Comprehensive understanding of structure- photosensitivity relationships of photochromic [2.2]paracyclophane-bridged imidazole dimers

The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and rapid fading in the dark. Experimental details for the enhancement of the photosensitivity and the unique photoisomerization of newly designed [2.2]paracyclophane-bridged imidazole dimers are demonstrated. We explored the structure-property relationships and demonstrated an efficient strategy for designing high-performance fast-photochromic molecules with increased photosensitivity to solar UVA radiation. The [2.2]paracyclophane-bridged imidazole dimer consists of two types of imidazole rings, Im1 and Im2. Im1 is characterized by a 6π electron system with an electron-donating characteristic, whereas Im2 is distinguished by a 4π electron system with an electron-withdrawing characteristic. The introduction of electron-donating substituents into the phenyl rings attached to the electron-withdrawing Im2 was proved to enhance the photosensitivity with the aid of the intramolecular charge transfer transitions. The unique photoisomerization resulting from the changes in the bonding manner between two imidazole rings was also investigated in detail.     

Photochromism of a water-soluble vesicular [2.2]paracyclophane-bridged imidazole dimer

Here we report the first photochromism of a newly designed [2.2]paracyclophane-bridged imidazole dimer in water. The photochromic dye with a hydrophilic and a hydrophobic substituent forms vesicles in water and shows instantaneous colouration upon UV light irradiation and successive rapid fading in the dark.     

Spectroelectrochemistry of a photochromic [2.2]paracyclophane-bridged imidazole dimer: clarification of the electrochemical behavior of HABI

The photochromic behavior of the imidazole dimers can be attributable to the photoinduced homolytic cleavage of the C-N bond between the two imidazole rings. On the other hand, although the simultaneous formation of the imidazolyl radical and imidazole anion by the one-electron reduction of an imidazole dimer was reported, no definitive evidence for this electrochemical reaction has been demonstrated. We report the first direct evidence for the electrochemical generation of the imidazolyl radical from the radical anion of the imidazole dimer by conducting the UV-vis-NIR spectroelectrochemical analysis of the [2.2]paracyclophane-bridged imidazole dimer.     

Photochromism in the arylaroylaziridine system

Photochromism of a number of arylaroylaziridines has been observed in the solid state or in the rigid matrix. The photochromic behavior is dependent on the intensity of the incident radiation, the reaction medium, the wavelength of light used and on the relationship of the substituents on the three-membered ring. From the visible absorption spectra it is clear that cis-arylaroylaziridines give different colored species than do the trans isomers. These results suggest that the coloration is due to an extensive electrical interaction between the bent bonds of the aziridine ring and the π orbitals of the benzoyl and phenyl groups. Attempts to trap the colored species by co-irradiation with substrates containing a multiple bond failed. A reaction has been found to occur upon heating various arylaroylaziridines with dimethylacetylene dicarboxylate in inert solvents.     

Substituent Effects on the Photochromic Properties of Benzothiophene‐Based Derivatives

Five diarylethene photochromic derivatives, the structures of which incorporate a central benzothiophene unit, a left‐hand thiazole group, and a right‐hand benzothiophene group, have been prepared. The compound with a thiazole unit with no substituent on the reaction‐center carbon atom reveals an unprecedented transformation upon light irradiation. When the 4‐position of thiazole is protected by a methyl group, the compounds show high photosensitivity and photochromic properties. In this case, light irradiation affords new compounds with [5]helicene structures featuring the highest redshifted absorption maxima reported to date.     

AZOALDOLASE PHOTOSENSITIVITY

Abstract —The chemical modification of rabbit muscle aldolase by coupling with diazotized p -amino benzoate results in the preparation of a photosensitive enzyme. The photosensitivity is realized with the presence of arene diazothioether chromophores, due to the substitution on cysteine residues (eight groups per enzyme molecule). By absorption and emission spectroscopy it has been shown that at neutral p H visible light causes E→Z isomerization of the extrinsic chromophore. The reaction is thermally reversed with τ= 2.3 min at 24.3°C. Fading of the chromophore is observed after long irradiation time, especially in alkali solution; at p H 13 some of the diazo groups are exchanged with histidine residues to form azo derivatives of the imidazole anion. Optimum condition for photochromic behaviour of azoaldolase is irradiation with visible light centered at 450 nm, by using neutral solutions of the protein. Within the photochromic cycle slight differences have been observed in the gel electrophoretic behaviour of the azoprotein.     

Helical Oligophenylene Linked with [2.2]Paracyclophane: Stereogenic π-Conjugated Dye for Highly Emissive Chiroptical Properties

A stereogenic π-system based on dimer ( 2 ) and trimer ( 3 ) of [2.2]paracyclophane (PC) and biphenyl was prepared and its structural, photophysical, and chiroptical properties were investigated. X-ray analysis revealed that the quaterphenyl moieties in 2 adopt a double helical structure anchoring [2.2]PC from both sides. Furthermore, 3 forms a isosceles triangle structure with a large chiral cavity. A homodesmotic reaction using DFT calculations revealed that 2 has a larger strain energy than 3 owing to its highly twisted phenylene linkers. Electronic and circular dichroic (CD) spectra were recorded in CH₂Cl₂ solution. The spectra of both 2 and 3 are similar, and their longest absorption band accompanying a remarkable Cotton effect is attributed to the transition from HOMO to LUMO, which is delocalized to the quaterphenyl moiety. These compounds exhibit fairly high fluorescence quantum yields (ϕ=0.70–0.83) and moderate dissymmetry factor (|g_CPL|=1.6×10⁻³) in circularly polarized luminescence (CPL).     

π-Fused bis-BODIPY as a candidate for NIR dyes

Benzene-fused bis-(borondipyrromethene)s (bis-BODIPYs) were synthesized by retro-Diels-Alder reaction of the corresponding bicyclo[2.2.2]octadiene-fused (BCOD-fused) bis-BODIPYs, which were, in turn, prepared from 4,8-ethano-4,8-dihydropyrrolo[3,4-f]isoindole derivatives. The π-fused bis-BODIPY chromophores were designed to show intensive absorption and strong fluorescence in the near-infrared region and not to have any strong absorption in the visible region. A 6,10-dibora-5a,6a,9a,10a-tetraaza-s-indaceno[2,3-b:6,5-b']difluorene derivative (syn-bis-benzoBODIPY) obtained by a thermal retro-Diels-Alder reaction of the corresponding BCOD-fused BODIPY dimer has strong absorption and emission bands at 775 and 781 nm, respectively. The absolute quantum yield is 0.36. The absorption is more than 5.0 times stronger than other absorptions observed in the visible region. In the case of 6,15-dibora-5a,6a,14a,15a-tetraaza-s-indaceno[2,3-b:6,7-b']difluorene derivatives (anti-bis-benzoBODIPY), the absorption and emission maxima exceed 840 nm.     

Chiral,Thermally Irreversible and Quasi-Stealth Photochromic Dopant to Control Selective Reflection Wavelength of Cholesteric Liquid Crystal

A chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. Since the absorption maximum wavelengths of both thermally stable photoisomers are nearly in the UV region (quasi-stealth photochromism), it can be exposed to visible light without inducing photochromic reactions. Therefore, when the photoswitching molecule is added to a permanent cholesteric liquid crystal whose reflection light wavelength is in the visible region, the UV light-induced photochromic reaction of the photoswitching molecule changes the wavelength of the reflection light in the visible light region. We have succeeded in regulating the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. Attempts to introduce this system in polymer dispersed cholesteric liquid crystals are also described.     

Fluorescence modulation with photochromic switches

Photochromic compounds change their color under illumination. In most instances, a colorless state switches to a colored one upon ultraviolet irradiation. The photogenerated species reverts to the original one either by thermal means or upon visible irradiation. These reversible transformations are accompanied by pronounced structural and electronic modifications, which often alter the ability of the photochromic compound to emit light. Under these conditions, the photoinduced and reversible interconversion of the colorless and colored states results in the modulation of the fluorescence intensity. Alternatively, fluorescence modulation can be implemented by attaching covalently a fluorescent group to a photochromic compound. Photoinduced changes in the dipole moment or conjugation of the photochromic component can then be designed to alter the emissive behavior of the fluorescent appendage. Similarly, photoinduced shifts in the redox potential or absorption wavelength of the photochromic fragment can be engineered to activate electron or energy, respectively, transfer pathways. Both processes can efficiently quench the fluorescence of the emissive component. Furthermore, the reversible absorption changes of a photochromic compound can effectively filter the emission of a compatible, but separate, fluorophore as long as the emission bands of the latter overlap the absorption bands of one of the two states of the former. When this design requirement is satisfied, fluorescence modulation can be achieved even if the two functional components are operated in distinct environments. Thus, either one of these ingenious mechanisms can be exploited to regulate the emissive behavior of collections of molecules in solution or even in rigid matrixes. In fact, the investigation of these fascinating systems can eventually lead to novel photoresponsive materials for photonic applications, while contributing to advance our basic understanding of the photochemical and photophysical properties of organic compounds.     

Absorption and Rayleigh scattering and resonance Rayleigh scattering spectra of [HgX<Subscript>2</Subscript>]<Subscript>n</Subscript> nanoparticles

The composition and existing species ot the reaction production ot Hg ana X (X= Cl, Br and I) under different conditions, and their absorption, Rayleigh scattering (RS) and resonance Rayleigh scattering (RRS) spectra have been studied. The results show that the products exist in the form of nanoparticles as [HgX2]n aggregates under suitable conditions, and their average diameters increase with the increase of X- diameters. The diameters of [HgCI2]n, [HgBr2]n and [Hgl2]n are less than 4 nm, equal to 9 nm and 70 nm respectively. There are bathchromic shifts gradually with the increase of X- diameters in their absorption spectra. The absorption bands of [HgCI2]n and [HgBr2]n locate at ultraviolet region. However, the absorption band of [Hgl2]n is obvious in visible light region. Among three particles, only [Hgl2]n exhibits a strong RRS and its scattering peak is at 580 nm. The main reasons leading to the enhancement of resonance scattering are the large size of nanoparticle, the formation of the interface     

Kinetics of photochemical reactions of a new biphotochromic compound upon photolysis with light of different wavelengths

Using the method of microsecond flash photolysis with UV and visible light, the spectral and kinetic characteristics of intermediate products of photolysis in toluene and methanol solutions of a new biphotochromic compound have been studied, in which two photochromic moieties, spironaphthoxazine and azomethine, are linked to each other in such a way that there is conjugation between π-electronic systems of the moieties in the ground state of the molecule. Two intermediate products have detected, whose relative efficiency of formation substantially depends on the solvent and the wavelength of excitation light.     

Photochromism of dihydroindolizines. Part 16: Tuning of the photophysical behavior of photochromic dihydroindolizines in solution and in polymeric thin film

In this work, photochromic materials based on the dihydroindolizine (DHI) system were synthesized in multistep reactions using chemical and photochemical methods. Some of the synthesized photochromic dihydroindolizine derivatives were substituted on the fluorene (region A) and pyridazine (region C) moieties in order to provide the appropriate functionality for optimal tuning of the photochromic properties of the system. Irradiation of the photochromic DHIs with polychromatic light led to ring opened colored betaines, which underwent thermal 1,5-electrocyclization. The red to green colored betaines produced after UV irradiation returned back through 1,5-electrocyclization to the corresponding DHIs with different rate constants depending on the substituents in both fluorene and pyridazine regions. The kinetic measurements of the thermal 1,5-electrocyclization under different temperatures that ranged from −10 to 25 °C showed that the half-lives of the colored betaines fall in the second to hours domain. Interestingly, these materials showed a very good photochromic behavior not only in solution but also in the PMMA matrix. Irradiation of a slide prepared by the deep-coating method led to the formation of the colored betaine and the kinetics of the thermally reversible 1,5-electrocyclization and the AFM image of the film has been recorded. Indeed, the chemical and thermal stability of the investigated betaines in polymer (PMMA) will render such species useful for a plethora of new of applications.     

Oxidation products of 2-arylphenanthro-[9,10-d]imidazoles—II : Structure and photochromism of oxidation products of 2-(4-methoxyphenyl)-phenanthro-[9,10-d]imidazole

By ferricyanide oxidation of 2-(4-methoxyphenyl)phenanthro[9,10-d]imidazole, three new photochromic compounds were obtained. These were 4-ethoxy-4H-, 4-methoxy-4H- and 2-methoxy-2H-phenanthro[9,10-d]imidazoles. These compounds gave the 2-(4-methoxyphenyl)-phenanthro[9,10-d]imidazolyl radical and acetaldehyde or formaldehyde by light irradiation. The imidazolyl radical dimerized gradually in the dark and the dimer dissociated to the imidazolyl radical on heating.     

Photochromic,Thermochromic, and Fluorescent Spirooxazines and Naphthopyrans: A Spectrokinetic and Thermodynamic Study

The photochromic and thermochromic behavior of four commercially available Reversacol dyes are presented. The compounds studied belong to the class of spirooxazines and naphthopyrans, which are typically thermoreversible photochromic molecules. On stimulation with UV light, these compounds become colored and exhibit spectra which extend over the whole visible region. Increasing the temperature causes spontaneous coloration (thermochromism). Herein, absorption and fluorescence spectra, molar absorption coefficients of the colorless and colored forms, fluorescence and photochemical quantum yields, and kinetic parameters of thermal bleaching (rate constant, frequency factor, and activation energy) are determined in acetonitrile solution. The thermal ground‐state reaction is exhaustively described in terms of thermodynamic parameters (equilibrium constant, free energy, enthalpy, and entropy). Temperature effects on photochemical and thermal colorabilities are evaluated. The results indicate that the two spirooxazines are good photochromes below room temperature, whereas they are efficient thermochromic compounds above room temperature. Naphthopyrans are better photochromes but worse thermochromic compounds than spirooxazines.     

On the photochromism of spiro

The recently synthesized spiro[cyclohexadiene-dihydroacridines] consisting of perpendicularly arranged aroylcyclohexadiene and N -methyl-dihydroacridine moieties were found to have photochromic properties. The reversible photoisomerization from the spiro compound toward a colored merocyanine caused by C–C bond cleavage in the cyclohexadiene was studied by stationary and time-resolved measurements of their optical spectra. The course of the absorption under UV and visible irradiation, respectively, and HPLC analysis of the photoproducts result in the determination of excitation energy-dependent quantum yields for the merocyanine formation and, in reverse, the ring closure, as well as degradation. Whereas the thermal back reaction completely recovers the spiro compound ( k ∼ 6.8 × 10⁻⁴ s⁻¹, T = 22°C), degradation of the merocyanine under irradiation at 480 nm has a probability of about 6%. Picosecond-resolved measurements of the fluorescence and the transient absorption show that photoisomerization occurs via the first excited singlet state within 100 ps depending on the activation barrier.     

Photochromic Properties of β-Cyclodextrin Dimer Linked by Dithienylethene

A fi-cyclodextrin dimer tethered by photoswitchable dithienylethene moieties was synthesized as a potentially tunable receptor. The dimer exhibits pronounced photochromic properties. Irradiation of the dimer in open form with UV light at 254 nm resulted in immediate photocyclization to the pink closed form; the colorless open form could be regenerated by irradiation with visible light of wavelength >460 nm. The reaction kinetics of the forward photoprocess were also studied. To investigate the binding ability of the dimer in open and closed forms, a fluorescence titration was performed. It was found that the stability constant for the binding of TSPP (meso-tetrakis (4-sulfonatophenyl)porphyrin) by the closed form of the dimer is a factor of 5 higher than that of the open form.     

Cucurbit[8]uril-induced diarylethene thermally activated delay fluorescence supramolecular switch for sequential energy transfer

Constructing molecular switches based on supramolecular assembly strategy is a research hotspot. In this work, we constructed an all visible-light-regulated supramolecular photo-switch based on pyridinium-modified diarylethene derivative (DTE-Me) and cucurbit[8]uril (CB[8]). CB[8] not only accelerated the photochromic process under 365 nm ultraviolet light but also shifted the absorption of open formed DTE-Me to the visible region, which led to efficient photocyclization under 450 nm visible light irradiation, while DTE-Me and DTE-Me/CB[7] remained unchanged under the same irradiating condition. Moreover, the complexation with CB[8] could induce the strong thermally activated delayed fluorescence (TADF) of guest molecular at 550 nm, which further shifted to 670 nm through two-step sequential energy transfer with sulforhodamine B (SRB) and Cy5. This energy transfer process could also be regulated with visible light, and the application for information encryption was also demonstrated. This assembly provides a convenient approach to construct all visible light-regulated TADF photo-switch.     

New photochromic symmetrical and unsymmetrical bis(heteroaryl)maleimides: A spectrokinetic study

The photochromic behavior of two newly synthesized diarylethenes is here presented. The compounds studied are the symmetrical (1-benzyl-3,4-bis(2-methylbenzo[b]thiophen-3-yl)-[1H]-2,5-dihydropyrrol-2,5-dione) and the unsymmetrical (1-benzyl-3-(2-methylbenzo[b]thiophen-3-yl)-4-(2,5-dimethyl-3-thienyl)-[1H]-2,5-dihydropyrrol-2,5-dione). Upon stimulation with UV or violet light, these compounds become red-colored due to photocyclization and cyclorevert to the light yellow open form upon irradiation with visible light. In this work, absorption and fluorescence spectra, molar absorption coefficients of the ethenic and cyclized forms, fluorescence quantum yields and photochemical quantum yields of both the forward and back photoreactions were determined. Temperature, excitation wavelength and solvent effects were explored. The symmetrical compound was found to be a bistable photochrome. In contrast, the unsymmetrical molecule resulted unsuitable as photochrome because of side degradation processes occurring in competition with cyclization.     

Covalently linked dimer of chlorophyll-a derivative with an amide bond and its folded conformer

A chlorophyll-a derivative homo-dimer covalently linked with a flexible methylene-amino-methylene group at the 3-position was benzoylated to give the dyad bearing a CH₂N(COPh)CH₂ linker. The synthetic dyad with the relatively rigid spacer showed red-shifted visible absorption bands and was oxidized more readily, in comparison with the corresponding monomer. The optical and electrochemical properties of the dimer are ascribable to the partial π-stacking of chlorin moieties in the benzoylated dyad, mimicking the special dimeric species in reaction centers of photochemical systems in natural phototrophs.