Synthesis , Characterization , Thermochromism , and Photochromism of Aromatic Aldehyde Hydrazone Derivatives

The Schiff bases N-(5-phenylthiazole-2-yl)-2-hydroxylnaphthaldehydehydrazone (1), N-(4-chloro-5-phenylthiazole-2-yl)-2hydroxylnaphthaldehydehydrazone (2), and N-(4-nitro-5-phenylthiazole-2-yl)-2-hydroxylnaphthaldehydehydrazone (3) were synthesized. These compounds were characterized by using IR, HNMR, CNMR, and MS. The photochromism of the compounds was investigated by IR and UV-visible spectrometry which is time variable under irradiation of 254 nm UV light. The thermochromism of the compounds was studied using temperature-variable IR, UV-visible spectrometry, TG, and differential scanning calorimetry (DSC). The results suggested that compound 2 showed thermochromism properties and compounds 2 and 3 displayed photochromism properties. The relationship between the substituents species and photochromic or thermochromic properties of these compounds was revealed as well.


Introduction
Of the many Schiff bases, o-hydroxylnaphthaldehydehydrazone had been found to be unique classes of Schiff bases with reversible thermochromism, photochromism, and other interesting properties.It was known that Schiff bases experience reversible phototransformations or thermal transformations between two forms, in particular, between enolimine form and keto-amine form [1]. The tautomerism of the Schiff bases played an important role in their photochromic and thermochromic characteristics [2,3].When heated or receiving ultraviolet radiation, the hydrogen atom of the O-H or N-H transferred and attached to the N atom of C=N or O atom of C=O (Scheme 2) leading to - charge transfer.The tautomerization between enol-imine and ketoamine achieved by proton migrating thermally or through irradiation induction, so orthohydroxyl played an important part on both thermochromism and photochromism of such Schiff bases [4,5].A link between molecular planar structures and thermochromism or photochromism has been proposed [6,7].Planarity of the molecule made it possible for the proton to transfer with small energy requirement [8,9].Substituents on the aromatic ring might also have an influence on the proton transfer [10,11].
In this study, three Schiff bases (1-3) were synthesized (Scheme 1).The thermochromism or photochromism of the compounds were also studied.The results suggested that although the compounds (1-3) were all o-hydroxyl Schiff bases, compound 1 showed no thermochromism or photochromism, so o-hydroxyl was not the only factor work for the thermochromic or photochromic properties; the substituent species on the phenyl ring had great effect on such properties.were measured using a TU-1901 ultraviolet spectrophotometer.DSC analysis was performed by a DZ3335 differential scanning calorimeter (DSC) and TG analysis was detected by a STA449-F5TAQ600 analyzer.ZF-6 UV-light analyzer (254 nm, 365 nm) was used as the radiation source.
The reaction mixture was refluxed for 30 minutes.The mixture became clear in a moment.Five minutes later, a great deal of solid appeared.Solid (compound 1, 2, or 3) was obtained after suction filtration and recrystallized with ethanol-DMF (volume ratio, 1 : 1).

Results and Discussion
3.1.Thermochromism 3.1.1.IR Spectroscopic and TG Studies.The color of compound 2 changed from gray purple to brown after being heated to 229 ∘ C and restored the original color after cooling.To investigate the thermochromism of the compounds thoroughly, KBr tablets of 1-3 were heated to certain temperatures on the electric hot plate; then the IR spectra were measured promptly.The IR spectra of 2 were shown in Figure 1(a).Compound 2 had the characteristic stretching band of hydroxyl group (O-H) at 3333 cm −1 .When compound 2 was heated to 140 ∘ C, the absorption peak intensity of hydroxyl group absorption peak decreased partly the result of a conversion of the hydroxyl group to the carbonyl group (Scheme 2).When the temperature reached 200 ∘ C, the band almost disappeared.When compound 2 was heated to 60 ∘ C, a small absorption peak appeared at 1763 cm −1 .This was due to the stretching vibration of the carbonyl of the keto-amine, which was produced from enol-imine of compound 2 on heating [12][13][14].
In fingerprint region (Figure 1(b)), small new absorption peaks appeared at 756 cm −1 and 695 cm −1 , while the peak at 652 cm −1 diminished.All of these changes were ascribed to the bending vibration of the C-H bond of the losing aromaticity ring of the keto-amine [12][13][14].
Figure 2 was the TG curve of compound 2. The TG curve suggested that there was a loss weight in the beginning of the test.The loss weight had relations to the loss of the crystal water in the molecule.Because the band of crystal water in IR spectra overlapped that of hydroxyl group, the reduction of the intensity at 3333 cm −1 in Figure 1(a) was about more than the loss of crystal water.Similar intensity changes could be observed in Figure 7.By photoirradiation, the absorption peak intensity at 3333 cm −1 of unheated compound 2 became smaller gradually without the loss of crystal water.So the intensity decreased partly because of the tautomerism and Figure 4 confirmed the discussion as  peaks for compounds 1 and 3 were 201 ∘ C and 258 ∘ C, respectively.There was another endothermic peak at 602 ∘ C for compound 3 which was a decomposition peak.

UV-Visible Spectroscopic Studies.
The UV-visible spectra of orthohydroxylated Schiff bases existing mainly as enolimine structure indicated the presence of bands at <400 nm.However, the compounds existing as either keto-amine or mixture of enol-imine/keto-amine forms showed a new band at >400 nm [15].Photochromism or thermochromism was the phenomenon of chromatic change due to the reversible photoisomerization or thermomerization between the two tautomers which showed different absorption bands and absorbance changes of the bands [16][17][18][19][20]. Figure 4 showed the UV-visible absorption spectra change of compound 2 on heating.The yellow DMF solution of compound 2 (concentration: 1.0 × 10 −5 mol/L, path length: 1 cm) transformed to light orange.The intensity of the absorption peak at 378 nm decreased, and a new band appeared at 484 nm.The absorption peak intensity of the new band became greater with temperature rising.The heated solution was left to rest for enough time to cool down to room temperature, and the color of the solution came back to yellow.UV-visible absorption spectra were tested again.The result was showed in Figure 4 as well.The absorption curve of the cooled compound 2 was compared with that of the unheated compound 2 and they agreed well.These indicated that compound 2 had reversible thermochromism.5 and 6 showed the UV-visible absorption spectra changes of compounds 2 and 3 upon irradiation of 254 nm UV light, respectively.The yellow DMF solution of compound 2 (concentration: 1.0 × 10 −5 mol/L, path length: 1 cm) transformed to pale orange under the UV light.The intensity of the absorption peak at 378 nm decreased and a new band appeared at 480 nm which kept increasing as the exposure time went on.

UV-Visible Spectroscopic Studies. Figures
The orange red DMF solution of compound 3 transformed to orange yellow under the UV light (concentration: 1.0 × 10 −5 mol/L, path length: 1 cm).Furthermore, the intensity of the absorption peak at 469 nm became smaller, while the absorption peak at 377 nm increased gradually.The changes of the absorption peaks of compounds 2 and 3 supported that the two compounds underwent tautomerism.Because the ratio of keto-amine form of compound 2 increased under UV irradiation, the absorbance of the peak at greater than 400 nm increased.For the same reason, the ratio of the enol-imine form of compound 3 increased while the keto-amine decreased after irradiation of UV light [21].
The UV light was removed after the solutions exposure to it for two hours.The solutions were let to stand for 24 hours and the UV-visible spectra of the compounds were detected again.The spectra of the restored compounds 2 and 3 were also showed in Figures 5 and 6, respectively.The absorption curves were of close resemblance to the spectra of the compounds without irradiation with UV light.Meanwhile, the color of the solutions was nearly back, so both of compounds 2 and 3 were reversible photochromic.

IR Spectroscopic Studies.
The IR spectra of 2 and 3 were taken on each exposure time by the 254 nm UV light.The time-variable behavior of the IR spectra of compounds 2 and 3 exposure to UV light was shown in Figures 7  and 8, respectively.In Figure 7, The hydroxyl group (O-H) stretching band was observed at 3333 cm −1 for compound 2. Upon irradiation with ultraviolet (UV) light, the intensity of the stretching band decreased.It was confirmed that the enolimine form of compound 2 changed partially to keto-amine, while in Figure 8 the hydroxyl group (O-H) stretching band of compound 3 at 3309 cm −1 became greater which confirmed that part of the keto-amine turned to enol-imine form when exposed to UV light.

The Effect of Substituents on Thermochromism or Photochromism.
Based on what had been discussed above, compounds 2 and 3 showed thermochromism and compound 2 exhibited photochromism, both phenomena being associated with an intramolecular proton transfer which led to the tautomerism between the enol-imine and the keto-amine [7].But compound 1 had no similar properties.Orthohydroxyl was essential for the intramolecular proton transfer.Compounds 1, 2, and 3 were all orthohydroxyl Schiff bases and their structures were similar except for the difference in the substituents on the phenyl groups, so the substituent species effected the proton transfer as well.Figures 9 and 10 were the UV-vis spectra of compounds 1-3 and those of the compounds exposure to the UV light for two hours, respectively.The results showed that compound 1 existed as enol-imine form and changed little after irradiation.Compound 2 existed as enol-imine form and transformed to keto-amine form, and the conversion ( = (%− 0 %)/ 0 %;  was conversion;  0 % and % were the absorbance of a compound before and after irradiation by UV light) was nearly 11% at 378 nm.As for compound 3, the conversion of keto-amine to enol-imine was about 36% at 469 nm.
Hammett parameters (paranitro: +0.778, para-Cl: +0.227, H: 0.0) were important basis for judging substituents effects on the reaction of benzene ring.For example, the alkalinity of p-nitroaniline was weaker than aniline due to the charge population on N atom being less and proton capacity weakened since the nitro group was a strong electron-withdrawing group.But p-nitroaniline and p-nitrophenylhydrazine were only alkaline which accept proton rather than lose proton.
But here it was evident that the molecule of compound 3 was a system containing several aryl rings and amino group was not isolated.If the proton on N atom could transfer, a double bond formed between the N atom and the neighboring C atom.Meanwhile, the unsaturated cyclic ketone changed to a much more stable benzene ring.Furthermore, the benzene ring and C=N formed conjugated system and the whole molecule became more stable.So the proton transfer could take place [20,[22][23][24][25][26][27][28][29].
The paranitro group made less charge population on N atom of amino group due to inductive effect and the shared electron pair deviate from H atom and turn to N atom which is advantageous to the hydrogen proton on N atom going away.
The substituent of compound 2 was a chlorine atom which was also an electron-withdrawing group.The compound tended to turn enol-imine into keto-amine form.Whatever the direction of the tautomerism, intramolecular proton in compounds 2 and 3 transferred more readily than compound 1.

Conclusion
Investigations of three naphthaldehyde hydrazone derivatives were conducted.It was found that not all of the o-hydroxyl Schiff bases exhibited thermochromism and photochromism.Compounds 1, 2, and 3 had similar structures, but the substituted species on phenyl ring were different.Compound 2 showed thermochromism and compounds 2 and 3 showed photochromism.As for compound 1, it showed no such properties.So the substituted species was a very important prerequisite in these properties.Results of UV-visible and IR analyses showed that compound 2 existed as enol-imine form.Heated to a certain temperature or exposed to UV light for a period of time, it partially transformed to keto-amine.Part of the keto-amine tautomer of compound 3 changed to enol-imine tautomer upon UV-light irradiation.Of these three compounds, the electron-withdrawing substituents on the benzene ring seemed to help the tautomerism.

Figure 1 :
Figure 1: The IR spectra of compound 2 on heating in the whole spectra range investigated (a) and in the narrowed range (b).

Figure 4 :
Figure 4: The UV-vis spectra of compound 2 on heating.

Figure 5 :Figure 6 :
Figure 5: The UV-vis spectra of compound 2 after a period of exposure to UV light.

Figure 7 :Figure 8 :
Figure 7: The IR spectra of compound 2 after a period of exposure to UV light.

Figure 10 :
Figure 10: The UV-vis spectra of compounds 1, 2, and 3 exposure to UV light for 120 min.