Photochemistry of a crown ether styryl dye adsorbed on silica gel and in acetonitrile solution: a comparative flash photolysis study

The photochemical behaviour of a photochromic crown ether styryl dye (BOB) adsorbed on silica gel has been found by a steady-state technique to be similar to that observed in a fluid solution of high polarity. The intermediate spectra obtained by diffuse reflectance laser flash photolysis of BOB on silica gel in a nitrogen atmosphere have been preliminarily attributed to the triplet-triplet (T-T) absorption of BOB and the absorption of BOB cation radicals. For comparison, the absorption spectrum of BOB triplets with a lifetime of ∼0.8 μs in acetonitrile solution has been obtained using biphenyl as a sensitizer.     

Spectral and photochemical properties of hybrid organic—inorganic nanosystems based on CdS quantum dots and a styrylquinoline ligand

The kinetics of photolysis of a styrylquinoline (SQ) derivative as the photochromic ligand in organic—inorganic hybrid nanosystems (HNSs) with the core composed of CdS quantum dots (QDs) has been studied for the first time as a function of the number of ligand molecules in the HNS shell, which varied from 1 to 10. The hybrid nanosystems have been synthesized in the microwave-assisted mode according to the single-step injection-free procedure. It has been shown that high quantum yields of photoisomerization of the SQ ligand are conserved in the HNS. In the early stages of the photolysis, regardless of the number of SQ ligand molecules in the HNS shell, the kinetics obeys the equation for the photolysis of the monomolecular system (model SQ photochrome) with allowance for the absorption due to QDs as an inert shutter. During the course of long-term photolysis, the quantum dots undergo photodegradation to be completely decomposed. According to the principal component analysis data, several photoproducts with different absorption spectra are formed at the intermediate times of the HNS photolysis.     

Synthesis of Photochromic Dyes Based on Annulated Coumarin Systems

Novel 2H‐chromenes derived from hydroxycoumarins were synthesized, and their photochromic behaviour was studied under flash‐photolysis conditions, showing a wide absorption range in the visible region. All the compounds exhibit low fluorescence, which apparently has no negative effect on their photochromic properties.     

Transient absorption spectra and photochromic mechanism of schiff bases

The transient absorption spectra of salicylideneaniline (SA) and salicylidenebenzylamine (SBA) have been investigated by means of the nanosecond laser flash photolysis. It has been found that the intermediates from the two compounds have different properties. According to the properties of the intermediates for compound SA, the primary photoproduct is a zwitterion produced by the excited singlet state, rather than a trans- keto isomer. The intermediate of compound SBA also was found to be a zwitterion, but produced not only by the excited singlet state but also by the excited triplet state. The photochromic mechanisms of the two compounds are proposed and discussed respectively.     

Untersuchung reversibler Photoprozesse durch Blitzlichtphotolyse IV. Triplettzustände des 1, 3, 6′, 8′- tetramethyl-dehydrodianthrons

By means of the flash photolysis technique, transient absorption spectra attributed to tetramethyl-dehydrodianthrone (TMD) in both the photochromic and triplet states have been investigated in polymethylmethacrylate matrices and in the solvent triacetin. In polymethylmethacrylate matrices and in rigid glasses of triacetin the triplet state of TMD is heavily populated. Triplet-triplet absorption and phosphorescence measurements show that below 180°K the triplet decay follows first order kinetics with the decay constant k=11,3 ± 0,1 s^?1. In incompletely solidified triacetin glass it is possible to monitor the transient absorption of the photochromic and the triplet state simultaneously. It is shown that the photochromic state ¹A₁* is not generated via the triplet state. Therefore the authors suggest a kinetic scheme characterised by a direct singlet state - photochromic state transition.     

Influence of polarity of medium on formation of intermediates in photolysis of alkylaminobenzophenones in solutions

Flash photolysis has been used in a study of the absorption spectra and decay kinetics of the intermediates in photolysis of 4,4′-bis-dimethylaminobenzophenone, 4,4′-bis-diethylaminobenzophenone, and 4-dimethylamino-benzophenone in various types of solvents. It has been established that flash photolysis of solutions of alkylaminobenzophenones in polar solvents produces not only alkylamine or ketyl radicals, but also an intermediate that absorbs in the 470 nm region, the decay curves of which are described by first-order kinetic equations. In the presence of tertiary amines, the yield of this product is lower. The addition of sodium perchlorate to the solution increases the lifetime of this intermediate species. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of the Ukraine, 31 Nauka Prospect, 252039 Kiev, Ukraine. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 1, pp. 27–31, January–February, 1997.     

Laser photolysis study of 1-aryl-3-methyl-5-(benzothiazole-2-yl) formazan phototransformation

Photochromism of 1-aryl-3-methyl-5-(benzothiazole-2-yl)formazans in toluene has been investigated by the laser flash photolysis method. It was established that the photochromic transformations involve the triplet state of the initial chelate form of formazans. The intermediate short-lived products of phototransformation were detected that could be related to one of the conformers or to the tautomers of the formazan molecule.     

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.     

Laser induced photochromic mechanism of 3-phenyl-3-[1,2-dimethylindol-3-YL]-3H-naphtho[2,1-B]pyran

The photochromic process of 3-phenyl-3-[1,2-dimethylindol-3-yl]-3H-naphtho[2,1-b]pyran [I] has been examined with nanosecond laser flash photolysis techniques in cyclohexane and acetonitrile respectively. Both excited singlet state and triplet state are involved in the photocoloration process. The decay kinetics of photoproducts are also studied. The maximum absorption wavelength and lifetime of the transient species are solvent dependent.     

The time-resolved spectra of a crowned indolinospiroben-zopyran

The photochromic process of an indolinospiropyran with a crown ether fragment (BN-BIPS) was studied by nanosecond laser photolysis technique. The results show that quinonic merocyanine B was formed via an excited singlet state from BN-BIPS; in acetonitrile solution, the transient absorption of merocyanine B showed an obvious decay while a new transient absorption at 440nm (from isomer C) was observed simultaneously. The decay of merocyanine B and the formation of isomer C (at 440nm) were accelerated in the presence of alkali metal cation. In contrast, the formation of isomer C was not observed in spiropyran without a crown ether fragment: BIPS.     

Photochromic fulgides and spirooxazines: mechanism and substituent effect on photoreactions

Molecular design, synthesis and photochromic properties of spirooxazines and fulgides are described. In the case of fulgides, the change of the substituents may lead to different photochromic properties and different photoreactions of the kind of compounds. In photochromic process of pyrryl-substituted fulgides, the excited singlet state is the mam species, but the excited triplet state is also involved. However, no excited triplet state has been observed in cyclization of aryl-substituted fulgides. In the case of spirooxazines, the substituents at 2'-position have great effect on the formation of photoproduct and on the mechanism of photoreaction. The increase of steric hindrance of the 2'-position substituent gives rise to the decrease of the quantum yield for the formation of photochromic merocyanine (PMC) and the increase of the relative quantum yield for the charge separated twist intermediate (CT).     

Photochemistry of tetrasulfur tetranitride: laser flash photolysis and quantum chemical study

The photochemistry of tetrasulfur tetranitride (1) was studied in hexane solution by the laser flash photolysis technique (LFP). The experimental findings were interpreted using the results of previous matrix isolation studies (Pritchina, E.A.; Gritsan, N.P.; Bally, T.; Zibarev, A.V. Inorg. Chem. 2009, 48, 4075) and high-level quantum chemical calculations. LFP produces two primary intermediates, one of which is the boat-shaped 8-membered cyclic compound (2) and the other is the 6-membered S(3)N(3) cyclic compound carrying an exocyclic (S)-N═S group (3). The primary products 2 and 3 absorb a second photon and undergo transformation to the 6-membered S(3)N(3) cycle carrying an exocyclic (N)-S≡N group (4), which is very unstable and converts back to intermediate 3. The quantum yield of the primary product formation is close to unity even though the quantum yield of photodegradation of 1 is low (~0.01). Thus, 1 is a photochromic compound undergoing in solution the thermally reversible photochemical isomerization. The mechanism of the photochromic process was established, and the rate constants of the elementary reactions were measured.     

Inclusion complex of spironaphthoxazine with gamma-cyclodextrin and its photochromism study

An environmentally benign transparent photochromic film, 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'-[3H]-naphtho[2,1-b][1,4]oxazine] (SPO) /gamma-cyclodextrin (gamma-CD), was prepared using the cast-coating method, and the nanocavity effect of gamma-CD on the photochromism of SPO was studied. The film mainly consists of the inclusion complex SPO@gamma-CD, which has been verified by EA, TGA, XRD, MS, and ICD to comprise a 1:1 host-guest stoichiometry. The film shows normal photochromism. The decoloration of photomerocyanine (PMC) fits biexponential decay: PMCs located in the cavity of gamma-CD decay with a rate constant of 6.0 x 10(-2) s(-1), which is nearly one order faster than those PMCs outside of the cavity.     

A new photochromic 8pi-system based on an azaheptatriene-tetrahydroazepinoisoquinoline electrocyclization

A new photochromic family of tetrahydroazepinoisoquinolines (THAI) 4a-i has been prepared. It undergoes light-induced ring opening from spiro compounds 4 to betaines 3 that decolorize in a very fast cyclizing reaction. Depending on substitution of the precursors 1 and 6, the photochromic styryltetrahydroindolizines (THI) 5k-q are formed in a periselective way. The conformation and configuration of the new photochromic THAI 4 and THI 5 were investigated by NMR and the structure of both compounds was proven by X-ray analysis. The photochromic properties were studied by laser flash photolysis, which afforded the lifetime of the colored form 3 and 3' in the micro- or nanosecond range.     

Photochromic and luminescence properties of isomeric forms of the hydronaphthyl radicals

The optical properties of the hydronaphthyl radicals produced upon radiolysis and photolysis of rigid naphthalene solutions in boric acid at 300 K and alcohol, ether and methylcyclohexane at 77 K were investigated. The ESR, absorption and luminescence spectra of the two different isomeric forms of hydronaphthyl radicals have been observed. The estimated values of the lowest electronic transition in α- and β-hydronaphthyl radicals are, respectively: E_α = 2.42 ± 0.03 eV, E_β = 2.66 ± 0.05 eV. It has been found that a reversible photo-induced interconversion between two hydroradicals takes place as well as a dark conversion from the β- to the α-isomer. Analogous photochromic behaviour of the absorption bands of biphenyl, phenanthrene, chrysene and fluorene rigid solutions have also been identified as reversible interconversion between isomeric hydroradicals.     

Photochromism of 2-(2,4-dinitrobenzyl)benzimidazole in solution

Spectral and kinetic parameters of photochromic 2-(2,4-dinitrobenzyl)benzimidazole in various solvents were studied by flash photolysis. The absorption band of the photoinduced colored form of 2-(2,4-dinitrobenzyl)benzimidazole shifts red with rise in the relative dielectric constant of the solvent. The rate of bleaching of the photoinduced species in ethanol, propan-2-ol, acetone, acetonitrile, and dimethyl sulfoxide is lower by 2–3 orders of magnitude than in benzene, toluene, and hexane due to formation of intermolecular hydrogen bond with the solvent.     

Photochromism of tetrahydroindolizines. Part XIV: synthesis of cis-fixed conjugated photochromic pyridazinopyrrolo[1,2-b]isoquinolines incorporating carbon-rich linkers

New carbon-rich photochromic tetrahydroindolizines (THIs) bearing dihydroisoquinoline derivatives as heterocyclic bases (region B) have been synthesized via different chemical and photochemical pathways. Three alternative pathways for the synthesis of the target photochromic THI-based pyridazinopyrrolo[1,2-b]isoquinolines via in situ trapping with hydrazine hydrate have been established. In order to obtain high product yields, different Sonogashira-mediated coupling reactions have been optimized. Low temperature multichannel UV–vis and flash photolysis techniques were used to detect the photochromic and kinetic properties of the synthesized system.     

Dynamics and mechanisms of the multiphoton gated photochromic reaction of diarylethene derivatives

The cycloreversion (ring-opening) process of one of the photochromic diarylethene derivatives, bis(2-methyl-5-phenylthiophen-3-yl)perfluorocyclopentene, was investigated by means of picosecond and femtosecond laser photolysis methods. The drastic enhancement of the reaction yield was observed only by the picosecond laser exposure. The excitation intensity effect of the reaction profiles revealed that the successive multiphoton absorption process leading to higher excited states opened the efficient cycloreversion process with a reaction yield of (50 +/- 10)%, while the one-photon absorption directly pumped to a higher excited state did not lead to the efficient cycloreversion reaction. These results indicate that not the energy of the excitation but the character of the electronic state takes an important role in the enhancement of the cycloreversion reaction.     

Synthesis, characterization and photochromic studies in film of heterocycle-containing spirooxazines

A series of novel heterocycle-containing spirooxazines have been designed and synthesized, and their photochromic properties were investigated under flash photolysis and continuous irradiation in particular regard to the fatigue resistance, the lifetime of the colored merocyanine form in various solutions and polymers. Especially, the characteristics of two UV-sensitive spirooxazines dispersed polymethylmethacrylate thin-films were extensively studied. Detailed studies showed that general significant shifts in the λ_max of the absorption spectra of the open forms, interesting fatigue resistances and emission fluorescene properties were observed.     

A photochromic phenoxyquinone based cyanide ion sensor

We have developed a new chemosensor system for cyanide ion that is based on a photochromic material. We observed that addition of cyanide anion to a UV irradiated solution of a phenoxynaphthacenequinone derivative brought about a significant change in the absorption spectra that enabled detection of cyanide ion in a selective and sensitive manner. A carbanion intermediate was shown to be responsible for the long wavelength absorption band (630-940 nm) that is generated by cyanide addition.