (TEM) and electron diffraction (ED) experiments indicated that amorphous molecular aggregates are formed in the mixtures (Fig. S7, ESI †). 19Evidently, PIBIP is an excellent AIE chromophore that could effectively suppress non-radiative decay to produce intense emission in the solid state.In summary, a smart dinuclear Ir(III) Schiff base complex PIBIP shows simultaneous reversible PCL behaviour and AIE-activity.An obvious bathochromic shift occurs in the solid state upon mechanical grinding, and this new emission reverts to the original state after wetting with DCM solvent.Anti-counterfeiting trademark and data encryption devices have been successfully constructed by combining PIBIP and fluorescent MASK through the piezochromic and solvatochromic properties of PIBIP.Therefore, these findings indicate that PIBIP could expand the applications of PCL materials to anti-counterfeiting, information storage and data security protection.
The work was funded by NSFC (No.51473028), the key scientific and technological project of Jilin province (20150204011GX, 20160307016GX), the development and reform commission of Jilin province (20160058).Work in Durham was funded by EPSRC grant EP/K039423/1.

X-ray crystallographic data
The molecular structure of PIBIP was confirmed by X-ray crystallographic analysis of single crystals.Diffraction data were collected on a Bruker SMART Apex CCD diffractometer using k(Mo-K) radiation (k = 0.71069 Å).Cell refinement and data reduction were made by the SAINT program.The structure was determined using the SHELXTL/PC program.The crystallographic data have been deposited with the Cambridge Crystallographic Data Centre with CCDC deposition number 1527325.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Fig. 1
Fig. 1 Emission spectra of unground "as synthesized" sample (P), ground sample (G), heated ground sample (H) and ground sample wetted with DCM (D).Inset: emission image of P, G, H and D under 365 nm UV illumination.

Fig. 2
Fig. 2 Powder X-ray diffraction patterns (a) and the DSC traces (b) of the corresponding samples.

Fig. 4
Fig. 4 Photographic images of (a) first-level anti-counterfeit trademark (b) second-level anti-counterfeit trademark and (c) information encryption and decryption device.Note the appearance of the yellow letters 'AIPE' within the bottom 'NENU'.

Fig. 5
Fig. 5 Emission spectra of PIBIP in THF-water mixtures with different water fractions (0-90% v/v) at room temperature.Inset: emission image of PIBIP in pure THF solution and THF-water mixture (50% water fraction) under 365 nm UV illumination.

Fig. S2 1
Fig. S2 1 H NMR spectrum of MASK in CDCl 3 at room temperature.

Fig. S5
Fig. S5 Photographic images of anti-counterfeit trademark with several reversible spraying and grinding processes under (a) daylight and (b) UV light

Fig. S8
Fig. S8 Molecular packing of PIBIP in the crystal.Color code: Ir purple; N blue; O red.

Table S3
Crystal data and structure refinement for PIBIP.