Lili Wang,a Yuanyuan Li,b Xuejiao You,a Kui Xu,a Qi Feng,a Jinmin Wang,a Yuanyuan Liu,a Kai Li,*a and  Hongwei Hou*a

Luminescent molecules with photochromic properties show strong potential in molecular switches, molecular logic gates,  photo-controllable materials, bio-imagine, anti-counterfeiting, photo-patterning, etc. However, there is still rare of  research in such molecule exhibiting reversible photo-controllable color and fluorescence change in solid states, which due  to the aggregation-caused quenching (ACQ) effect of most luminogens. In this work, a reversible photochromic molecule  (1) with aggregation-induced emission (AIE) property was developed. Compound 1 exhibits typical AIE properties as a  result of restricted intramolecular rotations (RIR) and excited-state intramolecular proton transfer (ESIPT) processes. As a  photochromic molecule, compound 1 shows reversible color and fluorescence changes upon UV light irradiation with good  fatigue resistance. More importantly, conversion rate from its irradiated form to initial form is controllable by temperature  and long wavelength light irradiation, which make it suitable for photo-patterning material with erasable property.  

1 Introduction

Luminescent materials have attracted much attention due to  their diverse applications in chemistry, physic, material  science, and biology.1 However, most of their practical uses  are usually limited by the notorious aggregation-caused  quenching (ACQ) phenomenon: luminogens exhibit good fluorescence in dilute solutions but weak or even no  fluorescence in concentrated solution or solid state as a result  of the self-quenching effect of adjacent molecules.2 In 2001, a  novel luminescence phenomenon of aggregation-induced  emission (AIE) was first observed by the group of Tang, which  is exactly opposite to the ACQ effect: the emission of  luminogens were very weak in solution but became intense  when aggregates formed.3 Since then, a variety of AIE systems  (tetraphenylethenes (TPE), hexaphenylsilole (HPS),  cyanostilbenes and salicyladazine (SA), etc.) were reported in  succession, which have been widely applied in the field of  organic light-emitting diodes (OLEDs), chemical sensors and  multifunctional luminescent material.4

Recently, Tang and co-workers have successfully expand the  application of AIE luminogens by developing a photoactivatable AIE system based on a caged TPE derivative.5 The  fluorescence emission of the AIE group was partially or  completely quenched by the quencher but can be recovered  upon cleavage of the quencher under UV stimuli. A following  work of another photo-activatable AIE system based on caged  SA derivatives were reported by Xiang and Tong’s group. 6 These novel photochromic systems show great potential of  application in photo-patterning, photo-activatable imaging and  anti-counterfeiting related areas. However, the  photochromism of these caged fluorophores are irreversible,  which limit their applications.7 To overcome this challenge,  some new strategies in the design of irreversible photocontrollable AIE system have been introduced such as photoinduced redox reactions and photo-induced cyclization  reactions. Nevertheless, the fatigue resistances of these  photochromic systems are still unsatisfactory.8

According to the reports, salicylaldehyde hydrazone is a  reversible photochromic group with good fatigue resistance.9 Upon UV irradiation, there is a reversible tautomerism  between its enol-form and keto-forms (Scheme 1). Notably,  the enol-form of salicylaldehyde hydrazone is beneficial for  fluorescence emission due to an excited-state intramolecular  proton transfer (ESIPT) process.10 The keto-forms of  salicylaldehyde hydrazone, in contrast, is a typical fluorescence  quenching unit due to its strong electron-withdrawing  property.11 These unique properties of salicylaldehyde  hydrazone suggest that it could be used as a photocontrollable quencher in the design of photo-controllable  luminescent molecules.

In this work, a reversible photochromic AIE molecule was  developed by connecting salicylaldehyde hydrazone with a  typical AIE moiety of TPE (Scheme 2). This specially designed  AIE molecule of 4-(1,2,2-triphenylvinyl)aniline salicylaldehyde  hydrazone (1) exhibited reversible photochromic property with  significant fluorescence change. Before UV irradiation, 1 was  yellow and exhibited strong fluorescence emission in  aggregative state. After UV irradiation, 1 turned to red and the  fluorescence emission declined significantly. The good fatigue  resistance property of 1 enables it to be applied in reversible  photo-patterning and anti-counterfeiting related applications.

2 Experimental  

2.1. Materials.

Salicylaldehyde, aniline, (2-bromoethene-1,1,2- triyl)tribenzene, 4-aminobenzeneboronic acid hydrochloride,  o-anisaldehyde, tetrabutylammonium bromide (TBAB) were  purchased from J&K Chemical Co., Beijing, China. All the other  materials were purchased from Sinopharm Chemical Reagent  Beijing Co., Beijing, China. In these experiments, all the  materials of analytical grade were used without further  purification.

2.2. Characterizations.

1H and 13C NMR spectra were recorded on a Bruker 400  Avance NMR spectrometer operated at 400 MHz. UV-Vis absorption spectra were recorded on a JASCO V-750  spectrophotometer. UV-Vis diffuse reflectance spectra were  measured by Agilent Cary 5000 UV-Vis-NIR Spectrometer,  BaSO4 was used as reference. Fluorescence spectra were  obtained on a Hitachi F-4600 spectrometer. Fluorescence  lifetimes and quantum yields were recorded on Edinburgh FlS- 980 fluorescence spectrometer. Dynamic light scattering (DLS)  experiments were performed by a NanoPlus-3 dynamic light  scattering particle size/zeta potential analyzer at room  temperature. ESI-MS spectra were obtained on Agilent  Technologies 6420 triple quadrupole LC/MS without using the  LC part. Elemental analysis experiments were carried out on a  Flash EA 1112 automatic element analyzer. Single-crystal X-ray  diffraction intensity data were collected on a Rigaku Saturn  724 CCD diffractometer with Mo Kα radiation (λ= 0.71073 Å)  at room temperature. X-ray Powder Diffraction (XRD) patterns  were obtained by a PANalytical X'Pert PRO diffractometer. The  photos and videos were carried with Nikon D5500 camera.  Laser of 405 nm used in Video 1 was produced by LD-T405F00  laser pointer (power output 20 mW). Visible light used in Video  1 and light sources used in Table S1 and Figure 10 were  produced by a CEL-HXF300/CEL-HXUV300 xenon light source  with different optical filters.

2.3. Synthesis.

Compounds 1-3 were synthesized according to the synthetic  routes shown in Scheme 3. The new compounds were  characterized by NMR spectra and ESI-MS spectra. Detailed  synthetic procedures and characterization data are shown  below and in Supplementary Information.

4-(1,2,2-triphenylvinyl)aniline salicylaldehyde hydrazone  (1). 4-(1,2,2-triphenylvinyl)aniline (1a) was prepared through a  reported procedure.12 Salicylaldehyde (0.73 g, 6 mmol) and  compound 1a (1.73 g, 5 mmol) was dissolved in 30 mL  absolute ethanol in a 50 mL flask. The stirred mixture was  heated to 90 o C for 2 h. Then the solvent was concentrated to  10 mL under reduced pressure and cooled to 4 o C. After that, 1  (2.08 g, 4.6 mmol) was obtained as a yellow precipitate with a  yield of 92%. 1 H NMR (400 MHz, CDCl3 ), δ (ppm): 6.90 (m, 1H),  7.07 (m, 20H), 7.35 (m, 2H), 8.58 (s, 1H). 13C NMR (400 MHz,  CDCl3 ), δ (ppm): 117.34, 119.31, 120.50, 127.70, 131.34, 131.37, 131.40, 140.14, 141.50, 142.90, 143.54, 143.59, 143.63,  161.92. ESI-MS spectrometry: m/z calcd. for [M+H]+ : 452.2;  found: 452.2. Elemental analysis, calcd. For C33H25NO: C, 87.77;  H, 5.58; N, 3.10, found: C, 88.24; H, 5.59; N, 3.10.

Aniline salicylaldehyde Schiff-base (2). Compound 2 was  prepared through a reported procedure.13

4-(1,2,2-triphenylvinyl)aniline-2-methoxybenzaldehyde  hydrazone (3). 2-methoxybenzaldehyde (0.20 g, 1.5 mmol) and  compound 1a (0.35 g, 1 mmol) was dissolved in 20 mL  absolute ethanol in a 50 mL flask. The stirred mixture was  heated to 90 o C for 2 h. Then the solvent was concentrated to  10 mL under reduced pressure and cooled to 4 o C. After that, 3  (0.35 g, 0.75 mmol) was obtained as a yellow precipitate with a  yield of 75%. 1 H NMR (400 MHz, CDCl3 ), δ (ppm): 3.92 (s, 3H),  6.97 (d, 1H), 7.12 (m, 20H), 7.46 (t, 1H), 8.16 (d, 1H), 8.94 (s,  1H). 13C NMR (400 MHz, CDCl3 ), δ (ppm): 55.60, 111.16, 120.66, 120.96, 126.46, 126.53, 127.57, 127.74, 127.88, 131.45, 132.23,  140.67, 140.92, 141.48, 143.89, 150.71, 155.86, 159.53. ESI-MS  spectrometry: m/z calcd. for [M+H]+ : 466.2; found:  466.2.Elemental analysis, calcd. for C34H27NO: C, 87.71; H, 5.85;  N, 3.01, found: C, 88.57; H, 5.88; N, 3.00.

3 Results and discussion

3.1 The AIE characteristics of 1.

The fluorescence characteristics of 1 was firstly investigated  in H2O/EtOH (H2O is poor solvent for 1 while EtOH is good  solvent for 1, water fraction (fw) from 0% to 99%, v/v) buffered  by 10 mmol/L PBS at pH 7.0. As shown in Figure 1, the  fluorescence emission of 1 at 539 nm was rather weak in  ethanol and barely increased before the fw reached 50%.  When fw reached to 60%, the fluorescence emission suddenly  enhanced, which was almost 5-fold greater than that in  ethanol solution and even larger when fw varied from 70% to  99%. The enhanced fluorescence emission could be  contributed to the poor solubility of 1 in water than that in  EtOH, which indicated that 1 exhibited typical AIE  characteristics. The fluorescence spectra of 1 in solid state is  similar to that of 1 in poor solvent (fw = 99%), as can be seen in  Figure 2, which further supported the AIE character

3.2 The origination of the AIE characteristics of 1.

It is worth noting that the maximum fluorescence emission  wavelength of 1 is remarkably longer than that of TPE  monomer (λmax = 453 nm),12 but similar with that of SA (λmax =  542 nm).13 This result suggested that the AIE property of 1 may  be not only originated from the TPE moiety but also from the  salicylaldehyde hydrazone moiety. To confirm this assumption,  anilinesalicylaldehyde hydrazone (2) and 4-(1,2,2- triphenylvinyl)aniline-2-methoxybenzaldehyde hydrazone (3)  were chosen as control compounds. As shown in Scheme 3,  compound 2 was designed in lack of the TPE part as compared  to 1. As expected, 2 exhibited no fluorescence in both EtOH  and aqueous solution (fw= 99%), which indicated that the TPE  part in compound 1 is indispensable for its AIE property (Figure  S1).

It is well known that the AIE properties for TPE and its  analogues are come from the restricted intramolecular  rotations (RIR) process: In good solvents, the molecules in  single state and the conjugate plane of phenyl rings undergo  dynamic intramolecular rotations, resulting in excited state  non-radiative transition and fluorescence quenching. In poor  solvents or solid state, the molecules are aggregated and the  rotations are greatly restricted, which make the radiative  transition to be the primary pathway for the excited state  electrons back to ground state.14 To further verify that the RIR  process contributed to the AIE property of 1, the fluorescence  emissions of 1 in solvent with high viscosity (ethanol/glycerin) were investigated. As shown in Figure 4, the fluorescence  emission of 1 enhanced gradually with the increase of glycerin  fraction (fg) because the high viscosity solvent restricted  intramolecular rotations. Meanwhile, no level-off tail could be  observed in their absorption spectra, which suggested there  was no aggregate of 1 in glycerin. These results indicate that  the RIR process is one of the main causes for the AIE property  of 1. Moreover, the RIR process is also supported by the crystal  structure of 1. Single crystal of 1 was grown from  methanol/ethanol mixtures by slow solvent evaporation and  was characterized by X-ray crystallography. The crystal of 1  was stable to moisture or atmosphere, and is soluble in  common organic solvents such as dichloromethane,  tetrahydrofuran, dimethylsulfoxide, and so on, but insoluble in  water. As shown in Figure 5A, all the carbon/nitrogen atoms in  the salicylaldimine moiety of 1 are in the same plane, which  indicates possible intramolecular hydrogen. Meanwhile, the  intermolecular distance of 1 is 5.7 Å (far over 3.8 Å15), which  suggest that there is no face-to-face-stacking in the crystal  (Figure 5B). All of these structural features indicated that 1 had  potential of AIE properties by RIR process in aggregate state.

According to the reports, the AIE properties of SA and its  analogues originated from RIR and ESIPT process: On one hand,  the intramolecular hydrogen-bond in salicylaldimine moiety  stabilizes the conjugated plane structure, which lead to a  possible RIR process.13, 16On the other hand, a fast four-level  (E-c-E*-c-K*-K, Figure 6) cycle occurs immediately after photo  excitation of the SA molecule through the intramolecular  hydrogen-bond, resulting in an ESIPT fluorescence.10a, 17 Thus,  when the intramolecular hydrogen-bond was broken, the AIE  properties of SA and its analogues should disappear. Control  compound 3 containing a methoxy group instead of the  phenolic hydroxyl group was synthesized to demonstrate the  role of salicylaldehyde hydrazone moiety in the AIE property of  1. As shown in Figure S1, no fluorescence could be detected  for 3 both in good and poor solvents. This result clearly  demonstrated that the salicylaldehyde hydrazone moiety is  indispensable for the AIE property of 1. In addition,  characteristic large Stokes shift (176 nm) could be observed in  compound 1 (Figure 2), which is larger than that of the  reported TPE monomer (about 60 nm12), suggesting a  reasonable ESIPT process.10c, 18 Therefore, ESIPT process is  another main cause for the AIE effect of 1 besides the RIR  process. Interestingly, compound 1 exhibits unique stability in  its trans-keto form compared with conventional ESIPT molecules,17, 19 which make it a potential photochromic  material. The photochromic property of 1 will be further  discussed in the following pages.

3.3 The reversible photochromic property of 1.

Compound 1 was supposed to exhibited reversible  photochromic property based on our design in Scheme 1,  which was characterized and supported by ultraviolet-visible  diffuse reflectance spectra (UV-DRS) and fluorescence spectra.  As shown in Figure 7 and Video 1, 1 was yellow and there was  no absorption band above 550 nm before UV light irradiation.  It turned to red and exhibited a broad absorption band from  450 nm to 600 nm after UV light irradiation. When the UV light  was removed, compound 1 restored to its original state  gradually. In contrast, as can be seen in Figure 7B, the  fluorescence emission of 1 around 545 nm was intense before  UV light irradiation (fluorescence quantum yields ϕ = 4.88%),  while it decreased significantly after UV light irradiation  (ϕ = 2.03%). The flfluorescence of UV-irradiated 1 also gradually  recovered in dark.

These photochromic properties of 1 might be attributed to  the tautomerism of salicylaldehyde hydrazone from its enolform (1-Enol) to trans-keto-form (trans-1-Keto) (Scheme 2). To  support this hypothesis, the photo-response performance of  control compounds 2 and 3 were measured. As shown in  Figure S2, compound 2 exhibited similar photochromic  property with 1 due to its salicylaldehyde hydrazone structure:  the UV light irradiation turned compound 2 from 2-Enol into  trans-2-Keto.9 In contrast, compound 3 exhibited no  photochromic property in lack of the salicylaldehyde  hydrazone moiety. Therefore, it could be deduced that the  salicylaldehyde hydrazone moiety of 1 was indispensable for  its photochromic property.  Interestingly, compound 1 exhibits tunable thermal  bleaching rate. At room temperature without light, the  patterns recorded on the film of 1 can stay for hours (Figure 8).  Elevated temperature would accelerate the conversion from  trans-1-Keto back to 1-Enol (Figure 9). The light source to  promote the photochromism of 1 was investigated, which is at  least in the range from 365 nm to 475 nm (Table S1). Light with  longer wavelength will not promote the photochromism  process but even accelerate the recovery rate. As shown in  Video 1, the letters were printed in a film of 1 by a household  laser point. After being irradiated by a visible light for about 10  s, the letters on the film could be erased completely. The  effect of light irradiation intensity and wavelength to the  recovery rate were investigated and the results were shown in  Figure 10 and Table S1. The fluorescence intensity of 1 at 545  nm before and after excess UV light irradiation is 141 and 56,  respectively. After keeping 1 in dark for 1 min, the intensity  increased to 59. When the recovery experiments were taken  under visible light of 500, 550, 600 and 650 nm (light  irradiation intensity was about 2 mW/cm2 ), the intensity  reached to 87, 110, 62 and 61, respectively.  (To ensure the veracity, all of the measurements were  repeated for 3 times). It can be seen that the visible light  around 550 nm will accelerate the recovery rate  most effectively. These results are consistent with the  reported ones.20

According to the reports,9, 20a, 21 the transformation between  cis-keto form and trans-keto form is the major cause of the  high stability of UV-irradiated salicylaldehyde hydrazone  derivatives. To investigate the configuration of UV-irradiated 1,  the photochromic properties of 1 in different organic solvents  (dispersed state) were investigated first. As shown in Figure S3,  compound 1 exhibited no photochromic property in its  dispersed state, which suggested that the aggregation of  molecule is indispensable to the stability of UV-irradiated 1:  Since there is no steric hindrance in dispersed state, 1 is more  inclined to exist as a lower energy state of cis-keto-form by  thermal motion, which can easily transform to its enol-form. In  aggregated state, however, the steric hindrance of contiguous  molecules will limit conformational change, which is beneficial  for the stability of trans-keto-form. Therefore, the UVirradiated 1 should be trans-form in aggregation state. The  trans-to-cis transformation is the rate determination step for  the recovery reaction. That is why the UV-irradiated 1 can  persist for hours at room temperature in dark (Figure 8).   The crystalline packing of molecules has an important effect on  their optical properties. According to the reports, salicylaldehyde  hydrazone can be classified into two categories, i.e., thermochromic  molecules and photochromic molecules.20b, 22 The thermochromic  molecules were usually observed as planar structures that are  closely packed in the crystal lattice. The dihedral angle between the  two aromatic rings is smaller than 25o . In contrast, the  photochromic molecules were usually found as twisted structures  with long intermolecular distances, and the dihedral angle between  the two aromatic rings is larger than 25o. 22c, 23 Interestingly, 1 exhibited typical photochromic but not obvious thermochromic  properties, although the dihedral angle between the two aromatic  rings in 1 is 7.66o (Figure 5), which is much smaller than 25o . This  uncommon phenomenon can be attributed to the loose packing of  the TPE moiety, which enlarged the intermolecular distance  effectively. This packing is beneficial to the isomerization from cisketo to trans-keto form under light irradiation.

It is known that the carbonyl moiety in trans-1-Keto is an  efficient fluorescence quencher due to its strong electronwithdrawing property.11 Therefore, the fluorescence intensity  of trans-1-Keto was much weaker than that of 1-Enol.  Furthermore, the absorbance of trans-1-Keto at 450-600 nm is  significantly higher than that of 1-Enol (Figure 7A). Therefore,  the self-absorption of trans-1-Keto is another reason for its  weaker fluorescence intensity.

Further information about the fluorescence change of 1 was  obtained from fluorescence dynamics measurement (Figure  11). The fluorescence lifetime (τ) of 1 before UV light  irradiation is 0.83 ns, which is analogous to that of 1 after UV  light irradiation (0.75 ns). Radiative decay rate constant (kr )  and non-radiative decay rate constant (knr) were calculated  according to the definition of τ = 1/(kr +knr) and ϕ = kr/(kr +knr).24 Before UV light irradiation, kr and knr of 1 were  5.88×107  s -1 and 1.15×109  s -1, respectively. While kr reduced to  2.57×107 s -1 and knr barely changed (1.23×109  s -1) after UV light  irradiation. The analogous non-radiative decay process of 1- Enol and trans-1-Keto suggest their similar microstructure in  solid state. To confirm this assumption, X-ray powder  diffractions (XRD) of 1 before and after UV light irradiation  were carried out (Figure 12). It can be seen that the XRD  spectra of 1 before and after UV light irradiation were almost  the same, which suggest their analogous crystal structures.

To gain further insight into the photochromic properties, the  calculations were performed on 1-Enol, cis-1-Keto and trans-1- Keto with Gaussian 09 program. The geometries were  optimized with RM062X hybrid density function and the basis  set used was 6-31+G(d,p). Theoretically, energy gap between  HOMO and LUMO (EHL) is correlated with the wavelength of  absorption: Smaller EHL enables the electron transit easier,  resulting in a red-shift of absorption wavelength.25 As shown in  Figure 13, the EHL of 1-Enol is 5.53 eV and the EHL of trans-1- Keto is 4.97 eV. These calculated results agree well with the  red-shifted absorption wavelength obtained by experiments  (Figure 7A). Meanwhile, compared with 1-Enol, the HOMO and  LUMO of cis-1-Keto and trans-1-Keto distributed mainly in the  area near to the carbonyl moiety, which visually demonstrated  the electron-withdrawing property of carbonyl moiety. In  addition, the total energy (sum of electronic and zero-point  energies) of cis-1-Keto and trans-1-Keto was calculated to be - 38137.6774 eV and -38137.2176 eV, respectively. The higher  total energy of trans-1-Keto indicates that the trans-form is a  metastable form of UV-irradiated 1. 1 is more inclined to exist  as a lower energy state of cis-1-keto in dispersed state, which  can easily transform to its 1-Enol. Thus, the photochromic  property is difficult to be observed in dispersed state.  Meanwhile, the total energy of cis-1-Keto and trans-1-Keto are  both higher than that of 1-Enol (-38137.9985eV), suggesting  that the compound have gained energy from UV light after  irradiation.

As a reversible photochromic material, 1 was capable of  serving as a material for photo-patterning with erasable  property. As shown in Figure 14, letters, 2-dimensional bar  code and other patterns could be recorded on the same film of  1. The process was rather simple as discussed below. Preorganized pattern was firstly printed in transparencies, then  UV light was allowed to pass through it onto the film of 1 and  the pattern was subsequently recorded. Other patterns could  be recorded on the same film after the former pattern  disappeared gradually in dark or was erased quickly by long  wavelength light irradiation. The fatigue resistance of 1 was  investigated by toggled repeatedly between 1-Enol and trans- 1-Keto for 10 times. As shown in Figure 15, the fluorescence at  545 nm stayed almost constant without degradation,  indicating a good fatigue resistance of 1. The reversible,  controllable photochromic properties, as well as the good  fatigue resistance of 1 indicate that it can be served as  promising solid material for erasable photo-patterning.

In conclusion, a specially designed AIE molecule of 1  connecting TPE and SA moieties has been developed.  Compound 1 exhibits typical AIE properties due to RIR and  ESIPT process. Compare with TPE monomer, compound 1 has  longer fluorescence emission wavelength of 545 nm and larger  Stokes shift of 176 nm. As a photochromic molecule, compound 1 shows reversible color and fluorescence changes  upon UV light irradiation. More importantly, recovery rate  from trans-1-Keto back to 1-Enol is controllable by  temperature and long wavelength light irradiation. 1 also  shows good fatigue resistance, which makes it an excellent  candidate for photo-patterning with erasable property.  Compared with the conventional caged photo-activatable  system, photo-induced redox system and photo-induced  cyclization system, this work provides a brand new strategy in  the design of reversible photo-controllable luminescent  system.