Photochromism of Diarylnaphthopyrans

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.

Synthesis and Photochromism of Novel Pyridyl-Substituted Naphthopyrans

Multitarget synthetic strategies to access novel photochromic 3H-naphtho[2,1-b]pyrans decorated with pyridyl units are described. The new pyridyl-substituted 3H-naphtho[2,1-b]pyrans display good photochromic properties with reversible generation of photomerocyanines, which exhibit mainly orange/red hues. Photochromic parameters including photocolorability and persistence of color vary tremendously on structural modification of the naphthopyran core.

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.

Base-Mediated Ring-Contraction of Pyran Systems Promoted by Palladium and Phase-Transfer Catalysis

A study of the ring-contraction of a model 3H-naphtho[2,1-b]pyran is described to elucidate and optimize the ring-contraction of naphthopyrans. Two efficient base-mediated protocols to access multiple naphthofurans, naphthodifurans, and a benzo-fused indole in generally good yields are reported. Furthermore, a protocol to selectively prepare (hetero)aryl-substituted naphthofurans via a Suzuki-coupling-ring-contraction process is presented. An additional protocol that allows Suzuki cross-coupling reactions to be performed on bromo-substituted naphthopyrans without the ring-contraction side reaction is reported.

Photochemistry of Photochromic Benzopyrans Studied by Time-Resolved Absorption Spectroscopy

Photochromic reactions of 2H-benzopyrans have been extensively studied to design new materials for commercial applications such as optical memories. Photochromism of 2H-benzopyrans proceeds from a C–O bond cleavage of the colorless closed form to give the colored open forms, which can thermally revert back to the original closed form. From time-resolved absorption spectroscopy, the ring opening of 2H-benzopyrans, 2,4-diphenyl-and 2,2,4-triphenyl-2H-benzopyran, is found to occur via the excited singlet state within 2 ps to produce vibrationally excited open forms in the ground electronic state. In the subnanosecond to submillisecond time domain, several decay components are observed. These components are assigned to respective stereoisomers with respect to two double bonds and one single bond of the open enone forms. As revealed from pump-laser power dependencies of the yields of the open forms, the photocleavage of the benzopyran molecules gives at first only the open forms revertible to the closed form by a single-bond rotation, and the photoexcitation of the first generated open forms gives rise to other open forms that need a double-bond rotation for reversion to the original closed form. Such a two-step two-photon photochromism can be expected to have a wide range of application.

Synthesis and Photochromic Properties of Conjugated Naphthopyran Derivatives

Three novel types of naphthopyran derivatives containing alkoxyl and phenyl moieties, 13-butyl −6,11-dimethyl-3,3-diphenyl-3,13-dihydrobenzo[h]indeno[2,1-f]chromen-13-ol (NP), 13-butyl-3,3-bis(6-methoxy-[1,1′-biphenyl]-3-yl)-6,6-dimethyl-3,13-dihydrobenzo[h] indeno[2,1-f]chromen-13-ol (NP1) and 13-butyl-3,3-bis(6-methoxy-4′-naphthalenyl-[1,1′-biphenyl]-3-yl)-6,11-dimenthyl-3,13-dihydrobenz[h]indeno[2,1-f]chromen-13-ol (NP2), were synthesised and characterised. Measurements indicated that they possessed an excellent photochromic response, a rapid thermal bleaching rate and good fatigue resistance. Meanwhile, the UV maximum absorption wavelengths of NP1 (562 nm) and NP2 (597 nm) appeared bathochromically shifted compared with NP (536 nm). The bleaching rate of NP1 was found to be approximately twice that of NP. The results indicated that the UV maximum absorption wavelength and bleaching rate increase with increasing length of the phenyl side chain moieties. In PMMA film, the compounds showed almost the same photochromic properties in the solid state as in dichloromethane solution. The UV absorbance with high-contrast switching in solid film could be beneficial for practical applications.

Efficient Photochemical Merocyanine-to-Spiropyran Ring Closure Mechanism through an Extended Conical Intersection Seam. A Model CASSCF/CASPT2 Study

A mechanism of the thermal and photochemical bleaching of merocyanine to spiropyran is proposed on the basis of CASSCF/CASPT2 calculations on the 6-(2-propenyliden)cyclohexadienone model system. Our results suggest that this photochemical transformation takes place in two steps. First, the initially pumped 1(pi-pi) S2 undergoes radiationless decay to 1(n-pi) S1 via an extended S2/S1 conical intersection seam that runs approximately parallel to the trans-to-cis isomerization coordinate, a few kilocalories per mole higher in energy. Thus, S2 --> S1 internal conversion is possible at all values of the S2 trans-to-cis reaction coordinate. Second, on the S1 potential energy surface, there is a barrierless ring closure reaction path from the S1 cis minimum that leads to a peaked S1/S0 conical intersection where the deactivation to the ground state takes place. The inertia of the moving nuclei then drives the system toward the ground-state minimum of the 2H-chromene product. Thus, the extended seam topology of the S2/S1 conical intersection and the coordinate of the branching space of the S1/S0 conical intersection are essential to explain the efficiency and high speed of this reaction.

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

The kinetic and structural behavior of a photochromic compound, 3-(2-fluorophenyl)-3-phenyl-3H-naphtho[2,1-b]pyran (F-Py), was investigated using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. Upon irradiation, the four theoretically predicted photomerocyanines appear along with a fifth form X, whose final structure has not been elucidated. This last form and two of the photomerocyanines are thermally labile, whereas the other two do not show any signs of decay. The system has been analyzed by NMR spectroscopy. This led to the structural assignment of each photomerocyanine. The kinetics of the thermal bleaching were monitored by directly and separately measuring the concentrations of each species at regular time intervals using 19F NMR spectroscopy. We therefore propose a plausible reaction mechanism. On the basis of this mechanism, the mathematical treatment and the study of the effects of temperature led to the determination of the kinetic and thermodynamic parameters (rate coefficients, enthalpy and entropy of activation) of this photochromic system. The leading role of the labile intermediate X on the formation of trans-transoid-cis (TTC) and cis-transoid-cis (CTC) photomerocyanines is pointed out.

One-Pot synthesis of photochromic naphthopyrans in the solid state

p-TsOH-catalyzed condensation reactions of 1,1-diaryl-2-propyn-1-ol (1) and 2-naphthol (2) in the solid state gave 3, 3-diaryl-3H-naphtho[2,1-b]pyran (6) via Claisen rearrangement. Similar reactions of 1 with 2,6- (7) and 2,7-dihydroxynaphthalenes (8) afforded naphthodipyrane derivatives 9 and 10, respectively. Bis-naphthopyran derivatives 12 were also obtained by the reaction of 1,1,6,6-tetraaryl-2,5-hexadiyn-1,6-diol (11) and 2-naphthol (2) in the solid state.