Tunable Long-Range Interactions between Self-Trapped Beams driven by the Thermal Response of Photoresponsive Hydrogels

Abstract

Spiropyran-functionalized hydrogels are a promising platform for nonlinear optical materials. These gels show an unprecedented combination of reversibility and processability while requiring low laser power. Long-range beam interactions are especially intriguing for applications in all-optical computing or sensing. Here we show that thermal swelling or deswelling of the gel accompanies the light-driven spiropyran isomerization and that these thermal effects play a significant role in both single-beam selftrapping and long-and short-range interactions of multiple beams. The direction and magnitude of a hydrogel's thermal response are controlled by the gel composition, which allows the behavior of single and multiple beams in these gel materials to be programmed over a vast chemical design space. We demonstrate this potential by comparing two-beam interactions in spiropyran-functionalized poly(acrylamide-co-acrylic acid) gels to those in poly(Nisopropylacrylamide-co-acrylic acid) gels, which undergo swelling or contraction with the increase in temperature, respectively. The presence of a secondary field that influences beam behavior leads to repulsive long-range interactions in the former gels, while the latter lack long-range repulsive interactions and show behaviors indicative of short-range attractive interactions, such as spiraling and beam fusion. This tunable diversity of behaviors suggests the enormous potential for hydrogels as versatile, soft, and nonlinear optical materials.

Spiropyran Photoisomerization Dynamics in Multiresponsive Hydrogels

“…These results have important implications for the design and optimization of light-responsive hydrogels for small length scales, where the quadratic dependence of the time scale of poroelastic diffusion on the characteristic length scale can lead to a relatively fast response of the hydrogel. For example, at the micrometer scale of microfluidics, − mesoporous hydrogels, , or self-trapped beams, , the time scale of poroelastic diffusivity can be seconds or less. In these cases, the tens of seconds we observe to be required for complete photoswitching even under high-intensity focused laser light becomes limiting to the full photoresponse.…”

“…Photoresponsive hydrogels have found applications in a wide variety of fields, including microfluidics, − soft robotic walkers, tools for mechanical interactions with biological matter, switchable surfaces, − data storage, drug delivery, and as a nonlinear optical material. , The spiropyran photoswitch is commonly used to impart light sensitivity to hydrogels due to the reversible changes in molecular structure, charge, and hydrophobicity it undergoes in response to visible light irradiation . Commonly, spiropyran pendant moieties are directly tethered to the polymer backbone. − Under acidic aqueous conditions these moieties exist primarily in the charged, protonated, ring-open merocyanine (MCH + ) form.…”

“…However, the rational design of such multifunctional hydrogels requires an understanding of how the choice of specific co-monomers affects the interconversion dynamics between different states of the spiropyran moieties. While significant work has been done to understand the effects of the spiropyran molecular structure on its photochemistry in solution ,,,− and on the effects of incorporation into a dense polymer matrix, less work has been done to explore the effects that co-monomers in an aqueous hydrogel environment have on spiropyran dynamics. , Additionally, the high-intensity, focused laser light conditions relevant to the nonlinear optics of spiropyran-functionalized hydrogels , constitute an environment in which spiropyran switching kinetics have rarely been studied. As part of our ongoing efforts to codify design principles for hydrogels that achieve self-regulation via opto-chemo-mechanical feedback behaviors, ,,− we used laser scanning confocal fluorescence microscopy and UV–vis spectroscopy to probe the effects of different co-monomers on the dynamics of light-driven and thermal isomerization of the spiropyran moieties within four different commonly used hydrogel materials: temperature-responsive pNIPAAm gels, pH-responsive pAAc and poly­(2-(dimethylamino)­methacrylate) (pDMAEMA) gels, and nonresponsive polyacrylamide (pAAm) gels.…”

From Appendage to Crosslinker – Unusual Swelling Behavior in Spiropyran-Modified Hydrogels

“…2) in the context of our efforts to design optically active, self-regulated hydrogels. 44,45 To this end, spiropyran variants 1 and 2 were synthesized (SI Sections 1 and 2) and co-polymerized into methylenebisacrylamide-crosslinked polyacrylamide hydrogels (SI Section 3, Tables S3.1-S3.3, verification of attachment by FTIR in SI Section 5.1). The spiropyran-free, control hydrogel, p(AAm-co-AA) polymerized at a 1:1 AAm:AA molar ratio and crosslinked with 1.1 mol% methylenebisacrylamide (BIS), swells in aqueous media with a swelling ratio (SR) increasing from ~2 to ~11 as pH increases from pH 5.5 to 7.5, due to the accumulation of negative charges on the polymer backbone resulting from the deprotonation of carboxylic acid moieties (pKa ~4.7, Fig.…”

“…Finally, we note that the presence of spiropyran aggregates may have significant effects on non-linear optical phenomena such as beam-trapping, which are under investigation in our laboratory. 44,45…”