Structure and photochromic properties of a novel diarylethene containing a 9-fluorenone hydrazone Schiff base moiety

Results and discussion

The synthetic route toward 1o is shown in Scheme 2. Condensation of diarylethene derivative 2 ¹⁴ and fluoren-9-ylidene-hydrazone resulted in the formation of diarylethene 1o in 66% yield.

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Scheme 2.

Synthetic route to 1o.

X-ray crystallographic analysis is helpful to elaborate relationships between the structural conformation and photochromic behavior in the crystalline phase. ¹⁵ , ¹⁶ The X-ray crystallographic analysis data are listed in Table 1, and the ORTEP (Oak Ridge Thermal Ellipsoid Plot) drawings and photochromic processes in the crystalline phase are shown in Figure 1. For 1o, the structure has a typical anti-parallel conformation, with a distance of 3.592 Å between the photoactive carbons (C10···C18), which meets the prerequisites for a molecule to undergo photochromism^6,7 from colorless to green in the crystalline phase.

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Table 1.

Crystallographic parameters of 1o.

	1o
Formula	C35H22F6N2S2
Formula weight	648.66
Temperature (K)	296(2)
Crystal system	Monoclinic
Space group	P21/n
Unit cell dimensions
a (Å)	7.7931(10)
b (Å)	28.972(4)
c (Å)	13.4514(18)
α (°)	90.00
β (°)	102.178(3)
γ (°)	90.00
Volume (Å3)	2968.7(7)
Z	4
Density (calcd) (g/cm3)	1.451
Goodness of fit on F2	1.096
Final R indices (I/2σ(I))
R 1	0.0859
ωR1	0.1906
R indices (all data)
R 1	0.1290
ωR1	0.2126

w = 1 / [ σ 2 ( F o 2 ) + ( 0 . 1626 P ) 2 + 1 . 8249 P ] , where P = ( F o 2 + 2 F c 2 ) / 3 .

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Figure 1.

ORTEP drawing of 1o (ellipsoids are drawn at the 50% probability level) and the photochromism in the crystalline phase.

Compound 1o includes four types of planar ring and they can give rise to three dihedral angles between every two adjacent planar rings. The dihedral angles between the central cyclopentene ring and the two adjacent thiophene rings are 34.7° and 46.9°, respectively. The dihedral angle between the thiophene ring and the adjacent benzene ring is 5.1°, indicating that the two fragments tend to coplanarity. ¹⁷ Moreover, the intermolecular hydrogen bonds C–H···F and C–H···S connect molecules with each other, which may strengthen the stability of the framework structure (Table 2).

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Table 2.

Hydrogen bond lengths (Å) and bond angles (°).

D–H···A	D–H	H···A	D···A	D–H···A
C(5)–H(5A)···S(1)	0.93	2.70	3.115(6)	108
C(20)–H(20A)···F(6)	0.93	2.43	2.911(7)	112
C(25)–H(25A)···N(1)	0.93	2.50	2.972(8)	111

D: donor group; A: acceptor group.

Diarylethene 1o showed reversible photochromic properties upon alternating irradiation with UV and visible light in methanol, as shown in Figure 2. Irradiation of 1o with 297 nm UV light resulted in a new absorption band centered at 601 nm due to the formation of the closed-ring isomer 1c, with a color changed from colorless to blue. The blue color was entirely bleached upon irradiation with visible light (λ > 510 nm), and the absorption spectrum returned to its initial state 1o.

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Figure 2.

Absorption spectra and color change of 1o with stimulation by light in methanol (2.0 × 10⁻⁵ mol L⁻¹).

Diarylethene 1o possessed excellent thermal stability as a solution in methanol and in the crystal phase. Storing methanol solutions in the dark in the presence of air after 30 days revealed no changes in the UV-Vis spectra of either 1o or 1c. No decomposition was detected when crystals of 1o were exposed to air for more than 2 years, and the crystal still maintained normal photochromism. In addition, the thermal cycloreversion rate of 1c in methanol was evaluated by UV-Vis spectroscopy at 298, 308, and 318 K, respectively. As shown in Figure 3(a), the maximum intensity of 1c decreased by 1% at 298 K, 6% at 308 K, and 14% at 318 K over 50 min. Therefore, the thermal cycloreversion rate increased with the increment of temperature. The fatigue resistance of 1 was examined in methanol at room temperature and the results are presented in Figure 3(b). The cyclic change in absorption upon alternating irradiation with UV and visible light was repeated for nearly 2 h for 1o, and no obvious decomposition was detected by UV-Vis absorption spectroscopy.

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Figure 3.

(a) The thermal fading of 1c at 298, 308, and 318 K, respectively. (b) The absorbance at 601 nm as a function of time during multiple forward and backward conversion cycles initiated from the open-ring isomer in methanol (2.0 × 10⁻⁵ mol L⁻¹).

The fluorescence of 1o in methanol (2.0 × 10⁻⁵ mol L⁻¹) was measured at room temperature. When excited at 335 nm, the emission peak of 1o was observed at 471 nm. As observed for most of the reported diarylethenes, ³ compound 1o showed notable fluorescent switching properties in methanol (2.0 × 10⁻⁵ mol L⁻¹). Upon irradiation with 297 nm light, the emission intensity of 1o notably decreased due to the formation of the weakly fluorescent closed-ring isomer 1c. The back irradiation with appropriate visible light recovered the original emission intensity. As shown in Figure 4, the emission intensity of 1o was quenched to ca. 31% in the photostationary state. Thus, the fluorescent modulation efficiency of 1o in the photostationary state was 69%.

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Figure 4.

Emission intensity changes of 1 upon irradiation with 297 nm UV light in methanol (2.0 × 10⁻⁵ mol L⁻¹) excited at 335 nm.

Conclusion

A 9-fluorenone hydrazone Schiff base moiety was introduced to form a novel diarylethene derivative. The novel diarylethene was structurally characterized by X-ray diffraction analysis. The compound possessed excellent thermal stability and fatigue resistance, and it showed excellent photo/switching properties in methanol and in the single crystalline phase. Moreover, the results reported here are expected to be helpful for investigation of the interaction between similar structures and metal ions or anions.

Experimental

The melting point was measured with a WRS-1B melting point apparatus. Mass spectra were measured using a Bruker amaZon SL spectrometer. Infrared spectra were recorded on a Bruker Vertex-70 spectrometer. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AV-400 spectrometer with tetramethylsilane (TMS) as an internal reference and CDCl₃ as the solvent. Elemental analysis was measured with a PE CHN 2400 analyzer. Absorption spectra were obtained with an Agilent 8453 UV-Vis spectrophotometer. Photo-irradiation was carried out with an SHG-200 UV lamp, a CX-21 UV fluorescence analysis cabinet, and a BMH-250 visible lamp. The required wavelength was isolated using an appropriate light filter. The fluorescence properties was measured with a Hitachi F-4600 spectrophotometer. Chemical reagents were purchased from Acros Corp. and used without further purification. All solvents used were spectroscopic grade and were purified by distillation before use. Reactions were monitored by analytical thin-layer chromatography on plates coated with 0.25 mm silica gel 60 F254. Crystals of 1o were obtained by slow evaporation from a dichloromethane–hexane co-solvent system. It was mounted on the top of glass fiber. X-ray diffraction data (Table 1) were collected on a Bruker SMART-1000 APEX II CCD diffractometer equipped with graphite-monochromated MoKα radiation (0.71073 Å) using a ω/φ scan mode at 296(2) K. The structure was solved by direct methods and refined by full-matrix least-squares on F² values using the program SHELXL-2014. ¹⁸ , ¹⁹ All non-hydrogen atoms were subjected to anisotropic refinement. Hydrogen atoms were generated theoretically and ridden on their parent atoms in the final refinement. For the full-matrix least-squares refinements (I > 2σ(I)), the unweighted and weighted agreement factors of R 1 = ∑ ( F o − F c ) / ∑ F o and w R 2 = [ ∑ w ( F o 2 − F c 2 ) 2 / ∑ w F o 4 ] 1 / 2 were used. Further details on the crystal structures have been deposited in the Cambridge Crystallographic Data Centre as supplemental publication CCDC 1867793 for 1o. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033 or email: deposit@ccdc.cam.ac.uk).

1-(2-methyl-5-phenyl-3-thienyl)-2-{2-methyl-5-(9-fluorenone-hydrazone)-3-thienyl}perfluorocyclopentene (1o): Compound 2 (0.14 g, 0.30 mmol) was dissolved in ethanol (25 mL) and the resulting solution treated fluoren-9-ylidene-hydrazone (0.058 g, 0.30 mmol). The mixture was refluxed for 6 h until no compound 2 was detected by thin-layer chromatography (TLC). After the solvent was removed by vacuum evaporation to afford a yellow solid in 66% yield. The title compound crystallized from hexane–dichloromethane co-solvent at room temperature producing the colorless crystals suitable for X-ray analysis. M.p.: 178−180°C. ¹H NMR (400 MHz, CDCl₃, TMS), δ (ppm): 1.91 (s, 3H, –CH₃), 2.00 (s, 3H, –CH₃), 7.21−7.25 (m, 4H, thiophene-H, phenyl-H), 7.30−7.37 (m, 5H, phenyl-H), 7.48 (d, 2H, J = 7.6 Hz, phenyl-H), 7.54 (t, 2H, J = 8.0 Hz, phenyl-H), 7.82 (d, 1H, J = 8.0 Hz, phenyl-H), 8.41 (d, 1H, J = 7.6 Hz, phenyl-H), 8.61 (s, 1H, –C=N); ¹³C NMR (100 MHz, CDCl₃, TMS), δ (ppm): 13.52, 14.10, 118.84, 118.88, 121.26, 122.05, 124.65, 127.03, 127.11, 127.28, 128.04, 129.93, 130.30, 130.53, 130.86, 132.23, 135.79, 136.99, 140.71, 141.53, 141.66, 146.22, 152.61, 160.52; infrared (IR) (ν, KBr, cm⁻¹): 1702 (–C=N), high-resolution mass spectrometry (HRMS), ESI⁺ m/z 649.1189 (MH⁺, C₃₅H₂₂F₆N₂S₂ found 648.1129).

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