Date:
Friday, March 14, 2025 - 10:00am
Location:
E2 1519 | https://ucsb.zoom.us/j/5937498325?omn=81122108450 | Meeting ID: 593 749 8325
Speaker:
Sara Sandlass
Title: Fundamental characterization of donor acceptor Stenhouse adducts in photoactive polymers towards applications in soft photoactuation
Abstract
Polymeric materials that display photoresponsive properties are at the core of many diverse research fields, including optical sensing, photoactuation, photopharmacology, optical data storage/transmission, and surface/interface engineering. To synthesize these “smart” materials, a polymer host is embedded with a photosensitizing agent that absorbs the incident radiation and converts this energy into a combination of thermal and chemical bond energy through nonradiative relaxation to drive a change in the chemical or physical properties of the chromophore/polymer system. Organic photoswitches are a class of chromophores that are well suited for this particular application because they undergo reversible absorption-induced isomerization, resulting in conformational changes that can be leveraged into macroscopic structural responses when exposed to light.
Donor-acceptor Stenhouse adducts, or DASAs, are a class of organic photoswitches that are ideal for photoactive material applications because they are visible light active (ε ~ 100,000 M-1cm-1), become transparent when irradiated (i.e., negatively photochromic), are kinetically and optically tunable through facile synthetic modifications, and can respond to multiple orthogonal stimuli simultaneously including light, heat, polarity, pH, hydration, ionic strength, and stress. These unique stimuli responsive properties arise from the mechanism of DASA photoswitching, which proceeds through an initial actinic (photochemical) step, followed by a series of thermal isomerizations occurring on the ground state potential energy surface (PES). This ground state PES is highly susceptible to the influence of external stimuli and to interactions between DASA and the host polymer, presenting both an opportunity for enhanced functionality and a fundamental challenge to characterize and control.
This work addresses the challenge of designing and implementing photoactive polymers from both a fundamental and applied perspective. The fundamental exploration focuses primarily on quantifying how polarizing and viscosity-driven environmental interactions shape the photoswitching kinetics and optical properties of DASA in polymers by leveraging a range of spectroscopic techniques. A novel optical technique called frequency modulated pump probe spectroscopy, or FMPPS, was developed in this work and combined with UV-Vis and polarized light absorption spectroscopy to measure dynamic processes such as photoswitching and stress-induced chromophore alignment. These improvements make it possible to measure the environmental sensitivity of the metastable intermediate isomer of DASA, even though this species is often too unstable to accumulate in concentrations detectable by standard methods (< 0.05 a.u.). Zooming out from the molecular level to the macroscopic scale, this work concludes by demonstrating two different applications in light powered actuation for photoresponsive polymers that are relatively simple in their chemical makeup, the first being a photothermally actuated fluid lens and the second being a DASA-based photothermally actuated bilayer cantilever. These examples aim to demonstrate that while molecular level fundamental insights are valuable for designing novel photoresponsive materials, engineering optimization is responsible for taking these materials from conceptual to functional.
Event Type:
General Event
