Naphthopyran molecular switches and their emergent mechanochemical reactivity

Abstract

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.

Fig. 1. Mechanophores covalently incorporated into polymer chains undergo productive chemical reactions in response to mechanical force. The mechanochromic transformation of naphthopyran is illustrated.

Scheme 1. Examples of mechanochromic mechanophores that generate color upon mechanochemical activation via homolysis (top) or a retro-Diels–Alder reaction (bottom).

Scheme 2. Poly(ladderene) undergoes a force-induced unzipping reaction to produce conjugated polyacetylene.

Scheme 3. Examples of mechanochromic mechanophores that undergo ring-opening reactions upon mechanochemical activation to produce thermally reversible colored dyes.

Scheme 4. Ring-opening reaction of a naphthopyran molecular switch based on the 3H-naphtho[2,1-b]pyran skeleton to reversibly generate a colored merocyanine dye.

Fig. 2. Molecular scaffolds depicting the three diaryl naphthopyran regioisomers with atomic positions identified. The different naming conventions are illustrated for each structure.

Scheme 5. Typical synthesis of naphthopyrans via acid-catalyzed condensation of naphthols and diaryl propargyl alcohols that proceeds through a multistep pathway. The synthesis of a 3H-naphtho[2,1-b]pyran is illustrated.

Scheme 6. Side reactions encountered in the synthesis of naphthopyrans via the acid-catalyzed route including the Meyer–Schuster rearrangement (top pathway, blue arrows) and conjugate addition to the propargyl cation at the 4-position of the 1-naphthol (bottom pathway, red arrows).

Scheme 7. Reaction conditions for the acid-catalyzed synthesis of naphthopyrans developed by Zhao and Carreira.

Scheme 8. Alternative titanium-mediated synthesis of angular naphthopyrans from naphthols and substituted cinnamaldehydes.

Scheme 9. Solvent-free reaction conditions for the synthesis of naphthopyrans via grinding or ball-milling.

Scheme 10. Exemplary photochemical ring-opening reaction of a 3H-naphthopyran and alkene isomerization to generate principal cis-transoid (CT) and trans-transoid (TT) isomeric merocyanine products.

Fig. 3. Representative reaction pathway for the photochemical ring-opening reaction of naphthopyran.

Scheme 11. The ring-opening reactions of 3H- and 2H-naphthopyrans generate merocyanine isomers with varying degrees of steric demand, which strongly influences absorption properties and reversion kinetics.

Fig. 4. UV-vis absorption spectra of 2,2-bis(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran (2H-NP) and 3,3-bis(4-methoxyphenyl)-3H-naphtho[2,1-b]pyran (3H-NP) in toluene before and after irradiation with UV light. Adapted with permission from ref. . Copyright 2006, Elsevier.

Fig. 5. Naphthopyrans have been developed extensively as UV light-responsive coloring (i.e., photochromic) agents in plastic ophthalmic lenses.

Fig. 6. Electron-donating para-substituents result in bathochromically shifted (longer wavelength) merocyanine absorption and increase the rate of thermal reversion. Values shown were measured in toluene at 20 °C.

Fig. 7. Merocyanine color can be finely tuned by modulating the steric demand of the group adjacent to a para-pyrrolidine substituent. The λ_max and half-life (t_1/2) are given for merocyanines with various meta-substituents (X). Values shown were measured in toluene at 20 °C.

Scheme 12. Amine substituents can stabilize 3H-naphthopyran-derived merocyanines through resonance.

Scheme 13. (a) Substituents at the 2- and 10-positions of 3H-naphthopyrans result in severe steric repulsion in the trans merocyanine. (b) Hydrogen bond acceptors at the 10-position of 3H-naphthopyrans stabilize the cis merocyanine stereoisomer.

Scheme 14. Ring fusion at the 4- and 5-positions of 2H-naphthopyrans prevents isomerization of the cis merocyanine isomer to the more thermally stable trans configuration.

Fig. 8. (a) Regioisomer specific mechanochemical activity of 3H-naphthopyran. Three naphthopyran regioisomers with varying polymer attachment positions all exhibit photochromic behavior, but only regioisomer NP5 is mechanochemically active due to effective force distribution across the labile C–O pyran bond. (b) Photographs of polydimethylsiloxane samples covalently crosslinked with the three different naphthopyran regioisomers subjected to uniaxial tension or UV light irradiation. Reproduced with permission from ref. . Copyright 2016, American Chemical Society.

Fig. 9. (a) Different 3H-naphthopyran mechanophores with varying substitution affecting merocyanine color and reversion rate. (b) Photographs of polydimethylsiloxane samples covalently incorporating the naphthopyran crosslinkers before and after being subjected to uniaxial tension and subsequent stress relaxation, or UV light irradiation, demonstrating the modularity of the naphthopyran mechanophore for designing mechanochromic materials. Adapted from ref. .

Fig. 10. Isomeric 2H- and 3H-naphthopyran mechanophores generate merocyanine dyes with distinct coloration and thermal stability. Photographs illustrate PDMS materials covalently incorporating the different naphthopyran crosslinkers with 2H-naphthopyran generating a red merocyanine that reverts relatively slowly compared to the more transient yellow merocyanine produced from a similarly substituted 3H-naphthopyran mechanophore.

Fig. 11. In contrast to the reversible photochemical reaction, mechanochemical activation of a 2H-naphthopyran mechanophore with polymer attachment via an ester linkage at the 10-position generates a persistent merocyanine dye stabilized by an intramolecular hydrogen bonding interaction that is established upon scission of the C(O)–O ester bond.

Fig. 12. Force-dependent multicolor mechanochromism from a bis-naphthopyran (BNP) mechanophore. (a) Unlike the sequential ring-opening reactions observed upon photochemical activation, mechanical force directly converts the BNP mechanophore to the purple bis-merocyanine species, which thermally reverts to the yellow monomerocyanine. Changing the amount of force delivered to the mechanophore biases the merocyanine distribution resulting in force-dependent coloration. (b) UV-vis absorption spectra of a 330 kDa poly(methyl acrylate) polymer with a chain-centered BNP unit illustrating sequential-ring opening behavior upon irradiation with UV light. Ultrasound-induced mechanochemical activation of (c) the same 330 kDa polymer, and (d) an analogous 40 kDa polymer that experiences lower force, illustrating non-sequential ring opening behavior. Insets show photographs of the final sonicated solutions. Adapted with permission from ref. . Copyright 2019, American Chemical Society.

Fig. 13. (a) Divergent photochemical and mechanochemical reactivity of a 3H-naphthodipyran (NDP) moiety incorporated near the center of a poly(methyl acrylate) chain. (b) UV-vis-near-infrared absorption spectra before and after photoirradiation of the polymer with UV light or ultrasound-induced mechanochemical activation. Inset shows a photograph of the two solutions after activation. (c) Absorption spectra recorded during ultrasonication of the polymer and time-dependent absorption profiles at 625 and 820 nm characteristic of monomerocyanine NDP_O–C·nBF₃ and dimerocyanine NDP_O–O·nBF₃, respectively. Adapted with permission from ref. . Copyright 2022, American Chemical Society.

Fig. 14. Complex force-responsive materials incorporating a naphthopyran mechanophore. (a) Blending a diarylbibenzofuranone and naphthopyran mechanophore into different phases of a silica/polymer composite material enables multicolor mechanochromism under different stimulation conditions. (b) Multicolor mechanochromism is achieved in polydimethylsiloxane elastomers by blending two naphthopyran mechanophores that generate merocyanine dyes with distinct colors and reversion kinetics. Portions of the figure were adapted from ref. .