Incorporation of Acrylate Based Spiropyran Monoliths in Micro-Fluidic Devices for Photo-Controlled Electroosmotic Flow

Spiropyran photochromic compounds can be switched using light exposure between a non-polar spiro form (SP) and a zwitterionic merocyanine form (MC) that is subject to protonation (MC-H+). It has recently been demonstrated by Walsh et al. that, under acidic conditions, electroosmotic flow (EOF) generated in vinyl based spiropyran monoliths can be modulated using light irradiation [1]. In this paper, we report a spiropyran-modified acrylate based monolith which is particularly sensitive to protonation in the MC form, producing a positively charged surface that converts to the unpolar SP form by exposure to white light. When the MC-H+ form is dominant, it produces a charged surface which enables a relatively high flow rate (up to 1.6 μl/min) to be generated under electroosmotic conditions. Upon exposure to white light, the concentration of MC-H+ decreases due to the photo-conversion to the uncharged SP form, with up to 20% reduction of the EOF. The process is reversible, and removal of the light source results in a flow increase back to the original rate. The ability to alter flow rates in micro-fluidic channels using light has very significant implications, as it could dramatically simplify the manner in which micro-flow systems are controlled.

Synthesis and Photochromic Properties of Poly(4-vinylpyridine-co-styrene) with Pendant Spiroxazine Moieties

An ionic polymer with pendant photochromic moieties was prepared by the reaction of poly(4-vinylpyridine-co-styrene) and 1,3,3-trimethyl-6′-bromohexyloxyspiro[2H]-indol-2,3′-[3H]-naphth[2,1-b][1,4]oxazine. The photochromic reaction in question is caused by the reversible heterolytic cleavage of the C(spiro)—O bond under UV irradiation, yielding the colored form that can return to the colorless form by ring closure under visible light irradiation or in dark. The visible range absorption of the corresponding polymer increased gradually by UV irradiation which is ascribed to the generation of the open merocyanine form from the closed spiro form. The ionic conductivity of polymer increased upon UV irradiation, which brought about the generation of zwitterions form, and subsequently decreased in dark, which brought about the generation of closed spiro form. Sufficient reversibility was found in this polymer and this response was completely synchronized with that in the absorbance changes.

Light-induced change of the molecular charge in a spironaphthoxazine compound

To evaluate the experimental conditions for the light-induced change of the molecular charge, the dependence of the photochromic reaction behaviour of the indolino spiro naphthoxazine compound 1,3,3-trimethyl-spiro[2H-indol 2,3′-[3H]naphth[2,1-b]-[1,4]oxazine] on the pH value of the solution is investigated. In the absence of UV light an acid–base equilibrium between the spiro form and a protonated closed form is established. By irradiation under appropriate acidic conditions the spiro form can be transformed into an open cationic form through the merocyanine form. Between the two open forms an acid–base equilibrium is established. Except for the closed protonated form the structures are confirmed by mean of NMR. The kinetics of the thermal ring-closing reaction of both forms are investigated by flash photolysis. It is shown that the ring-closing reaction proceeds exclusively via the merocyanine form. The pK values determined for both equilibria give the pH range, in which the light-induced change of the molecular charge occurs.