Photochromism Emergence in N‐Salicylidene p‐Aminobenzenesulfonate Diallylammonium Salts

N‐Salicylidene p‐aminobenzenesulfonate salts were prepared by in situ condensation of p‐aminobenzenesulfonate diallylammonium salt and salicylaldehyde. Modulation of thermo‐ and photochromism was achieved by varying the alkyl chain length of the diallylammonium counter‐cation. A structural–optical properties investigation reveals that both crystal packing and dihedral angle between aromatic rings of the N‐salicylidene aniline switch are not sufficient to predict the occurrence of photochromism in the solid state. The available free space around the N‐salicylidene p‐aminobenzenesulfonate, in addition to the flexibility of the nearby environment, is shown to be of major importance for the cis→trans isomerisation to occur as well as for the stabilisation of the trans‐keto form. Emergence of photochromic properties was determined from the diallylhexylammonium cation within the series of investigated counter‐cations. High stability is observed for the trans‐keto form of one polymorph of N‐salicylidene p‐aminobenzenesulfonate diallylhexylammonium salt (k=2.4×10^?7 s^?1).     

Insights into the Origin of Solid‐State Photochromism and Thermochromism of N‐Salicylideneanils: The Intriguing Case of Aminopyridines

All light and no heat? Structure–property relationships for photochromic and thermochromic N‐salicylideneanils have been revised (see picture). One derivative has a packed crystal structure, is photochromic and exhibits an infinitely slow thermal back relaxation of technological interest. Complexation of the anils to transition metals has also been studied.

     

Synthesis,spectroscopic/electrochemical characterization and DNA interaction study of novel ferrocenyl‐substituted porphyrins

We report the synthesis and characterization of two new meso‐ferrocenylporphyrins, with the ferrocenyl unit attached at the para position of a C₆F₄ group by a C? N bond with ? NH(CH₂)₂NH? ( 2a ) and ? NH(CH₂)₄NH? ( 2b ) spacers. Compounds 2a and 2b were characterized through elemental analyses, electrospray ionization high‐resolution mass spectrometry, ¹H NMR, UV–visible and fluorescence spectroscopies, electrochemistry and spectroelectrochemistry. The free base porphyrin 2 was included for comparison purposes. The binding ability of 2 and the porphyrin derivatives 2a and 2b with calf thymus DNA was determined using UV–visible and fluorescence spectroscopies. The results suggest that the interaction of these systems most probably occurs through π‐stacking by non‐classical modes involving the partial insertion of the C₆F₅ ring between adjacent base pairs on DNA and possible hydrogen interaction with the aliphatic ? NH(CH₂)_nNH? (n = 2 or 4) groups with calf thymus DNA. Moreover, we also demonstrate that porphyrins generate singlet oxygen species and show good photostability after irradiation.     

Temperature‐Controlled Switchable Photochromism in Solid Materials

A novel strategy to achieve thermally switchable photochromism in solid materials is reported, which relies on the preparation of polymeric core–shell capsules containing solutions of photochromic dyes in acidic phase‐change materials. Upon changing the phase (solid or liquid) of the encapsulated medium, one of the two photochromic states of the system is selectively stabilized on demand, allowing for reversible interconversion between direct and reverse photochromism when thermally scanning through the melting temperature of the phase‐change material. This strategy, which does not require the addition of external agents or chemical modification of the dyes, proved to be general for different spiropyran photochromes and to be applicable to the fabrication of a variety of functional materials by simply embedding the capsules obtained into a solid matrix of choice.     

Polymorphism versus thermochromism: interrelation of color and conformation in overcrowded bistricyclic aromatic enes

The nature of the thermochromic form of overcrowded bistricyclic aromatic enes (BAEs) has been controversial for a century. We report the single-crystal X-ray structure analysis of the deep-purple and yellow polymorphs of 9-(2,7-dimethyl-9H-fluoren-9-ylidene)-9H-xanthene (11), which revealed the molecules in a twisted and a folded conformation, respectively. Therefore, the deeply colored thermochromic form B of BAEs is identified as having a twisted conformation and the ambient-temperature form A as having a folded conformation. This relationship between the color and the conformation is further supported by the X-ray structures of the deep-purple crystals of the twisted 9-(9H-fluoren-9-ylidene)-9H-xanthene (10), and of the yellow crystals of the folded 9-(11H-benzo[b]fluoren-11-ylidene)-9H-xanthene (12). Based on this conclusive crystallographic evidence, eleven previously proposed rationales of thermochromism in BAEs are refuted. In the twisted structures, the tricyclic moieties are nearly planar and the central double bond is elongated to 1.40 A and twisted by 42 degrees . In the folded structures, the xanthylidene moieties are folded by 45 degrees and the fluorenylidene moieties by 18-20 degrees . Factors stabilizing the twisted and folded conformations are discussed, including twisting of formal single or double bonds, intramolecular overcrowding, and the significance of a dipolar aromatic "xanthenylium-fluorenide" push-pull structure.     

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.     

A photochemical route to ferrocenyl‐substituted ferrapyrrolinone complexes

In the presence of iron pentacarbonyl, photochemical reaction between phenylisocyanate and ferrocenylacetylene results in ferrapyrrolinone complex [Fe₂(CO)₆(μ₂‐η³‐FcC═C(H)C(O)NPh)] ( 1 ) and maleimide 3‐ferrocenyl‐1‐phenyl‐1H ‐pyrrole‐2,5‐dione ( 2 ). Under similar experimental conditions, ferrocenyl−/phenyl‐substituted butadiyne primarily shows the activation of only one C☰C bond and results in ferrapyrrolinone complexes [Fe₂(CO)₆(μ₂‐η³‐FcC═C(C☰CR)C(O)NPh)] ( 3 , R = Fc; 3a , R = Ph), maleimides 3‐ferrocenyl‐1‐phenyl‐4‐(ferrocenylethynyl)‐1H –pyrrole‐2,5‐dione ( 5 ) and 3‐ferrocenyl‐1‐phenyl‐4‐(phenylethynyl)‐1H –pyrrole‐2,5‐dione ( 5a ) and [Fe₂(CO)₆(μ₂‐η³‐FcC═C(R)C(O)NPh)] ( 4 ; R  = 3‐ferrocenyl‐1‐phenyl‐1H ‐pyrrole‐2,5‐dione). Compound 4 consists of ferrapyrrolinone and a maleimide unit, formed by the activation of both C☰C bonds of diferrocenylbutadiyne. Activation of both C☰C bonds in a substituted butadiyne is a rare observation. Formation of the ferrapyrrolinone compounds is an advance over the earlier reported methods which generally use internal alkynes and involve prior synthesis of other clusters.     

Metallocenyl‐[2H]naphtho[1,2‐b]pyrans: metal effect on the photochromic behaviour

Previous studies have shown that the substitution by a ferrocenyl group in the 2‐position of naphthopyrans has a specific and an original effect on the photochromic behaviour. In this work, the synthesis and the photochromic properties of new naphthopyrans substituted in the 2‐position by three different metallocenyl groups (ferrocenyl, ruthenocenyl and osmocenyl) are presented. Whereas the ferrocenyl‐substituted derivatives under UV irradiation show two absorption bands, the ruthenocenyl and osmocenyl derivatives are characterized by only one absorption band under the same condition. The photochromic behaviour of these compounds is compared with that of their parent alkyl or phenyl 2‐substituted [2H]‐naphtho[1,2‐b]pyrans. Copyright © 2002 John Wiley & Sons, Ltd.     

Photo-and thermochromic properties of 1′,3′,3′-trimethyl-6-nitro-8-pyridiniomethylspiro[2H-[1]benzopyran-2,2′-indoline] chloride in the crystalline state

1′,3′,3′-Trimethyl-6-nitro-8-pyridiniomethylspiro[2H-[1]benzopyran-2,2′-indoline] chloride (1) was synthesized. Single crystals (crystal hydrates) of the open merocyanine form of compound 1 were obtained under the dark conditions, and their crystal structure was studied. On irradiation with visible light compound 1 crystallizes from propan-2-ol in the polycrystalline closed form. The spectral and photochemical properties of compound 1 in the polycrystalline state and the kinetic characteristics on thermal transformation of the closed form to the open form were studied. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1548–1553, September, 2006.     

X-ray single crystal study of reversible thermochromism and the pecularities of atomic thermal motion in n,n-diphenylguanidinium(1+) hexabromotellurate (IV)

Peculiarities of the atomic structure of N,N-diphenylguanidium (1+) hexabromotellurate (IV), (C₁₃H₁₄N₃)₂[TeBr₆], which has reversible thermochromic properties, have been studied by X-ray single crystal method at different temperatures. It has been revealed that temperature-dependent distortions of the geometry of the thermochrom compound are not expressed anyhow substantially; therefore, the temperature behavior of thermal parameters of Br atoms was studied in detail. For the first time the interrelation between the thermochromic properties of the complex and peculiarities of atomic thermal vibrations has been disclosed: abnormally great increase in the deflection angle of the axis σ₃ of the ellipsoid of thermal vibrations of apical bromine atoms from Te-Br bond direction under the temperature reduction reflects the decrease of vibronic interaction between the ground and the first excited state of Te(IV) (the display of Jahn-Teller dynamic effect of the second order) and, correspondingly, results in asymmetry lessening of the band A in the absorption spectrum and causes the change in compound color determined by it. There has been revealed the presence of C-H…Br hydrogen bond via hydrogen atoms of phenyl rings of diphenylguanidium. Basing on the analysis of atomic thermal parameters, it has been established that the apical atom Br(4) having the shortest distance in the octahedron is linked with the central atom relatively weaker than other atoms.     

Formation of a leuco Spirolactone from 4‐(2‐Carboxyphenyl)‐7‐diethylamino‐4′‐dimethylamino‐1‐benzopyrylium: Design of a Phase‐Change Thermochromic System Based on a Flavylium Dye

A phase‐change thermochromic system was designed through the reversible transformation of the 4‐substituted flavylium dye 4‐(2‐carboxyphenyl)‐7‐diethylamino‐4′‐dimethylamino‐1‐benzopyrylium into its leuco form, in the presence of a developer (ethyldiisopropylamine) and a suitable solvent (e.g., acetonitrile, n‐pentadecanonitrile). The leuco form of the flavylium‐based dye is a spirolactone species whose ring opens at low temperature (below the solvent melting point) to form the blue flavylium cation. Decarboxylation of the lactone to give 4‐phenyl‐7‐diethylamino‐4′‐dimethylamino‐1‐benzopyrylium was observed upon irradiation of the system with UV light, erasing the thermochromic effect.     

Novel Bisthienylethene Containing Ferrocenyl‐Substituted Naphthalimide: A Photo‐ and Redox Multi‐Addressable Molecular Switch

A new photochromic bisthienylethene system (BTE? NAFc) is reported in which the ferrocene unit (Fc) is incorporated into a naphthalimide chromophore as the central ethene bridging unit. The incorporated Fc unit in the photochromic system of BTE? NAFc has several effects on optical properties, such as fluorescence‐modulation through photoinduced electron transfer (PET), a decrease in the photochromic cyclization quantum yield, and a selective two‐step oxidation process. The ability to drive ring‐opening and ring‐closing reactions with a secondary redox‐modulation provides increased functionality to the photochromic system. Based on these meaningful photo‐ and redox‐modulation properties, five unprecedented multi‐addressable states (BTE? NAFc, BTE? NAFc⁺, c‐BTE? NAFc, c‐BTE? NAFc⁺, and BTE⁺? NAFc⁺) and gated photochromism are successfully obtained within the unimolecular BTE platform, thus providing deeper insight into photochromic systems as multifunctional outputs.     

A Versatile Dithienylethene‐Functionalized Ph‐Diazabutadiene (DAB) Ligand: From Photoswitchable Main‐Group Molecules to Photochromic Polymers

Within the past decade photochromic materials, specifically dithienylethenes (DTEs), have received increased interest because of their ability to function as potential photoswitchable molecular devices and optical memory storage systems. Current research in this area has focused on incorporating organic architectures to functionalize the DTE framework and alter the resulting photophysical properties; however, their syntheses are often not trivial. In this context, we have designed a simple and versatile diimine ( 2 ) containing adjacent 2,5‐dimethyl(thienyl) rings in the backbone. This redox active diimine ( 2 ) acts as a precursor to a novel photochromic ligand and has been used to coordinate to both boron and phosphorus elements, along with the synthesis of a phosphorane‐side‐chain functionalized polymer without further functionalization to the parent DTE framework. A study of the resulting photochromic properties of these compounds revealed that 1) the UV‐visible absorption spectra of the closed‐ring isomer were dependent of the element present in the N,N′‐chelating pocket and 2) incorporating the dithienylethene into a side‐functionalized phosphorane polymer greatly increased the closed‐/open‐ring reversibility and decreased the formation of by‐products.     

N‐Substituted Dicyanomethylphenyl Radicals: Dynamic Covalent Properties and Formation of Stimuli‐Responsive Cyclophanes by Self‐Assembly

Dynamic covalent bonds and their chemistry have been of particular interest both from a fundamental and materials science aspect. Demonstrated herein is that triphenylamine (TPA) and carbazole (Cz), substituted with a dicyanomethyl radical, are useful motifs for dynamic covalent chemistry as they have the appropriate bond strength between monomer units as well as high stability and synthetic simplicity. TPA and Cz units substituted by two dicyanomethyl radicals formed macrocyclic oligomers classified as novel types of azacyclophanes, and in particular, the TPA‐based diradical gave a cyclic dimer in almost quantitative yield. The cyclic oligomers exhibited thermo‐ and mechanochromic behavior resulting from the generation of radical species by intermonomer C?C bond cleavage.     

A Dithienylethene‐Based Rewritable Hydrogelator

Dithienylethene photochromic switching units have been incorporated into a hydrogelating system based on a tripeptide motif. The resulting hybrid system provided both a photochromic response and the ability to gelate water under acidic and neutral conditions. Fluorescence spectroscopy shows that the dithienylethene units are in sufficient proximity to each other to stack in gel fibers, with the tripeptide unit determining solubility. TEM measurements provided insight into the microscopic structure of the fibers formed.     

Regioselective complexation in the 2H‐chromene series and studies of their photochromic properties

The regioselectivity of the complexation of chromenes with tricarbonyl(trispyridine)chromium [Cr(CO)₃Py₃] can be controlled by the addition of BF₃·Et₂O boron trifluoride etherate. Depending on the molar ratio of the Lewis acid, different complexes can be prepared. The spectrokinetic properties of the so‐formed complexes were studied and it was shown that the thermal bleaching constants are either more or less reduced, depending on the position of ­the chromium tricarbonyl moiety. Copyright © 2000 John Wiley & Sons, Ltd.     

A Fast,Visible‐Light‐Sensitive Azobenzene for Bioorthogonal Ligation

Azobenzenes have been used as photoresponsive units for the control of numerous biological processes. Primary prerequisites for such applications are site‐selective incorporation of photoswitchable units into biomolecules and the possibility of using non‐destructive and deep‐tissue‐penetrating visible light for the photoisomerization. Here we report a push–pull azobenzene that readily undergoes a Staudinger–Bertozzi ligation with azide groups, that can be addressed with visible light (>440 nm) and exhibits the solvato‐ and acidochromism typical for push–pull systems. The thermal relaxation in aqueous environment proceeds on the low‐millisecond timescale, thus enabling control over biological processes on similar timescales. The approach is demonstrated in the modification of a quartz surface and in the incorporation of an azobenzene unit into a functional peptide, the third zinc finger in the mammalian factor Sp1.     

A Multi‐Addressable Switch Based on the Dimethyldihydropyrene Photochrome with Remarkable Proton‐Triggered Photo‐opening Efficiency

A series of photochromic derivatives based on the trans‐10b,10c‐dimethyl‐10b,10c‐dihydropyrene (DHP, “closed form”) skeleton has been synthesized and their photoisomerization leading to the corresponding cyclophanediene (CPD, “open form”) isomers has been investigated by UV/Vis and ¹H NMR spectroscopies. Substitution of the DHP core with electron‐withdrawing pyridinium groups was found to have major effects on the photoisomerization efficiency, the most remarkable examples being to enhance the quantum yield of the opening reaction and to allow fast and quantitative conversions at much lower radiant energies. This effect was rationalized by theoretical calculations. We also show that the reverse reaction, that is, going from the open form to the closed form, can be electrochemically triggered by oxidation of the CPD unit and that the photo‐opening properties of pyridine‐substituted DHPs can be efficiently tuned by protonation, the system behaving as a multi‐addressable molecular switch. These multi‐addressable photochromes show promise for the development of responsive materials.