Synthesis and the effect of alkyl chain length on optoelectronic properties of diarylethene derivatives

Photochromic symmetrical diarylethene derivatives 1a-6a bearing different long alkyl chains at 2-position of thiophene rings have been synthesized and their structures have been determined by single-crystal X-ray diffraction analysis. The effect of alkyl chain length on their optoelectronic properties, such as photochromism in solution as well as in the crystalline phase and electrochemical performance was investigated in detail. These diarylethenes have showed good photochromic behavior both in solution and in the single crystalline phase. Introduction of the long alkyl chains at 2-position of bis(5-formyl-3-thienyl)perfluorocyclopentene increased the absorption coefficients of both open- and closed-ring isomers and induced bathochromic shifts of the maximal wavelength absorption of the closed-ring isomers. The long alkyl chains can also decrease the cyclization/cycloreversion quantum yields and the oxidation potentials. The cyclic voltammetry indicated that the band gap of these diarylethene derivatives was significantly affected by the alkyl chain length.     

Synthesis and photochromic reactivity of a diarylethene dimer linked by a phenyl group

A diarylethene dimer linked by a phenyl group was synthesized and the photochromic behavior was examined. Upon irradiation with ultraviolet light (λ=313 nm), a hexane solution of the diarylethene dimer (1a) turned purple blue. Upon further prolonged irradiation the color changed to blue. The purple-blue and blue colors are due to the formation of a dimer having one open- and one closed-ring forms (1b) and a dimer having two closed-ring forms (1c), respectively. Both 1b and 1c returned to 1a by irradiation with visible light (λ>500 nm). The photochromic reactivity was evaluated by measuring quantum yields of the photocyclization and photocycloreversion reactions. The photocyclization quantum yield was 0.50. The cycloreversion quantum yield from 1c to 1b (0.0026) was lower than that from 1b to 1a (0.0094).     

Synthesis and photochromic properties of novel yellow developing photochromic compounds

Two 1-thiazolyl-2-thienylcyclopentene derivatives, 1a and 2a, and a 1-thiazolyl-2-vinylcyclopentene derivative 3a have been synthesized in an attempt to obtain photochromic compounds which change the color from colorless to yellow, and have low photocycloreversion quantum yields and high absorption coefficients of the colored isomers. All of these compounds underwent reversible photochromic reactions. Compounds 1a and 2a in toluene solutions changed the color upon 313 nm light irradiation from colorless to orange and pink, in which absorption maxima were observed at 494 nm (ε=10,000 M⁻¹ cm⁻¹) and 525 nm (ε=8500 M⁻¹ cm⁻¹), respectively. On the other hand, the colorless toluene solution of 3a turned yellow upon irradiation with 313 nm light, in which the absorption maximum was observed at 416 nm (ε=17,100 M⁻¹ cm⁻¹). The photocyclization/cycloreversion quantum yields of 3 were 0.19 and 0.0014, respectively. The conversion from the open- to the closed-ring isomer of 3 in the photostationary state under irradiation with 313 nm light was close to 100%.     

Systematic study on the thermal cycloreversion reactivity of diarylethenes with alkoxy and alkyl groups at the reactive carbons

The relationship between the thermal cycloreversion reactivity of diarylethenes and the bulkiness of the substituents at the reactive carbons was systematically investigated. Two photochromic diarylethenes, 1,2-bis(2-isobutoxy-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-neopentoxy-5-phenyl-3-thienyl)perfluorocyclopentene (2a), were newly synthesized and their optical properties and thermal cycloreversion reactivity were examined, because there is insufficient data for diarylethenes with alkoxy groups at the reactive carbons. The steric substituent constant was employed to correlate the relationship between the thermal cycloreversion reactivity of diarylethenes with alkyl and alkoxy groups at the reactive carbons and the bulkiness of the substituent. A good correlation was obtained for the substituent constant using CH₂ instead of oxygen in the alkoxy groups. The results indicate that this is a very useful strategy for the design of novel diarylethenes with desired thermal cycloreversion reactivity.     

New photochromic diarylethenes with a six-membered aryl unit

A new class of photochromic diarylethenes bearing a six-membered aryl unit has been developed. They underwent reversible cyclization and cycloreversion reactions upon alternating irradiation with UV and visible light both in solution and in PMMA film. However, diarylethenes 1, 2, and 4 showed no photochromism in the crystalline phase although they packed with an anti-parallel conformation and the distances between the two reactive carbon atoms were both less than 4.2 Å.     

Substituent effects on the properties of photochromic diarylethenes

Photochromic symmetrical diarylethenes 1o-5o bearing different electron-donating or electron-withdrawing substitutents have been synthesized, and the structures of 1o, 2o, 4o, and 5o were determined by single-crystal X-ray diffraction analysis. Substitutent effects on their optoelectronic properties, including photochromism, fluorescence, and electrochemical properties were investigated in detail. The electron-withdrawing substituents can shift significantly the absorption maxima of the diarylethenes to a longer wavelength and increase their cyclization quantum yield, while the molar absorption coefficients increased with an increasing electron-donating ability. Diarylethenes 1, 2, and 4 show good photochromism both in solution and in the single crystalline phase; however, diarylethenes 5 show no photochromism in the crystalline phase because the distance between the reactive carbons become larger than 4.2 Å. Diarylethenes 1-3 exhibited good fluorescent switching upon alternating irradiation with UV and visible light, and their fluorescent conversions in the photostationary state were all larger than 80% in hexane. In addition, cyclic voltammetry tests showed that different electron-donating and electron-withdrawing substituents had a remarkable effect on the electrochemical behaviors of these diarylethenes.     

Syntheses and optoelectronic properties of four photochromic dithienylethenes

Four photochromic dithienylethene compounds, 1,2-bis(2-methyl-5-naphthalene-3-thienyl)perfluorocyclopentene 1a, 1,2-bis[2-methyl-5(p-fluorophenyl)-3-thienyl]perfluorocyclopentene 2a, 1,2-bis[2-methyl-5(p-ethoxyphenyl)-3-thienyl]perfluorocyclopentene 3a, and 1,2-bis[2-methyl-5(p-N,N-dimethylaminophenyl)-3-thienyl]perfluorocyclopentene 4a were synthesized, and their optoelectronic properties, such as photochromism in solution as well as in poly-methylmethacrylate (PMMA) amorphous films, fluorescences and electrochemical properties were investigated in detail. These dithienylethenes have shown good photochromic behavior both in solution and in PMMA amorphous film. All of them exhibited relatively strong fluorescence and gave a bathochromic shift upon increasing concentration in THF. The irreversible anodic oxidation of 1a, 2a and 4a was observed by performing cyclic voltammetry experiments.     

Synthesis, crystal structures, and photochromic properties of 6,6′ or 7,7′ or 6,7′-dimethyl-[2,2′-bi-1H-indene]-3,3′-diethyl-3,3′-dihydroxy-1,1′-diones

New photochromic title compounds 1, 2, and 3 have been prepared starting from 4-methylphthalic anhydride. Compounds 3a and 3b are a pair of enantiomers and were obtained as a racemic mixture (numbered as 3). Compounds 1, 2, and 3 were successfully separated from the isomeric mixture product through fractional crystallization, and their structures are confirmed by X-ray crystallographic analysis. UV-vis absorption and photochromic properties of 1, 2, and 3 have also been investigated. Results reveal that the substituents, even like the simple methyl, on the benzene rings of biindenylidenedione could considerably affect the photochromic property, as well as other properties of this kind of compounds.     

Asymmetric 1,3-dipolar reactions of 3-sulfinylfuran-2(5H)-ones with 11H-dibenzo[b,e]azepine 5-oxide. Synthesis of pyrroloazepines via isoxazoloazepines

The addition of morphanthridine N-oxide (1) to homochiral 3-p-tolylsulfinylfuran-2(5H)-ones (2a and 2b) under mild conditions affords furoisoxazoloazepines (3a and 3b) in high yields and with complete regioselectivity. The π-facial and endo-selectivities are also complete from 2a, which yields anti-3a-endo as the only diastereoisomer, whereas cycloreversion determines that the anti-3b-endo adduct can be almost exclusively isolated from 2b. Proper manipulation of the furoisoxazoloazepines allows the synthesis of the optically pure isoxazoloazepines and pyrroloazepines.     

New cyano-substituted organic dyes containing different electrophilic groups: aggregation-induced emission and large two-photon absorption cross section

Three aggregation-induced emission active dyes (3a–c) were synthesized and their one- and two-photon absorption properties have been investigated. They were all found to be weakly fluorescent in THF solution, while they exhibited dramatic fluorescence enhancement in water/THF mixtures. The solid fluorescence of 3a–c was recorded and their fluorescence quantum efficiency (Φ_F) values were determined to be 8.0%, 8.1%, and 16.4%, respectively. Moreover, the two-photon absorption (2PA) cross-sections (σ) of 3a–c were measured and 3a showed the highest value of 702 GM. The excellent aggregation-induced emission and 2PA properties provide a promising alternative for biophotonic materials.     

Synthesis and photochromic properties of quinoxalino[e]-annelated dimethyldihydropyrene with planar chirality

9,10-Diacetyl-2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene annelated with a quinoxaline unit (q-DM-DHP, 3) was prepared from 5,13-di-tert-butyl-8,16-dimethyl[2.2]metacyclophane-1,2,9,10-tetraoxide (2) and o-phenylenediamine in two steps. The photochromic isomers of q-DM-DHP 3, closed form 3a and open form 3b, were separated by reverse-phase HPLC, and their isolated UV–visible spectra were estimated. The chiral HPLC analyses of q-DM-DHP 3 indicated their planar chirality, and their CD spectra were measured. The photochromic properties and thermodynamic properties of 3 were also discussed based on their UV–visible, ¹H NMR, CD, and fluorescence spectra.     

Functionalized BF2 chelated azadipyrromethene dyes

Fluorescent molecules that emit in the near infrared are potentially useful as probes for biotechnology. A relatively under-explored design for probes of this type are the aza-BODIPY dyes; this study was performed to enhance our understanding of these materials and ways in which they may be used in dye cassette systems. Thus, the aza-BODIPY dyes 1a-g were prepared. An advanced intermediate toward an eighth compound in the series, 6h, was made but it could not be complexed with boron effectively to give 1h. Spectroscopic properties of these compounds were recorded, and correlations between substituent effects, UV absorbance, fluorescence emissions, and quantum yields were made. Compound 1a was coupled with a fluorescein-alkyne derivative to give the energy transfer cassettes 2 and 3. Both these compounds gave poor energy transfer and the possible reasons for this were discussed.     

Photophysical and electrochemical properties of π-extended molecular 2,1,3-benzothiadiazoles

The reaction of 4,7-dibromo-2,1,3-benzothiadiazole with arylboronic acids (phenyl, 1-naphthyl, 4-methoxyphenyl, 4-chlorophenyl and 4-trifluoromethylphenyl) in the presence of catalytic amounts of a NCP-pincer palladacycle affords photoluminescent π-extended 4,7-diaryl-2,1,3-benzothiadiazoles 4a-e in high yields. These 4,7-diaryl-2,1,3-benzothiadiazoles exhibit high fluorescent quantum yields, high electron affinities and adequate band gap values for testing as OLEDs. The 4,7-bis-naphthyl-2,1,3-benzothiadiazole 4b presents two different lifetimes (bi-exponential decay) due to the presence of two atropisomers. The Sonogashira coupling reaction of 4,7-diethynyl-2,1,3-benzothiadiazole 6 with the corresponding halo-aryl compounds (iodobenzene, 1-bromonaphthalene, 4-iodoanisole, 4-bromo-N,N-dimethylaniline and 2-bromopyridine) afforded the photoluminescent π-extended 4,7-bis-alkynylaryl-2,1,3-benzothiadiazoles 7a-e, also in high yields. These 4,7-diethynyl-2,1,3-benzothiadiazoles also present high fluorescent quantum yields, high electron affinities and adequate band gap values for testing as OLEDs. The 4,7-disubstituted-2,1,3-benzothiadiazoles 4a-e and 7a-e exhibit different electrochemical behavior. The presence of two ethynyl spacers in 2,1,3-benzothiadiazoles 7a-e shifts the reduction potentials to less cathodic values and also results in two well-defined and distinct reduction processes.     

Synthesis and characterization of 2H-pyrano[3,2-c]coumarin derivatives and their photochromic and redox properties

A series of 2H-pyrano[3,2-c]chromen-5-one derivatives were synthesized and characterized. Their photochromic and redox properties were investigated by the UV-vis absorption spectroscopy. While compounds with one or two phenyl groups incorporated at the 2-position were present in both ring-opened (5a and 10a) and ring-closed (6a and 11a) forms, the incorporation of an N,N-dimethylamino group on either side of the aromatic ring resulted in formation of the ring-opened (5b and 10b) forms only. The ring-closed forms 13 and 18 with a methyl substituent at the 3-position of the pyran moiety failed to exhibit photochromic behavior. Compound 23 with an N,N-dimethylamino group on the aromatic ring displayed increasing shoulder absorption in the visible region and a distinct change of color upon UV irradiation. The non-fluorescent 10b instantly changed from dark red to colorless, when treated with sodium borohydride. The reduced 28 was blue fluorescent with a quantum yield of 0.46 and could be returned to its original color via DDQ oxidation.     

Synthesis of polyfunctionalized furans from 3-acetyl-1-aryl-2-pentene-1,4-diones

The BF₃-catalyzed cyclization of 3-acetyl-1-aryl-2-pentene-1,4-diones 1a-e in the presence of water in boiling tetrahydrofuran gave bis(3-acetyl-5-aryl-2-furyl)methanes 2a-e in 26-79% yields along with a small amount of 3-acetyl-5-aryl-2-methylfurans 3a-e. The exact structure of 2a was determined by X-ray crystallography. The use of a half volume of the solvent for the reaction of 1a resulted in the formation of 2,4-bis(3-acetyl-5-phenyl-2-furfuryl)-3-acetyl-5-phenylfuran (4) together with 2a and 3a. A similar reaction of 1a was carried out in the presence of 3-acetyl-5-(4-methylphenyl)-2-methylfuran (3d) to afford 4-(3-acetyl-5-phenyl-2-furfuryl)-3-acetyl-5-(4-methylphenyl)-2-methylfuran (5) in 49% yield. The BF₃-catalyzed reaction of 1a with 2,4-pentanedione in dry tetrahydrofuran at 23°C gave 3-(3-acetyl-5-phenyl-2-furfuryl)-4-hydroxy-3-penten-2-one (6a) and 3-(3-acetyl-2-methyl-4-phenyl-5-furyl)-4-hydroxy-3-penten-2-one (7a) in 66 and 24% yields, respectively. The product distribution depended on the reaction temperature. A similar reaction of 1b-e also yielded the corresponding trisubstituted furans 6b-e and tetrasubstituted furans 7b-e in good yields. These results suggested the presence of the furfuryl carbocation intermediate A during the reaction. The one-pot synthesis of 6a and 7a was also achieved by a similar reaction using phenylglyoxal. The deoxygenation of 1a with triphenylphosphine gave 3a in 88% yield, while 1a was treated with concentrated hydrochloric acid to yield 3-acetyl-2-chloromethyl-5-phenylfuran (8) which was quantitatively transformed in ethanol into 3-acetyl-2-ethoxymethyl-5-phenylfuran (9) and in water into 3-acetyl-5-phenylfurfuryl alcohol (10), respectively. In addition, the Diels-Alder reaction of cyclopantadiene with 1a gave the corresponding [4+2] cycloaddition products 11 and 12.     

Reactions of azulenes with 1,2-diaryl-1,2-ethanediols in methanol in the presence of hydrochloric acid: comparative studies on products, crystal structures, and spectroscopic and electrochemical properties

Although reaction of guaiazulene (1a) with 1,2-diphenyl-1,2-ethanediol (2a) in methanol in the presence of hydrochloric acid at 60 °C for 3 h under aerobic conditions gives no product, reaction of 1a with 1,2-bis(4-methoxyphenyl)-1,2-ethanediol (2b) under the same reaction conditions as 2a gives a new ethylene derivative, 2-(3-guaiazulenyl)-1,1-bis(4-methoxyphenyl)ethylene (3), in 97% yield. Similarly, reaction of methyl azulene-1-carboxylate (1b) with 2b under the same reaction conditions as 1a gives no product; however, reactions of 1-chloroazulene (1c) and the parent azulene (1d) with 2b under the same reaction conditions as 1a give 2-[3-(1-chloroazulenyl)]-1,1-bis(4-methoxyphenyl)ethylene (4) (81% yield) and 2-azulenyl-1,1-bis(4-methoxyphenyl)ethylene (5) (15% yield), respectively. Along with the above reactions, reactions of 1a with 1,2-bis(4-hydroxyphenyl)-1,2-ethanediol (2c) and 1-[4-(dimethylamino)phenyl]-2-phenyl-1,2-ethanediol (2d) under the same reaction conditions as 2b give 2-(3-guaiazulenyl)-1,1-bis(4-hydroxyphenyl)ethylene (6) (73% yield) and (Z)-2-[4-(dimethylamino)phenyl]-1-(3-guaiazulenyl)-1-phenylethylene (7) (17% yield), respectively. Comparative studies of the above reaction products and their yields, crystal structures, spectroscopic and electrochemical properties are reported and, further, a plausible reaction pathway for the formation of the products 3-7 is described.     

New photochromic chemosensors for Hg and F

Two new chemosensors (1a and 1b) based on photochromic dithienylcyclopentene were designed and synthesized, and their spectral behaviors toward various metal ions and anions were investigated in detail. Compounds show excellent optical properties and distinguish Hg²⁺ and F⁻ in CH₃CN. Job’s plot reveals that the presence of Hg²⁺ induces the formation of a 1:1 complex between 1a or 1b and Hg²⁺. From the spectral responses and ¹H NMR analysis, the deprotonation of the thioamide protons is proposed to explain the sensing mechanism for 1a and 1b toward F⁻. It is found that 1a and 1b exhibit ring-opening and ring-closing photoisomerization with UV-vis light irradiation. Furthermore, their photochromic properties can be modulated by Hg²⁺ and F⁻ ions. Moreover, 1a and 1b in photostationary states become promising sensors for Hg²⁺ and F⁻ with high selectivity.     

Synthesis and photophysical properties of porphyrins with fluorenyl pendant arms

Symmetrical-A₄-porphyrins bearing four fluorene donor moieties TOFP (5,10,15,20-tetra(4-(2 methyloxyfluorenyl)phenyl)porphyrin) as well as eight fluorene arms OOFP (5,10,15,20-octa(3,5-(2-methyloxyfluorenyl)phenyl)porphyrin) were synthesized and characterized. Preliminary photophysical properties are reported. In comparison to the reference tetraphenylporphyrin TPP, the luminescence properties are slightly improved. The fluorescence quantum yields of tetrafluorenylporphyrin TOFP (1) and octafluorenylporphyrin OOFP (2) are 0.10 and 0.13, respectively.     

Conjugated C3 symmetric aryl tripyrroles and aryl bipyrroles: synthesis,optical and electronic properties

Conjugated C₃ symmetric molecules have been pursued for their fluorescent and electrochemical properties. The synthesis of conjugated C₃ symmetric aryl tripyrroles has been performed by a route featuring three steps from trimethyl 1,3,5-benzene tricarboxylate 6: copper-catalyzed cascade addition of vinylmagnesium bromide to convert the carboxylates into γ,δ-unsaturated ketones, Tsuji–Wacker olefin oxidations to form tris(1,4-dione) 9, and Paal–Knorr condensation with ammonia and different amines to furnish the final tripyrroles (30–60% yields). In addition, incomplete reaction of vinylmagnesium bromide to 6 provided benzoate 8 possessing two γ,δ-unsaturated ketones, which were similarly converted to 3,5-dipyrrolylbenzoates 11. The absolute fluorescence quantum yields (Φ_fl) and electrochemical properties of 5a–c and 11a–c were investigated. The Φ_fl are the first reported for such compounds and they ranged between 2 and 40%, contingent on structure and solvent polarity. Cyclic voltammetry revealed that the compounds could be both oxidized and reduced, albeit irreversibly. The oxidation potentials (E_ox) varied between 0.73 V and 1.2 V and the reduction potentials (E_red) varied from −0.83 V to −1.36 V. The lowest redox processes were measured for the NH–pyrrole 5a. Moreover, tripyrrole 5a was air stable and on oxidative doping with ferric chloride exhibited a 50 nm bathochromic shift in its absorbance spectrum.     

Synthesis of novel fluorescent 3-aryl- and 3-methyl-7-aryl-[1,2,3]triazolo[1,5-a]pyridines by Suzuki cross-coupling reactions

Two series of compounds, 3-aryl- (series A, compounds 2a-j) and 3-methyl-7-aryl-[1,2,3]triazolo[1,5-a]pyridines (series B, compounds 3a-j) have been synthesized by Suzuki cross-coupling reactions, with a triazolopyridine halide and an aryl or heteroaryl boronic acid in moderate to good yields. All compounds obtained are fluorescents, the quantum yields, particularly those of compounds 3f-j, are very high.