Abstract
We identify unique features of a highly-absorbing negatively photochromic molecular switch, donor acceptor Stenhouse adduct (DASA), that enable its use for self-regulating light-activated control of fluid flow. Leveraging features of DASA’s chemical properties and solvent-dependent reaction kinetics, we demonstrate its use for photo-controlled Rayleigh-Bénard convection to generate dynamic, self-regulating flows with unparalleled fluid velocities (~mm s−1) simply by illuminating the fluid with visible light. The exceptional absorbance of DASAs in solution, uniquely controllable reaction kinetics and resulting spatially-confined photothermal flows demonstrate the ways in which photoswitches present exciting opportunities for their use in optofluidics applications requiring tunable flow behavior.
Highlights
We identify unique features of a highly-absorbing negatively photochromic molecular switch, donor acceptor Stenhouse adduct (DASA), that enable its use for self-regulating light-activated control of fluid flow
Though DASA-CF3-PI switches from its colored, open form to a bleached, closed form (Figs. 1a, 2a) in both toluene and chloroform, the tunability of this behavior arises from solventand concentration-dependent photoswitching kinetics
The following studies highlight the dynamic control of flow behavior by varying solvent, concentration, and light intensity. This DASA derivative exhibits a faster backward photoreaction in toluene than in chloroform stabilized with 0.75% ethanol, as evidenced by reverse rates of reaction of 0.046 s−1 and 0.035 s−1 in toluene and chloroform, respectively, at 10 μM (Fig. 2b, Supplementary Note 1). This difference in kback is intensified at high concentrations—as DASA-CF3-PI exhibits a stronger concentration-dependence in toluene than in chloroform—leading to an even faster back reaction[29]
Summary
We identify unique features of a highly-absorbing negatively photochromic molecular switch, donor acceptor Stenhouse adduct (DASA), that enable its use for self-regulating light-activated control of fluid flow. The system operates as follows: by irradiating a solution containing DASA with visible light (Fig. 1b) a controlled bleaching front (i.e., a growing non-absorbing region) is generated, which enables the local manipulation of solution temperature (Fig. 1c) which in turn incites a timevarying thermal gradient and Rayleigh-Bénard convection (Fig. 1d) These convective flows are inherently self-regulated via the complex interplay between heat transfer within the fluid and to the external environment, the photochemical conversion kinetics, and the high absorption—enabling a novel method of control over fluid motion. This degree of control is presently inaccessible via other methods and, using a negatively photochromic molecular photoswitch such as DASA opens new avenues for the use of high-absorbing photoswitches in applications for self-regulated fluid pumping and mixing
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