Photocontrolled multiple-state photochromic benzo[b]phosphole thieno[3,2-b]phosphole-containing alkynylgold(I) complex via selective light irradiation

Photochromic materials have drawn growing attention because using light as a stimulus has been regarded as a convenient and environmental-friendly way to control properties of smart materials. While photoresponsive systems that are capable of showing multiple-state photochromism are attractive, the development of materials with such capabilities has remained a challenging task. Here we show that a benzo[b]phosphole thieno[3,2‑b]phosphole-containing alkynylgold(I) complex features 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. More importantly, the exclusive photochemical reactivity of the thieno[3,2‑b]phosphole moiety of the gold(I) complex can be initiated upon photoirradiation of visible light. Stepwise photochromism of the gold(I) complex has been made possible, offering an effective strategy for the construction of multiple-state photochromic materials with multiple photocontrolled states to enhance the storage capacity of potential optical memory devices.

O wing to the growing demands of smart materials that can alter their physical and chemical properties for potential applications, stimuli-responsive compounds with possible stimuli-controlled functions have drawn enormous attention over decades [1][2][3][4] . In particular, the photoresponsive system is amongst one of the most attractive and important candidates in stimuliresponsive systems [5][6][7][8][9] because the stimulus of light has been regarded to be environmentally benign, convenient to access and orthogonal to electronic components of devices. Upon varying the wavelength and intensity, remote control using light could be accomplished in a qualitative and quantitative manner, respectively 5 . In addition, it is easier to deliver light as a stimulus to achieve remote control on the smart materials over time and space with high precision when compared to other stimuli 6 . The potential applications of photoresponsive materials, such as in optical memory storage 10 and photoswitchable molecular devices 11 , have continuously provided the motivation for the rapid development of new photoresponsive materials with promising properties. Among the photoresponsive molecules, diarylethene has been recognized as one of the most popular molecules [12][13][14][15][16][17][18][19][20][21][22][23] because of the excellent thermal stability of the photogenerated colored closed form that leads to high thermal irreversibility, robust photostability over multiple photoswitching cycles that offers its high fatigue resistance, distinctly different photophysical properties of the thermodynamically stable open and photogenerated closed forms, and relatively high photoswitching efficiency [12][13][14][15][16][17] .
In general, photoresponsive molecules should ideally undergo reversible photochemical reactions upon excitation of light, providing a bistable system coded with ON and OFF states 12 . Furthermore, a photoresponsive system that is able to offer more than two states by photocontrol so as to enrich the diversity and the versatility and to enhance the storage capacity of optical memory devices, would be much more appealing than the conventional bistable photoresponsive systems [24][25][26] . However, the development of multiple-state photochromic systems with photocontrolled multistates has remained a major challenge, with only a limited number of compounds capable of successfully exhibiting multiple photoinduced color changes 24 . Since 2003, Irie and coworkers have reported the pioneering works of a dimer 27 and a trimer 28 of organic fused-diarylethenes with multiple photochromism that display up to four colors. A decade later, Chen and coworkers reported a few examples of Au(I) 29 , Pt(II) 30 , and Ru(II) 31 complexes by the incorporation of different diarylethene-based ligands to undergo multiple photocyclization reactions with multistates in four colors. Except for the abovementioned examples, most of the other systems with more than one diarylethene unit, have only shown a single photochromic reaction [32][33][34][35][36][37][38] because of the presence of intramolecular energy transfer quenching of the photoactive open form excited states by the photogenerated closed form (Fig. 1a) 34 . Another challenge in the design of multiple-state photochromic systems is the need for orthogonality of the photochromic and photophysical properties of the diarylethene units 24 . It has commonly been observed that for a system consisting of more than one diarylethene unit, all the open forms could undergo photocyclization reactions simultaneously and similarly, the closed forms could undergo photocycloreversion reactions at the same time (Fig. 1b) [39][40][41][42][43][44][45][46] , resulting in a single photoinduced color change, unless the photocyclization and photocycloreversion reactions of different diarylethene units could be orthogonal and triggered selectively by photoexcitation at different wavelengths. Despite multiple photochromism demonstrated by the aforementioned examples, they are realized mainly by the simultaneous photocyclization reactions of all open form moieties nonselectively upon UV excitation (Fig. 1c) [27][28][29][30][31] , followed by subsequent photocycloreversion reactions of each closed form unit. Therefore, there is a strong need for the design of multiple photochromic systems that can allow selective and versatile control of a photocyclization reaction of the desired open form unit in an exclusive manner for potential applications of multiple-state photochromic materials.
Luminescent gold(I) complexes have been extensively explored and utilized for potential applications 47 including chemosensors for the detection of various metal ions 48 , supramolecular selfassembly materials [49][50][51][52][53][54][55] , aurophilicity-induced stimuli-responsive materials 56 and catalysis 57 . With the simple preparation methods, relatively good thermal-and photo-stabilities as well as unique linear geometry 58 , alkynylgold(I) complexes represent one of the most interesting members among the family of organogold(I) complexes. Recent work by us showed that alkynylgold(I) complexes could be used to construct multi-stimuli-responsive materials 59 , in which photochromic and mechanochromic moieties could be engineered into the same molecule with the two functions independent of each other via gold(I) complexation. It is believed that the gold(I) center can play an important role in isolating the different stimuli-responsive moieties through appropriate design 59 .
Considering the promising photochromic properties of phosphole-fused dithienylethene-based materials [59][60][61][62] , together with the rare examples of multiple-state photochromic materials to afford photocontrolled multistates 24 , we envision that the incorporation of two dithienylethene-based photoswitches of the UV-driven benzo[b]phosphole 60 and the visible-light-driven thieno[3,2-b]phosphole 61 to gold(I) via metal complexation could realize a successful demonstration of multiple photochromism. More interestingly, the gold(I) metal center plays an important role in separating two photoactive units that lead to the suppression of intramolecular quenching processes of the excited states 63 . Furthermore, it is also anticipated that an exclusive photochemical reactivity could be switched on only on the visible-light-driven thieno [3,2-b]phosphole rather than the UVdriven benzo[b]phosphole moieties upon photoexcitation of visible light, allowing an occurrence of a single photocyclization reaction of the selective photoswitchable unit (Fig. 1d). Besides, the individual photocycloreversion reaction of the closed forms of benzo[b]phosphole and thieno [3,2-b]phosphole ligands would be initiated independently upon photoexcitation of visible light at different wavelengths. With the continuous interest in exploring various sophisticated photochromic systems [34][35][36][37][38]46,[59][60][61][62][64][65][66][67][68][69][70] , in this work, we report the multiple-state photochromic benzo[b] phosphole thieno [3,2-b]phosphole-containing alkynylgold(I) complex 1, including its synthesis and characterization, together with the electrochemical, photophysical and multiple-state photochromic properties, in which reversible multistates can be photocontrolled qualitatively by an external stimulus of light with different wavelengths. More importantly, complex 1 has been found to show 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 good photoswitching efficiency, rendering it an important candidate of multiple-state photochromic organometallic systems with multiple states for the potential applications of optical memory devices with enhanced storage capacity in the foreseeable future.

Results and discussion
Synthesis and structural characterization. The multiple-state photochromic benzo[b]phosphole thieno[3,2-b]phosphole-containing alkynylgold(I) complex 1 was synthesized by a modified literature procedure for the synthesis of benzo[b]phosphole alkynylgold(I) complexes (Fig. 2) 59 . Complex 1 has been readily prepared by mixing the benzo[b]phosphole chlorogold(I) precursor (BzP-AuCl), TMS-protected thieno[3,2-b]phosphole-containing alkyne (TMS-ThP), tetra-n-butylammonium fluoride (TBAF) and sodium hydroxide in dichloromethane solution at room temperature overnight. Complex 1 was obtained after column chromatography and recrystallization as a yellow solid with good yield and is shown to be stable to air and moisture. Interestingly, complex 1 can be dissolved in a wide range of nonpolar to polar solvents such as hexane, benzene, dichloromethane, ethyl acetate, and methanol. Complex 1 was characterized by 1 H and 31 P{ 1 H} NMR spectroscopy, HR-ESI-MS and elemental analysis. Owing to the presence of diastereomeric mixtures with the existence of a chiral center at the phosphorus atom and the helical structure rendered by the disposition of the two photoactive bisthienyl rings, the benzo[b]phosphole moiety of complex 1 is found to feature two sets of 1 H and 31 P{ 1 H} NMR signals (Supplementary Figs. 1 and 2) even at high temperature (70°C). This has also been observed in the previously reported benzo[b]phosphole derivatives 60 . In contrast, the thieno[3,2-b]phosphole moiety shows only one set of signals in the 1 H and 31 P{ 1 H} NMR spectra at high temperature (70°C) 61 , leading to the observation of two sets of 31 Table 1). With a potential value similar to that of the oxidation of the thiophene-containing alkyne 37 Table 2). Given the insensitivity to oxygen, relatively small Stokes shift ( Supplementary Fig. 7), emission lifetime shorter than a submicrosecond, together with the similarity of the emission spectra of the alkynylgold(I) complex with TMS-ThP ( Supplementary Fig. 6), the fluorescence of complex 1 is originated from the singlet gold(I)-perturbed [π−π*] thieno[3,2-b]phosphole-containing alkynyl ligand-centered excited state with intraligand charge transfer character. Complex 1 is found to exhibit much lower luminescence quantum yield (ϕ lum = 3.3%) than the reported phosphole-based compounds 71 showing color changes from orange to purplish-gray with the appearance of a new low-energy absorption band at ca. 580 nm (Fig. 3b). Upon subsequent photoirradiation of visible light at ca. 500 nm, photocycloreversion reaction of the closed form of thieno[3,2-b]phosphole moiety could occur predominantly (1-cc → 1-co), in which color changes from purplish-gray to greenishgray, with a drop of the low-energy absorption band at ca. 500 nm, are observed (Fig. 3c). Eventually, a recovery of the yellow solution from the greenish-gray solution has been realized upon photoexcitation at ca. 580 nm, with a drastic reduction of the lowest-energy absorption band at ca. 580 nm (Fig. 3d) (Fig. 3b); while the changes at 390 nm are found to be much larger during photocycloreversion reaction of the thieno[3,2-b]phosphole moiety (Fig. 3c), leading to conversion from 1-cc to 1-co. Since the molecular structure of complex 1 consists of two photochemically active ligands of dithienylethene-based benzo[b]phosphole and thieno [3,2-b] phosphole units linked together via gold(I) complexation, it is anticipated that complex 1 is capable of performing multiple photochromism upon photoirradiation of selective wavelengths (Fig. 4) Fig. 10). Furthermore, the generation of 1-co could also be realized by photoexcitation at ca.  59 . In addition, complex 1 displays a photoinduced decline in the intensity of the green emission (λ em = 550 nm) with an 80% decrease during the photocyclization reactions with UV excitation ( Supplementary  Fig. 15). Similarly, BzP-AuCl and TMS-ThP also show emission spectral changes with a drop of the emission intensity upon photocyclization reactions (Supplementary Figs. 16 and 17).

(d) This Work: Stepwise Photochromic Reactions via Selective Photocyclization Reactions
Complex 1 is shown to exhibit promising photochromic properties, including excellent thermal irreversibility, robust fatigue resistance under ambient conditions, high conversion of the photogenerated closed form from the open form at the PSS, and good photochemical cyclization and cycloreversion quantum yields to offer good photoswitching efficiency. Intriguingly, both the closed forms of benzo[b]phosphole and thieno [3,2-b]phosphole moieties of complex 1 feature negligible thermal backward ring-opening reactions (<0.1%) even at a higher temperature of 50°C over 4000 min (Supplementary Figs. 18 and 19). Moreover, the closed forms of complex 1 also display over ten photochromic cycles without significant photodegradation (<2. 2% Table 3), respectively. Considering the potential application of photochromic materials for optical memory devices with enhanced storage density, solidstate photochromism of complex 1 on thin film has been explored to provide multiple photocontrolled state of different colors. Upon photoirradiation at selective wavelengths, four isomers (1-oo, 1-oc, 1-cc, 1-co) of the alkynylgold(I) complex 1 on the PMMA thin film with yellow, orange, brown and bronze olive in colors can be successfully obtained (Fig. 4).
Computational study. To gain more insight into the multiplestate photochromic properties of complex 1, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been performed on 1-oo, 1-oc, 1-cc, and 1-co (Figs. 5, 6 and Supplementary Fig. 22). The lowestenergy absorption band of 1-oo, computed at ca. 441 nm (Supplementary Fig. 23 and Supplementary Table 4), is attributed to the HOMO → LUMO+1 excitation, where the HOMO and the LUMO+1 are the π and π* orbitals, respectively, predominantly localized on the dithienylethene-containing thieno[3,2-b]phosphole alkynyl ligand (Fig. 5a). Therefore, the lowest-energy absorption band can be assigned as IL [π → π*] transition of the open form of the thieno[3,2-b]phosphole alkynyl ligand. The  Table 5). This band is contributed by the HOMO → LUMO excitation, where the HOMO and LUMO are the π and π* orbitals, respectively, localized on the methyl-substituted 1,8-thia-as-indacene condensed ring (Fig. 5b), and thus it can be assigned as IL [π → π*] transition of the closed form of the thieno [ ( Fig. 5c), and therefore, it can be assigned as IL [π → π*] transition of the closed form of benzo[b]phosphole. Moreover, upon photocycloreversion of the closed form of the thieno [3,2-b] phosphole moiety in 1-cc, the lowest-energy absorption band at ca. 608 nm remains, but the singlet-singlet transition computed at ca. 514 nm in 1-cc (S 0 → S 3 ) is found to disappear in 1-co (Supplementary Fig. 26 and Supplementary Table 7), which is consistent with a drop of the low-energy absorption band at ca. 500 nm observed in the photochromic study. Moreover, 1-co displays a low-energy absorption band at ca. 608 nm that is originated from the HOMO → LUMO excitation. As the HOMO and LUMO are the π and π* orbitals localized on the phenylsubstituted 1,8-thia-as-indacene condensed ring (Fig. 5d) Table 8), which are almost identical to those in 1-oo. The Gibbs free energies of 1-oc, 1-cc, and 1-co, with respect to 1-oo, are 0.67, 11.90, and 10.71 kcal mol −1 , respectively (Supplementary Table 8), revealing that 1-oc is more stable than 1-co and 1-cc. The low-energy absorption band at 441 nm of 1-oo is contributed by the HOMO → LUMO+1 excitation, where the electron density of the LUMO+1 is localized on the open form of the thieno [3,2-b] phosphole moiety. The signs of the lobes on the p orbitals of the reactive carbon atoms are preorganized for the formation of a carbon-carbon σ bond via a conrotatory fashion (Fig. 6a), providing a geometric requirement of the molecular orbital for 1-oo to undergo photocyclization to give 1-oc upon photoirradiation at ca. 440 nm. Regarding 1-oc, the transitions computed at 514 nm (HOMO → LUMO) and 395 nm (HOMO −1→LUMO and HOMO → LUMO + 2) would not be able to bring about a ring closure, as there is no electron density localized on the reactive carbon atoms of the open form of the benzo [b] phosphole moiety in the LUMO as well as the lack of possible bond formation for a carbon-carbon σ bond in a conrotatory fashion in the LUMO + 2. The photocyclization at the open form of the benzo[b]phosphole moiety is instead contributed by the S 0 → S 15 transition at 318 nm, which corresponds to the HOMO − 5→LUMO+1 excitation. This is because, in the LUMO+1, the signs of the lobes on the p orbitals of the reactive carbon atoms are preorganized to enable the formation of the carbon-carbon σ bond in a conrotatory fashion (Fig. 6b).  orbitals of the reactive carbon atoms in LUMO+1 are preorganized to allow the formation of the carbon-carbon σ bond in a conrotatory fashion (Fig. 6c). From the perspectives of molecular geometry, energetic features and virtual orbitals of complex 1, the realization of multiple-state photochromic behaviors has been strongly supported by the computational study. The Cartesian coordinates of the optimized S 0 geometries of 1-oo, 1-oc, 1-cc, and 1-co are given in Supplementary Tables 9-12, respectively. Furthermore, the spin-orbital coupling (SOC) constants of 1-oo, 1-oc, and 1-co have been determined using Amsterdam Density Functional (ADF) program, in order to gain more insights into the mechanism of the multiple-state photochromic reaction in 1. These involve the estimation of the rate of intersystem crossing (ISC) in 1-oo, 1-oc, and 1-co, which would be revealed by the SOC constants between the singlet excited states (S n ) and the triplet excited states (T m ) that are energetically close to S n (Supplementary Table 13 ), suggesting the inefficient ISC processes in 1-oc and 1-co. Moreover, the energy difference between S 4 and T 9 in 1-oc and that between S 3 and T 4 in 1-co are estimated to be 0.18 and 0.30 eV, respectively, which are not negligibly small. Since the energy difference between S n and T m has an exponential effect on the rate of the ISC, together with the relatively low value of SOC constants in 1-oo, 1-oc, and 1-co, inefficient ISC between the S n and T m states is anticipated. Therefore, the first photocyclization in 1-oo and the second photocyclizations in 1-oc and 1-co would be mainly attributed to their singlet excited states, though the involvement of the triplet state via ISC to some "dark" triplet states cannot be completely ruled out. However, given the small SOC constants and the non-negligible energy differences between the S n and T m states, we believe this is less likely. Inefficient ISC in gold(I) systems is not unprecedented 73 . Given the inefficient ISC process associated with the relatively small SOC constants in 1-oo, 1-oc, and 1-co, it is anticipated that the photocyclization of the open forms of benzo[b]phosphole and thieno [3,2-b]phosphole moieties would occur within picosecond timescale via the singlet excited state, similar to that found for the photocyclization of the organic counterparts 74 .
In this work, a newly designed benzo[b]phosphole thieno[3,2-b]phosphole-containing alkynylgold(I) complex has been successfully synthesized to display multiple photoinduced color changes upon selective photoexcitation of light at specific wavelengths. With the rare examples of multiple-state photochromic organometallic complexes, the present work has not only successfully illustrated that the multiple photoinduced color changes can 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. 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 will 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.

Methods
Materials, spectroscopic measurements and instrumentation, and computational details are available in the Supplementary Information.

Data availability
The data that support the plots within this paper and other findings of this study are available from the manuscript, its Supplementary Information, or from the corresponding author upon reasonable request. The electrochemical, photophysical, photochromic, and computational data generated in this study are provided in the Supplementary Information.