Achieving T-Type Photochromism through Generating Copper(I) Metallacyclopentadiene Biradical

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE14 Feb 2022Achieving T-Type Photochromism through Generating Copper(I) Metallacyclopentadiene Biradical Xu Zhang, Jin-Yun Wang, Liang-Jin Xu, Xu-Yuan Jin, Xin Yang, Li-Yi Zhang and Zhong-Ning Chen Xu Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 , Jin-Yun Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 , Liang-Jin Xu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 , Xu-Yuan Jin State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100039 , Xin Yang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100039 , Li-Yi Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 and Zhong-Ning Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 https://doi.org/10.31635/ccschem.021.202101675 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Photochromism due to organometallic ring-closure through C−C linkage has never been attained. In this work, we report a preliminary approach to achieve T-type photochromism by creating a metallacyclopentadiene biradical under light irradiation. A class of CuPt2 phenylacetylide compounds was prepared with distinguished photochromic recyclability and durability, which involved the formation of thermally sensitive copper(I)-butadienyl, five-membered ring through copper(I)-participated ring-closure. Upon UV light irradiation, the metallacyclopentadiene with copper(I) center chelated by 1,3-butadiene-1,4-diyl biradical was generated through photochemical C–C linkage of two face-to-face oriented ethynyl units in close proximity, giving rise to a drastic change in UV–vis absorption spectrum. When UV light irradiation was ceased, decoloration occurred through thermal cycloreversion, which endowed the photochromic CuPt2 complexes with self-recovery characteristics. Electron paramagnetic resonance (EPR) spectral study confirmed a biradical feature of a ring-closed isomer. The thermal decoloration half-lives at ambient temperature were regulated from seconds to hours by increasing F substituents in phenylethynyl ligands. Download figure Download PowerPoint Introduction Photochromic compounds show extensive applications in the fields of optical information storage, anticounterfeiting, decoration, and protection.1–5 T-type photochromism is particularly attractive because one of the isomers is present in T-type photochromic molecules6–12 exhibit thermal decoloration characteristics, useful in smart glass, photochromic lense, anticounterfeiting, and so on. Manipulating thermal bleaching reaction rates of colored isomers while preserving photochromic colorability and durability is still a major challenge for the applications of T-type photochromism in various fields.13,14 The coloration/decoloration of photochromic compounds arises primarily from photoinduced cyclization, isomerization, intramolecular proton transfer, group transfer, electron (charge) transfer, and so on.1–13 Particularly, photocyclization represents one of the most important and ubiquitous mechanisms for photochromism.1–5 Surprisingly, to the best of our knowledge, photochromism due to metal-participated photocyclization through C−C linkage has not yet been reported, although ring-closing reactions commonly operate in photochromic organic compounds such as diarylethenes, spiropyrans, spirooxazines, and others.1–5,15,16 Photochromic substances can be divided into inorganic, organic, and inorganic–organic hybrid compounds based on compositions and structures. For inorganic–organic hybrid compounds, photochromism, in most cases, originates from inherent photoresponsive characteristics of individual organic or inorganic components.17,18 To improve thermal and photochemical stability, as well as photoresponsive sensitivity, the combination of organic and inorganic constituents in inorganic–organic hybrid compounds provides a viable approach for fostering advantages and circumventing weaknesses of individual organic and inorganic components. In fact, the incorporation of photoresponsive ligands to suitable metal ions through coordination bonds has been extensively utilized to improve photochromic performance and modulate intramolecular electronic and magnetic interactions through simple metal coordination without altering photochromic moieties.16–18 Alternatively, photochromic properties in metal–organic compounds can be achieved by structural rearrangements of metal–ligand and/or metal–metal bonds.19 Nevertheless, photochromism has never been attained through organometallic photocyclization to generate metallacyclopentadiene biradical. In this research, we performed preliminary experiments to attain T-type photochromism in CuPt2 heterotrinuclear phenylacetylide complexes (Scheme 1) through the formation of thermally sensitive copper(I) metallacyclopentadiene. Thereby, we have established a new T-type photochromic system through photodriven C–C linkage of two parallelly oriented ethynyl units in close proximity to generate 1,3-butadiene-1,4-diyl biradical. Decoloration of the metallacyclopentadiene was conducted through thermal cycloreversion of copper(I) metallacyclopentadiene at Cβ–Cβ′ bond (Scheme 1). Since thermal decoloration reaction rates are highly dependent on the character of ethynyl ligands, we introduced electronegative F atoms to phenylacetylides to artificially manipulate the rate of thermal cycloreversion, achieving half-lives from seconds to hours so that these photochromic complexes could exhibit unusual self-recovery characteristics with excellent application potential in anticounterfeiting, encryption, smart glass, lense, and others. Scheme 1 | Photochromic CuPt2 complexes 1−4. Download figure Download PowerPoint Experiment Section Synthesis of [Pt2Cu(μ-dpmp)2(C≡CC6H5)4](ClO4) (1) To a CH2Cl2 (20 mL) solution of dpmp (dpmp = bis(diphenylphosphinomethyl)phenylphosphine) (50.6 mg, 0.1 mmol) was added [Cu(MeCN)4](ClO4) (16.3 mg, 0.05 mmol) with stirring to obtain a clear solution. Upon the addition of Pt(PPh3)2(C≡CC6H5)2 (92.2 mg, 0.1 mmol), the solution turned pale yellow upon stirring at ambient temperature for 4 h. After concentration, the solution was chromatographed on a silica gel column to collect the yellow band using CH2Cl2-acetone (v∶v = 10∶1) as eluent. Yield: 75%. Anal. Calcd for C96H78ClCuO4P6Pt2: C, 58.51; H, 3.99. Found: C, 58.39; H, 4.07. Electrospray ionization high-resolution mass spectrometry (ESI-HRMS) m/z (%): 1870.3123 (100) [M−ClO4]+ (calcd 1870.3127). Infrared (IR) (KBr, cm−1): 2115 (w), 1101 (s). 1H NMR (CD2Cl2, ppm): 8.02–7.98 (m, 4H), 7.89–7.84 (m, 8H), 7.52–7.49 (t, 4H, J = 7.3 Hz), 7.44–7.35 (m, 10H), 7.26–7.17 (m, 12H), 7.13–7.10 (t, 4H, J = 7.5 Hz), 7.03–7.00 (t, 8H, J = 7.6 Hz), 6.93–6.81 (m, 8H), 6.69–6.65 (t, 4H, J = 7.8 Hz), 6.53–6.51(d, 4H, J = 7.2 Hz), 6.04–6.02 (d, 4H, J = 7.9 Hz), 3.95–3.80 (m, 4H), 3.71–3.63 (m, 4H). 31P NMR (CD2Cl2, ppm): 11.2 (m, 4P, JP-P = 37.2 Hz, JPt–P = 2548 Hz), −13.4 (m, 2P, JP–P = 41.0 Hz). Synthesis of [Pt2Cu(μ-dpmp)2(C≡CC6H4F-4)4](ClO4) (2) This compound was prepared by the same synthetic procedure as that of 1 except for the use of Pt(PPh3)2(C≡CC6H4F-4)2 instead of Pt(PPh3)2(C≡CC6H5)2. Yield: 75%. Anal. Calcd for C96H74ClCuF4O4P6Pt2: C, 56.45; H, 3.65. Found: C, 56.17; H, 3.68. ESI-HRMS m/z (%): 1942.2743 (100) [M−ClO4]+ (calcd 1942.2750). IR (KBr, cm−1): 2114 (w), 1100 (s). 1H NMR [dimethyl sulfoxide (DMSO)-d6, ppm]: 8.15–8.02 (m, 4H), 7.93–7.79 (m, 8H), 7.60–7.35 (m, 14H), 7.35–7.20 (m, 12H), 7.15–6.99 (m, 12H), 6.65 (t, 4H, J = 8.94 Hz), 6.48 (t, 4H, J = 8.88 Hz), 6.26 (dd, 4H, J1 = 8.68 Hz, J2 = 5.64 Hz), 5.89 (dd, 4H, J1 = 8.78 Hz, J2 = 5.66 Hz), 4.41–4.20 (m, 4H), 3.78–3.60 (m, 4H); 31P NMR (DMSO-d6, ppm): 11.2 (m, 4P, JP–P = 37.0 Hz, JPt–P = 2554 Hz), −12.4 (m, 2P, JP–P = 39.7 Hz). 19F NMR (DMSO-d6, ppm): −103.1 (s, 2F), −105.2 (s, 2F). Synthesis of [Pt2Cu(μ-dpmp)2(C≡CC6H2F3-2,4,6)4](ClO4) (3) This compound was prepared by the same synthetic procedure as that of 1 except for the use of Pt(PPh3)2(C≡CC6H2F3-2,4,6)2 instead of Pt(PPh3)2(C≡CC6H5)2. Yield: 79%. Anal. Calcd for C96H66ClCuF12O4P6Pt2: C, 52.73; H, 3.04. Found: C, 52.32; H, 3.11. ESI-HRMS m/z (%): 2086.1989 (100) [M−ClO4]+ (calcd 2086.1996). IR (KBr, cm−1): 2119 (w), 1101 (s). 1H NMR (DMSO-d6, ppm): 8.00–7.89 (m, 4H), 7.89–7.77 (m, 8H), 7.53–7.31 (m, 14H), 7.15 (q, 8H, J = 6.36), 7.09–6.94 (m, 8H), 6.83 (t, 12H, J = 7.94), 6.65 (t, 4H, J = 8.42), 4.60–4.37 (m, 4H), 3.88–3.74 (m, 4H); 31P NMR (DMSO-d6, ppm): 9.38 (m, 4P, JP–P = 35.4 Hz, JPt–P = 2547 Hz), −10.4 (m, 2P, JP–P = 37.9 Hz). 19F NMR (DMSO-d6, ppm): −105.2 (d, 4F, J = 5.9), −106.6 (t, 2F, J = 6.0 Hz), −107.7 (d, 4F, J = 5.0 Hz), −110.5 (t, 2F, J = 4.8 Hz). Synthesis of [Pt2Cu(μ-dpmp)2(C≡CC6F5)4](ClO4) (4) This compound was prepared by the same synthetic procedure as that of 1 except for the use of Pt(PPh3)2(C≡CC6F5)2 instead of Pt(PPh3)2(C≡CC6H5)2. Yield: 74%. Anal. Calcd for C96H58ClCuF20O4P6Pt2: C, 49.48; H, 2.51. Found: C, 49.32; H, 2.66. ESI-HRMS m/z (%): 2230.1259 (100) [M−ClO4]+ (calcd 2230.1242). IR (KBr, cm−1): 2130 (w), 1101 (s). 1H NMR (DMSO-d6, ppm): 8.04–7.95 (m, 4H), 7.94–7.81 (m, 8H), 7.60–7.32 (m, 14H), 7.17–7.00 (m, 16H), 7.93–6.85 (m, 8H), 4.79–4.54 (m, 4H), 3.88–3.68 (m, 4H). 31P NMR (DMSO-d6, ppm): 10.0 (m, 4P, JP–P = 35.5 Hz, JPt–P = 2483 Hz), −9.65 (m, 2P, JP–P = 37.6 Hz). 19F NMR (DMSO-d6, ppm): −137.7 (dd, 4F, J1 = 5.9 Hz, J2 = 23.7 Hz), −140.2 (dd, 4F, J1 = 6.9 Hz, J2 = 24.4 Hz), −155.4 (t, 2F, J = 21.9 Hz), −158.3 (t, 2F, J = 22.4 Hz), −162.9 (dt, 4F, J1 = 6.6 Hz, J2 = 23.4 Hz), −163.9 (dt, 4F, J1 = 6.8 Hz, J2 = 23.3 Hz). Single-crystal growth Pale yellow single crystals of 1·3CH2Cl2, 2·CH2Cl2·dimethylformamide (DMF), 3·4CH2Cl2, and 4 suitable for X-ray diffraction were grown by layering diethyl ether onto the corresponding dichloromethane or dichloromethane/DMF solutions in the dark for 1–2 weeks. Green single crystals of 4c were obtained by layering n-hexane onto a dichloromethane solution of complex 4 under continuous irradiation of UV light (365 nm) at ambient temperature for 1 week. Results and Discussion Synthesis and characterization Complexes 1− 4 (Scheme 1) were prepared by reacting 1 equiv Cu(MeCN)4ClO4, 2 equiv dpmp, and 2 equiv Pt(PPh3)2(C≡CR)2 in dichloromethane solutions at ambient temperature. They were purified by silica gel column chromatography to afford pale yellow solid products in 74−79% yields. Complexes 1− 4 were characterized by ESI-HRMS, 1H, 31P, and 19F NMR spectroscopy ( Supporting Information Figures S1−S11), and X-ray crystallography ( Supporting Information Tables S1 and S2). The ring-closed isomers were generated in situ by UV light irradiation of dichloromethane solutions of complexes 1− 4 at ambient temperature. To structurally characterize the photochromic isomer, single green crystals of ring-closed complex 4c, suitable for X-ray diffraction, were obtained by layering n-hexane to a dichloromethane solution of complex 4 upon continuous irradiation under UV light (365 nm) at ambient temperature for 1 week. The perspective views of CuPt2 complex 4 (Figure 1a) and its ring-closed isomer 4c (Figure 1b) from X-ray crystallography are shown in Figure 1, and those of CuPt2 complexes 1− 3 are provided in Supporting Information Figure S12. The cationic CuPt 2 complexes consisted of two trans-Pt(C≡CR)2 moieties, one Cu(I) atom, and two dpmp ligands, respectively. Two trans-Pt(C≡CR)2 moieties were arranged at the two sides, whereas the Cu(I) atom was in the middle. The CuPt2 centers were doubly linked by dpmp and acetylides and further stabilized by significant Pt(II)−Cu(I) (dPt−Cu = 2.9573(5)−3.0211(14) Å) interaction.20,21 The Pt−Cu−Pt (159.74(5)−165.74(7)°) arrays deviated significantly from linearity. The Pt(II) centers exhibited a square-planar coordination environment composed of trans-oriented C2P2 donors. Two square-planar PtC2P2 moieties gave a dihedral angle of 59.4−65.4° in CuPt2 complexes 1− 4. The copper(I) center was coordinated to two P and two C donors to give a highly distorted tetrahedral geometry. Figure 1 | Perspective views of CuPt2 complex 4 (a) and ring-closed complex 4c (b) from X-ray crystallography. The perchlorates, phenyl rings on phosphorus atoms, and H atoms are omitted for clarity. Download figure Download PowerPoint As shown in Figure 1a, the Cu−acetylide (dCu−C = 2.261(4)−2.427(17) Å) bonding enforced two phenylacetylide ligands to be arranged face-to-face in close proximity with intense π–π contacts between two phenyl rings (dPh−Ph = 3.45−3.67 Å), while the other two phenylacetylide moieties were far from each other. Intriguingly, the π–π interaction between two face-to-face arranged benzene rings followed 1 → 2 → 3 → 4 ( Supporting Information Figure S13), showing a stepwise enhancement, as reflected by the gradually reduced distance between two phenyl rings in the order of 3.67 Å ( 1) → 3.55 Å ( 2) → 3.52 Å ( 3) → 3.45 Å ( 4). Noticeably, the face-to-face oriented benzene rings were parallel to each other but did not totally overlap, with a staggering angle to some extent ( Supporting Information Figure S13). Interestingly, the staggered angle followed 1 (31.9°) → 2 (23.9°) → 3 (10.3°) → 4 (3.5°), decreasing progressively. Compared with triclinic P 1 ¯ the space group of complex 4 (pale yellow), the crystal symmetry of ring-closed complex 4c (green) improved dramatically with an orthorhombic Pnc2 space group. As shown in Figure 1b, the ring-closed CuPt2 complex 4c exhibited a C2-axis symmetry along the Cu center and the midpoint of the C34−C34a bond. As the C33−C34 (dC33–C34 = 1.330(19) Å) and C34−C34a (dC34–C34a = 1.502(3) Å) bonds provided similar bond lengths to those of 1,3-diene (ca. 1.34 and 1.46 Å), the five-membered metallacycle was generated, composed of copper(I) center chelated by 1,3-butadiene-1,4-diyl moiety, also called as metallacyclopentadiene.22 The sp2 hybrid character of C33 and C34 was further confirmed by the sum of three angles around C33 (359°) and that around C34 (360°). As a metal-hybridized five-membered ring analogue to cyclopentadiene, the copper(I) metallacyclopentadiene was a perfect coplanar since the sum of the internal bond angle was 540° with one rather sharp C–Cu–C (78.7(8)°) angle and two much obtuse Cu–C=C (119.0(3)°) angles. The Cu−P length in ring-closed complex 4c (2.277(3) Å) was a little longer than those in ring-open isomer 4 (2.204(4) and 2.209(4) Å). In striking contrast, the Cu−Cvinyl lengths (dCu1−C33 = 1.947(13) Å) in ring-closed complex 4c were significantly shorter than the corresponding Cu−Cacetylide (dCu1−C65 = 2.389(16) and dCu1−C89 = 2.427(17) Å) distances in the ring-open isomer 4, but they were in the normal range of Cu−Cvinyl (1.91−2.08 Å) distances for 1,3-butadienyl organocopper(I) compounds.23–25 Undoubtedly, the Cu(I)−C bonds were substantially stabilized by dπ–pπ bonding interaction in metallacyclopentadiene.22 Most notably, when two parallelly arranged ethynyl groups in complex 4 were photochemically linked to generating a 1,3-butadiene-1,4-diyl moiety in the ring-closed isomer 4c, the C34−C34a length in ring-closed isomer 4c (1.502(3) Å) was extremely shorter than the corresponding C66−C90 distance in complex 4 (3.45(2) Å). In contrast, the C33−C34 length (1.330(19) Å, double bond) in the ring-closed complex 4c was remarkably longer than the corresponding C65−C66 and C89−C90 (1.20(2) and 1.22(2) Å, triple bonds) lengths in ring-open complex 4. The Pt−Cvinyl (dPt1−C33 = 2.036(13) Å) length in ring-closed isomer 4c was obviously longer than the corresponding Pt−Cacetylide (dPt1−C65 = 1.978(17) Å and dPt2−C89 = 1.984(18) Å) distances in complex 4. Nevertheless, another two Pt−Cacetylide (dPt1−C41 = 1.996(13) Å) lengths in ring-closed 4c were similar to those in 4 (dPt1−C73 = 2.006(16) and dPt2−C81 = 2.02(2) Å). Also, the Pt−P lengths in ring-closed complex 4c (2.299(3) and 2.318(3) Å) were comparable to those in complex 4 (2.310(4)−2.329(4) Å). Thus, upon ring-closure, the bond distances around two Pt centers did not show significant variations, except for the two Pt−C bonds with face-to-face oriented ethynyl units in close proximity, involved in photocyclization with the transformation from sp to sp2 hybrid. The Pt−Cu distance in 4c (3.0052(7) Å) was a little longer than those in 4 (2.986(2) and 3.007(2) Å), indicating that photocyclization exerted a slight influence on d8–d10 intermetallic interaction. UV–vis absorption properties The UV absorption bands of complexes 1− 4 (Figure 2) in CH2Cl2 solutions consisted of high-energy bands below 330 nm due to ligand-centered transitions, together with low-energy bands that peaked at ca. 360–380 nm, tailing to 550 nm. Upon irradiation at 365 nm, pale yellow CH2Cl2 solutions of complexes 1− 4 turned purple (for 1 and 2) or green (for 3 and 4) owing to the formation of ring-closed isomers 1c− 4c (Scheme 1) through photocyclization reactions. A demonstration of photocyclization and thermal cycloreversion based on complex 1 was recorded (see the procedure Video S1 in Supporting Information). Since the solution color intensified instantaneously under UV light irradiation, complexes 1− 4 exhibited a sensitive and rapid photochromic response. The rapid coloration due to an occurrence of low-energy absorption bands in the visible region is typical of the formation of ring-closed products. Figure 2 | UV–vis absorption spectra and photographs of CuPt2 complexes 1 (a), 2 (b), 3 (c), and 4 (d) in CH2Cl2 (5 × 10−5 mol L−1) solutions before (black line) and after (red line) UV light irradiation at 365 nm at ambient temperature. Download figure Download PowerPoint When a CH2Cl2 solution of complex 1 (Figure 2a) or 2 (Figure 2b) was irradiated under UV light at 365 nm, weak broadband centered at 545 or 565 nm occurred, accompanied by the attenuation of the UV absorption band at 368 or 382 nm, respectively. Upon irradiation of complex 3 (Figure 2c) or 4 (Figure 2d) under UV light at 365 nm, the UV absorption bands (358 and 382 nm for 3 or 360 and 382 nm for 4) attenuate sharply, while the visible region showed low-energy broadband (peaked at 598 nm for 3 or 602 nm for 4), together with an occurrence of a strong peak (462 nm for 3 or 466 nm for 4) in the near UV region. Interestingly, once the UV light irradiation ceased, the colored solutions gradually faded with remarkably self-recovery characteristics at ambient temperature. Obviously, ring-closed isomers 1c− 4c (Scheme 1) were thermodynamically metastable and thermally sensitive such that reversible decoloration was conducted through thermal cycloreversion of copper(I) metallacyclopentadiene at Cβ–Cβ′ bond (Scheme 1). To evaluate the kinetic process of ring-closed products 1c− 4c in thermal decoloration at ambient temperature, the dependence of the absorbance at the absorption maxima on the thermal decay time was investigated ( Supporting Information Figures S14−S17). Since the thermal decoloration process obeyed first-order kinetics, the kinetic equation was expressed as ln(At/A0) = −kt by Beer–Lambert law, where k is the reaction rate, t is time, and A0 and At are the absorbance of closed-ring isomer at an initial and arbitrary time (t), respectively. As shown in 1, the rate of thermal decoloration reactions followed → → → 4c in whereas the half-lives were gradually longer in this Undoubtedly, in ring-closed isomers to the substituents in phenylacetylide ligands. Obviously, the electronegative F atoms in phenylacetylide the the thermal of the ring-closed isomer, such that a longer time for the thermal cycloreversion reaction to Thus, the rate of thermal decoloration is artificially by the electronic of phenylacetylide ligands. the half-lives of thermal decoloration from seconds to hours in the order → → = → 4c = by increasing the of F atoms in phenylacetylide ligands. 1 | The and of for Complexes in CH2Cl2 at 4c k × × × × 10−5 7.6 half-lives of thermal decoloration reactions are by = As in the structural since the distance and the staggered angle between face-to-face arranged benzene rings followed 1 → 2 → 3 → 4 by the π–π interaction showed a stepwise in this order so that the thermal decoloration reaction was and to in the order of → → → This with the order of the gradually whereas reduced rate occurred in the thermal cycloreversion process of ring-closed products 1c− the T-type photochromic properties of complexes 1− 4 showed perfect with structural characteristics. To the and of photochromism, the absorbance of complex 2 at 565 nm was by irradiation at 365 nm for 1 and thermal bleaching in CH2Cl2 solution at ambient temperature. As shown in Figure the photochromic CuPt2 complex without attenuation of the absorbance after photochromic been a distinguished photochromic and Figure 3 | The absorbance of complex 2 at 565 nm in CH2Cl2 solution at upon irradiation at 365 nm for and thermal bleaching for Download figure Download PowerPoint To into the of absorption characteristics with photochromism, were performed on complexes 1− 4 ( Supporting Information Tables and Figures and and ring-closed isomers 1c− 4c ( Supporting Information Tables The low-energy bands in complexes 1− 4 were to to and In contrast, the ring-closed absorption bands in ring-closed products 1c− 4c were to the copper(I) with some characteristics of to ring and Obviously, the low-energy absorption bands of ring-closed isomers in the visible region from copper(I) Photochromic To into the of photochromism, X-ray spectroscopy and electron paramagnetic resonance (EPR) were performed on complex 4 and its ring-closed isomer The spectra of complex 4 ( Supporting Information Figure and its ring-closed isomer 4c ( Supporting Information Figure a peak in the or region of the Cu atom, respectively. the of Cu atom in ring-closed isomer 4c for and for were to the of 4 for and for The of typical of at further the of or and confirmed the Cu(I) in ring-closed the for and for of Pt atoms in complex 4 and its ring-closed isomer 4c ( Supporting Information Figure were and in with the of other Pt(II) Thus, the confirmed that Cu(I) and Pt(II) atoms without the photochromic The spectra of complexes 1− 4 and the corresponding ring-closed isomers 1c− 4c (Figure 4) in CH2Cl2 were investigated at of complexes were before irradiation, at the of ca. upon UV light irradiation. the were to ring-closed products 1c− 4c generated in situ by UV irradiation of complexes 4 in the same As Cu(I) and Pt(II) are the were due to 1,3-butadiene-1,4-diyl most generated the photocyclization As shown in Figure 4, the of weak due to = 2 in CH2Cl2 at that the biradical were in The magnetic of ring-closed 4c at a temperature range of ( Supporting Information Figure confirmed the for the biradical. by at the that the of the biradical on two Cu−Cvinyl bonds for ring-closed products 1c− 4c, which were much at the two atoms than the copper(I) center as shown in Figures Figure 4 | The spectra of ring-closed complexes in CH2Cl2 before and after (green) UV irradiation (365 nm) at Download figure Download PowerPoint such as were by through thermal of To the interaction between of the the reaction were and that the could be reduced dramatically when two ethynyl units in close proximity in were parallelly arranged such that 1,3-butadiene-1,4-diyl could be generated at temperature through a and kinetic that upon at thermal of could be conducted by the formation of 1,3-butadiene-1,4-diyl with the highly dependent on the The in the substituents was for the and the reaction for Figure | The = of the in ring-closed complexes (a), (b), (c), and 4c (d) by at the Download figure Download PowerPoint In this work, 1,3-butadiene-1,4-diyl is generated through photochemical of face-to-face oriented phenylacetylides in close proximity, dramatically stabilized through the formation of copper(I) metallacyclopentadiene. that is dπ–pπ interaction in copper(I) metallacyclopentadiene a in 1,3-butadiene-1,4-diyl biradical. Most notably, the the two face-to-face arranged phenylacetylides the the

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