In-situ crystal structure analysis and control of photochromism with dual-mode photoreactive soft crystals

Photochromic compounds are of great interest currently owing to their potential applications. Their three-dimensional structure must be clarified to understand the photochromism mechanism. In-situ X-ray crystal structure analysis is a powerful tool for determining the structure directly after the photochromic reaction. In this review, I discuss about solid-state photochromic compounds, their direct X-ray observation in the crystal form, and in-situ control of photochromism in “dual photoreactive soft crystals”, which is a novel approach.     

Photochromic transformations of cymantrenyl amides

Photochemical properties of N-acyl-substituted aminomethyl- and 1-aminoethylcymantrenes were studied for the first time. These compounds form photochromic systems in solutions due to the intermolecular exchange of ligands at the manganese atom.     

Recent progress of luminescent metal complexes with photochromic units

Photo-responsive molecules have been studied extensively because of their light irradiation abilities that enable modulation of certain physical and chemical properties in emerging molecular electronic and photonic devices. For advanced photonic applications, photochromic metal complexes that have photochromic units as the photo-responsive ligand are highly desirable, as they allow improvement of the photochromic properties and their photo-switching functionality. This article focuses on recent progress in luminescent metal complexes with photochromic units. Luminescence-switching properties of photochromic metal complexes depend on characteristic electronic transitions. The electronic transitions of photochromic metal complexes can be divided into three categories: (1) π–π* transition of the ligand, (2) metal to ligand charge transfer (MLCT) in transition-metal complex, and (3) f–f transition in lanthanide complex. Luminescence modulation using various metal complexes with photochromic units has been studied extensively in recent years, and various applications for future molecular switching devices are expected in the field of advanced photonics. Based on the literature and our studies on luminescent metal complexes with photochromic units, we report on the recent progress of luminescent metal complexes with photochromic units.     

Colour switching with photochromic vinylidene-naphthofurans

A series of novel photochromic vinylidene-naphthofurans with extended conjugation, and a free hydroxyl function, were easily prepared using the Suzuki reaction. After silanization, these dyes were embedded in ormosil matrices affording solid and transparent materials that acquire different colourations (violet, green, bluish), reversibly, when exposed to the UV (Sun) light, for 2?min, at room temperature. The presence of an extra phenyl ring in some positions affects both the λ_max of absorption of the photochromic compounds in the uncoloured closed and open coloured form. After removal of the light source the materials lose progressively their colouration returning to the initial uncoloured state in less than 15?min?at room temperature.     

New binuclear copper(II) complexes with bishydrazones derived from spiropyran of the benzoxazinonium series

New bishydrazones were prepared from a photochromic spiropyran of the benzoxazinonium series, containing free functional groups in the 2[H]-chromene moiety. This bishydrazones form binuclear complexes with copper(II) ions. The structure of these complexes was studied by ESR spectroscopy and magnetic measurements. The magnetochemical study revealed antiferromagnetic exchange coupling in the complexes.     

Photochromism in p-methylbenzoylthioacetone and related β-thioxoketones

Irradiation of p-methylbenzoylthioacetone with UV or visible light brings about spectral changes characteristic for the photochromic behavior of β-thioxoketones. The nature of the initial species and of the photochromic product can be assigned based on spectral studies of the electronic and IR spectra in rare gas matrices combined with quantum chemical calculations. Additional arguments for the vibrational assignments are provided by using polarized light to induce the phototransformation, and by subsequent measurements of linear dichroism on partially aligned samples. Comparison with the results previously obtained for three related molecules: thioacetylacetone, p-methyl(thiobenzoyl)acetone, and monothiodibenzoylmethane reveals a common pattern of the photochromic reaction. In all four molecules, the initial species corresponds to an intramolecularly hydrogen-bonded enolic molecular structure, and the main photochromic product to an “open”, nonchelated enethiolic counterpart. On the basis of a computational TD-DFT study of ground and excited electronic states, possible mechanisms of the photochromic transformation are discussed.     

Synthesis and photochromic properties of substituted naphthopyran compounds

A group of 3,3-diaryl-3H-naphtho[2,1-b]pyran compounds with functional substituents at the 5- and 6-positions of the naphthopyran skeleton and the para positions at the 3-aryl moieties were prepared through condensation reactions between 2-naphthol derivatives and 1,1-diarylprop-2-yn-1-ol derivatives. The chemical structures of the compounds were confirmed by NMR and MS. The crystal structure of 3,3-diphenyl-6-morpholino-3H-naphtho[2,1-b]pyran (4b) was determined and the relationship between the pyran substructure and photochromism was discussed. The photochromic properties were studied as well, and decoloration kinetics of colored forms was fitted to the biexponential model. Among these compounds, 4b was considered to be the best one due to the large ΔOD of colored form, which is one of the most important properties used in the photochromic material.     

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.     

New Salt Spiropyran of Indoline Series with Fluorine Substituent

Synthesis, study of structure and photochromic properties of new salt spiropyran of indoline series containing a fluorine substituent in the 6′-position of the 2H-chromene moiety have been described in the work. The structure of the compound has been confirmed by ¹H NMR spectroscopy and X-ray diffraction analysis. Comparison of the structure of the compound with the previously studied analogue containing a methyl group instead of a fluorine atom in the 6′-position of the [2H]-chromene moiety has been made. This compound has been found to be photochromic, long-wavelength absorption maximum of its open form shows a bathochromic shift, and lifetime is increased three times as compared with the methyl analogue.     

New benzopyranocarbazoles: synthesis and photochromic behaviour

The synthesis of three new benzopyranocarbazoles (=[indole]naphthopyrans) from hydroxybenzo[a]carbazoles is described. The photochromic properties of these novel compounds were investigated under flash photolysis and continuous irradiation. Compared to known [indole]benzopyrans these new compounds showed a significant bathochromic shift in the spectra of the open forms, an increase in colourabilities and slower ring closure kinetics. The photochromic behaviour of compound 4 has been further investigated. Continuous near-UV irradiation led to the formation of one photoisomer (TC) that was subsequently partially converted, to the other (TT). Thermal reversion of the preirradiated system to the original form was only partial and followed a monoexponential decay involving the back-conversion of the TC-isomer to the uncoloured closed form (CF). The thermally stable TT-isomer could only be photobleached with visible light. This process was shown to proceed through a fast photoconversion TT→TC followed by the thermal path TC→CF. Thermal relaxation of the activated system was also studied at various temperatures.     

Synthesis, electrochemical and photochromic behaviour of a series of (1,4-dithiafulven-6-yl)substituted 3H-naphtho[2,1-b]pyran derivatives

The synthesis and electrochemical and photochromic properties of new 3,3-diphenyl-8-(1,4-dithiafulven-6-yl)-[3H]-naphtho[2,1-b]pyran derivatives containing differently substituted dithiafulvenyl units are described. An example of electrochemical dimerization is shown which gives access to electroactive bichromophoric systems. Such systems could allow the study of the interplay of photochromic and electrochemical properties.     

The allomorphism of a photochromic diarylethene

The photochromic diarylethene, 1,2-bis(2-methyl-5-(2,2′-dicyanovinyl)-thien-3-yl)perfluorocyclopentene (1a), was synthesized by a novel method. Two kinds of single crystals of the compound were obtained depending on the different recrystallization conditions and their structures were determined by X-ray crystallographic analysis. Allomorphism of the photochromic diarylethene was discovered. The compound underwent a photochromic reaction both in solution and in the single crystalline phase of the anti-parallel conformer.     

Unexpected formation of new photochromic compounds derived from 3,3-diphenyl-3H-naphtho[2,1-b]pyran-1-one

Two new unexpected photochromic compounds were obtained from naphtho[2,1-b]pyran-1-one 1. The reaction of this ketone with the silyl enol ether methyl trimethylsilyl dimethylketene acetal, catalyzed by TiCl₄, afforded the photochromic dihydronaphtho[2,1-b]pyranone 2. The Reformatsky reaction of ketone 1 with ethyl bromoacetate led to the formation of the expected alcohol that under acid treatment gave, unexpectedly, the novel photochromic benzocoumarin 6. UV irradiation compounds 2 and 6 in solution provided thermally stable photoproducts that returned to the initial uncoloured forms under visible irradiation. The photochromic behaviour of these compounds and the structures of the photoproducts formed in these reactions were characterized by 1D and 2D NMR.     

New photochromic polymers

New photochromic polymers were synthesized by quaternization of amino groups of N-methyldiethanolamine polyterephthalate with a mixture of bis(2,5-dimethyl-4-chloromethyl-3-thienyl)-1,3-dioxol-2-one and p-or m-xylylene dichloride. The spectral-kinetic studies of coatings based on the obtained polymers showed that these coatings have photochromic properties, the efficiency of which decreases with an increase in the sample thickness.     

Studies on photochromic benzimidazol[1,2a]pyrrolidin-2-ones from the condensation of 2-methyl-3-benzothienylethylidene-(isopropylidene)succinic anhydride with 1,2-diaminobenzenes

Novel photochromic benzimidazol[1,2a]pyrrolidin-2-ones, which give thermally stable highly coloured photochromes, have been synthesised by condensation of 2-methyl-3-benzothienylethylidene(isopropylidene)succinic anhydride with 1,2-diaminobenzene and its 4,5-dimethyl and 4,5-dimethoxy derivatives. The photochromic properties are reported.     

Synthesis and properties of 1,3,3-trimethylspiro[indoline-2,3′-naphtho[2,1-b][1,4]oxazin]-6′-amine, a novel, red colouring photochromic spirooxazine

The synthesis and photochromic properties of 1,3,3-trimethylspiro[indoline-2,3′-naphtho[2,1-b][1,4]oxazin]-6′-amine, a novel red-colouring photochromic spirooxazine and derivatives is reported. The synthesis proceeds via an unusual one-pot transamination, oxime formation, N-O bond cleavage, spirooxazine formation process.     

One-step synthesis and photochromic properties of a stable triangle terthiophene

A new photochromic compound based on the hexatriene backbone was prepared by one-step coupling. The photochromic backbone, composed of three thiophene ring, undergoes reversible ring-opening and ring-closing photoisomerization reactions when irradiated with ultraviolet and visible light, respectively. It exhibited fatigue resistance and thermally irreversible photochromic properties. The ring-open form exhibits appreciable fluorescence, while quenched by the ring-closed form.     

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.     

Photo-modulated wide-spectrum chromism in Eu3+ and Eu3+/Tb3+ photochromic coordination polymer gels: application in decoding secret information

Photo-switching emission of photochromic materials has paramount importance in the field of optoelectronics. Here, we report synthesis and characterization of a dithienylethene (DTE) based photochromic low molecular weight gelator (LMWG) and self-assembly with lanthanide (Eu³⁺ and Tb³⁺) ions to form a photochromic coordination polymer gel (pcCPG). Based on DTE ring opening and closing, the TPY-DTE gel shuttles from pale-yellow coloured TPY-DTE-O to dark blue coloured TPY-DTE-C and vice versa upon irradiating with UV and visible light, respectively, and both the photoisomers show distinct optical properties. Furthermore, integration of Eu³⁺ and Tb³⁺ lanthanides with TPY-DTE resulted in red and green emissive Eu-pcCPG (Q.Y. = 18.7% for the open state) and Tb-pcCPG (Q.Y. = 23.4% for the open state), respectively. The photoisomers of Eu-pcCPG exhibit photo-switchable spherical to fibrous reversible morphology transformation. Importantly, an excellent spectral overlap of the Eu³⁺ centred emission and absorption of DTE in the closed form offered photo-switchable emission properties in Eu-pcCPG based on pcFRET (energy transfer efficiency >94%). Further, owing to the high processability and photo-switchable emission, the Eu-pcCPG has been utilized as invisible security ink for protecting confidential information. Interestingly, mixed Eu³⁺/Tb³⁺ pcCPG exhibited photo-modulated multi-spectrum chromism reversibly where the colour changes from yellow, blue, and red to green and vice versa under suitable light irradiation.

A lanthanide based photochromic coordination polymer gel (pcCPG) material has been developed which showed photomodulated colour change based on pcFRET and has the potential to be employed for decoding secret information.     

Reinvestigation of prototropic photochromism: 3-benzoyl-2-benzylchromones

The synthesis of a series of new nine 3-benzoyl-2-benzylchromones is developed through a classical and an optimized Kostanecki-Robinson method involving an o-hydroxyphenyl-β-diketone and an acid anhydride. The different spectrokinetic studies carried out in solution and in polymer matrixes have highlighted the main structure/photochromic parameters relationships and the influence of the medium on the photochromic properties.