Photocontrolled Multiple-State Photochromic Benzo[b]phosphole Thieno[3,2-b]phosphole-Containing Alkynylgold(I) Complex via Selective Light Irradiation

The development of photoresponsive systems that show multiple-state photochromism has remained a challenging task. We present an alkynylgold(I) complex that features multiple photoinduced color changes, in which the gold(I) center plays an important role in allowing multiple photochromic reactions.
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In this work, we demonstrate that a benzo[b]phosphole thieno[3,2‑b]phosphole-containing alkynylgold(I) complex 1 shows multiple photoinduced color changes, in which the gold(I) metal center plays an important role in separating two photoactive units that leads to the suppression of intramolecular quenching processes of the excited states. Furthermore, an exclusive photochemical reactivity could be switched on only at the visible-light-driven thieno[3,2‐b]phosphole moiety rather than the UV-driven benzo[b]phosphole moiety upon photoexcitation of visible light, allowing an occurrence of a single photocyclization reaction of the selected photoswitchable unit. Besides, the individual photocycloreversion reaction of the closed forms of benzo[b]phosphole and thieno[3,2‐b]phosphole ligands could be initiated independently upon photoexcitation of visible light at different wavelengths.

The research interests of Yam’s group include inorganic and organometallic chemistry, supramolecular chemistry, photophysics and photochemistry, and metal-based molecular functional materials for sensing, organic optoelectronics and energy research. Yam’s group has been interested in developing photochromic heterocycle-fused diarylethenes for the construction of organometallic metal complexes with interesting photocontrolled functions. In 2004, Yam and coworkers reported the coordination of a newly designed 1,10-phenanthroline-containing diarylethene ligand to a rhenium(I) metal center to realize a visible-light photoswitch via metal-to-ligand charge transfer (MLCT) state photosensitization.1 On the other hand, Yam’s group has developed a number of photochromic systems containing more than one diarylethene moieties, including oligothienoacene,2 pyrrole,3 N-heterocyclic carbene palladium(II) and gold(I) complexes,4 diphosphine dialkynyl platinum(II) complex,5 β‐diketonate boron(III) complex,6 and tris(8-hydroxyquinolinato) aluminium(III) complex.7 However, most of the above-mentioned examples have only shown a single photochromic reaction because of the presence of intramolecular energy transfer quenching of the photoactive open form excited states by the photogenerated closed form.

With the continuous interest in the exploration of various photochromic heterocycle-fused diarylethenes,8-14 Yam’s group in 2013 reported a series of diarylethene-containing phospholes with readily tunable photochromism upon varying the P-substituent.15 In order to take advantage of the weak aromaticity of the phosphole ring, we synthesized a new series of benzo[b]phosphole-fused diarylethenes,16 which feature promising photochromic properties including excellent thermal irreversibility, robust fatigue resistance, and relatively high photoswitching efficiency under ambient condition, by the incorporation of the weakly aromatic phosphole ring as an ethene bridge directly attached to the photoactive bis-thienyl rings. Based on this system, we then reported heterocycle-fused thieno[3,2-b]phosphole oxides as visible light switches,17 benzo[b]phosphole alkynylgold(I) complexes with mechanochromic property to serve as multistimuli-responsive materials,18 and barbiturate pendant-containing benzo[b]phosphole oxides with co-assembly property and photoinduced morphological changes.19

 Based on our previous finding that the gold(I) center of benzo[b]phosphole alkynylgold(I) complexes with photochromic and mechanochromic behaviors can play an important role in isolating the different stimuli-responsive moieties,18 we envision that the incorporation of the UV-driven benzo[b]phosphole and the visible light-driven thieno[3,2‑b]phosphole to gold(I) via metal complexation (Figure 1) could realize a successful demonstration of multiple photochromism, in which the gold(I) center plays an important role in separating two photoactive units that leads to the suppression of intramolecular quenching processes of the excited states.

Figure 1.         The chemical synthesis that allows the incorporation of the UV-driven benzo[b]phosphole and the visible light-driven thieno[3,2‑b]phosphole to gold(I) via metal complexation.

Although a few examples of photochromic systems have shown multiple photochromism,20-24 they are usually realized by the simultaneous photocyclization reactions of all open form moieties non-selectively upon UV excitation, followed by subsequent photocycloreversion reactions of each closed form unit. We would like to achieve a selective and versatile control of a photocyclization reaction of the desired open form unit in an exclusive manner. The benzo[b]phosphole thieno[3,2‑b]phosphole-containing alkynylgold(I) complex 1 has been successfully synthesized to display multiple photoinduced color changes upon selective photoexcitation of light at specific wavelengths (Figure 2). A single photocyclization reaction of the visible light-driven open form of the thieno[3,2‑b]phosphole ligand (1-oo1-oc), with the color changes from yellow to orange, has been triggered exclusively by the irradiation of visible light at ca. 440 nm. Upon UV excitation, the UV-driven open form of the benzo[b]phosphole ligand would undergo photocyclization (1-oc1-cc), with color changes from orange to purplish grey. Photocycloreversion of the closed form of the thieno[3,2‑b]phosphole unit (1-cc1-co), with the color changes from purplish grey to greenish grey, would take place by photoirradiation at ca. 500 nm. Upon further photoirradiation at ca. 580 nm, the photocycloreversion of the closed form of the benzo[b]phosphole unit (1-co1-oo) would take place, with a recovery of the yellow color from the greenish grey color, returning the gold(I) complex back to its open form (1-oo). Therefore, four isomers of 1-oo, 1-oc, 1-cc and 1-co with different colors could be obtained via the stepwise photochromic reactions. On the other hand, both the open forms of benzo[b]phosphole and thieno[3,2‑b]phosphole ligands could undergo simultaneous photocyclization reactions upon UV excitation (1-oo1-cc), indicated by the color changes from yellow to purplish grey. However, shining visible light at ca. 550 nm on the closed forms could lead to complete photocycloreversion reactions (1-cc1-oo), featuring photoinduced color changes from purplish grey to yellow. The whole processes of each photocyclization and photocycloreversion reactions have been monitored by 1H and 31P{1H} NMR and UV-vis absorption spectroscopy. Moreover, this benzo[b]phosphole thieno[3,2‑b]phosphole-containing alkynylgold(I) complex has also been found to feature promising photochromic behaviors such as excellent thermal irreversibility even at high temperature, robust fatigue resistance at ambient condition, high conversion of the closed forms at the photostationary state (PSS) and relatively decent photoswitching efficiency.

Figure 2. Stepwise photochromic reactions of complex 1 upon photoexcitation of light at selective wavelengths. Insets show the multiple photoinduced color changes of complex 1 in solution and PMMA thin film.

With the rare examples of multiple-state photochromic organometallic complexes, the present work has not only successfully illustrated that the multiple photoinduced color changes could be accomplished by the incorporation of UV-driven and visible light-driven photoswitches with promising photochromic behaviors via gold(I) complexation, but also has demonstrated that the metal center of gold(I) has played an important role in assisting the photochemical reactivity of the remaining open form unit instead of suffering from the quenching process, which is supported by computational study, in which virtual orbitals are found to mainly localize on the photoactive open form moiety without any significant electronic communication with the closed form unit.

 It is envisaged that the molecular design of this new benzo[b]phosphole thieno[3,2‑b]phosphole-containing alkynylgold(I) complex that features multiple-state photochromic properties would provide an in-depth insight into the construction of photochromic organometallic complexes with photocontrolled multiple states using an easy preparation method, boosting the storage capacity of optical memory devices for potential applications in the foreseeable future.

 For further details, please refer to our article in Nature Communications: https://www.nature.com/articles/s41467-021-27711-9.

 For more stories of Yam’s research group, please visit: http://web.hku.hk/~wwyam/yam/homepage/ and https://twitter.com/vivianyam1

References:

  1. Yam, V. W.-W., Ko, C.-C. & Zhu, N. Photochromic and luminescence switching properties of a versatile diarylethene-containing 1,10-phenanthroline ligand and its rhenium(I) complex. J. Am. Chem. Soc. 126, 12734−12735 (2004).
  2. Ko, C.-C., Lam, W. H. & Yam V. W.-W. Photochromic oligothienoacene derivatives with photo-switchable luminescence properties and computational studies.  Chem. Commun. 5203−5205 (2008).
  3. Wong, H.-L., Ko, C.-C., Lam, W. H., Zhu, N. & Yam V. W.-W. Design and synthesis of a new class of photochromic diarylethene‐containing dithieno[3,2‐b:2’,3’‐d]-pyrroles and their switchable luminescence properties. Chem.–Eur. J. 15, 10005−10009 (2009).
  4. Yam, V. W.-W., Lee, J. K.-W., Ko, C.-C. & Zhu, N. Photochromic diarylethene-containing ionic liquids and N-heterocyclic carbenes.  J. Am. Chem. Soc. 131, 912−913 (2009).
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  6. Poon, C.-T., Lam, W. H. & Yam V. W.-W. Synthesis, photochromic, and computational studies of dithienylethene‐containing β‐diketonate derivatives and their near‐infrared photochromic behavior upon coordination of a boron(III) center. Chem.–Eur. J. 19, 3467−3476 (2013).
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