Anomalous photochromism and mechanochromism of a linear naphthopyran enabled by a polarizing dialkylamine substituent

Abstract

In contrast to common angular naphthopyrans that exhibit strong photochromic and mechanochromic behavior, constitutionally isomeric linear naphthopyrans are typically not photochromic, due to the putative instability of the completely dearomatized merocyanine product. The photochemistry of linear naphthopyrans is thus relatively understudied compared to angular naphthopyrans, while the mechanochromism of linear naphthopyrans remains completely unexplored. Here we demonstrate that the incorporation of a polarizing dialkylamine substituent enables photochromic and mechanochromic behavior from polymers containing a novel linear naphthopyran motif. In solution phase experiments, a Lewis acid trap was necessary to observe accumulation of the merocyanine product upon photochemical and ultrasound-induced mechanochemical activation. However, the same linear naphthopyran molecule incorporated as a crosslinker in polydimethylsiloxane elastomers renders the materials photochromic and mechanochromic without the addition of any trapping agent. This study provides insights into the photochromic and mechanochromic reactivity of linear naphthopyrans that have conventionally been considered functionally inert, adding a new class of naphthopyran molecular switches to the repertoire of stimuli-responsive polymers.

Scheme 1. Naphthopyran isomers and their stimuli-responsive behavior. (a) Angular naphthopyrans undergo ring-opening reactions with UV light or mechanical force. (b) Photochromism and mechanochromism of linear 2H-naphthopyran is enabled upon introduction of an electron donating dialkylamine substituent.

Fig. 1. Density functional theory (DFT) calculations using the constrained geometries simulate external force (CoGEF) method performed on a linear 2H-naphtho[2,3-b]pyran model containing a para-pyrrolidine substituent predict a ring-opening reaction upon mechanical elongation. The structure of the predicted merocyanine product is shown, corresponding to the position in the profile denoted by the arrow. The features at ∼5.5 and 6.5 Å displacement correspond to conformational changes. Calculations were performed at the B3LYP/6-31G* level of theory.

Scheme 2. Synthesis of poly(methyl acrylate) polymers incorporating a linear 2H-naphtho[2,3-b]pyran unit.

Fig. 2. UV-vis absorption spectra of PMA-PyLNP in CH₃CN with 0.5 mM BF₃·Et₂O during (a) photoirradiation at −30 °C with 311 nm UV light, and (b) continuous ultrasonication at −15 °C. Ultrasonication of PMA-Control results in negligible merocyanine formation. Insets show absorbance at 540 nm as a function of photoirradiation or sonication time.

Fig. 3. (a) PDMS networks covalently crosslinked with linear 2H-naphthopyran mechanophore Crosslinker-PyLNP (1.5 wt%) prepared via Pt-catalyzed hydrosilylation. (b) Photographs of the material before and after photoirradiation with 365 nm UV light for 120 s through a photomask, and after mechanical force applied via compression (2×) using an embossed stamp. Schematic representations of the photomask and stamp are shown.