English

• 0.   Introduction

  Photochromic diarylethene was first synthesized by M. Irie in 1988^[1]. It inherently exhibits two different chemical forms (open-form and closed-form), which are reversibly inter-converted in response to optical stimulation. The molecular structure changes from open-form to closed-form are reflected in color changes and can be applied to various photonic devices due to its outstanding thermal stability, excellent photo-fatigue resistance, high sensitivity, and characteristic bistability^[2-5]. In solution, the aryl groups at the center of the conjugated unit of diarylethene are pointing in either the same direction (parallel orientation) or opposite directions (anti-parallel orientation). The two orientations are inter-converted and only the anti-parallel orientation can take place in the photochemical reaction to form a closed-ring isomer upon light irradiation. Also, the distance between the two carbons where the new bond formed during photo-cyclization should be less than 0.42 nm^[6].

  Several attempts have been reported to modulate the photochromic properties of diarylethenes for practical applications. These attempts were mainly concerned with optimizing the substituent on the core unit of diarylethenes. So far, diarylethenes having various heteroaryl groups including thiophene, furan, indole, and pyridine have been synthesized and examined^[7-10]. Specifically, dithienylethenes (bearing two thienyl rings) are considered the most promising materials due to their lowest aromatic stabilization energies^[9] and attracted concerted research interests. Alternatively, constructing supramolecular architectures by coordination-driven self-assembly has been adopted as a smart strategy to acquire novel photo-physical and photochemical properties^[11-13]. The inorganic metal salts work as a protecting sheath to the organic dithienylethenes and collaborate with the organic ligands to modulate the complex′s properties. The strategy often requires the functionality of the thienyl rings with coordinating groups for bridging with metal ions. Our previous implementation of the strategy led to some new coordination frameworks assembled by dithienylethenes coupled with pyridyl^[14-15] and carboxyl acid^[16-17]. Most of these complexes showed retained and modified photochromism after coordination, with only a few exceptions^[18-19]. The facile modulation of absorption of dithienylethenes can be accomplished by simply adding the metal species^{[15-16, 18, 20]}. Very recently, we reported the syntheses of 1,2-bis-(2′-methyl-5′-(4″-cyanophenyl)-3′-thienyl) perfluorocyolopentene^[21] (BM-4-CP-3-TP, L_o) and it′s three Ag(Ⅰ) complexes^[22]. It′s concluded that the photo-switching of these complexes could be modulated conveniently and finely by varying the coordination anions accompanied by triggering by light irradiation.

  The above encouraging results prompted us to further investigate whether the metal coordination with closed-ring ligand could modify its absorption, which is helpful to understand the structure-property relationship of dithienylethene complexes deeply. Therefore, herein the photo-induced closed-ring form of BM-4-CP-3-TP (L_c) was separated successfully and characterized for the first time by X-ray single crystal diffraction coupled with ¹H NMR. In addition, the L_c was employed as a ligand to react with Ag(CF₃SO₃), which is a soft acid and especially suitable for coordination with ligands containing cyano groups to form stable hybrid frameworks. The photochromism of the closed-ring ligand in solution and solid state as well as the impact of metal coordination on the photo-isomerization in the solid state were examined and discussed by UV-Vis spectra and cyclic voltammetry (CV), respectively.

  1.   Experimental

  1.1   Materials and instruments

  All reagents were purchased from commercial sources and used as received without further purification. All reactions and manipulations during synthesis were carried out under a nitrogen atmosphere. Solvents were dried using standard procedures and distilled under a nitrogen atmosphere before use.

  FT-IR spectra in KBr (4 000-450 cm^-1) were recorded using an IR Prestige-21 spectrometer. ¹H NMR spectra were measured on Bruker INOVA-400 MHz spectrometer in CDCl₃ solution at room temperature (with tetramethylsilane as internal reference). Elemental analysis was conducted on a VarolEL Ⅲ elemental analyzer. ESI-MS spectrum was measured on AXIMA-CFR^TM plus MALDI-TOF with MeOH as a solvent in positive mode. Absorption spectra in THF solution (ca. 0.1 mmol·L^-1) and PMMA film were measured on a Hitachi U-3900H spectrometer. The PMMA films were prepared by dissolving BM-4-CP-3-TP in THF at 70 ℃ (0.5%) and draining on a quartz glass sheet. Absorption spectra in the solid state were measured by diffuse reflection using the Kubelka-Munk method on a SHIMADZU UV-3600 spectrometer, and barium sulfate was used as a reference. Photo-irradiation was carried out using a 100 W Xe lamp in the atmosphere, and light with appropriate wavelength was isolated by passing the light through RANYAN cut-off filters. CV was performed by means of a three-electrode configuration employing a platinum-wire working electrode (0.5 mm diameter disk) and an Ag-AgCl counter electrode, the reference electrode being a steel electrode. A Princeton ParStat 4000 scanning potentiostat was used. All experiments were performed under nitrogen at room temperature with solvents previously saturated with nitrogen. The electrolyte used was TBA(BF₄). The surface of the working electrode was polished before each measurement.

  Single crystals of L_c·EtOH (0.21 mm×0.20 mm× 0.19 mm) were used for data collection using BRUKER SMART APEX Ⅱ CCD with graphite monochromated Mo Kα radiation (λ=0.071 073 nm). The intensity data was collected using the multi-scan technique, and 7 233 reflections were collected at 296 K. The linear absorption coefficient μ for Mo Kα radiation was 0.24 mm^-1. The structure was solved by direct methods followed by subsequent Fourier calculations. The non-hydrogen atoms were refined anisotropically. The final cycle of the full-matrix least-squares refinement was based on 2 034 reflections, converged with the unweighted and weighted agreement factors of R= ∑|| F_o|-|F_c||/∑|F_o| and wR=[∑w (F_o²-F_c²)²/∑w(F_o²)²]^1/2. The atomic scattering factors and anomalous dispersion terms were taken from the International Tables for X-ray Crystallography, Vol. Ⅳ^[23]. All calculations were performed using the SHELXL-97^[24]. The final R= 0.076 6 and wR=0.139 9 for observed reflections with I > 2σ(I), and R=0.189 1, wR=0.170 4 for all data with S=1.049.

  1.2   Syntheses of ligand L_c and complex {[Ag(L_c)] (CF₃SO₃)}_n (1)

  BM-4-CP-3-TP was prepared according to the literature method^[22] (Scheme 1). Irradiation of the THF solution of BM-4-CP-3-TP with 254 nm light resulted in a blue solution followed by column chromatography over silica gel affording L_c as a deep blue powder. Single crystals (L_c·EtOH) suitable for X- ray measurements were obtained by re - crystallization from THF/EtOH (1∶1, V/V) solution at room temperature. Crystallographic data: triclinic (space system), P1 (space group), a= 1.022 5(7) nm, b=1.069 0(7) nm, c=1.537 8(11) nm, α =71.418(13)°, β =84.453(14)°, γ=67.192(13)°, V= 1.468 0(17) nm³, Z=2, D_c=1.532 g·cm^-3, M_r=616.63. IR (KBr pellet): 2 366 (m), 2 224 (m), 1 601 (m), 1 267 (m), 1 107 (s), 977 (s), 829 (s), 737 (m). ¹H NMR (CDCl₃, 400 MHz): δ 7.72 (d, 4H, J=8.4 Hz, cyanophenyl H), 7.66 (d, 4H, J=8.4 Hz, cyanophenyl H), 6.76 (s, 2H, thienyl H), 2.20 (s, 6H, —CH₃).

  Scheme 1

  

  Scheme 1.  Synthesis of ligand L_c

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  The reaction of Ag(CF₃SO₃) (0.039 0 g, 0.15 mmol) and BM-4-CP-3-TP (0.057 1 g, 0.1 mmol) in benzene/ethyl acetate (15 mL) resulted in a clear blue solution and followed slow evaporation for one week in dark give rise to complex 1 as blue microcrystals (Yield: 54.0%). Anal. Calcd. for C₃₀H₁₆AgF₉N₂O₃S₃(%): C, 43.53; H, 1.93; N, 3.39; S, 11.61. Found(%): C, 43.51; H, 1.94; N, 3.42; S, 11.65. IR (KBr pellet, cm^-1): 2 923 (w), 2 225 (w), 1 602 (w), 1 556 (w), 1 460 (w), 1 274 (w), 1 108(w), 1 029 (w), 972 (w), 827 (w). ¹H NMR (CDCl₃, 400 MHz): δ 7.71 (d, 4H, J=8.0 Hz, cyano H), 7.67 (d, 4H, J=8.0 Hz, cyano H), 6.94 (s, 2H, thienyl H), 2.51 (s, 6H, —CH₃). ESI-MS (MeOH, positive mode): [CF₃SO₃]⁺ (m/z 149.929 2), [Ag(CF₃SO₃) (C₆H₅CN)] ⁺ (m/z 359.117 5), [Ag(L_c) (CF₃SO₃)]²⁺ (m/z 413.166 2), [L_c-2CH₃)]⁺ (m/z 538.962 7), [Ag(L_c-2CH₃-CN)]⁺ (m/z 618.874 6), [Ag(L_c)(CF₃SO₃)(C₆H₅CN)]⁺ (m/z 968.870 5).

  2.   Results and discussion

  2.1   Structural characterization

  2.1.1   Crystal-structural characterization of L_c

  Fig. 1a depicts the crystal structure of L_o·THF determined previously by X-ray crystallographic analysis^[22] as a reference purpose. The two thienyl rings are fixed in a photo-reactive anti-parallel orientation with a dihedral angle of 58.86° between the two thienyl rings. The distance between reactive carbon atoms, C11 and C11A, is 0.365 nm. single crystalS of L_c were recrystallized from THF/EtOH and structurally determined by X-ray crystallographic analysis. Fig. 1b shows the solvent molecule, i. e. EtOH, is included in the crystals and F atoms are disordered. L_c is revealed to be an interesting racemic mixture of R,R and S,S enantiomer pairs (conformer A and conformer B, Fig. 1c and 1d). This is because, upon irradiation with UV light, the dithienylethene molecule undergoes a photochemical conrotatory cyclization producing two enantiomers originating from two asymmetric (chiral) carbon atoms^[25-28].

  Figure 1

  

  Figure 1.  Crystal structures of (a) L_o·THF and (b) L_c·EtOH; Conformers (c) A and (d) B in L_c·EtOH

  Symmetry code: A: -x, y, 1.5-z

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  The most significant change in the structure of L_c relative to that of L_o is the reduction of the C11—C13/C11A—C13A distance from 0.365 nm in L_o (C11—C11A) to 0.150 9(11)/0.164 6(12) nm. This is consistent with the formation of a new single bond originating from photo-cyclization. In another word, the open-ring isomer was transferred to the closed-ring isomer. In addition, the C16—C17, C15—C13/13A and C10—C11/11A distances (0.146 6(5), 0.156 5(8), and 0.155 3(8)/0.156 4(9) nm, respectively) are now consistent with single bonds, whereas the C15—C16 and C10 —C17 distances (0.133 0(5) and 0.135 4(5) nm) is now characteristic of double bonds. Other detailed differences in bond distances are presented in Table S1 (Supporting information). These changes in bond distances and types demonstrated unambiguously a structural transformation from 1,3,5-hexatriene in L_o to cyclohexadiene in L_c in response to appropriate optical stimulation.

  Besides, the two molecules L_o and L_c are significantly different in the dihedral angles between the perfluorocyclopentene ring and the thiophene ring. The dihedral angle in L_o is 51.26° and it decreases sharply to 2.43° and 2.35° (conformer A) as well as 4.73° and 4.94° (conformer B) in L_c, suggesting that π-electrons were delocalized through the molecule of L_c. This structural change relates closely to the new visible absorption band of closed-ring isomers in UV-Vis spectra.

  The structure of L_c generated in photo-cyclization can also be easily characterized by the ¹H NMR spectrum. The ¹H NMR spectrum of L_c showed four signals at δ of 7.72, 7.66, 6.76, and 2.20, which can be assigned to the protons of cyanophenyl, thienyl, and methyl, respectively. The δ at 2.20 and 6.76 are characteristic of a ring-closed product with time-averaged C₂ symmetry (Fig. 2). The two signals shifted to lower chemical shifts in comparison with those in open-ring isomer L_o (δ =2.00 (6H) and 6.40 (2H), respectively), reflecting the structural differences between the two isomers. In conclusion, the photo-induced closed-ring isomer of BM-4-CP-3-TP was verified for the first time by the combined evidence of crystal structure and ¹H NMR analysis.

  Figure 2

  

  Figure 2.  ¹H NMR spectra of BM-4-CP-3-TP in closed-form (a) and open-form (b)

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  2.1.2   Characterizations of complex 1

  The structure of complex 1 was characterized by elemental analysis, FT-IR, ¹H NMR, and ESI-MS analysis. In the FT-IR spectrum of 1 (Fig. S1), the typical peak of ν(C—F) from CF₃SO₃^- was observed at 1 029 cm^-1 (1 022 cm^-1 for free anions), suggesting the presence of the anion. The characteristic vibration peak of C—F in the ligand was found at 1 274 cm^-1 (1 267 cm^-1 for free ligand), together with the C≡N stretching vibration peak of the ligand found at 2 225 cm^-1 for 1, indicating the ligand is included in the framework. The non-significant shift of C≡N stretch was ca. 3 cm^-1 relative to the corresponding band in the free ligand (2 223 cm^-1), confirming a very small π-back donation effect of the cyano group^[17-20]. These observations proved the formation of complex 1.

  The solution ¹H NMR spectra of complex 1 showed a resonance at δ 2.51, characteristic of the methyl H protons of thienyl. Resonances at δ 7.71, 7.67, and 6.94 were observed with the predicted coupling patterns for each of the unique protons of the cyanophenyl and thienyl, respectively. These results confirmed the existence of the ligand in complex 1.

  The mass spectrum of 1 (Fig. 3) indicated a peak (m/z =149.929 2) corresponding to [CF₃SO₃]⁺, evidencing again the inclusion of the anion in complex 1. In addition, the appearances of [Ag(L_c)(CF₃SO₃)(C₆H₅CN)]⁺ at m/z=968.870 5 further suggest the polymeric structure of complex 1. The detailed assignments are summarized in Table 1.

  Figure 3

  

  Figure 3.  ESI-MS spectrum of complex 1

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  Table 1

  

  Table 1.  Assignments of ESI-MS data of complex 1

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  m/z	Assignment	m/z	Assignment
  149.929 2	[CF3SO3]+	538.962 7
  359.117 5		618.874 6
  413.166 2		968.870 5

  Based on the above data it can be concluded that CF₃SO₃^- is involved in the 1D chain structure of complex 1. Its formula is determined as {[Ag(L_c)](CF₃SO₃)}_n (Scheme 2) from which the calculated results of elemental analysis match well with the experimental values (see section 2.1).

  Scheme 2

  

  Scheme 2.  Proposed structure of complex 1

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  2.2   Photochromism

  Previous research reported the excellent thermal stability of closed-BM-4-CP-3-TP in toluene solution by ¹H NMR spectroscopy^[29-30]. The thermal conversion into the open form was found to be negligible (2%) after 14 h at 100 ℃. In this work, its photo conversion was further examined^[29]. It is fixed in closed-ring form and its photo-reversion is reasonable. However, whether the photo-induced open-ring isomer could occur in the photo-cyclization reaction is still uncertain and worthy of investigation. According to Woodward-Hoffman rules, the photo-cyclization reaction should proceed only from the photo-active anti-parallel conformer^[31]. In solution, the ratio of anti-parallel conformation was 50%. Whereas in solid state dithienylethene molecules are fixed in specific conformation, only the anti-parallel conformer could be converted into its closed-ring isomer. Based on the above consideration, the photochromic reactions of closed-BM-4-CP-3-TP were examined in solution and solid state, respectively.

  2.2.1   Photochromism of closed-BM-4-CP-3-TP in solution

  Before irradiation, the low-energy band of closed-BM-4-CP-3-TP appeared at 607 nm, which is characteristic of π→π^* transitions of the closed-ring isomers of dithienylethene (Fig. 4a). Irradiation with the light longer than 550 nm in wavelengths led to the visible band flattened gradually with blue color fading to colorless. The photo-stationary state (PSS) was reached upon light irradiation for 25 min. These findings point to the photo-reversion of closed-BM-4-CP-3-TP: i.e., closed-ring isomer to open-ring isomer. ε was calculated as 1.07×10⁴ L·mol^-1·cm^-1. The conversion of the ring-opening reaction was given as the molar fraction of open-ring isomer in PSS^[29] and determined as 88% by ¹H NMR (Fig.S2a), and further confirmed by comparing its PSS spectra with that of pure BM-4-CP-3-TP (Fig.S3a).

  Figure 4

  

  Figure 4.  Time-dependent absorption spectral changes of closed-BM-4-CP-3-TP in THF solution (0.10 mmol·L^-1) upon irradiation with light of (a) ≥550 nm and (b) 254 nm

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  Photo-cyclization was accomplished by irradiation with 254 nm light for 90 min (Fig. 4b). The absorbance of the visible band at 607 nm increased again inferring the regeneration of the ring-closed isomer. The color of the solution was returned to the initial blue. The conversion of ring-closing was estimated at 50% by ¹H NMR. Importantly, subsequent irradiation with visible light-triggered photo-reversion again, and thus the whole cycle can be repeated. These results indicated the reversible photochromism of closed-BM-4-CP-3-TP.

  The photo-cycloreversion kinetics of closed-BM-4-CP-3-TP and photo-cyclization kinetics of BM-4-CP-3-TP in solution were further determined by UV-Vis spectra upon alternating irradiation with visible light or UV light, respectively. During the photo-cycloreversion of L_c, the relationship between ln(c₀/c) and irradiation time (t) behave in perfect linearity (c₀: initial concentration, c: concentration at irradiation time t), indicating that the ring-opening process belongs to the first-order reaction (Fig. S4a). The slope of the line represents the reaction rate constant (k) in the solution. The k of the photo-cycloreversion process of L_c was obtained as 1.212×10^-3 s^-1 in solution. However, in the ring-closing process of L_o it could be seen that the relationship between the concentration and irradiation time (t) had good linearity, demonstrating that this process follows the zeroth-order reaction (Fig. S4b). The k of photocyclization was determined as 7.838×10^-9 mol·L^-1·s^-1.

  2.2.2   Photochromism of closed-BM-4-CP-3-TP in solid state

  The photochromism of closed-BM-4-CP-3-TP was further investigated in solid state and the maximum absorption in the visible band was 607 nm (Fig. 5), which showed a marked red shift as compared to our previously reported dithienylethene derivatives in PSS (λ_max=576 nm^[20], 580 nm^[15], and 597 nm^[18], respectively). Thus, the tuning of the absorption to a longer wavelength is realized successfully by incorporating cyano groups. The larger absorption wavelength is advantageous to the low-energy readout. It absorbs visible light (≥550 nm) and undergoes efficient conversion to its ring-open counterparts with the visual color change from blue to colorless. The absorbance of the band at 607 nm flattened along with prolonged light irradiation until reaching PSS after 15 min photo-irradiation. UV light (254 nm) irradiation for 50 min triggered the reverse photoreaction and regenerated the original species. The result infers that the closed-BM-4-CP-3-TP are fixed in photo-reactive anti-parallel conformation as convinced previously by X-ray diffraction. The maximum absorption of 607 nm in the solid state was the same as that in THF. However, the time to PSS was significantly reduced from 25 to 15 min in the solid state, suggesting faster photo-isomerization in the solid state. Besides, good reversibility and reproducibility of photochromism for closed-BM-4-CP-3-TP in PMMA films were observed after 10 cycles of coloration and decoloration (Fig.S5). The excellent fatigue resistance indicated its potential practical applications.

  Figure 5

  

  Figure 5.  UV-Vis absorption spectra of closed-BM-4-CP-3-TP in the solid state by irradiation with light of (a) ≥550 nm and (b) 254 nm

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  2.2.3   Photochromism of complex 1 in solid state

  The above results suggested the reversible photoswitching of ligand in the solid phase, thus the photochromism of complex 1 was investigated further in the solid state, as shown in Fig. 6. The photo-isomerizations between the closed-ring and open-ring forms of 1 occurred reversibly and effectively in the solid state. It takes 15 min for 1 to reach PSS and 40 min for the reversible reaction. In comparison, the corresponding times for the ligand were 15 and 50 min, respectively. These results suggest the photo-isomerization rates of 1 were slightly faster than those of the ligand. The maximum absorption of 607 nm of 1 was as same as that of the metal-free closed-ring ligand, indicating that the configuration of closed-BM-4-CP-3-TP in 1 is similar to that of the metal-free ligand. That is, after complexation, the structure of the closed-ring ligand does not change significantly. It is explained reasonably by the mutual restriction of the closed-ring ligand to rotate freely to satisfy the coordination with Ag(CF₃SO₃) while maintaining the rigid closed-ring conformation. And the maximum absorption of 1 was further compared to that of the photo-induced closed-ring form of the previous dinuclear complex using the same metal salt and ligand yet in the open-ring form^[22]. After photochemical ring closure, the referenced complex transformed to closed-ring form and the absorption red-shifted to 623 nm, indicating the significant impact of metal coordination. That is, when the ligand is in the open-ring form it has relatively larger coordination space for free rotation of thienyl rings to bridge with Ag(CF₃SO₃). However, when the ligand is fixed in the rigid closed-ring form, there are limited spaces for metal coordination. In addition, the quite different absorption values of 1 and referenced complex reflect their different structures (Scheme 3). In summary, metal coordination shows an unnoticeable influence on the absorption of the complex derived from the closed-ring ligand.

  Figure 6

  

  Figure 6.  UV-Vis absorption spectra of complex 1 in the solid state under irradiation with light of (a) ≥550 nm and (b) 254 nm

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  Scheme 3

  

  Scheme 3.  Schematic diagram of complex 1 and referenced complex

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  The thermal stability of complex 1 was further examined and the absorbance at 607 nm remained constant for more than 3 d at 80 ℃, indicating excellent thermal stability as the metal-free ligand.

  2.3   Electrochemical properties

  Because of the differences in the π-conjugation of the two isomers of dithienylethenes, the photoresponsive molecule also allows for changes in its electrochemical properties. The changes may be used for molecular-scale electronic switches. Also, the electrochemical properties of the ligand and complex 1 can provide useful information on the band gap. Therefore, we performed CV experiments of BM-4-CP-3-TP in MeCN (1.0 mmol·L^-1). Fig. 7 shows the CV curves of BM-4-CP-3-TP and complex 1 in both open-ring form and closed-ring form with a scan rate of 30 mV·s^-1. According to Eq. 1 and 2^[31], the energy parameters E_HOMO and E_LUMO can be calculated (they can be estimated by using the energy level of ferrocene (Fc) as a reference; 4.8 eV is the constant of the energy level of the ferrocene below the vacuum level).

  $ E_{\text {HOMO }}=-\left(E_{\text {onset }}{ }^{\text {Ox }}+4.8\right) $

  (1)

  $ E_{\text {LUMO }}=-\left({E_{\text {onset }}}^{\text {Red }}+4.8\right) $

  (2)

  Figure 7

  

  Figure 7.  CV curves of the ligand (a) and complex 1 (b) on a platinum wire electrode in an electrolyte solution of TBA(BF₄) in CH₃CN

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  As shown in Fig. 7a, both L_o and L_c exhibited reversible oxidation and reduction. The onset potentials (E_onset) of oxidation and reduction for L_c were initiated at 1.723 and -0.658 eV, respectively. Therefore, the values of E_HOMO and E_LUMO were calculated to be -6.52 and -4.14 eV, respectively. The band gap of L_c was determined as 2.38 eV. Similarly, the values of E_HOMO and E_LUMO of L_o were calculated as -6.54 and -4.09 eV, respectively. The band gap of L_o was determined as 2.46 eV. The smaller band gap of L_c than L_o was in good agreement with the bathochromic shift observed by UV-Vis spectroscopy.

  The above data shows the oxidation process for the L_o occurred at higher potentials than in the corresponding L_c. This is because the longer conjugation length of the closed-ring isomer generally leads to a less positive potential^[32-33]. After the cyclization reaction, the π-conjugation of the ring-closed isomer extends across the perfluorocyclopentene ring causing a lower oxidation onset. Similar results have been observed in other works^[31-33].

  Based on the same method, the band gap of complex 1 was determined as 2.004 eV, which was significantly smaller than that of the ligand. The smaller band gap can be used to well explain the faster photo-isomerization process (15 and 40 min) of complex 1 than those of the ligand (25 and 50 min).

  3.   Conclusions

  A crystal of BM-4-CP-3-TP in the closed-ring isomer was isolated and characterized structurally for the first time. The combined X-ray diffraction with ¹H NMR analysis proved ambiguously the structural transformations of the complex during photo-cyclization. The complex showed reversible photochromism in solution and solid state, however, its complexation with Ag(Ⅰ) ion did not modify the maximum absorption of closed-BM-4-CP-3-TP. This work suggests the coordination-driven strategy for modulation of the absorption is inapplicable for closed-ring dithienylethene. Nevertheless, the photo-isomerization of the complex was promoted as evidenced by its smaller band gap measured by cyclic voltammetry.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Scheme 1  Synthesis of ligand L_c

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  Figure 1  Crystal structures of (a) L_o·THF and (b) L_c·EtOH; Conformers (c) A and (d) B in L_c·EtOH

  Symmetry code: A: -x, y, 1.5-z

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  Figure 2  ¹H NMR spectra of BM-4-CP-3-TP in closed-form (a) and open-form (b)

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  Figure 3  ESI-MS spectrum of complex 1

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  Scheme 2  Proposed structure of complex 1

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  Figure 4  Time-dependent absorption spectral changes of closed-BM-4-CP-3-TP in THF solution (0.10 mmol·L^-1) upon irradiation with light of (a) ≥550 nm and (b) 254 nm

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  Figure 5  UV-Vis absorption spectra of closed-BM-4-CP-3-TP in the solid state by irradiation with light of (a) ≥550 nm and (b) 254 nm

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  Figure 6  UV-Vis absorption spectra of complex 1 in the solid state under irradiation with light of (a) ≥550 nm and (b) 254 nm

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  Scheme 3  Schematic diagram of complex 1 and referenced complex

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  Figure 7  CV curves of the ligand (a) and complex 1 (b) on a platinum wire electrode in an electrolyte solution of TBA(BF₄) in CH₃CN

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  Table 1.  Assignments of ESI-MS data of complex 1

  m/z	Assignment	m/z	Assignment
  149.929 2	[CF3SO3]+	538.962 7
  359.117 5		618.874 6
  413.166 2		968.870 5

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