Effects of the Peripheral Heteroaryl Substituents on the Photochromism of New Pyridine-Containing Diarylethenes

Three new unsymmetrical pyridine-containing diarylethenes with a variable peripheral heteroaryl unit were synthesized and their structures were determined by single-crystal X-ray diffraction analysis. The substituent effects of variable heteroaryl groups on the photochromic and fluorescence properties were systematically discussed. Thienyl and thiazyl could effectively enhance the cyclization quantum yields and photoconversion ratios in the photostationary state, as compared to the phenyl group. However, they had no obvious effect on the fluorescence behaviors. Moreover, X-ray structural analysis revealed that the N...S heteroatom-contact interactions resulted in a higher cyclization quantum yield, which may be ascribed to the effective stabilizing ability on the photoreactive formations in diarylethenes with a thienyl/thiazyl unit.


Introduction
][6][7] Generally, changes in heteroaryl moieties result in significant modification of the photochromic behaviors of photochromic diarylethenes.To date, the design and synthesis of diarylethene derivatives have mainly focused on the variation of two heteroaryl moieties, which stimulated character diversity of this class of molecules. 1or example, diarylethenes having two thiophene moieties exhibit outstanding thermal stability and outstanding fatigue resistance; 1,8 whereas symmetrical diarylethenes having two pyrrole groups are thermally unstable and return to the open-ring isomers even in the dark. 9In the case of six-membered heteroaryl rings, diarylethenes having two phenyl/naphthyl groups are thermally reversible with poor photochromism, 10,11 but some unsymmetrical diarylethenes having a six-membered heteroaryl ring are thermally irreversible with good photochromism. 12,138][19] In previous works, we reported some diarylethenes with a pyridine moiety directly linked at the central cyclopentene moiety and elucidated their specific photoactive characteristics. 20The results obtained have contributed to a broad understanding of the photochromism of pyridine-containing diarylethene derivatives.
Apart from the fact that heteroaryl rings have significant effects on the photochromic characteristics of diarylethenes, various functional substituents and their substituted positions have also remarkable effects on their properties, which can be used for the fine-tuning of the optoelectronic properties of photochromic diarylethenes.2][23] From these publications, it can be concluded that the reported substituents are usually simple functional groups.In this study, using variable heteroaryl rings (benzene, thiophene, and thiazole) as peripheral groups, we have synthesized three new pyridine-containing diarylethenes to elucidate the substituent effects of heteroaryl groups on the photochromic behaviors of these diarylethene derivatives (Figure 1).As far as we are aware, this is the first report where various heteroaryl rings were used as peripheral substituents to study the substituent effects on the photochromism of diarylethenes.

General methods
Chemical reagents were purchased from J&K Scientific Ltd. and used without further purification.All solvents used were spectroscopic grade and were purified by distillation before use.All solution-phase reactions were performed under an atmosphere of dry argon or nitrogen.Reactions were monitored by analytical thin-layer chromatography on plates coated with 0.25 mm silica gel 60 F254 and Al 2 O 3 .High-performance liquid chromatograpy (HPLC) analysis was performed by an Agilent 1200 instrument fitted with a ZORBAX SIL column (5 µm particle size, 4.6 mm, 25 cm); the wavelength in the detector of HPLC analysis was 297 nm.Melting points were measured with a WRS-1B melting point apparatus.Infrared (IR) spectra were recorded on a Bruker Vertex-70 spectrometer.Nuclear magnetic resonance (NMR) spectra were obtained with a Bruker AV-400 spectrometer with tetramethylsilane (TMS) as internal reference and CDCl 3 as solvent.Elemental analysis was carried out with a PE CHN 2400 analyzer.Mass spectra (MS) were measured with an Agilent 1100 ion trap MSD spectrometer.Absorption spectra were obtained with an Agilent 8453 UV-Vis spectrometer.Photoirradiation was carried out with an SHG-200 UV lamp, a CX-21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible lamp.The light intensity was 25 µW cm -2 and 13.5 mW cm -2 for UV and Vis lamps, respectively.The required wavelength was isolated by the use of the appropriate light filters.Fluorescence properties were measured with a Hitachi F-4600 spectrophotometer.X-Ray experiments of single-crystal were performed on a Bruker SMART APEX2 CCD area-detector equipped with graphite monochromatized Mo Ka radiation at room temperature (294 ± 2 K).Quantum yield was measured using 1,2-bis(2-methyl-5-phenyl-3-thienyl) perfluorocyclopentene as references for the cyclization and the cycloreversion reactions, respectively.The absorbances of the sample and reference at the irradiation wavelength (297 nm) were adjusted to be the same for the cyclization quantum yield measurement.The reaction rates of the sample and reference were measured in the same conditions and compared.For the cyclization quantum yield measurement, absorbance (A) at the absorption maximum of the closed-ring isomer was plotted against time.For the cycloreversion quantum yield measurement, -log A at the absorption of the irradiated wavelength was plotted against time.The measurement was carried out three times, and the value was determined by averaging.

2,6-Dibromo-3,5-dimethylpyridine (4)
To a s t i r r e d o l e u m s o l u t i o n ( 4 0 m L ) o f 3,5-dimethylpyridine (5.35 g, 50 mmol) at 273 K was added dropwise Br 2 (5.64 mL, 110 mmol).Stirring continued for 0.5 h at 273 K. Then the reaction mixture was heated to 353 K and refluxed for 36 h.The solution was cooled to room temperature, poured into water slowly, and neutralized by aqueous NaOH.After being extracted with dichloromethane, the organic layer was washed with water, and dried over anhydrous Na 2 SO 4 .The crude product was purified by column chromatography on silica gel using dichloromethane as eluent to afford 4 (10.71g, 80%) as a white solid.m.p. 102-103 °C; 1 H NMR (400 MHz, CDCl 3 ) d 7.28 (s, 1H, pyridine-H), 2.24 (s, 6H, -CH 3 ).
A stirred THF solution of 6 (0.2 g, 0.38 mmol), phenylboronic acid (0.068 g, 0.56 mmol), Pd(PPh 3 ) 4 , and Na 2 CO 3 was refluxed for 24 h under argon atmosphere.The reaction was slowly cooled to room temperature, and concentrated in vacuo.After being extracted with ethyl acetate, the organic layer was dried over anhydrous Na 2 SO 4 .The crude product was purified by column chromatography on Al 2 O 3 using petroleum ether as eluent and afforded 1o (0.12 g, 40%) as a colorless crystal.m.p. 122-123 °C; 1   A stirred anhydrous toluene solution of 6 (0.2 g, 0.38 mmol), 2-tributylstannylthiophene (1.42 g, 3.8 mmol) and Pd(PPh 3 ) 4 was refluxed for 24 h under argon atmosphere.The reaction was slowly cooled to room temperature, and concentrated in vacuo.After being extracted with ethyl acetate, the organic layer was dried over anhydrous Na 2 SO 4 .The crude product was purified by column chromatography on Al 2 O 3 using petroleum ether as eluent and afforded 2o (0.68 g, 34%) as a colorless crystal.m.p. 157-158 °C; 1   A stirred anhydrous toluene solution of 6 (0.2 g, 0.38 mmol), 2-tributylstannylthiazole (1.58 g, 4.3 mmol) and Pd(PPh 3 ) 4 was refluxed for 24 h under argon atmosphere.The reaction was slowly cooled to room temperature, and concentrated in vacuo.After being extracted with ethyl acetate, the organic layer was dried over anhydrous Na 2 SO 4 .The crude product was purified by column chromatography on Al 2 O 3 using petroleum ether as eluent and afforded 3o (0.21 g, 34%) as a colorless crystal.m.p. 150-151 °C; 1

Photochromism
Figure 3 shows the absorption spectral changes of diarylethene 1 and color changes of diarylethenes 1-3 induced by alternating irradiation with UV light (297 nm) and visible light (λ > 500 nm) in hexane.Upon irradiation at 297 nm UV light, the colorless solution of diarylethene 1o turned purple quickly and a new absorption band centered at 581 nm emerged due to the formation of the closed-ring isomer 1c.The colored solution of 1c was bleached entirely upon irradiation with visible light (λ > 500 nm) and its absorption spectrum returned to the initial state 1o.Similarly, diarylethenes 2 and 3 also exhibited favorable photochromism in hexane (Figure S1).Upon irradiation with 297 nm light, absorption bands in visible region appeared and the colorless solutions of 2o and 3o turned blue as a result of producing the closed-ring isomers 2c and 3c, respectively.In photostationary state, the absorption maxima were observed at 618 nm for 2c and 623 nm for 3c.The colored solutions of 2c and 3c could be decolorized upon irradiation with visible light (λ > 500 nm).When reaching photostationary state, the isosbestic points were observed at 302 nm for 1o, 320 nm for 2o, and 320 nm for 3o, indicating that there exists only a two-component photochromic reaction between the open-ring and the closed-ring isomers of these diarylethene derivatives. 24,25he photoconversion ratios of 1-3 from open-ring to closed-ring isomers were determined by HPLC analysis in photostationary state, with values of 73% for 1o, 86% for 2o, and 87% for 3o (Figure 4).In addition, 1-3 have remarkable fatigue resistance and good thermal stability.In hexane, the coloration and decoloration cycles of 1-3 could repeat more than 40 times with the degradation of 9% for 1c, 7% for 2c, and 5% for 3c (Figure S2).The thermal stability of 1o-3o and 1c-3c was checked in hexane at room temperature and at 341 K.The results showed that no decomposition was detected by UV-Vis spectroscopy for all of the hexane solutions.The thermal decay profiles of 1c at various temperatures are shown in Figure 5.The results indicated that the absorbance decay of 1c at the absorption maxima obeyed the first-order thermal cycloreversion kinetics.The thermal fading changes of 2c and 3c were also measured in the range of 298-343 K (Figure S3).According to the Arrhenius equation, the activation energies were determined as 18.7 kJ mol -1 for 1c, 38.4 kJ mol -1 for 2c, and 36.2 kJ mol -1 for 3c.The half-lifetimes at 298 K were 56 h for 1c, 126 h for 2c, and 67 h for 3c, respectively.Moreover, the thermal cycloreversion rates of 1c-3c were distinctly different at 298 K, and the values gradually increased in going from thienyl to thiazyl to phenyl groups.For each compound, its thermal cycloreversion rate accelerated with the increment of temperature.
The photochromic parameters of diarylethenes 1-3 in hexane are summarized in Table 1.It was clearly found that the different peripheral heteroaryl substituents had a significant effect on the photochromic properties of these diarylethenes, including the absorption maxima, molar absorption coefficients, quantum yields of cyclization and cycloreversion, and photoconversion ratios.For the three derivatives, the absorption maxima of both their open-ring and closed-ring isomers increased in the order of phenyl < thienyl < thiazyl substituent, and their molar absorption coefficients increased in the order of phenyl < thiazyl < thienyl substituent.The quantum yields of cyclization and   cycloreversion were measured using 1,2-bis(2-methyl-5phenyl-3-thienyl)perfluorocyclopentene as a reference. 26s shown in Table 1, the cyclization quantum yields of diarylethenes with a five-membered peripheral heteroaryl group (such as in compounds 2 and 3) notably enhanced, and their cycloreversion quantum yields decreased slightly, as compared to diarylethene with a six-membered peripheral heteroaryl group (such as in compound 1).In photostationary state, the photoconversion ratios of 2 and 3 were also much higher than that of 1.The results revealed that using five-membered heteroaryl rings (such as compounds 2 and 3) as peripheral substituents could effectively enhance the cyclization quantum yields and photoconversion ratios of these diarylethene derivatives.

Crystal structures and photochromism in the crystalline phase
Single crystals of 1o-3o were obtained via slow evaporation of their individual hexane solution, and the final structural conformations were provided by X-ray crystallographic diffraction analysis.The Oak Ridge thermal ellipsoid plot (ORTEP) drawings and photochromic processes in the crystalline phase are shown in Figure 6; the crystal data are summarized in Supplementary Information.1o-3o were packed in anti-parallel conformations in their individual crystalline phases.The corresponding dihedral angles and the distances between the reactive carbon atoms of 1o-3o are summarized in Table 2.The dihedral angle between pyridine and adjacent benzene was 64.1° for 1o, the dihedral angle between pyridine and adjacent thiophene was 17.7° for 2o and that between pyridine and adjacent thiazole was 5.6° for 3o, revealing that the coplanarity notably enhanced by the order of 1o < 2o < 3o.Consequently, the p-conjugation extension of the aromatic systems gradually increased in going from 1o < 2o < 3o, resulting in an evident increment in their absorption maxima.The distances between the two reactive carbons were 3.688 Å for 1o, 3.563 Å for 2o, and 3.651 Å for 3o, which were short enough for the occurring of photocyclization reaction in the hexatriene unit. 27,28As expected, the three crystals exhibited reversible photoisomerization reactions by photoirradiation in the crystalline phases.Upon irradiation with UV light, the colorless crystals of 1o-3o turned into the colored crystals 1c-3c.Reversely, the colored crystals could be bleached into the corresponding colorless ones upon irradiation with appropriate visible light.In addition, the X-ray structural analysis indicates that weak intramolecular interactions exist in their individual crystalline phases.The distances between N1 and adjacent F atoms were estimated to be 2.994 Å for 1o, 2.857 Å for 2o, and 2.889 Å for 3o, which are less than the sum of the van der Waals radii of the N (1.55 Å) and F (1.47 Å) atoms. 29The interatomic distances between N1 and S2 atoms were estimated to be 2.812 Å for 2o and 2.796 Å for 3o, which are also less than the sum of the van der Waals radii of the N (1.55 Å) and S (1.80 Å) atoms.The results revealed that the N … F and N … S heteroatom-contact interactions existed in 2o and 3o, which could be useful for stabilizing their photoreactive formations in both crystalline and solution phases. 29The experimental evidence also elucidated that the higher cyclization quantum yields of 2 and 3, compared to that of 1 (no N … S contact), were a result the synergistic effect of both N … F (conformational aspect) and N … S (energetic aspect) interactions. 20,29

Fluorescence
As observed for most of the reported diarylethenes, [30][31][32] diarylethenes 1-3 exhibited notable fluorescent switches by photoirradiation.The fluorescence changes of diarylethene 1 in hexane (5.0 × 10 -5 mol L -1 ) are shown in Figure 7.When excited at 300 nm, the emission peak of 1o was observed at 446 nm.Upon irradiation with 297 nm light, the emission intensity of 1o decreased notably due to the formation of the closed-ring isomer 1c.Reversely, the back irradiation with appropriate visible Table 1.Photochromic parameters of 1-3 in hexane (2.0 × 10 -5 mol L -1 ) at room temperature Compound λ o,max / nm a (ε / (L mol -1 cm -1 )) λ c,max / nm b (ε / (L mol -1 cm -1 )) light regenerated the open-ring isomer 1o and recovered the original emission intensity.When photostationary state was reached, the fluorescent modulation efficiency was 67% for 1.Just like 1, 2 and 3 also showed remarkable fluorescent switches in hexane upon alternating irradiation with UV and visible light (Figure S4); when excited at 300 nm, the emission peaks of 2o and 3o were observed at 439, and 444 nm, respectively.In the photostationary state, the fluorescent modulation efficiencies were 69% for 2, and 73% for 3.The residual fluorescence of 1-3 in photostationary state may be attributed to the incomplete cyclization reaction and the existence of parallel conformation. 13The results indicated that the heteroaryl unit linked to the pyridine moiety had no obvious effect on the fluorescence behavior of these diarylethene derivatives.a q 1 : dihedral angle between benzene and thiophene; q 2 : dihedral angle between thiophene and cyclopentene; q 3 : dihedral angle between cyclopentene and pyridine; q 4 : dihedral angle between pyridine and adjacent benzene, thiophene, thiazole, respectively.d: Distance between the reacting carbon atoms.Compared to the pyridine-containing diarylethenes with the similar molecular skeleton, 15 the fluorescent modulation efficiency of the three diarylethenes with a peripheral heteroaryl group enhanced notably during the process of photoisomerization, indicating that the diarylethenes are ideal for practical applications in certain fluorescent switching devices. 4,33,34

Conclusions
In summary, three new pyridine-containing diarylethenes with a variable peripheral heteroaryl unit have been synthesized to elucidate the effects of the peripheral heteroaryl substituents on the photochromic properties.These compounds exhibited notable photochromism and acted as an evident fluorescence switch in solution.Our experimental results showed that the cyclization quantum yields were enhanced by introducing thienyl and thiazyl to the 6-position of the pyridine moiety, which was ascribed to the N … S heteroatom-contact interactions.The results will provide a useful strategy for the design of new diarylethenes with tunable photochromic behaviors.

Figure 5 .
Figure 5. Thermal fading of 1c in hexane at various temperatures.

Table 2 .
Distances between the reacting carbon atoms and dihedral angles of 1o-3o