NMR structural and kinetic assignment of fluoro-3H-naphthopyran photomerocyanines

Naphthopyran molecular switches and their emergent mechanochemical reactivity

Naphthopyran molecular switches undergo a ring-opening reaction upon external stimulation to generate intensely colored merocyanine dyes. Their unique modularity and synthetic accessibility afford exceptional control over their properties and stimuli-responsive behavior. Commercial applications of naphthopyrans as photoswitches in photochromic ophthalmic lenses have spurred an extensive body of work exploring naphthopyran-merocyanine structure-property relationships. The recently discovered mechanochromic behavior of naphthopyrans has led to their emergent application in the field of polymer mechanochemistry, enabling advances in the design of force-responsive materials as well as fundamental insights into mechanochemical reactivity. The structure-property relationships established in the photochemical literature serve as a convenient blueprint for the design of naphthopyran molecular force probes with precisely tuned properties. On the other hand, the mechanochemical reactivity of naphthopyran diverges in many cases from the conventional photochemical pathways, resulting in unexpected properties and opportunities for deeper understanding and innovation in polymer mechanochemistry. Here, we highlight the features of the naphthopyran scaffold that render it a powerful platform for the design of mechanochromic materials and review recent advances in naphthopyran mechanochemistry.

Designing naphthopyran mechanophores with tunable mechanochromic behavior

Mechanochromic molecular force probes conveniently report on stress and strain in polymeric materials through straightforward visual cues. We capitalize on the versatility of the naphthopyran framework to design a series of mechanochromic mechanophores that exhibit highly tunable color and fading kinetics after mechanochemical activation. Structurally diverse naphthopyran crosslinkers are synthesized and covalently incorporated into silicone elastomers, where the mechanochemical ring-opening reactions are achieved under tension to generate the merocyanine dyes. Strategic structural modifications to the naphthopyran mechanophore scaffold produce dramatic differences in the color and thermal electrocyclization behavior of the corresponding merocyanine dyes. The color of the merocyanines varies from orange-yellow to purple upon the introduction of an electron donating pyrrolidine substituent, while the rate of thermal electrocyclization is controlled through electronic and steric factors, enabling access to derivatives that display both fast-fading and persistent coloration after mechanical activation and subsequent stress relaxation. In addition to identifying key structure-property relationships for tuning the behavior of the naphthopyran mechanophore, the modularity of the naphthopyran platform is demonstrated by leveraging blends of structurally distinct mechanophores to create materials with desirable multicolor mechanochromic and complex stimuli-responsive behavior, expanding the scope and accessibility of force-responsive materials for applications such as multimodal sensing.

Bridging the visible: the modulation of the absorption by more than 450 nm

The coexistence of 18 isomers, identified by NMR spectroscopy and gathered into six states with very different absorption properties, is observed upon irradiation with selective wavelengths of a triphotochromic system joining two naphthopyran entities through a dithienylethene bridge. The sequential electronic coupling of photochromic groups resulted in an absorption behavior reaching far into NIR.

Controlled Conversion of Isomers in a Hybrid Biphotochromic System

[reaction: see text] The photochromic performance of a hybrid system connecting naphthopyran and dithienylethene was investigated, and the photochemistry of eight different isomers was explored by choosing an appropriate wavelength of light.

Synthesis and NMR Spectroscopic Characterization of Some Fluoro-2H-1-Benzopyran Derivatives

The synthesis of some new fluoro-2H-1-benzopyran derivatives utilizing a reaction between titanium phenolates and β-phenylcinnamaldehydes in toluene is reported. These compounds were characterized by NMR and UV/visible spectroscopy as well as mass spectrometry. In solution all the compounds are photochromic. Complete assignment of the 1H and 13C resonances was achieved by concerted application of homonuclear (gs-COSY), proton-detected (C, H) one-bond (gs-HMQC), and long-range (gs-HMBC) heteronuclear two-dimensional chemical shift correlation experiments using a 500 MHz NMR spectrometer equipped with a cryoplatform and a 5 mm cryoprobe. The mass spectra of the different compounds were characterized by intense molecular and high fragment ions. The introduction of an atom of fluorine as a molecular probe is of interest in determining the mechanistic aspects of the photochemical process.

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.

NMR Kinetic Investigations of the Photochemical and Thermal Reactions of a Photochromic Chromene

The photochromic behavior of 2,2-di(4-fluorophenyl)-6-methoxy-2H-1-chromene has been investigated by 19F NMR spectroscopy. Photocoloration under UV irradiation at low temperature led to the formation of three interconverting photoisomers including two merocyanines and an unprecedented allenyl-phenol isomer. Photobleaching with visible light, which was known to lead to reversion to the initial closed chromene, was shown to increase allenyl-phenol concentration. Thermal relaxation of the preirradiated system was also studied at various temperatures. In each case (UV and visible irradiations, thermal isomerization), the kinetics of each of the four species was monitored. Numerical analysis of concentration vs time profiles enabled us to unequivocally establish the global mechanism occurring in each of the experimental conditions and to interpret the specific reactivity of each photoisomer. It has been shown that, among the 12 possible isomerization processes, only some paths were active. For the first time, it has been possible to determine their corresponding thermal activation parameters and photochemical quantum yield ratios.

Multinuclear NMR structural characterization of an unprecedented photochromic allene intermediate

[structure: see text] An o-allenyl-p-methoxyphenol intermediate (Int) was for the first time proved to be involved in the photochromic reaction of 2,2-di(4-fluorophenyl)-6-methoxy-2H-1-chromene (FC). Transoid-cis (TC) and transoid-trans (TT) photomerocyanines and the intermediate Int are generated by UV irradiation of FC. Visible light converts TC and TT into Int. The monitoring of the reactions was carried out on the basis of (1)H, (13)C, (19)F, and (2)H NMR spectroscopy.

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.