Photochromic Heterocycle-Fused Thieno[3,2 ... - ACS Publications

Article Cite This: J. Am. Chem. Soc. 2017, 139, 15142-15150

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Photochromic Heterocycle-Fused Thieno[3,2‑b]phosphole Oxides as Visible Light Switches without Sacrificing Photoswitching Efficiency Nathan Man-Wai Wu, Maggie Ng,† Wai Han Lam,† Hok-Lai Wong, and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials [Areas of Excellence Scheme, University Grant Committee (Hong Kong)] and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China S Supporting Information *

ABSTRACT: A series of novel photochromic thieno[3,2-b]phosphole oxides has been shown to demonstrate photochromism without the need for the use of high-energy ultraviolet (UV) light irradiation while maintaining promising photochromic properties such as excellent thermal irreversibility, robust fatigue resistance, and high photoswitching efficiency. Promising visible light-induced photochromic properties have been realized by the new molecular design, in which various π systems have been incorporated into the weakly aromatic phosphole backbone instead of the conventional modification of the peripheral diaryl units that usually leads to a drastic reduction of the photochromic quantum yields (ϕO→C and ϕC→O < 0.01) or even a loss of the photochromic behavior. Excellent fatigue resistance has been observed for a representative compound with no apparent loss of photochromic reactivity over ten photochromic cycles by alternate irradiation with violet (ca. 410 nm) and green (ca. 500 nm) light with high photochromic quantum yields (ϕO→C = 0.87 and ϕC→O = 0.44), rendering it a new promising candidate as visible light photoswitches for various potential applications.

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INTRODUCTION Visible light-driven photoswitchable systems have drawn tremendous attention recently for their potential applications in the fields of photoresponsive optoelectronics1,2 as well as photopharmacology,1,3 mainly because of the less destructive and less harmful nature of visible light toward optical devices and biological cells, respectively. Among those photoswitchable compounds, diarylethenes have drawn substantial attention over decades due to their excellent thermal irreversibility, robust fatigue resistance and rapid photoresponsiveness with multifunctional properties,4 such as light-driven molecular machines,5 photocontrolled supramolecular nanostructures6 and photomodulated catalysts.7 However, photochromic diarylethenes that do not require UV excitation to undergo photochromic reaction have been rarely reported and less explored.1h In general, the colorless open form of diarylethenes would undergo photochromic reaction to give the colored closed form upon UV irradiation. Thus, the conventional strategy to achieve visible light switches is to extend the πconjugation on the peripheral diaryl pendants to reduce the HOMO−LUMO energy gap of the open form. This strategy has been demonstrated by several pioneering works, in which extended π-conjugated systems, such as bithiophenes, 8 carotenoids,9 porphyrins10 and perylene monoimides,11 have been attached to the peripheral diaryl rings. However, most of the these diarylethenes with extended π-conjugation on the diaryl pendants suffered from extremely low photocyclization and photocycloreversion quantum yields (ϕO→C and ϕC→O < 0.01) with some even showing a loss in their photochromic behaviors. Such inefficient photoresponsiveness would restrict © 2017 American Chemical Society

their further developments and potential applications as visible light-driven photoswitches. Introducing triplet photosensitizers into diarylethene-containing ligands has provided another important strategy12 to address visible light switching by intramolecular metal-to-ligand charge-transfer (MLCT) excited state photosensitization. Apart from intraligand (IL) absorption bands, photoexcitation into the MLCT absorption bands (ca. 400−480 nm) is able to sensitize the photocyclization reaction. Several pioneering works involved the incorporation of ReI,13 PtII,14 RuII,15 and IrIII16 into the diarylethene system to successfully develop visible light switches, but the rather poor photocycloreversion quantum yields reported would limit their practical applications. Apart from triplet sensitization, the utilization of upconversion nanoparticles17 and multiphoton absorption18 of visible to near-infrared (NIR) light have been reported as alternative strategies. However, upconversion and multiphotonic processes involve the use of more sophisticated instrumentations, rendering the simple addition of extended π systems into photoswitches much more attractive. Phospholes have drawn enormous attention for over a decade due to their unique electronic structures and tunable photophysical properties for potential applications in optoelectronics.19 Very recently, new photochromic phospholecontaining dithienylthiophenes20 and a new photochromic benzo[b]phosphole oxide21 have been reported to exhibit excellent photochromic properties in both solution and thin Received: August 7, 2017 Published: October 13, 2017 15142

DOI: 10.1021/jacs.7b08333 J. Am. Chem. Soc. 2017, 139, 15142−15150

Article

Journal of the American Chemical Society film states under ambient conditions. These new types of photochromic phospholes have been regarded as ideal candidates for potential practical application in photoswitchable electronics as well as optical memory devices. In particular, promising photochromic properties have been envisioned to be beneficial by the minimization of the energy difference between the open form and the photogenerated closed form.4c The incorporation of weakly aromatic phosphole to the photochromic active bis-thienyl rings represents a judicious strategy to achieve enriched photochromic performances. With our continuing interest in designing and developing versatile photochromic compounds,7c,12−14,15c,20−22 we anticipate that the incorporation of the extended π system to the photochromic phosphole backbone rather than the peripheral pendant bis-thienyl rings would offer a new strategy for visible light photoswitch design without sacrificing the photochromic behavior or suppressing the photoresponsiveness dramatically in terms of the photochromic quantum yields. Although photochromic phospholes have been reported,20,21 to the best of our knowledge, the incorporation of various π systems into the phosphole backbone to achieve visible light photoswitchable materials has never been explored and reported. Herein, we report the synthesis and characterization of a series of novel thieno[3,2-b]phosphole oxide derivatives 1−6 (Scheme 1) along with their thermal, electrochemical, photo-

have also been structurally characterized by X-ray crystallography. The perspective views of 2 and 3 are shown in Figure 1.

Figure 1. Perspective view of thieno[3,2-b]phosphole oxides 2 (a) and 3 (b) with atomic numbering. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids were shown at the 30% probability level.

The crystal structures of 2 with R chiral center and 3 with S chiral center have been obtained. More importantly, the unexpected chemical structures of the annulated products have been further confirmed by X-ray crystallographic analysis, which suggests that the radical intermediates probably have undergone 4-exo-trig ring closure reaction to give the unexpected rearranged products.23 Furthermore, 2 and 3 in the crystalline solid state have been observed to adopt antiparallel conformations, with the distances between the two photochemically active carbon atoms determined to be less than 3.60 Å, rendering their solid state photochromism in the restricted spatial environment possible.5b The X-ray crystallographic data as well as the selected bond lengths and angles are summarized in Table S1 and S2 in the Supporting Information (SI). Unlike the previously reported benzo[b]phosphole derivatives,20 this series of thieno[3,2-b]phosphole oxides possess only one set of 1H and 31P{1H} NMR signals at room temperature. However, upon decreasing the temperature, the 1H NMR signals of the representative compound 2 would split into two sets of signals (Figure S1), suggesting the recovery of the chiral helical structure arising from the restricted rotation between the dimethyl bis-thienyl rings. Thermal, Electrochemical, and Photophysical Studies. The thermal stability of the thieno[3,2-b]phosphole oxide derivatives has been studied by thermogravimetric analysis (TGA). Compounds 1−6 are thermally stable with decomposition temperatures (Td) of higher than 248 °C (Table S3). The TGA thermograms of the compounds are depicted in Figure S2−S7. Compounds 1−6 with the exception of 4 show irreversible oxidation waves that range from +0.80 to +1.47 V vs saturated calomel electrode (SCE) (Table 1). The first irreversible oxidation wave is tentatively assigned as the dimethylthiophene oxidation with mixing of phosphole backbone oxidation for 1−3 and 5 because of its rather low sensitivity toward the various π systems. However, diphenylamine oxidation was assigned as the first oxidation wave for 4 and 6 due to their strong perturbation in the presence of the more electron-rich diphenylamine moieties. Furthermore, 1−6 show quasi-reversible reduction couples that range from −1.48 to −1.73 V vs SCE (Table 1). The heterocycle-fused phosphole oxide-centered reduction is believed to be responsible for the first quasi-reversible reduction couple of 1−6 owing to their strong dependence on the different π systems. More importantly, a less positive first oxidation wave of 5 (+1.32 V) < 3 (+1.33 V) < 1 (+1.38 V) and a less negative first

Scheme 1. Synthesis and Chemical Structure of 1−6

physical and photochromic properties, in which the photocyclization and photocycloreversion reactions occur when being irradiated by violet light at ca. 420 nm and green light at ca. 530 nm, respectively. More importantly, this molecular design strategy of utilizing different π systems fused to the phosphole backbone would offer a versatile method for the preparation of visible light photoswitches while maintaining the excellent thermal irreversibility, robust fatigue resistance and rapid photoresponsiveness in comparison to the previously reported diarylethene-based visible light photoswitches.8−11 The present work provides an alternative strategy to rationally design visible light photoswitches with excellent photochromic performances.

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RESULTS AND DISCUSSION Synthesis and Characterization. Photochromic thieno[3,2-b]phosphole oxides 1−6 were synthesized using a modified literature procedure for the synthesis of benzo[b]phosphole oxides,23 that involved a silver-mediated dehydrogenative annulation of thiophene-containing phenyl hydrophosphine oxides with the corresponding diarylacetylenes, as shown in Scheme 1. 1−6 have been characterized by 1H and 31 1 P{ H} NMR spectroscopy, as well as HR EI-MS, HR ESI-MS and elemental analysis. Thieno[3,2-b]phosphole oxides 2 and 3 15143


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Journal of the American Chemical Society Table 1. Electrochemical Data of 1−6a compound

oxidation E1/2b (V) vs SCE, (ΔEp/mV)c [Epad (V) vs SCE]

1 2 3 4 5 6

[+1.38] [+1.47] [+1.33] +1.02 (62), [+1.52] [+1.32] [+0.80]

reduction E1/2b (V) vs SCE, (ΔEp/mV)c [Epce (V) vs SCE] −1.73 −1.57 −1.61 −1.59 −1.55 −1.48

(89)f (60), [−1.93] (73), [−1.95] (64) (70), [−1.92] (52), [−1.77]

a

In CH2Cl2 with 0.1 M nBu4NPF6 as supporting electrolyte; glassy carbon as working electrode; scan rate, 100 mV s−1. bE1/2 = (Epa + Epc)/2; Epa and Epc are anodic and cathodic peak potentials, respectively. cΔEp = (Epa − Epc). dEpa is reported for irreversible oxidation wave. eEpc is reported for irreversible reduction wave. fScan rate, 200 mV s−1.

reduction wave of 5 (−1.55 V) > 3 (−1.61 V) > 1 (−1.73 V) were observed (Figure 2) upon introduction of a more

Figure 3. (a) Electronic absorption and (b) emission spectra of the open forms of 1−6 in degassed benzene solution at 298 K; asterisk represents an instrumental artifact.

400 nm of the degassed benzene solution of 1−6, the thieno[3,2-b]phosphole oxides display green to red emission at about 520−640 nm (Figure 3b). Similarly, the introduction of the extended π systems and the electron-rich diphenylamino moieties also lead to a bathochromic shift of the emission maxima, with emission energy in the order of 1 (523 nm) > 2 (543 nm) ≈ 3 (542 nm) > 5 (552 nm); 2 (543 nm) > 4 (562 nm); 5 (552 nm) > 6 (645 nm). The luminescence quantum yields of the photochromic thieno[3,2-b]phosphole oxides are found to be much lower than the reported thienophosphole oxides,19a,e probably due to the presence of the photochromic moiety that leads to opening up of the competing photochromic cyclization pathway in the excited state. Photochromic Studies. Upon violet light excitation at around 420 nm, the open forms of the thieno[3,2-b]phosphole oxides 1−6 are found to display color changes from pale yellow, yellow and yellowish orange to pale orange, deep orange and deep purple in degassed benzene solutions (Figure 4), respectively. The generation of the closed forms via the photocyclization reaction has been indicated by the growth of the new low-energy absorption bands at about 480−620 nm (Figure 5 and Figure S8−S11).1−17,20−22 Isosbestic points are observed in the UV−visible absorption spectral traces, indicating a clean photochemical transformation between the open forms and the closed forms, in which no obvious formation of byproducts was found in the 1H NMR spectra of 2 after photoexcitation (Figure S12). The photocycloreversion can be triggered by photoirradiation with green light at around 530 nm of the absorption bands of the closed forms, during which the reversal of the spectral changes is observed. In

Figure 2. Cyclic voltammograms for the (a) oxidation and (b) reduction scans of 1, 3 and 5 in CH2Cl2 (0.1 M nBu4NPF6).

extended π system into the phosphole backbone from compound 1 to 3, and to 5, indicating a reduction in the HOMO−LUMO energy gap from 1 to 3, and to 5. Compounds 1−6 are found to mainly absorb at about 300− 500 nm with moderate intensity in benzene solution (Figure 3a). The absorption bands are assigned as the intraligand π → π* transition, with the possible mixing of an intramolecular charge transfer transition from the peripheral bis-thienyl units to the central thieno[3,2-b]phosphole oxide moieties. A gradual bathochromic shift of the lowest-energy absorption maxima of the open forms with an energy trend of 1 (385 nm) > 2 (410 nm) ≈ 3 (414 nm) > 5 (421 nm) with increasing extent of πconjugation has been observed. Upon excitation at about 320− 15144


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Journal of the American Chemical Society

Figure 4. Photochromic reactions and color changes of 1, 2 and 6 in degassed benzene solution and 2 in PMMA thin film upon light excitation at 298 K.

Figure 6. Electronic absorption spectra of the closed forms of 1−6 in degassed benzene solution at 298 K.

Table 2. Photophysical Data of 1−6 in Degassed Benzene Solution at 298 K absorption compound

configuration

1

Open form Closed form Open form Closed form Open form Closed form Open form Closed form

2 3 4

5 6

Open form Closed form Open form Closed form

emission

λmax (nm) (ε/dm3 mol−1 cm−1)

λem (nm)a

ϕlumb

385 (5900) 361 (39500), 483 (4910) 333 (8100), 410 (6990) 371 (47600), 483 (5940) 327sh (7920), 414 (9750) 374 (45200), 485 (5310) 310sh (27500), 419 (9540) 306 (37300), 374 (55400), 420 (8940),490 (6250) 342 (6560), 421 (16400) 386 (54400), 496 (6610) 310sh (37300), 411 (13300) 320 (21700), 411 (36100), 542 (11700), 573 (11400)

523

0.064

543

0.032

542

0.048

562

0.011

552

0.056

645

0.006c

a

Corrected emission maxima. bRelative luminescence quantum yield is measured at room temperature using quinine sulfate in 0.5 M H2SO4 as the reference. c[Ru(bpy)3]Cl2 in degassed aqueous solution is used as the standard at room temperature.

the closed forms of 5 (496 nm) to 6 (573 nm) has been observed when the diphenylamino substituents are introduced onto the bis-thienyl rings, while a slight perturbation from 2 (483 nm) to 4 (490 nm) has been found when a diphenylamino substituent is incorporated into the backbone. Furthermore, the incorporation of extended π-conjugated systems into the phosphole backbone results in a larger bathochromic shift of the absorption maxima of the open forms than the closed forms, so that the photocyclizations of the open forms can be initiated by visible light upon increasing the πconjugation on the backbone. More importantly, the photochromic quantum yields of the thieno[3,2-b]phosphole oxide derivatives 1−6 are found to be much higher than those of previously reported visible light photoswitches,8−11 in which the introduction of π-conjugated systems into the photoactive diaryl units usually leads to the extremely low photochromic quantum yields, (ϕO→C and ϕC→O < 0.01) and even results in the loss of photochromic activities. For 2, the photocyclization and photocycloreversion quantum yields upon the respective photoirradiation of violet (ca. 407 nm) and green (ca. 500 nm) light are found to be 0.87 and 0.44 (Table 3), respectively, which are even greater than the reported fused-heterocycle-

Figure 5. UV−vis absorption spectral changes of (a) 2 and (b) 6 in degassed benzene solution upon visible light excitation at around 420 nm.

addition, the emission intensity of 1−6 is found to drop dramatically during the photocyclization reactions, as depicted in Figure S13−S18. The electronic absorption spectra of the closed forms of 1−6, with their lowest-energy absorption maxima ranging from 483−573 nm, are shown in Figure 6. The photophysical data of the open and closed forms are tabulated in Table 2. Apart from degassed benzene solutions, the representative compound 2 also displays photochromism in nondegassed acetonitrile, 50% water−acetonitrile mixture and in poly(methyl methacrylate) (PMMA) thin film under ambient conditions; the UV−visible absorption spectral changes of which are shown in Figure S19−S21. A significant bathochromic shift of the lowest-energy absorption maxima of 15145


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Journal of the American Chemical Society Table 3. Photochromic Data of 1−6 in Degassed Benzene Solution at 298 Ka photochromic quantum yield (ϕ)b

compound

photocyclization

1 2 3 4 5 6

0.50 0.87 0.77 0.35 0.23 0.35

c

photocycloreversion 0.10 0.44 0.43 0.042 0.084 0.087e

d

conversion at photostationary state (PSS) (%)f 88 51 85 78 83 84

Data with an uncertainty of ±10%. bFerrioxalate was used as the chemical actinometer. c407 nm was used as the excitation wavelength. d If not specified, 500 nm was used as the excitation wavelength. e530 nm was used as the excitation wavelength. f410 nm was used as the excitation wavelength. a

containing diarylethenes that commonly exhibit photocyclization and photocycloreversion quantum yields of less than 0.50 and 0.10, respectively.1−17,20−22 In addition, the excellent fatigue resistance and thermal irreversibility of the thieno[3,2b]phosphole oxides have been attributed to the direct attachment of photoactive bis-thienyl units to the phosphole backbone with weak aromaticity. 2 displays no apparent loss of photochromic activity over ten photochromic cycles initiated by excitation of alternate visible violet and green light, as depicted in Figure 7a, in which the photogenerated closed form can be photochemically converted back to open form completely. Interestingly, 2 also displays excellent fatigue resistance without severe loss of photochromic reactivity over five photochromic cycles in nondegassed acetonitrile, 50% water−acetonitrile solution and in PMMA thin film under ambient conditions (Figure S22−S24). Furthermore, 2 has been demonstrated to exhibit excellent thermal irreversibility in nitrogen-flushed 1,2dichlorobenzene solution, in which the thermal backward reaction of the closed form to the open form is observed to be insignificant (2000 min), as shown in Figure 7b. The half-life of the photogenerated closed form is estimated to be 398 days at room temperature with a thermal decay rate constant of 1.21 × 10−6 min−1. A long-term stability to the photomodulated properties can be provided given the superior thermal stability of the closed form of 2. Computational Studies. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been performed to gain further insights into the electronic structures and the origins of the electronic transitions of the open and closed forms of 1−6. The HOMO of the open forms corresponds to the π orbital of the peripheral bis-thienyl pendant units with mixing of the π orbital of the heterocycle-fused thieno[3,2-b]phosphole backbones, except for diphenylamine-containing 4 and 6, in which the HOMO mainly corresponds to the π orbital of the diphenylaminoaryl moieties, while the LUMO of the open forms corresponds to the π* orbital of the heterocycle-fused thieno[3,2-b]phosphole backbones, as shown in Figure 8a−b, respectively. The HOMO and LUMO of the closed forms are the π and π* orbitals that are delocalized over the condensed ring of the heterocyclefused thieno[3,2-b]phosphole systems, as depicted in Figure 9a−b, respectively. On the basis of the TDDFT calculations (Table S4 and S5), the S0 → S1 transition mainly corresponds to the HOMO → LUMO excitation for both the open and the

Figure 7. (a) UV−vis absorption changes of 2 in degassed benzene solution at 500 nm on alternate excitation at 410 and 500 nm over ten cycles at 298 K. (b) Thermal decay plot of ln(At/A0) versus time for the closed form of 2 monitored at 500 nm at 25 and 100 °C in nitrogen-flushed 1,2-dichlorobenzene solution. A0 and At denote the respective absorbance at time zero and t; theoretical linear fits are represented by solid lines. The expanded thermal decay plot at 25 °C is shown in the inset.

Figure 8. Spatial plots (isovalue = 0.03) of the HOMO and LUMO of the open forms of (a) 2 and (b) 6.

closed forms, in which the transition is assigned as π → π* transition. More importantly, upon increasing the π-conjugation on the phosphole backbone, a narrowing of the HOMO− LUMO gap of the open form has been achieved, with 5 (3.57 eV) < 3 (3.65 eV) < 1 (3.87 eV), by the lowering of the energy of the LUMO and the raising of the energy of the HOMO 15146


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2,3-Bis(2,5-dimethylthiophen-3-yl)-1-phenylthieno[3,2-b]-phosphole-1-oxide (1). 1,2-Bis(2,5-dimethylthiophen-3-yl)ethyne (123 mg, 0.50 mmol), phenyl(thiophen-2-yl) hydrophosphine oxide (208 mg, 1.00 mmol, 2 equiv), silver(I) oxide (232 mg, 1.00 mmol, 2 equiv) and distilled DMF were stirred at 100 °C in the dark overnight. The reaction mixture was cooled down to room temperature, followed by dilution using ethyl acetate (40 mL) and filtration to remove the insoluble solid. Deionized water (50 mL) was used to wash the organic layer, which was subsequently dried and filtered. The solvent was removed under reduced pressure to dryness. Column chromatography with silica gel (70−230 mesh) was then employed to purify the crude product by using hexane−ethyl acetate mixture as eluent. Recrystallization from a concentrated solution of 1 by layering hexane afforded a yellow solid. Yield: 72 mg, 0.16 mmol; 32%. 1H NMR (400 MHz, DMSO-d6, 358 K, δ/ppm) δ 1.79 (s, 3H, −CH3), 1.97 (s, 3H, −CH3), 2.27 (s, 3H, −CH3), 2.43 (s, 3H, −CH3), 6.52 (s, 1H, thienyl), 6.86 (s, 1H, thienyl), 7.29−7.31 (m, 1H, thienyl), 7.45−7.49 (m, 2H, phenyl), 7.54−7.63 (m, 3H, phenyl), 7.72−7.73 (m, 1H, thienyl). 31P{1H} NMR (162 MHz, DMSO-d6, 298 K, δ/ppm) δ 26.46. HRMS (Positive ESI) calcd for C24H21OP32S3 m/z = 452.0492, found 452.0468 [M]+. Elemental analyses calcd (%) for C24H21OPS3·0.5MeOH: C 62.80, H 4.95; found (%): C 62.97, H 4.70. 2,3-Bis(2,5-dimethylthiophen-3-yl)-1-phenylbenzo[b]thieno[3,2b]phosphole-1-oxide (2). This was prepared by a similar synthetic procedure as that for 1 except phenyl(benzo[b]thiophen-2-yl) hydrophosphine oxide (210 mg, 0.81 mmol) was used in place of phenyl(thiophen-2-yl) hydrophosphine oxide. Yield: 57 mg, 0.11 mmol; 27%. 1H NMR (400 MHz, DMSO-d6, 358 K, δ/ppm) δ 1.85 (s, 3H, −CH3), 2.00 (s, 3H, −CH3), 2.29 (s, 3H, −CH3), 2.46 (s, 3H, −CH3), 6.57 (s, 1H, thienyl), 6.94 (s, 1H, thienyl), 7.38−7.43 (m, 2H, phenyl), 7.45−7.50 (m, 2H, phenyl), 7.55−7.58 (m, 1H, phenyl), 7.64−7.73 (m, 3H, phenyl), 8.06−8.08 (m, 1H, phenyl). 31P{1H} NMR (162 MHz, DMSO-d6, 298 K, δ/ppm) δ 28.97. Positive EI-MS, m/z 502. HRMS (Positive EI) calcd for C28H23OP32S3 m/z = 502.0649, found 502.0638 [M]+. Elemental analyses calcd (%) for C28H23OPS3·0.5H2O: C 65.73, H 4.73; found (%): C 65.48, H 4.45. 2,3-Bis(2,5-dimethylthiophen-3-yl)-1-phenylthieno[2,3-d]thieno[3,2-b]phosphole-1-oxide (3). This was prepared by a similar synthetic procedure as that for 1 except phenyl(thieno[3,2-b]thiophen-2-yl) hydrophosphine oxide (208 mg, 0.85 mmol) was used in place of phenyl(thiophen-2-yl) hydrophosphine oxide. Yield: 57 mg, 0.11 mmol; 26%. 1H NMR (400 MHz, DMSO-d6, 358 K, δ/ ppm) δ 1.84 (s, 3H, −CH3), 2.00 (s, 3H, −CH3), 2.28 (s, 3H, −CH3), 2.44 (s, 3H, −CH3), 6.54 (s, 1H, thienyl), 6.91 (s, 1H, thienyl), 7.47− 7.54 (m, 3H, phenyl and thienyl), 7.57−7.60 (m, 1H, phenyl), 7.64− 7.70 (m, 3H, phenyl). 31P{1H} NMR (162 MHz, DMSO-d6, 298 K, δ/ ppm) δ 25.83. Positive EI-MS, m/z 508. HRMS (Positive EI) calcd for C26H21OP32S4 m/z = 508.0213, found 508.0200 [M]+. Elemental analyses calcd (%) for C26H21OPS4·0.5MeOH: C 60.66, H 4.42; found (%): C 60.73, H 4.17. 2,3-Bis(2,5-dimethylthiophen-3-yl)-1-phenyl-7-(diphenylamino)benzo[b]thieno[3,2-d]phosphole-1-oxide (4). This was prepared by a similar synthetic procedure as that for 1 except phenyl(5(diphenylamino)benzo[b]thiophen-2-yl) hydrophosphine oxide (273 mg, 0.64 mmol) was used in place of phenyl(thiophen-2-yl) hydrophosphine oxide. Yield: 61 mg, 0.09 mmol; 28%. 1H NMR (400 MHz, DMSO-d6, 358 K, δ/ppm) δ 1.84 (s, 3H, −CH3), 1.99 (s, 3H, −CH3), 2.28 (s, 3H, −CH3), 2.45 (s, 3H, −CH3), 6.55 (s, 1H, thienyl), 6.91 (m, 1H, thienyl), 6.98−7.00 (m, 4H, phenyl), 7.03−7.08 (m, 3H, phenyl), 7.17−7.18 (m, 1H, phenyl), 7.25−7.29 (m, 4H, phenyl), 7.42−7.47 (m, 2H, phenyl), 7.52−7.59 (m, 3H, phenyl), 7.93−7.95 (m, 1H, phenyl). 31P{1H} NMR (162 MHz, DMSO-d6, 298 K, δ/ppm) δ 26.66. HRMS (Positive ESI) calcd for C40H32NOP32S3 m/z = 669.1384, found 669.1355 [M]+. Elemental analyses calcd (%) for C40H32NOPS3·0.5MeOH: C 70.92, H 5.00, N 2.04; found (%): C 71.17, H 4.79, N 1.95. 2,3-Bis(2,5-dimethylthiophen-3-yl)-1-phenylbenzo[4,5]thieno[2,3-d]thieno[3,2-b]phosphole-1-oxide (5). This was prepared by a similar synthetic procedure as that for 1 except phenyl(benzo[4,5]thieno[2,3-d]thiophen-2-yl) hydrophosphine oxide (326 mg, 1.04

Figure 9. Spatial plots (isovalue = 0.03) of the HOMO and LUMO of the closed forms of (a) 2 and (b) 6.

(Table S6), with the trend in good agreement with results from the electrochemical and photophysical measurements. The spatial plots of the HOMO and LUMO of the other thieno[3,2b]phosphole oxide derivatives are summarized in Figure S25 and S26.

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CONCLUSIONS In conclusion, photochromic thieno[3,2-b]phosphole oxides have been rationally designed and developed as novel visible light photoswitchable materials with excellent photochromic properties. Furthermore, the promising performances such as excellent thermal irreversibility, robust fatigue resistance as well as high photocyclization and photocycloreversion quantum yields have been demonstrated by a molecular design strategy, in which extended π-conjugated systems are incorporated into the weakly aromatic phosphole backbone to lower the HOMO−LOMO energy gaps of the open forms to extend the excitation wavelength to the visible region. This design strategy outperforms the more widely adopted method of introducing the π systems into the photoactive diaryl units that often results in rather low switching efficiency. This work would provide guiding principles for the future developments of visible light-triggered photoswitches, offering less destructive photocontrollable functions for potential applications of photoresponsive electronics and photoactive pharmacological agents. Further developments of the photochromic phospholes with versatile functions to address multifunctional photomodulated materials are now in progress.

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EXPERIMENTAL SECTION

Materials and Reagents. N,N-Dimethylformamide (DMF, Arkonic Scientific, AR) was purified by distillation over calcium hydride, followed by deaeration with prepurified nitrogen gas before used. Thiophene-containing phenyl hydrophosphine oxides and diarylacetylenes were prepared using a modified literature procedure.20,24 All reagents (analytical grade) were used without further purification. Synthesis of Thieno[3,2-b]phosphole Oxides 1−6. All the compounds were prepared by a modified literature procedure for the synthesis of benzo[b]phosphole oxide derivatives using diphenyl hydrophosphine oxides and diarylacetylenes with silver(I) oxide under anhydrous condition using standard Schlenk technique.23 15147


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average values measured just before and after each photolysis experiment of ferrioxalate actinometry.25a,27 In the determination of photochromic quantum yields of 1−6, the sample solutions with absorbance slightly greater than 2.0 at the excitation wavelength were prepared and were subjected to photoirradiation.25a,27 The photocyclization and photocycloreversion quantum yields were measured at a small percentage of conversion by observing the initial rate of absorbance changes (ΔA/Δt) at the low-energy absorption maxima of the closed form. The percentage conversions of the closed forms at their photostationary states (PSS) were quantified using a combination of 1H NMR and UV−vis spectroscopic studies of irradiated sample with known concentration. Computational Details. Gaussian 09 software package was used for the calculation.28 The ground-state geometries of 1−6 including the open and the closed forms were fully optimized in benzene solution by using DFT calculations at the PBE0 level29 in conjunction with the conductor-like polarizable continuum model (CPCM) using benzene as the solvent.30 Only the photochromic-active open forms with antiparallel conformation was considered. Vibrational frequencies were calculated for all stationary points to verify that each was a minimum (NIMAG = 0) on the potential energy surface. On the basis of the ground state optimized geometries, time-dependent density functional theory (TDDFT) method31 at the same level associated with CPCM (benzene) was employed to compute the singlet−singlet transitions in the electronic absorption spectra of the open and closed forms of 1−6. For all the calculations, the 6-31G(d,p) basis set was employed to describe all the atoms.32 The DFT and TDDFT calculations were performed with a pruned (99 590) grid for numerical integration.

mmol) was used in place of phenyl(thiophen-2-yl) hydrophosphine oxide. Yield: 64 mg, 0.11 mmol; 21%. 1H NMR (400 MHz, DMSO-d6, 358 K, δ/ppm) δ 1.86 (s, 3H, −CH3), 2.02 (s, 3H, −CH3), 2.29 (s, 3H, −CH3), 2.47 (s, 3H, −CH3), 6.57 (s, 1H, thienyl), 6.98 (s, 1H, thienyl), 7.42−7.46 (m, 1H, phenyl), 7.48−7.53 (m, 3H, phenyl), 7.58−7.62 (m, 1H, phenyl), 7.68−7.73 (m, 2H, phenyl), 8.00−8.05 (m, 2H, phenyl). 31P{1H} NMR (162 MHz, DMSO-d6, 298 K, δ/ ppm) δ 25.68. Positive EI-MS, m/z 558. HRMS (Positive EI) calcd for C30H23OP32S4 m/z = 558.0366, found 558.0369 [M]+. Elemental analyses calcd (%) for C30H23OPS4: C 64.49, H 4.15; found (%): C 64.64, H 4.20. 2,3-Bis(5-(N,N-diphenylamino)-2-methylthiophen-3-yl)-1-phenylbenzo[4,5]thieno[3,2-d]thieno[2,3-b]phosphole-1-oxide (6). This was prepared by a similar synthetic procedure as that for 5 except 1,2-bis(2-methyl-5-(N,N-diphenylamino)-thiophen-3-yl)ethyne (115 mg, 0.21 mmol) was used in place of 1,2-bis(2,5-dimethylthiophen3-yl)ethyne. Yield: 50 mg, 0.058 mmol; 28%. 1H NMR (400 MHz, DMSO-d6, 358 K, δ/ppm) δ 2.06 (s, 3H, −CH3), 2.16 (s, 3H, −CH3), 6.41 (s, 1H, thienyl), 6.86 (s, 1H, thienyl), 6.94−6.97 (m, 4H, phenyl), 7.03−7.07 (m, 3H, phenyl), 7.09−7.12 (m, 5H, phenyl), 7.26−7.29 (m, 4H, phenyl), 7.32−7.36 (m, 4H, phenyl), 7.42−7.53 (m, 4H, phenyl), 7.58−7.62 (m, 1H, phenyl), 7.65−7.71 (m, 2H, phenyl), 8.00−8.05 (m, 2H, phenyl). 31P{1H} NMR (162 MHz, DMSO-d6, 298 K, δ/ppm) δ 25.97. Positive EI-MS, m/z 864. HRMS (Positive EI) calcd for C52H37ON2P32S4 m/z = 864.1526, found 864.1512 [M]+. Elemental analyses calcd (%) for C52H37ON2PS4: C 72.19, H 4.31, N 3.24; found (%): C 72.15, H 4.38, N 3.34. Physical Measurements and Instrumentation. 1H NMR and 31 1 P{ H} NMR spectra were recorded by a Bruker DPX-400 (400 MHz) Fourier transform NMR spectrometer and a Bruker AVANCE 400 (162 MHz) Fourier transform NMR spectrometer, respectively. Tetramethylsilane (Me4Si) and 85% phosphoric acid (H3PO4) were used as standard for the determination of the chemical shifts (δ, ppm) of 1H and 31P{1H} NMR, respectively. Electron impact (EI) and electrospray ionization (ESI) mass spectra were recorded on a Thermo Scientific DFS High Resolution Magnetic Sector mass spectrometer and Bruker maXis II High Resolution Liquid Chromatography Quadrupole-Time of Flight (LC-QTOF), respectively. A Carlo Erba 1106 elemental analyzer was employed for the determination of the elemental analyses of the new compounds (Institute of Chemistry, Chinese Academy of Sciences, Beijing.) If not specified, samples for photophysical and photochromic measurements were degassed with no less than four freeze−pump−thaw cycles on the high-vacuum line prior to measurements. A Varian Cary 50 UV−vis spectrophotometer was used for UV−vis absorption measurements. The monochromatic light was generated by passing the light source from 300 W Oriel Corporation Model 60011 Xe (ozone-free) lamp through an Applied Photophysics F 3.4 monochromator. The kinetics experiments of the thermal backward reaction of the closed form of 2 at various temperatures were carried out by using a Varian Cary 50 UV−vis spectrophotometer with a single cell Peltier thermostat. Spex Fluorolog-3 Model FL3-211 spectrofluorometer with a R2658P PMT detector was used for the measurements of the steady-state emission spectra of 1−6. Optical dilute method was used for the determination of relative luminescence quantum yields using an aqueous solution of quinine sulfate in 0.5 M sulfuric acid (ϕlum = 0.546, λex = 365 nm) or a degassed aqueous solution of [Ru(bpy)3]Cl2 (ϕlum = 0.042, λex = 410 nm) as the standard at 298 K.25,26 A CH Instrument, Inc. model CHI620 electrochemical analyzer was used for the cyclic voltammetric measurements. Electrochemical properties of 1−6 were studied in deaerated dichloromethane solution in the presence of 0.1 M nBu4NPF6. A glassy carbon (CH Instrument) electrode and a Ag/AgNO3 (0.1 M in acetonitrile) electrode were used as the working electrode and reference electrode, respectively. The counter electrode was a platinum wire that was separated from the glassy carbon (CH Instrument) electrode by a sintered-glass frit in the electrolytic cell. The internal reference was the ferrocenium/ferrocene couple (FeCp2+/0).26 Ferrioxalate actinometry was used for the determination of photochemical quantum yields.25a The intensities of incident light at different wavelength were obtained from the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08333. Crystal structure determination, selected bond lengths and angles; UV−vis and emission spectral changes; fatigue resistance study; 1H and 31P{1H} NMR spectra; computational details; selected singlet−singlet transitions and orbital energies, and Cartesian coordinates of optimized structures; characterization (PDF) Crystal data (CIF) Crystal data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Vivian Wing-Wah Yam: 0000-0001-8349-4429 Author Contributions †

M. Ng and W. H. Lam contributed equally to the computational studies.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V. W.-W. Y. acknowledges UGC funding administered by The University of Hong Kong for supporting the Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry Facilities under the Support for Interdisciplinary Research in Chemical Science, and the support from the URC Strategic Research Theme on New Materials of The University of Hong Kong. This work was supported by the French National Research Agency (ANR)/Research Grants Council (RGC) Joint Research Scheme (A-HKU704/12), the Research Grants 15148


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Council (RGC) General Research Fund (GRF) of the Hong Kong Special Administrative Region, P. R. China (HKU 17305614) and the University Grants Committee (UGC) Areas of Excellence Scheme (AoE/P-03/08). N. M.-W. W. acknowledges the receipt of a postgraduate studentship from The University of Hong Kong. We are grateful to Ms. H.-S. Chan at The Chinese University of Hong Kong for the assistance in X-ray crystal structure data collection and determination, and the Information Technology Services (ITS) of The University of Hong Kong for providing computational resources.

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