A coating of photoresponsive spiropyran molecules covalently bound to a glass surface along with a mixture of organosilanes to control the surface environment was prepared. The relatively nonpolar spiropyran can be reversibly switched to a polar, zwitterionic merocyanine isomer that has a much larger dipole moment by UV light, and back again by visible light. The contact angle was 11°-14°lower for dry, spiropyrancoated surfaces after irradiation with UV than that for dry, spiropyran-coated surfaces after irradiation with visible light. The light-induced changes observed in the surface energy were correlated to the switching of the surface-bound spiropyran molecule between polar and nonpolar forms by means of fluorescence spectroscopy and epifluorescence microscopy. Water in capillary tubes coated with the photosensitive layer was observed to rise when the wavelength of incident light was switched from visible to UV. The UVinduced rise was of the order of 2.8 mm for a 500 µm diameter capillary. This microfluidic actuation of water in an enclosed capillary or microchannel using light is termed "photocapillarity". Contact angle hysteresis prevented the water from flowing back down the capillary when the light was switched from UV back to visible.
A comprehensive theoretical study of the reaction mechanisms for the conversion between spiropyrans (SPs) and the open form of merocyanines (MCs) has been conducted by theoretical calculations. The reaction mechanisms on the ground-and triplet-state potential energy surfaces (PESs) were investigated using the density functional method. Time-dependent density functional theory (TD-DFT) calculations using the CIS optimized excited-state geometries were carried out to study the reaction mechanisms on the lowest excited singlet-state PES. Two possible reaction mechanisms for the thermal conversion between SPs to MCs were found on the ground-state PES. The geometrical parameter, BLA (Bond Length Alternation), which correlates the strengths of the substituents and the polarities of solvents, was used to explain the changes in the reaction mechanism induced by the different donor-acceptor pairs and solvents. In addition, the reaction mechanisms of spiropyran/merocyanine conversion on the triplet and the lowest excited singlet potential energy surfaces were also studied; several possible reaction mechanisms on the excited-state PESs were proposed. A comprehensive mechanistic view of the ultrafast photochemistry of spiropyrans was revealed and interpreted in terms of the strengths of substituents and the polarity of solvents.
A rough surface morphology is shown to significantly amplify the light-induced change in water contact angle of a photoresponsive surface. Smooth Si surfaces and fractally rough Si nanowire surfaces grown on a Si substrate were studied, both coated with a hydrophobic monolayer containing photochromic spiropyran molecules. Under visible irradiation the spiropyran is in a closed, hydrophobic form, whereas UV irradiation converts the spiropyran to a polar, hydrophilic form, reducing the contact angle. The superhydrophobic nanowire surface both amplifies the light-induced contact angle change by a factor of 2 relative to a smooth surface and reduces the contact angle hysteresis. As a result the UV-induced advancing contact angle is lower than the receding contact angle under visible irradiation, allowing water drops to be moved solely under the influence of a UV−visible light gradient. The amplification of the reversible light-induced wetting angle change was predicted using the Wenzel model for fractally rough surfaces. The model and amplification effects are expected to apply to other types of stimuli-induced contact angle changes such as that by heat or electrical potentials.
Microgels with photo-, thermally, and pH-responsive properties in aqueous suspension have been synthesized and characterized using dynamic light scattering and UV-visible spectroscopy. The new route involved first preparing poly(N-isopropylacrylamide) (PNIPAM)-allylamine copolymer microgels and a spiropyran photochrome (SP) bearing a carboxylic acid group. Then the functionalized spiropyran was coupled to the microgel via an amide bond. The dark-equilibrated gel particles feature spiropyran molecules in the polar, merocyanine form. After irradiation of visible light, the particle size becomes smaller because spiropyran changes to the relatively nonpolar, closed spiro form. The PNIPAM-SP microgels undergo a volume phase transition in water from a swollen state to a collapsed state with increasing temperature under all light conditions. However, the transition temperature range of the PNIPAM-SP is much broader than that for the PNIPAM without SP. The PNIPAM-SP microgels are monodisperse and self-assemble into a crystalline lattice while in suspension. The UV-visible spectra of an aqueous suspension of PNIPAM-SP microgel in the dark-adapted, merocyanine form showed both an absorption peak around 512 nm due to the merocyanine (giving a reddish color to the suspension) and two sharp peaks from Bragg diffraction of colloidal crystallites. Upon visible irradiation, the 512-nm band bleached significantly due to spiropyran photoisomerization. The spiropyran photoisomerization and accompanying color changes of the suspension were reversible upon alternating dark, UV, and visible light irradiation. Due to the residues of amine groups, the swelling capability of PNIPAM-SP microgels reduces as the pH value is changed from 7 to 10.
Spiropyrans are a group of organic molecules that undergo a reversible photoinduced transformation (i.e., photochromism) from a colorless, nonplanar spiropyran form to a colored, planar merocyanine form. Photochromism is accompanied by a large change in the structure and in the dipole moment. These changes suggest that such molecules might be useful in light-controlled, “smart surface” applications. This study examines the effect of the microenvironment near the surface-bound spiropyran on its photochemistry. The surfaces were designed to exhibit a mixture of hydrophobic and hydrophilic components by using a mixed silane chemistry on a glass substrate, and the spiropyran was covalently bound to the surface via amide linkages. The solvatochromic behavior of spiropyran derivatives was studied in solution using UV−vis absorption spectroscopy and fluorescence spectroscopy for comparison with the surface-bound species. Spiropyrans in solution and on the surface both exhibited negative solvatochromism. Correlations between emission maxima of the spiropyrans and Reichardt's E T(30) polarity scale revealed that the surface-bound spiropyran experienced lower polarity than a solution model in solvents of low and medium polarities. Linear solvation energy relationships using the Kamlet−Taft polarity scales showed that hydrogen bonding played a prominent role in solvent stabilization of surface-bound spiropyrans in hydrogen-bonding solvents. The surface design used causes the spiropyran to interact significantly with the surface in solvents of lower polarity and to behave as if it were dissolved in solution in more polar, hydrogen-bonding solvents.
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