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Abstract

A novel aldehyde-substituted acceptor-DTE-acceptor-type dithienylethene derivative, in which the difluoroboron β-diketonate and aldehyde groups at the termini of the dithienylethene function as electron acceptors, has been developed. The structure was confirmed by ¹H NMR, ¹³C NMR, ¹⁹F NMR, high-resolution mass spectrometry (electrospray ionization) and infrared. The derivative displays solvent-dependent photophysical properties, visible light–triggered near-infrared photochromism, and fluorescent switching behavior in solvents of different polarity. Moreover, it can be utilized as a versatile building block to construct novel near-infrared photochromic materials.

Keywords

aldehydes difluoroboron β-diketonate dithienylethene near-infrared photochromism visible light

Introduction

In recent years, photochromic materials have drawn tremendous attention due to their application in photoswitchable molecular devices and optical memory storage systems.^1,2 Photochromic compounds can undergo reversible photochemical reactions by irradiation with light.^3–5 Among them, dithienylethene has been extensively studied by virtue of its irreversible thermal stability, high sensitivity, and remarkable fatigue resistance.^6–11 However, one serious restriction for dithienylethene compounds in future applications is the necessity to trigger photochromism by ultraviolet (UV) light, which is often damaging to cells and penetrates only partially through most media in vivo.¹² Thus, there is a need to develop near-infrared (NIR) photochromic materials which are triggered by visible light for future applications in optoelectronics and photopharmacology.^13–16 In the past few decades, significant progress has been made in visible-light-controlled photochromic compounds.^17,18 The most commonly used method to achieve visible light switches is to extend the π-conjugation system,^19–21 which can reduce the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) energy gap of the ring-opened isomer and hence achieve NIR photochromic activity.²² Recently, several alternative approaches have also been explored, such as metal complex systems,^23,24 energy up-conversion,²⁵ electron transfer,²⁶ multiphoton absorption,²⁷ and donor-DTE-acceptor systems.²⁸ Furthermore, the introduction of electron-withdrawing groups on both sides of the dithienylethene provides a versatile route to fabricate acceptor-DTE-acceptor (A-DTE-A)-type photochromic materials, in which the acceptor group can reduce the energy of the ring-opened isomer and make bathochromical shift absorbance.²⁹ However, we still know very little about these photochromic compounds, thus searching for new scaffolds with visible light–triggered dithienylethenes with A-DTE-A structures is highly desirable.

Difluoroboron β-diketonate (BF₂bdk) complexes are a class of highly fluorescent organoboron compounds which have received increasing attention due to their intriguing photophysical properties, such as strong emission in solution and solid state, two-photon-excited fluorescence, large extinction coefficients, intramolecular charge transfer (ICT) character, and environment-sensitive emission.^30–34 In addition, the aldehyde group as an electron acceptor (A) unit is widely used in photoelectric materials. Furthermore, it can also function as a classical functional group for further transformations, such as reduction, oxidation, aldol condensation, and Knoevenagel condensation. Herein, we present a novel aldehyde-substituted A-DTE-A-type dithienylethene 1 (Scheme 1), in which BF₂bdks group function as both fluorophore and electron acceptor (A), and the aldehyde group acts as electron acceptor (A) and provides a versatile reaction site for the construction of novel NIR photochromic materials. As expected, dithienylethene 1 displayed solvent-dependent photophysical properties, visible light–triggered NIR photochromism and fluorescent switching behavior in solvents of different polarity.

Scheme 1.

Synthetic route and design rationale of dithienylethene 1.

Result and discussion

Synthesis and characterization

The stepwise synthesis of aldehyde- and BF₂bdk-substituted A-DTE-A-type dithienylethene derivative 1 is outlined in Scheme 1. 1,2-Bis-(5-chloro-2-methylthien-3-yl) cyclopent-1-ene 2 as the starting material was treated with n-BuLi followed by the addition of N,N-dimethylformamide (DMF) to give 4,4′-(cyclopent-1-ene-1,2-diyl)bis(5-methylthiophene-2-carbaldehyde) (3). Subsequently, the target dithienylethene 1 was obtained by a Knoevenagel condensation reaction between di-CHO intermediate 3 and 1-phenylbutane-1,3-dione (4) in a yield of 52%. To avoid Knoevenagel condensation at the C-2 atom of 1-phenylbutane-1,3-dione (4), the β-diketone moiety had to be fixed into the enol form by the formation of a BF₂ complex between 1-phenylbutane-1,3-dione (4) and BF₃. Therefore, 4 is first reacted with BF₃ at 65 °C for 2 h, followed by successive addition of 3, B(OBu)₃ and n-BuNH₂, in which B(OBu)₃ was used to scavenge water generated during the reaction, while n-BuNH₂ is a typical base used as a catalyst for this type of reaction. The structure of the target dithienylethene 1 was characterized by standard spectroscopic methods (¹H NMR, ¹³C NMR, ¹⁹F NMR, high-resolution mass spectrometry (HRMS), and infrared (IR)). In the ¹H NMR spectrum of dithienylethene 1, the coupling constants of the signals at ca. 8.09 and 6.40 ppm were 16.0 Hz, revealing the trans-form of the vinyl group.

Photophysical properties

The photophysical properties of dithienylethene 1 in solvents with different polarity were first investigated before visible light irradiation, as described in Figure 1 and Table 1. An absorption band at 460–476 nm appeared, which was ascribed to intramolecular π-π* transitions.³⁵ For example, the maximum absorption wavelength of dithienylethene 1 was located at 468 nm (ε = 6.25 × 10⁴ cm⁻¹ M⁻¹) in toluene, which showed an obvious bathochromic shift with increasing polarity of the solvent, reaching 476 nm in dimethyl sulfoxide (DMSO; Figure 1(a)). Accordingly, it exhibited distinct solvatochromic behavior (Figure 1(c)). As illustrated in Figure 1(b) and (d), it displayed strong fluorescence emission in a nonpolar or less polar solvent (such as toluene, CH₂Cl₂, tetrahydrofuran (THF), and EtOAc), and its fluorescence emission bands were red-shifted significantly on increasing the solvent polarity. For instance, 1 exhibited a strong green emission in toluene with the maximum emission wavelength at 528 nm, which was red-shifted to 559 nm in CH₂Cl₂ (orange fluorescence). However, very weak fluorescence emission was observed in acetone and DMSO. Meanwhile, its fluorescence quantum yields (Ф_F) in different solvents were determined using rhodamine 6G as reference, and it was found that the Ф_F values decreased on the increasing solvent polarity, from 0.48 in toluene to 0.01 in DMSO (Table 1), which may be due to a nonradiative decay process induced by the dipole–dipole interaction between excited molecules and the highly polar solvent. Consequently, dithienylethene 1 showed distinct solvent-dependent photophysical properties, which were attributed to an ICT transition from the dithienylethene moiety to the BF₂bdk group.

Figure 1.

(a) Normalized UV-Vis absorption spectra of 1 in different solvents (2.0 × 10⁻⁵ mol/L); (b) normalized fluorescence emission spectra of 1 (λ_ex = 460–476 nm) in different solvents (2.0 × 10⁻⁵ mol/L); (c) the corresponding solution color of 1 in different solvents before photoirradiation (Tol: toluene; EA: EtOAc; Ace: acetone); and (d) a fluorescent photograph of 1 showing the fluorescence in different solvents under 365 nm irradiation (Tol: toluene; EA: EtOAc; Ace: acetone).

Table 1.

The photophysical data of 1 in different solvents at 298 K (2.0 × 10⁻⁵ mol/L).

Solvent	λ_abs^a (nm)	ε × 10^{4 b} (cm⁻¹ M⁻¹)	λ_em^c (nm)	Φ_f^d
Toluene	468	6.25	528	0.48
CH₂Cl₂	474	7.32	559	0.45
THF	464	6.99	546	0.39
EtOAc	460	5.07	548	0.27
Acetone	464	7.03	562	0.08
DMSO	476	6.32	554	0.01

THF: tetrahydrofuran; DMSO: dimethyl sulfoxide.

Absorption maximum.

Extinction coefficients calculated at the absorption maxima.

Fluorescence emission maxima.

Fluorescence quantum yield determined by a standard method with rhodamine 6G in water (Φ_f = 0.75, λ_ex = 488 nm) as reference.

Subsequently, the photoisomerization behaviors of dithienylethene 1 in various solvents were explored at room temperature. It was found that 1 could undergo a photoisomerization reaction between the ring-opened isomer and the ring-closed isomer upon alternating the visible light irradiation between >402 and >600 nm, as illustrated in Scheme 1. It exhibited NIR photochromic behavior with a gradually increasing low-energy absorbance band at 670–708 nm in the different solvents. As shown in Figure 2(a), the absorption maximum of 1 in CH₂Cl₂ was observed at 474 nm (ε = 7.32 × 10⁴ cm⁻¹ M⁻¹). Upon irradiation with >402 nm visible light, a new low-energy absorption band centered at 704 nm (ε = 4.04 × 10⁴ cm⁻¹ M⁻¹) appeared concomitant with an obvious color change from yellow to green, due to the formation of the corresponding ring-closed isomer 1 (c). Furthermore, an obvious isosbestic point was observed at 528 nm, indicating that ring-opened isomer 1 (o) was cleanly turned into the photocyclized product. The green ring-closed isomer 1c underwent a cycloreversion reaction and returns to the initial 1 (o) upon irradiation with λ > 600 nm visible light. In particular, it showed good reversibility after repeating the photochromic process six times, indicating excellent fatigue resistance (Supplemental Figure S1). The cyclization and cycloreversion quantum yields of 1 were 0.64 (φ_o-c) and 0.0089 (φ_c-o) in CH₂Cl₂, respectively. Similar photochromic properties were obtained when other solutions of dithienylethene 1 (toluene, THF, EtOAc, acetone, and DMSO) were irradiated with the same visible light, as shown in Supplemental Figures S2–S10 and Table 2. Furthermore, the photochromic parameters for 1 in different solvents were compared, and it was found that dithienylethene 1 in a nonpolar or less polar solvents (toluene, CH₂Cl₂, THF, and EtOAc) reached photostationary state (PSS) more efficiently than that in highly polar solvents (acetone and DMSO). Accordingly, dithienylethene 1 displayed photochromic properties with solvent polarity dependence, which was in accordance with BF₂bdk-containing photochromic dithienylethenes reported recently.^36,37

Figure 2.

(a) Absorption spectral changes of dithienylethene 1 with >402 and >600 nm light irradiation in CH₂Cl₂ (2.0 × 10⁻⁵ mol/L) at 298 K; (b) normalized absorption spectra of ring-closed isomer 1 (c) in various solvents (2.0 × 10⁻⁵ mol/L); (c) corresponding color changes of dithienylethene 1 in various solvents upon photoirradiation (>402 nm) (Tol: toluene, EA: EtOAc, Ace: acetone).

Table 2.

Absorption characteristics and photochromic quantum yields of 1 in different solvents at 298 K.

Solvent	λ_abs^a (nm)	ε × 10^{4 b} (cm⁻¹ M⁻¹)	φ_o-c^c	φ_c-o^d
Toluene	688	1.98	0.51	0.0085
CH₂Cl₂	704	4.04	0.64	0.0089
THF	680	3.57	0.47	0.0076
EtOAc	670	2.34	0.43	0.0075
Acetone	690	1.17	0.17	0.0033
DMSO	708	0.39	0.03	0.0014

THF: tetrahydrofuran; DMSO: dimethyl sulfoxide.

Absorption maximum of ring-closed isomers.

Extinction coefficients calculated at the absorption maxima.

Quantum yields of ring-opened isomers.

Quantum yields of ring-closed isomers.

The photochromic behavior of dithienylethene 1 was examined by means of ¹H NMR spectrometry. As shown in Figure 3, the resonances of numerous protons in the ring-closed isomer 1 (c) showed an obvious changes in chemical shifts (such as those of H_b′, H_c′, H_d′, H_e′, H_g′, H_h′, and H_i′) upon irradiation with >402 nm visible light. For example, the proton signals of the trans-vinyl group linking the dithienylethene and BF₂bdk moieties for 1 (o) appeared at 6.40 ppm (H_b) and 8.09 ppm (H_c), while those for 1 (c) appeared at 6.48 ppm (H_b′) and 7.95 ppm (H_c′), respectively. Moreover, the photocyclization yield was 36% on reaching the PSS, according to ¹H NMR analysis.

Figure 3.

Partial ¹H NMR spectral changes of 1 upon irradiation with >402 nm visible light in CDCl₃ at room temperature.

The fluorescent switching behavior of dithienylethene 1 induced by visible light irradiation in the above solvents was also explored at room temperature. As illustrated in Figure 4(a), the emission intensity in CH₂Cl₂ gradually decreased along with an obvious orange fluorescence fading upon irradiation with λ > 402 nm light, which was due to the formation of the ring-closed isomer. The fluorescence intensity of 1 was quenched by ca. 89% on reaching the PSS. Furthermore, photoirradiation with >600 nm light returned the original emission as a result of the formation of the ring-opened isomer. Similar results were obtained when other solutions of 1 were irradiated with the same wavelength light (Supplemental Figures S11–S15). These results implied that dithienylethene 1 may be applied as a novel fluorescence switch in future optoelectronic materials.

Figure 4.

(a) Emission spectral changes of of dithienylethene 1 with >402 and >600 nm light irradiation in CH₂Cl₂ (2.0 × 10⁻⁵ mol/L) at 298 K and (b) the variation curve of fluorescent intensity of dithienylethene 1 upon irradiation with >402 nm light in CH₂Cl₂.

Conclusion

In summary, a novel aldehyde- and BF₂bdk-substituted dithienylethene derivative 1 with an A-DTE-A structure was synthesized. It exhibited solvent-dependent photophysical properties prior to irradiation. Moreover, it showed solvent-dependent NIR photochromic properties and fluorescent switching behavior with excellent fatigue resistance upon irradiation with visible light. Accordingly, dithienylethene 1 may be applied not only as a novel photochromic compound in future optoelectronic materials, but also as a versatile building block to construct NIR photochromic materials.

Experiments

General methods

All manipulations were carried out under a nitrogen atmosphere by standard Schlenk techniques unless otherwise stated. THF and toluene were distilled from sodium-benzophenone. DMF was dried with magnesium sulfate and then distilled under vacuum. 1,2-Bis(5-chloro-2-methylthiophen-3-yl)cyclopent-1-ene (2) and 4,4′-(cyclopent-1-ene-1,2-diyl)bis(5-methylthiophene-2-carbaldehyde) (3) were prepared by the reported methods.³⁸ All other starting materials were obtained commercially as analytical grade and used without further treatment. The relative quantum yields were determined by comparison with the known yields of the compound 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene.^{39
¹}H and ¹³C NMR spectra were collected on a Bruker AVANCE III 400 MHz spectrometer (all the chemical shifts are reported relative to tetramethylsilane (TMS)). High-resolution mass spectra were obtained on a SCIEX X500R QTOF (electrospray ionization (ESI) mode). All the absorption spectra were collected on a Shimadzu UV-2600 UV-Vis spectrophotometer, and the fluorescence spectra were obtained on a Hitachi Model F-4500 fluorescent spectrophotometer. In the photochromic experiments, the visible light irradiation experiment was carried out using a photochemical reaction apparatus with a 500-W Hg lamp and a CEL-HXF300 14 V 50 W xenon lamp with a cut-off filter.

Synthesis of (E)-(2-{5-[2-(2,2-difluoro-6-phenyl -2H-1λ, 3, 2λ-dioxaborinin-4-yl)vinyl}-2- methyl-thiophen-3-yl]cyclopent-1-en-1-yl)-5-methylthiophene-2-carbaldehyde (1)

A solution of 1-phenylbutane-1, 3-dione (4) (163 mg, 1.0 mmol) and BF₃·OEt₂ (0.19 mL, 1.5 mmol) in anhydrous toluene (4 mL) was stirred at 65 °C for 1 h under N₂. Next, a solution of 3 (316 mg, 1.0 mmol) in toluene (3 mL) was added followed by tributyl borate (0.54 mL, 2.0 mmol), and the mixture reacted for a further 30 min, then n-Butylamine (50 μL, 0.5 mmol) was added slowly, and the resulting solution was stirred at 65 °C for 12 h. After cooling to room temperature, the solvent was removed under vacuum. The crude product was purified by on a silica gel column using petroleum ether/dichloromethane (2:1 v/v) as the eluent to give the target compound 1 as an orange solid (yield: 52%); m.p.:173.2–174.6 °C. IR (KBr, cm⁻¹): 2920, 2846, 1740, 1648, 1537, 1458, 1384, 1078, 780, 640. ¹H NMR (400 MHz, CDCl₃): δ = 9.75 (s, 1H), 8.06–8.11 (m, 3H), 7.66 (t, J = 8.0 Hz, 1H), 7.51–7.54 (m, 2H), 7.43 (s, 1H), 7.14 (s, 1H), 6.56 (s, 1H), 6.40 (d, J = 16.0 Hz, 1H), 2.80–2.85 (m, 4H), 2.05–2.16 (m, 2H), 2.08 (s, 3H), and 2.01 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 182.46, 181.32, 180.25, 176.98, 175.51, 146.41, 143.43, 141.21, 140.14, 137.82, 137.53, 137.09, 136.19, 135.95, 134.93, 134.82, 132.02, 129.06, 128.67, 118.13, 38.41, 29.69, 22.87, 15.42, and 15.23. ¹⁹F NMR (376.5 MHz, CDCl₃): δ = −141.52 (d, J = 22.6 Hz, 2F). HRMS (ESI-TOF): m/z: [M + Na]⁺ calcd for C₂₇H₂₃BF₂O₃S₂Na: 531.1043; found: 531.1038.

Supplemental Material

Suppl_file – Supplemental material for Aldehyde-substituted acceptor-DTE-acceptor-type dithienylethene as a versatile building block for near-infrared photochromic materials

Supplemental material, Suppl_file for Aldehyde-substituted acceptor-DTE-acceptor-type dithienylethene as a versatile building block for near-infrared photochromic materials by Yangyang Wang, Ming Qin, Yufei Song, Yingying Pei, Yijie Dai, Yongjie Ma and Ziyong Li in Journal of Chemical Research

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors acknowledge financial support from the Key Scientific Research Project of Higher Education of Henan Province (grant nos. 18A150012 and 18A550012) and the Science and Technology Project of Henan Province (grant no. 182102310668).

ORCID iD

Ziyong Li

Supplemental material

Supplemental material for this article is available online.

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