Localization-Induced Crystal-to

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Molecular Orbital Delocalization/Localization Induced Crystal-to -Crystal Photochromism of Schiff Bases without Ortho-Hydroxyl Groups Sheng Ding, He Lin, Yuming Yu, Lang Liu, Caiming Deng, Jianzhang Zhao, and Dianzeng Jia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07408 • Publication Date (Web): 09 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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The Journal of Physical Chemistry

Molecular Orbital Delocalization/Localization Induced Crystal−to−Crystal Photochromism of Schiff Bases without Ortho−Hydroxyl Groups Sheng Ding,1,# He Lin,1,# Yuming Yu,2 Lang Liu,*,1 Caiming Deng,1 Jianzhang Zhao,2,3 Dianzeng Jia*,1 1 Key Laboratory of Energy Materials Chemistry, Ministry of Education; Institute of Applied Chemistry, Xinjiang University, Urumqi, 830046 Xinjiang, People’s Republic of China 2 School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, 830046 Xinjiang, People’s Republic of China 3 State Key Laboratory of Fine Chemicals, Dalian University of Technology, E−208 West Campus, 2 Ling−Gong Road, Dalian 116024, People’s Republic of China

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Abstract: Solid−state photochromic compounds can be used in molecular switch and memory, smart window and etc. However, new photochromic mechanism was rarely reported. Photochromic Schiff bases without ortho−hydroxyl groups (thioamide hydrazones) have been prepared, which are drastically different from the traditional photochromic Schiff bases (e.g. salicylal anils, pyrazolone thiosemicarbazones). Systematic experimental and theoretical studies of thioamide hydrazone confirm the crystal−to−crystal photochromism and thermo−enhanced photochromism. The faint yellow original form changes to yellow after UV irradiation and can further bleached by visible light irradiation. Besides, the decolored form can automatically change to yellow in dark at room temperature or by heating. Structures of the three forms (the original form, the colored form and the decolored form) show that the intramolecular torsion and vibration lead to the increase/decrease of the donor (nitrogen atom) and acceptor (sulphur atom) distance of the intermolecular hydrogen bond (N−H…S). It further results in the delocalization/localization of HOMO, which is verified by the density functional theory calculation. Thus, the absorption intensity increase/decrease is attributed to the reversible switch between π→π*/n→π* transition. The conclusion was further confirmed by an analogue compound without the thiocarbonyl group. The switch of transition is accompanied by intermolecular orbital dehybridization/hybridization during the photochromism induced by light due to the primitive cell contains more than one molecules.


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INTRODUCTION Photochromic compounds have been widely applied in molecular devices,1-4 non−linear optics,5-9 organic electronics,10-14 self−healing or self-assembling system,15-18 etc. Up to now, typical photochromic molecules mainly focus on diarylethene,19-24 azobenzene,25-28

Stenhouse adduct,29-30

spiropyran,31-34 Schiff base,35-40 and so on. Among them, Schiff bases have been received extensive attention since salicylal anils were firstly reported by Senier et al. and further studied by Cohen and co−workers.41-42 Especially, some Schiff bases containing ortho−hydroxyl groups show photochromism or thermochromism in the solid−state. Traditionally, the photochromism mechanism is due to the intramolecular proton transfer induced by light or heat,43-44 leading to the reversible transformation between two meta−stable states of the molecules, the enol form (OH form) and the keto form (NH form). During the process, the hydrogen atom bonded to the oxygen atom transfers to the nitrogen atom of C=N through intramolecular hydrogen bond. The formation of keto form results in an absorption peaked at longer wavelength than that of the enol form.45 In the past several decades, the photochromism of Schiff bases on salicylal anils was confined to this mechanism. New photochromic mechanism is rarely reported. In recent years, our group found that pyrazolone semicarbazone/thiosemicarbazone derivatives (with ortho−hydroxyl groups) exhibit solid−state photochromism.46-50 The isomerization originates from the proton transfer via intermolecular hydrogen bonds, which result in the transformation between the enol form and the keto form of the molecules. The two different forms give different absorption at 450 nm and leads to the color switch between white and yellow. However, the mechanism of these compounds still belongs to the traditional proton transfer. Up to now, most of the photochromic processes of Schiff bases, including salicylal anils and pyrazolone derivatives, are highly dependent on the presence of 3 ACS Paragon Plus Environment


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ortho−hydroxyl group. To the best of our knowledge, Schiff bases without ortho−hydroxyl groups have been rarely reported to show the photochromism. Herein, we replaced pyrazolone moiety with thiophene ring while keep the molecular skeleton of thiosemicarbazide, resulting in (E)−2−(thiophen−2−ylmethylene)hydrazine−1−carbothioamide (1− −H). The thioamide hydrazone still exhibits photochromic properties and thermo−enhanced photochromism in the solid state (Scheme 1). Combined X−ray single crystal diffraction with theoretical computation, it can be found that an intramolecular orbital delocalization/localization results in the photochromism. This

conclusion

was

also

confirmed

by

non-photochromic

amide

hydrazone,

(E)−2−(thiophen−2−ylmethylene)hydrazine−1−carboxamide (2− −O). More importantly, intermolecular orbital dehybridization/hybridization has been reported for the first time due to two unsymmetric molecules contained in the primitive cell. Scheme 1. Molecular structures of the thioamide/amide hydrazone

Methods Materials and Reagents. Thiophene−2−carbaldehyde, thiosemicarbazide and semicarbazide were purchased from Aldrich Company, USA. Other reagents came from commercial sources and were used directly without further purification. Synthesis. Synthesis of (2E)−2−[(thiophen−2−yl)methylidene]hydrazine−1−carbothioamide (1− −H). Thiophene−2−carbaldehyde (2.0 mmol) and thiosemicarbazide (1.5 mmol) were dissolved in ethanol (3 4 ACS Paragon Plus Environment

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mL) containing glacial acetic acid (Catalytic equivalent). The mixture was stirred and refluxed at 80 °C for 0.5 h (Scheme 1). Pale yellow microlite precipitated after standing overnight and was filtrated. The crude product was recrystallized from ethanol to yield the pale yellow product 1− −H (85.60%). The other compound was synthesized with the same method as 1− −H. The synthetic route of the compounds is shown in Scheme 1. 1, (E)−2−(thiophen−2−ylmethylene)hydrazine−1−carbothioamide, 1− −H (faint yellow powder, yield: 85.60%). m. p. 190.3−191.5 °C. 1H NMR (DMSO−d6, 400 MHz): δ 11.44 (s, 1H), 8.42 (s, 1H), 8.20 (s, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.55 (s, 1H), 7.45−7.44 (m, 1H), 7.12 (dd, J = 8.0, 4.0 Hz, 1H). 13C NMR (DMSO−d6, 100 MHz): δ 177.81, 138.77, 137.76, 130.71, 129.00, 128.08 ppm. HRMS (ESI) Calcd for C6H7N3S2 [M + H] +: 186.01542, found: 186.01529. 2, (E)−2−(thiophen−2−ylmethylene)hydrazine−1−carboxamide, 2− −O (faint yellow powder, yield: 86.30%). m. p. 217.9−218.9 °C. 1H NMR (DMSO−d6, 400 MHz): δ 10.22 (s, 1H), 8.04 (s, 1H), 7.55 (d, J = 5.1 Hz, 1H), 7.32 (d, J = 3.5 Hz, 1H), 7.18 – 6.94 (m, 1H), 6.25 (s, 2H). 13C NMR (DMSO−d6, 100 MHz): δ 156.44, 139.56, 134.94, 128.83, 127.78, 127.50 ppm. HRMS (ESI) Calcd for C6H7N3OS [M + H] +: 170.03826, found: 170.03787. General Characterization. ESI-MS spectra were made on an Ultimate 3000/Q-Exactive UPLCMS/MS. 1H NMR,

13

C NMR spectra were measured with an INOVA−400 NMR spectrometer with

DMSO−d6 as solvent. UV−Vis spectra were performed on a Hitachi UV−3900 spectrometer equipped with an integrating sphere accessory. Melting points were measured with a B−540 melting point apparatus without correction. Irradiation of 365 nm UV light and visible light over 400 nm were provided by an ultraviolet lamp (15 W) and a tungsten lamp (40 W) with a 400 nm band−pass filter for coloration and decoloration, respectively. The distance between the lamp and samples is about 2 cm.



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Two− −photon Excitation. The two−photon excitation was induced by output pulses at 730 nm (5 ns pulse width), which were reduced by neutral density filter. The optical parametric oscillator (Rainbow OPO) was pumped with a pulsed Nd:YAG laser (Brilliant B, 532 nm) at a repetition rate of 10 Hz.

43

The beam was focused on about 3 mm diameter and the crystals were irradiated for 30 min at room temperature. Single Crystals and the X− −ray Single Crystal Diffraction. The single crystals for X−ray single crystal diffraction were harvested after slow evaporation of ethanol in ambient condition for 3−10 days. The structures of single crystals were determined with a BRUKER SMART APEX II X−ray single crystal diffractometer equipped with a graphite monochromated Mo Kα radiation (λ = 0.71073 Å) by using

ω−2θ scan technique at room temperature. The structures of different forms were solved with SHELXS using direct methods, refined on F2 using all data by full matrix least−squares procedures with SHELXL−2014, within Olex−2.51-53 All non−hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to N1B, N2B, C2B, C4B in the crystal structure of 1− −H (Figure 5) after two−photon excitation were fixed using the HFIX command in SHELXTL. Positions of other hydrogen atoms were located from difference electron density maps. Crystallographic data for 1− −H form I, form II and 2− −O form I, form II were deposited at the Cambridge Crystallographic Data Centre with CCDC reference number 1566912, 1855414, 1854544, 1855423. Computational studies. Based on the crystal structures determined by the X−ray single crystal diffraction, our calculations were performed within DFT using the Perdew−Burke−Ernzerholf (PBE) functional54 and the plane wave pseudopotential scheme as implemented in the Quantum−ESPRESSO package.55 The kinetic energy cutoff was set to 36 Ry, and the convergence of energy was 1×10−6 Ry. K−point sampling was restricted to the Г point. Optical absorption spectra were computed by TDDFT using the Liouville–Lanczos approach.56-57 6 ACS Paragon Plus Environment

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RESULTS AND DISCUSSIONS Photochromic and Thermo− −enhanced Photochromic Properties. The UV−Vis spectra for the powder of 1− −H upon irradiation of 365 nm UV and 400 nm visible light at room temperature are shown in Figure 1. There is a weak absorption band between 375 nm and 500 nm for the original faint yellow powder (Figure 1a). A gradual increase of the absorptivity with irradiation of 365 nm light was observed. Meanwhile, the faint yellow powder (form I) gradually changed to yellow, no more spectral evolution was observed after 70 min of irradiation. It indicates that the photo−stationary state (PSS) (form II) is obtained. As the yellow powder was exposed to 400 nm visible light, decrease of band absorbance (375−500 nm) can be observed with increasing irradiation time (Figure 1b). The yellow powder simultaneously reverted to faint yellow (form III). However, its original absorbance was not fully recovered and it reached the third photo−stationary state. When the bleached powder was re−exposed to the UV light, it turned to yellow again. Figure 2a displays the fatigue resistance of the compound under the alternative irradiation of UV light and visible light. The memory effect of the compound is remarkable after more than ten cycles of coloration/decoloration. The above results indicate that 1− −H exhibits the photochromic properties in the solid state, but its fatigue resistance is unsatisfactory.



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Figure 1. (a) UV−Vis absorption spectra of as−synthesized 1− −H in the solid state with irradiation of 365 nm light (R.T.), (b) the colored 1− −H upon irradiation with 400nm visible light (R.T.), (c) the bleached 1− −H changes with the time of solid powder kept in darkness at R.T., the inset: variation of the absorbance at λ = 425 nm with the time. (d) UV−Vis spectra variation of colored 1− −H with irradiation of 400 nm visible light (R.T.). It is noteworthy that only the decolored state (form III) of 1− −H can automatically reverse to yellow (form II) by itself after we leave it in darkness at room temperature or heating for a period of time (Figure 1c). The yellow powder could still bleach to faint yellow through irradiation of 400 nm visible light (Figure 1d). This process of 1− −H was named as thermo−enhanced photochromism.58 Figure 1c demonstrates analogous absorption spectra changes of the decolored 1− −H in darkness at room temperature. The absorbance of the band around 375−500 nm increased similar to Figure 1a with standing of the compound in darkness and it turned to yellow by itself (form II). The inset reveals the absorbance at 425 nm with the time. Enhancement of absorbance was observed in the initial 10 min and practically reached PSS after 60 min in darkness. The thermal half-time of the process under room temperature is 11.7 min (Figure S1). When the colored powder (form II) was irradiated for 30 min by visible light again, the absorbance of band (375−500 nm) gradually reduced until reaching PSS (form III) (Figure 1d). Compared with Figure 1b, the decoloration rate is faster.


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Figure 2. (a) Reversible change in absorption at λ = 425 nm of as−synthesized 1− −H during alternative 365 /400 nm light irradiation cycles. (b) Reversible change in absorption at λ = 425 nm of decolored 1− −H in alternative darkness (R.T.)/upon 400 nm visible light irradiation. We also performed the fatigue resistance study of the thermo−enhanced process. In Figure 2b, the coloration/decoloration cycles of 1− −H was displayed. The recycling process was much stable than the photochromic process in Figure 2a and repeated well over 10 cycles. The switch process demonstrates good reversibility and stability. In addition, when the UV irradiation duration was shortened, the memory effect of 1− −H could be reduced obviously and the fatigue resistance was largely improved. Scheme 2. Photochromism and thermo−enhanced photochromism of 1− −H

1− −H can switch between different colors by being placed in darkness or exposed to sunlight irradiation at daytime (Scheme 2). Thus, the compound is a potential material for the fabrication of smart window. Moreover, form II of 1− −H can transform to form I through evaporating the form II methanol solution into solid. More interestingly, the newly synthesized faint yellow powder form I did not change its color by heating directly. However, if the faint yellow powder was irradiated by 365 nm light for 10 minutes and we could observe the color of the powder deepened a little, then that even without the completely formation of the colored form II, the heat could induce the fast change of its color. The result indicates that 1− −H exhibits the thermally enhanced photochromic properties. The heat induced colored form reversed into the faint yellow form III after irradiation of the visible light. The switch process could continue by heating and the visible light irradiation, which did not need further UV irradiation.



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As shown in Figure 3, the absorbance at 425 nm increased as the faint yellow powder form I was irradiated for 10 min by 365 nm light. Then by replacing the UV irradiation with heating at 353 K for 10 min, the absorption intensity continued to increase till the stationary state form II was reached. Similarly, the absorbance decreased until it reached form III after the colored form was irradiated by the visible light. The cycles proceeded over 10 cycles under the alternative heating and irradiation of visible light. Its fatigue resistance is better than that under the alternative irradiation of UV light and visible light.

Figure 3. The absorbance changes of 1− −H at λ = 425 nm (purple dotted line, 365 nm UV irradiation for 10 min; green solid line, heated the sample at 353K; orange dashed line, 400 nm visible light irradiation). Kinetics and Thermodynamics of the Photochromic Processes. The corresponding first−order rate constants of coloration and decoloration of the compound were determined by fitting the experimental data to the following equation,59

kt = ln [(A∞−A0)/(A∞−At)]/t

(1)

where kt represents the first−order rate constant, and A0, At, and A∞ are the observed absorption intensity measured at the beginning, time t, and at the end of the reactions. The first−order kinetic curves of 1− −H 10 ACS Paragon Plus Environment

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for different transition processes are shown in Figure 4a. According to the slopes, the kinetic rate constants for the UV irradiation (U), the visible light irradiation (V) are 4.54×10−2 min−1 and 3.90×10−2 min−1, respectively. ‡ ‡ kB T ∆S -∆H kt = e R e RT h

(2)

To obtain the activation parameters of the reversible photochromic process, the photocromism was studied at different temperatures (from 40−120 °C). The experimentally determined rate constants kt were used in an Eyring analysis (eq. 2), where kB represents the Boltzmann constant (kB = 1.381×10−23 J K−1), h is the Planck constant (h = 6.626×10−34 J s−1), the gas constant R is 8.314 J K−1 mol−1. Through plotting ln(k/T) versus 1000/T, Eyring plot (Figure 4b) reveals enthalpy of activation ∆H‡ = 17.5 kJ mol−1 and entropy of activation ∆S‡ = −183.8 J K−1 mol−1 for 365 nm UV irradiation and ∆H‡ = 8.1 kJ mol−1 and ∆S‡ = −209.6 J K−1 mol−1 for 400 nm visible light irradiation, respectively. The negative entropies of activation illustrate the transition states are in higher ordered form than in the ground states. Other parameters including the Gibbs free energies changes at 298 K are show in Table 1.

Figure 4. (a) First−order kinetic plots of 1− −H at room temperature. U: coloration by 365 nm UV irradiation, V: decoloration by 400 nm visible light irradiation. (b) Eyring plot for coloration rate of 1− −H (blue, UV light irradiation (365 nm), 40−120 °C) and its decoloration rate (red, 400 nm visible light irradiation, 40−120 °C).

Table 1. Activation Parameters and Rates Constants of Photochromic Process 11 ACS Paragon Plus Environment


∆H‡ (kJ mol−1)

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∆S‡ (J K−1 mol−1) ∆G‡ (298K, kJ mol−1) k×10−2 (298K, min−1)

365 nm UV

17.5

−183.8

72.3

2.2

400 nm vis

8.1

−209.6

70.5

4.5

Crystal− −to− −crystal Photochromic Mechanism. Based on the above research, our products exhibit interesting photochemical properties. However, several questions still remain: (1) How does the structure change during the photochromic process? (2) How does the structural change affect the electronic properties? (3) And what is the relationship between the electronic structures and the absorption spectra? Herein, the photochromic mechanism was systematically studied. To clarify the structural changes during the photochromism, it is preferable to determine the coloration/decoloration crystal structures by X−ray single crystal diffraction. However, since 365 nm UV irradiation is absorbed only by the molecules on the surface of the crystal, two−photon excitation (730 nm) and 400 nm visible light were employed to induce the molecular structural changes deep inside the single crystal.18 Herein, the 1− −H single crystal without defects was used in the single crystal diffraction study to have a deep understanding of the changes of the molecular structures during the photochromism. The faint yellow single crystal obtained by the evaporation of its ethanol solution is the original form I. After two−photon excitation of the form I crystal, the obtained yellow single crystal was denoted as the form II. The form III represented the bleached single crystal, which was obtained after the visible light irradiation of the form II. The crystal structures of the three forms were measured by X−ray single crystal diffraction. All the data were collected from the same single crystal of 1− −H and related structural refinement are shown in Table S1. The crystal of the form I belongs to the monoclinic crystal system with the P21/n space group. The crystal system and space group were kept very well after two−photon excitation and 12 ACS Paragon Plus Environment

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visible light irradiation. However, the cell dimensions have increased after the photoisomerization, especially the volume of the unit cell. Then the cell dimensions and volume reversed back to the ones in the form I upon the visible light irradiation. Molecule structures of 1− −H in the primitive cell and the crystal packing are shown in Figure 5 (thermal ellipsoid plot). The primitive cell contains two molecules (A and B), which lie in different sub−lattices. To compare the molecular structures during the photochromism, the bond lengths of the two molecules in different forms are listed in Table S2. Interestingly, the structural changes of the molecule B are more remarkable than those of the molecule A. The changes of intramolecular bond variation are displays in Supporting information, Figure S2. The ordinate represents the difference between different forms of the same bond, while the abscissa is corresponding to the indices of each bonds. We can clearly observed that the variation of bond length changes in molecule B are dramatically large than molecule A. Moreover, if a bond length, for example bond 4, decreased from form I → form II, then it increased during the process from form II → form III. This indicates that the bond lengths of molecule B experienced the reversible elongation and shortening during the coloration and decoloration. The reversible variation of bond lengths is similar to the changes in cell dimensions.



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Figure 5. Thermal ellipsoid plot (50% probability level) of the molecular structure of 1− −H in primitive cell and the crystal packing. Based on the above results, we supposed that the photochromsim has a strong connection to the molecular structural changes during the light irradiation. Thus, the related torsion angles were compared. The variation of torsion angles of S1A−C1A−N2A−N3A (α) and S1B−C1B−N2B−N3B (β) in different forms are demonstrated in Table 2. The two−photon excitation (from I → II) resulted in the increase of the angle of α by 0.015°, while the decrease of the one of β by 1.579°. Besides, the corresponding angles reversed back by the decrease of α by 0.077° and the increase of β by 1.117° in the form III, respectively. The results indicate that the changes of the molecule B (β) are more obvious than the molecule A (α). The intramolecular torsion leads to the space position changes of atoms and further influences the intermolecular interaction, like hydrogen bond. All the hydrogen bonds of 1− −H in different forms are list in Table S3 and we found several hydrogen bonds disappeared after two−photon excitation. Furthermore, the distance between donor and acceptor (d (D…A)) of the remaining hydrogen bonds also experienced more or less reversible changes during light irradiation. In brief, the D…A distance variation are mainly due to the intramolecular torsion (Table 2) and vibration (Supporting information, Table S2, Figure S2) result from the two−photon excitation/visible light irradiation. Table 2. Variation of Torsion Angles of 1− −H in Different Forms Form

α, °

∆, ° (α)

β, °

∆, ° (β)

I

176.605

/

170.402

/

II

176.620

+0.015

168.823

−1.579

III

176.543

−0.077

169.940

+1.117

The transform of molecular structure directly leads to the changes in electronic structures, which contributes to the absorption variation. To further explore the effect of the structure changes on the electronic properties, we performed DFT calculation. Figure 6 shows the calculated projected density of 14 ACS Paragon Plus Environment

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states (PDOS, a1–a3), the highest occupied molecular orbital (HOMO, b1–b3) and the lowest unoccupied molecular orbital (LUMO, c1–c3) of the three forms. In the form I, Figure 6a1 shows an overlap between the HOMOs of the molecule A and molecule B, suggesting an intermolecular hybridization. In comparison to the states of the molecule A ranging from −0.5 eV to 0 eV in the form I, the ones in the form II are shifted to lower energies spanning from −0.6 eV to −0.2 eV (Figure 6a2). This indicates that the electrons of molecule A are no longer available to contribute to the HOMO, leaving the electrons of molecule B in the HOMO alone, which leads to the dehybridization between the molecule A and molecule B. After 400 nm visible light irradiation, the hybridization between molecule A and molecule B was recovered in the form III (Figure 6a3). The LUMOs of the molecule A and molecule B are spanning from 2.0−3.0 eV, which do not shift during the photochromic process. Thus, the two−photon excitation/visible light irradiation induced the dehybridization/hybridization between the molecule A and molecule B. To the best of our knowledge, photochromism related intermolecular dehybridization/hybridization was first reported. Furthermore, the HOMO is basically localized on the thioamide sulfur atoms (S1A and S1B) before photoisomerization (Figure 6b1). The localization of HOMO in form I is mainly assigned to the electrophilicity of the proton in the intermolecular hydrogen bond (N1A−H…S1B). These results suggest that the weak absorption at 425 nm of the original state can be ascribed to n→π* transition. Due to the fact that the LUMO is little affected by the light irradiation, we concentrated on the occupied states in the following discussion. In Figure 6b2, we found the HOMO of form II in the molecule B is no longer focused on S1B, which can be interpreted as the delocalization of the molecular orbital. This is mainly resulted from the D…A distance of N1A−H…S1B increases from 3.477 Å to 3.524 Å in the form II, the proton electrophilicity weakens and further leads to the HOMO delocalization. Thus the increase of absorption at 425 nm can be assigned to the transformation from n→π* transition to π→π* 15 ACS Paragon Plus Environment


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transition. In the form III, the D…A distance of the intermolecular hydrogen bond decreased while the HOMO recovered (Figure 6b3) and the absorption decreased accordingly. Overall, the 365 nm/400 nm light irradiation induced the increase/decrease of the D…A distance of the intermolecular hydrogen bond, which led to the delocalization/localization of the HOMO. The switch between π→π*/n→π* transition finally resulted in the variation of absorbance.

Figure

6.

Photo−induced

delocalization/localization

and

the

intermolecular

orbital

dehybridization/hybridization. PDOS (a1−a3, left), HOMO (b1−b3, middle) and LUMO (c1−c3, right) for 1− −H: (1) form I, (2) form II, and (3) form III. The absorption spectra of 1−H were also calculated using time−dependent density functional calculation (TDDFT) and shown in Supporting information, Figure S3b, where they were compared to the corresponding UV−Vis spectra (Supporting information, Figure S3a). It was observed that the computed spectra showed a peak at the absorption edge (~400 nm), which was basically consistent with the UV−Vis spectra. In addition, during the process of the form I → the form II → the form III, the intensity of the band increased, then reversed to the original one. Such variation trends were in agreement with the intensity change of the experimental data, although the original intensity was not fully recovered in experiments. To examine quantitatively to which extent the UV /visible light irradiation modifies the band gap of 1− −H, the energy differences between the highest occupied level and 16 ACS Paragon Plus Environment

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lowest unoccupied level for three forms were calculated using DFT. The calculated band gaps are 2.24 eV, 2.19 eV, and 2.23 eV for the form I, II and III, respectively (Supporting information, Figure S4). Note that the differences in band gaps for various investigated species are small, and we also did not find obvious blue−shift or red−shift in UV−Vis absorption spectra of 1− −H (Supporting information, Figure S3a, S3b). In order to further confirm our postulation that thioamide sulphur atom plays a key role during the photochromsim, we synthesized (E)−2−(thiophen−2−ylmethylene)hydrazine−1−carboxamide (2− −O, Scheme 1) which replaced the thioamide sulphur atom with oxygen atom (Supporting information, Figure S5) and performed control experiments to verify the delocalization/localization of HOMO focused on the thioamide sulphur atoms. The UV−Vis spectra upon 365 nm UV irradiation or heating at 353K are shown in Supporting information, Figure S6. We observed that the absorbance between 240−800 nm does not change during irradiation or heating process. Neither photochromism nor thermochromism were found in 2− −O upon outside stimuli. In Supporting information, Figure S7, we can observe that π→π* transition does not change during irradiation process. This rationalizes the reason that the absorbance of 2− −O does not increase after UV light irradiation.

Conclusions A novel intramolecular orbital delocalization/localization induced crystal−to−crystal photochromism of thioamide hydrazone was reported. Reversible changes of crystal structures during the photochromic process were directly observed, which include the variation of the unit cell parameters, some bond lengths and torsion angles etc. The intramolecular torsion and vibration directly lead to the increase/decrease of the D…A distance of intermolecular hydrogen bond and further result in the switch between delocalization (π→π* transition)/localization (n→π* transition) of the HOMO. Furthermore, we first discovered the light induced dehybridization/hybridization during photochromism between 17 ACS Paragon Plus Environment


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different molecules in the primitive cell. The photochromism mentioned above has not been observed in the compound which replaced the sulphur atom with oxygen atom. Thus the thioamide sulfur atom and the intermolecular hydrogen bond originated from thiosemicarbazide contribute to the solid state photochromism of thioamide hydrazone. The original form (form I) of thioamide hydrazone changes to yellow (form II) through UV irradiation and it can be bleached by visible light irradiation. The bleached form (form III) can automatically turns into yellow (form II) by placing the sample in darkness at R.T. or heating. The original form (form I) can also be changed to yellow through heating at 353 K after just 10 min UV irradiation. Thus, the three states photochromic compound has the potential applications in anti-counterfeiting, information storage, swart window, etc.

■ ASSOCIATED CONTENT Supporting Information. NMR spectra, related crystalline data, calculated absorption spectra and band gaps of 1− −H, molecular structures, UV− −Vis spectra and orbitals of 2-O, 1− −H Forms.cif, 2− −O Forms.cif. The Supporting Information is available free of charge on the ACS Publications website.

■ AUTHOR INFORMATION Corresponding Author * [email protected], [email protected](Liu, L.); [email protected](Jia, D.) Author Contributions # These authors contributed equally.

■ ACKNOWLEDGMENTS


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This work was supported by the National Natural Science Foundation of China (21362037) and Xinjiang University Doctoral Innovative Research Program (XJUBSCX-2015011). J. Zhao thanks the support of Department of Education of the Xinjiang Uyghur Autonomous Region (Tianshan Scholar Chair Professor). He Lin thanks the support of Xinjiang University Doctoral Research Foundation (BS160256) and the Scientific Research Program of the Higher Education Institution of Xinjiang (XJEDU2017M004). We also thank Guido Fratesi for fruitful discussions.

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Scheme 1. Molecular structures of the thioamide/amide hydrazone 41x20mm (600 x 600 DPI)

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Figure 1. (a) UV-vis absorption spectra of as-synthesized 1-H in the solid state with irradiation of 365 nm light (R.T.). (b) The subsequent UV-vis spectra changes of the colored 1-H upon irradiation with 400nm visible light (R.T.). (c) UV-vis absorption spectra of the bleached 1-H changes with the time of solid powder kept in darkness at R.T., the inset: variation of the absorbance at λ = 425 nm with the time in darkness. (d) UV-vis spectra variation of colored 1-H with irradiation of 400 nm visible light (R.T.). 54x36mm (300 x 300 DPI)


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Figure 2. (a) Reversible change in absorption at λ = 425 nm of as-synthesized 1-H during alternative 365 nm/400 nm visible light irradiation cycles. (b) Reversible change in absorption at λ = 425 nm of decolored 1-H in alternative darkness (R.T.)/upon 400 nm visible light irradiation treatments. 29x10mm (300 x 300 DPI)



Scheme 2. Photochromism and thermo-enhanced photochromism of 1-H 17x3mm (600 x 600 DPI)


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Figure 3. The absorbance changes of 1-H at λ = 425 nm (purple dotted line, 365 nm UV irradiation for 10 min; green solid line, heated the sample at 353K; orange dashed line, 400 nm visible light irradiation). 61x45mm (300 x 300 DPI)



Figure 4. (a) First-order kinetic plots of 1-H at room temperature. U: coloration by 365 nm UV irradiation, V: decoloration by 400 nm visible light irradiation. (b) Eyring plot for coloration rate of 1-H (blue, UV light irradiation (365 nm), 40-120 °C) and its decoloration rate (red, 400 nm visible light irradiation, 40-120 °C). 31x12mm (300 x 300 DPI)


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Figure 5. Thermal ellipsoid plot (50% probability level) of the molecular structure of 1-H in primitive cell and the crystal packing. 64x50mm (300 x 300 DPI)



Figure 6. Photo-induced delocalization/localization and the intermolecular orbital dehybridization/hybridization. PDOS (a1-a3, left), HOMO (b1-b3, middle) and LUMO (c1-c3, right) for 1-H: (1) form (I), (2) form (II), and (3) form (III). 53x34mm (300 x 300 DPI)


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Table of Contents 33x13mm (300 x 300 DPI)


Localization-Induced Crystal-to

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