Indolino-Oxazolidine Acido- and Photochromic System Investigated by NMR and Density Functional Theory Calculations.

Three addressable indolino-oxazolidine units connected through an isomerizable double bond to a substituted thiophene represent a smart example of a multiaddressable system whose reversible responses could be selectively activated on demand. Experimental and theoretical approaches to push forward the understanding of the system mechanism and set pathways to design optimized compounds for suitable application are here presented. NMR and UV-visible spectroscopies are used for structural and kinetic studies, while density functional theory (DFT) calculations pave the way to highlight energetic and electronic processes that are involved. Substitution and solvent effects toward the reactivity of the compounds are experimentally studied and combined with theoretical calculations. The most efficient and selective stimuli to travel between the four possible states resulting from the ring-opening of indolino[2,1- b]oxazolidine (generally referenced as BOX) derivatives and the trans-cis isomerization of the ethylenic junction are elucidated.

Control of the Switching Speed of Photochromic Naphthopyrans through Restriction of Double Bond Isomerization.

An efficient synthesis of photochromic fused-naphthopyrans was developed. UV-vis or sunlight irradiation of these uncolored compounds in solution led to the formation of a single colored photoisomer along with an unusual and uncolored bicyclic compound formed through an intramolecular photochemical Diels-Alder reaction. Both species faded thermally in the dark to the initial form. A mechanism for this transformation is proposed based on NMR studies of irradiated solutions. The new fused-naphthopyrans have been incorporated into hybrid organic-inorganic matrices affording light-yellow materials that develop intense red colorations under UV light and return to the initial uncolored state in just a few seconds, in the dark, at room temperature. These results are useful for the development of fast switching materials used in the production of photochromic lenses.

Fast Color Change with Photochromic Fused Naphthopyrans.

Photochromic molecules can reversibly develop color upon irradiation with UV light. These smart molecules, mainly in the naphthopyran family, have been applied with success to ophthalmic lenses that darken quickly under sunlight and revert to the uncolored state after several minutes in the dark. This slow adaptation to the absence of light is one of the limitations and is due to the formation of an unwanted photoisomer. We have designed a new naphthopyran with a bridged structure which prohibits the formation of the undesirable, persistent photoisomer and thus shows a very fast switching between the uncolored and colored states. UV irradiation of a hybrid siloxane matrix doped with the new fused naphthopyran leads to the formation of a pink coloration bleaching in a few milliseconds, in the absence of light, at room temperature. This new fused naphthopyran is easily prepared in three steps from readily accessible precursors and is amenable to structural modifications to tailor color and lifetime of the colored photoisomer.

A simple molecule-based octastate switch.

Dithienylethene oxazolidine hybrid system connected through an isomerizable double bond exists under eight molecular states on demand. Combinations of electrocyclization of dithienylethene, Z/E isomerization and acid-base oxazolidine change cause selective addressabilities. Two intricate gated photochromic performances allow the execution of an 8-step molecular switch, which renders this molecular system the most complex known up to date.

A multifunctional photoswitch: 6π electrocyclization versus ESIPT and metalation.

A terthiazole-based molecular switch associating 6π electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady-state and ultrafast absorption spectroscopy and rationalized by DFT/TDDFT calculations. The switch behaves as a biphotochrome with time-dependent photochemical outcome and displays efficient ESIPT-based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.

Acid-catalyzed domino reactions of tetraarylbut-2-yne-1,4-diols. Synthesis of conjugated indenes and inden-2-ones.

The reaction of tetraarylbut-2-yne-1,4-diols with electron-rich aromatic compounds at room temperature, under p-TsOH catalysis, affords substituted polycyclic aromatic indene derivatives through a domino reaction involving the formation of a cationic allenylium intermediate. This species can undergo a series of competitive intramolecular cascade reactions, leading to a conjugated inden-2-one. This simple method allows the efficient synthesis of substituted indenes and inden-2-ones, in two steps, from aromatic ketones.

Synthesis of 1-vinylidene-naphthofurans: a thermally reversible photochromic system that colors only when adsorbed on silica gel.

A set of new 1-vinylidene-1,2-dihydro-naphtho[2,1-b]furans were unexpectedly obtained in the reaction of 2-naphthol with readily available 1,1,4,4-tetraarylbut-2-yne-1,4-diols. Surprisingly, when adsorbed in silica gel, these compounds exhibit photochromism at room temperature, whereas not in solution and in the solid state. UV or sunlight irradiation leads, in a few seconds, to the formation of intense pink/violet to green colors that bleach completely in a few minutes in the dark. These new compounds also exhibit reversible acidochromic properties in solution: addition of TFA leads to the opening of the furan ring and addition of a proton to the allene function, leading to a conjugated and violet tertiary carbocation that returned immediately to the uncolored allenic structure upon addition of a weak base.

Photochromic C2-symmetric chiral diarylethene: from the initial state to the final state.

The behavior of 1,2-bis[(R)-2-(1-methoxymethoxyethyl)-3-benzo[b]thienyl] hexafluorocyclopentene before and upon UV irradiation and during thermal evolution of the photoirradiated solution has been thoroughly investigated by multinuclear NMR spectroscopy. A dynamic NMR study of the initial state was performed, providing a detailed description of the perceived conformational processes in the system. Before irradiation, three open conformations are in equilibrium, whereas UV irradiation generated the two expected cyclized diastereomers. The minor diastereomer was thermally less stable than the major one, thus leading to an unexpected increase in the diastereoselectivity when raising the photoirradiation temperature. In addition, a long thermal evolution induced slow rearrangement of the stable diastereomer into byproducts that were identified.

Photochromic fused-naphthopyrans without residual color.

A series of new photochromic fused-naphthopyrans with an alkyl bridge between the pyran ring and the naphthalenic core was synthesized in several steps from 4-(bromomethyl)benzocoumarin. The presence of the alkyl bridge in these new fused-naphthopyrans prevents the formation of one long-lived photoisomer and therefore has a dramatic effect on their photochromic properties: UV irradiation of common naphthopyrans gives rise to two isomeric colored photoisomers, one of which fades very slowly and is responsible for a persistent residual color. UV excitation of these new uncolored fused-naphthopyrans leads to the formation of only one colored photoisomer that fades completely to the uncolored state in few seconds/minutes following a monoexponential decay law, thus avoiding the problem of the residual coloration typically observed with naphthopyrans.

Reactant-induced photoactivation of in situ generated organogold intermediates leading to alkynylated indoles via Csp2-Csp cross-coupling.

Photosensitization of organogold intermediates is an emerging field in catalysis. In this context, an access to 2,3-disubstituted indoles from o-alkynyl aniline and iodoalkyne derivatives via a gold-catalyzed sequence under visible-light irradiation and in the absence of an exogenous photocatalyst was uncovered. A wide scope of the process is observed. Of note, 2-iodo-ynamides can be used as electrophiles in this cross-coupling reaction. The resulting N-alkynyl indoles lend themselves to post-functionalization affording valuable scaffolds, notably benzo[a]carbazoles. Mechanistic studies converge on the fact that a potassium sulfonyl amide generates emissive aggregates in the reaction medium. Static quenching of these aggregates by a vinylgold(I) intermediate yields to an excited state of the latter, which can react with an electrophile via oxidative addition and reductive elimination to forge the key C-C bond. This reactant-induced photoactivation of an organogold intermediate opens rich perspectives in the field of cross-coupling reactions.

Light-Controlled Release and Uptake of Zinc Ions in Solution by a Photochromic Terthiazole-Based Ligand.

We have synthesized and fully characterized a photochromic zinc complex with a terphenylthiazole-based ligand with a salen-like cavity. The solution stability of the complex was found to be greatly dependent on the state of the photochromic ligand and an interesting photo-triggered release and uptake of zinc ions was found as well as monitored by its fluorescence. The contrasting stability difference of the zinc complex between its two isomeric states was rationalized by DFT calculations.

Comprehensive photokinetic and NMR study of a biphotochromic supermolecule involving two naphthopyrans linked to a central thiophene unit through acetylenic bonds.

A photophysical and photochemical study of a biphotochromic compound where two naphthopyran units are linked by an acetylene-thiophene-acetylene bridge has been carried out in toluene. Both fluorescence and intersystem crossing to the triplet manifold were found to compete with the photocoloration process. Two photoproducts (transoid-trans and transoid-cis stereoisomers), absorbing at approximately 480 nm and corresponding to the opening of a single photochromic unit, were detected by spectrophotometric analysis after short irradiation time in diluted solution and identified by 1H-nuclear magnetic resonance (NMR) spectroscopy. After prolonged irradiation at 228 K of highly concentrated solutions (up to 3 x 10(-3) mol dm(-3)), two additional isomers, absorbing at approximately 550 nm, were formed. Their NMR spectra indicate the opening of both photochromic units. An interesting effect of selective vibronic excitation was found, showing that the photoreaction is favored at excited vibronic levels to the detriment of the radiative relaxation.

Insights into the recombination of radical pairs in hexaarylbiimidazoles.

The (13)C NMR characterization of the 2,2'- and the 4,4'-hexaarylbiimidazoles is reported for the first time as resulting from the recombination of the radical pairs produced upon irradiation of the 1,2'-dimer, without any isolation processes, directly in a 500 MHz NMR spectrometer coupled with in situ laser irradiation.

The control of photochromism of [3H]-naphthopyran derivatives with intramolecular CH-π bonds.

The photochromism of [3H]-naphthopyran derivatives can be switched from T-type to inverse- or P-type through the manipulation of relative thermodynamic stabilities of open isomers with intramolecular CH-π bonds.

Preventing the formation of the long-lived colored transoid-trans photoisomer in photochromic benzopyrans.

A new photochromic fused benzopyran presenting a bridge between the pyran double bond and the benzenic ring was prepared. While the UV irradiation of usual benzopyrans leads to the formation of two colored photoisomers with very different thermal stabilities, studies by laser flash photolysis showed that the presence of this particular bridge prevents the formation of the undesirable long-lived colored TT isomer and therefore after laser irradiation the colored solution fades following a fast monoexponential decay.

Wavelength-dependent reactivity of a quinolinone: toward a photochromic three-state system.

The photochemical reactivity of the quinolinone 3 was investigated using NMR by monitoring its reactions under appropriate irradiation wavelengths. Besides the irreversible formation of degradation products which were structurally identified, the reversible formation of the enol 4 and cyclobutenol 5 was also observed. The enol and cyclobutenol can be switched or reversed back to the quinolinone 3, resulting in a photochromic three-state system in which the relative ratio of the three components largely depends on the irradiation wavelength used.

Studies of polyphotochromic behaviour of supermolecules by NMR spectroscopy. Part 2. A bis-[3H]naphthopyran with a (Z)-ethenic bridge between each moiety.

The behaviour of a new biphotochromic molecule (CC-Z), formed by two naphthopyran moieties linked through a (Z)-ethenic double bond, has been studied by 1H and 19F NMR spectroscopy. To facilitate the investigations, a naphthopyran (CHR) and a model constituted by a naphthopyran entity linked through a double bond spacer to a methoxynaphthalene nucleus (C-Z) were also studied. At first, the studies were performed at low temperature (228 K) to extract the 1H and 19F NMR chemical shifts characterising photomerocyanines. Thereafter, the behaviour at ambient temperature was checked, identifying a major reaction of (Z-E) isomerisation as well as an intra-molecular cyclisation reaction. An unexpected thermal opening of the naphthopyran entities in the cyclised form was also observed, leading to an equilibrium between the different stereoisomers of the photomerocyanines.

Studies of polyphotochromic behaviour of supermolecules by NMR spectroscopy. Part 1. A bis-spirooxazine with a (Z)-ethenic bridge between each moiety.

The photoproducts resulting from the irradiation of a closed colourless biphotochromic molecule formed by two spironaphthoxazine entities linked through an ethenic bridge and its model compound, a singular spironaphthoxazine joined by a double bond to a 2-methoxynaphthalene nucleus in the Z configuration have been studied by high resolution 1H NMR spectroscopy. At low temperature, a clean conversion between closed and open structures has been observed with the demonstration of mono and bi opening of the biphotochromic molecule. In contrast, studies at ambient temperature have implied an irreversible loss of the photochromic properties, with the formation of diastereoisomers, resulting from the cyclisation-oxidation reaction of the systems. But, by carrying out experiments on degassed samples, a significant reversion to closed initial structures was observed.

NMR characterisation of photo-electrocyclised structures of a spirooxazine derivative.

Three electrocyclised structures (C-DPh, TTC-DPh and CTC-DPh), produced after UV irradiation of a spirooxazine linked by a (Z)-ethenic bridge to a 2-methoxynaphthalene nucleus, have been identified by NMR spectroscopy.