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.

Biohybrid antenna systems have been constructed that contain synthetic chromophores attached to 31mer analogues of the bacterial photosynthetic core light-harvesting (LH1) β-polypeptide. The peptides are engineered with a Cys site for bioconjugation with maleimide-terminated chromophores, which include synthetic bacteriochlorins (BC1, BC2) with strong near-infrared absorption and commercial dyes Oregon green (OGR) and rhodamine red (RR) with strong absorption in the blue-green to yellow-orange regions. The peptides place the Cys 14 (or 6) residues before a native His site that binds bacteriochlorophyll a (BChl-a) and, like the native LH proteins, have high helical content as probed by single-reflection IR spectroscopy. The His residue associates with BChl-a as in the native LH1 β-polypeptide to form dimeric ββ-subunit complexes [31mer(-14Cys)X/BChl](2), where X is one of the synthetic chromophores. The native-like BChl-a dimer has Q(y) absorption at 820 nm and serves as the acceptor for energy from light absorbed by the appended synthetic chromophore. The energy-transfer characteristics of biohybrid complexes have been characterized by steady-state and time-resolved fluorescence and absorption measurements. The quantum yields of energy transfer from a synthetic chromophore located 14 residues from the BChl-coordinating His site are as follows: OGR (0.30) < RR (0.60) < BC2 (0.90). Oligomeric assemblies of the subunit complexes [31mer(-14Cys)X/BChl](n) are accompanied by a bathochromic shift of the Q(y) absorption of the BChl-a oligomer as far as the 850-nm position found in cyclic native photosynthetic LH2 complexes. Room-temperature stabilized oligomeric biohybrids have energy-transfer quantum yields comparable to those of the dimeric subunit complexes as follows: OGR (0.20) < RR (0.80) < BC1 (0.90). Thus, the new biohybrid antennas retain the energy-transfer and self-assembly characteristics of the native antenna complexes, offer enhanced coverage of the solar spectrum, and illustrate a versatile paradigm for the construction of artificial LH systems.

The challenge of creating both pigment building blocks and scaffolding to organize a large number of such pigments has long constituted a central impediment to the construction of artificial light-harvesting architectures. Light-harvesting (LH) antennas in photosynthetic bacteria are formed in a two-tiered selfassembly process wherein (1) a peptide dyad containing two bacteriochlorophyll a molecules forms, and(2) the dyads associate to form cyclic oligomers composed of 8 or 9 dyads in LH2 and 15 or 16 in LH1 of purple photosynthetic bacteria. While such antenna systems generally have near-quantitative transfer of excitation energy among pigments, only a fraction of the solar spectrum is typically absorbed. A platform architecture for study of light-harvesting phenomena has been developed that employs native photosynthetic peptide analogs, native bacteriochlorophyll a, and synthetic near-infrared-absorbing bacteriochlorins. Herein, the syntheses of 10 lipophilic bacteriochlorins are reported, of which 7 contain bioconjugatable handles (maleimide, iodoacetamide, formyl, carboxylic acid) for attachment to the peptide chassis. The bioconjugatable bacteriochlorins typically exhibit a long-wavelength absorption band in the range 710 to 820 nm, fluorescence yield of 0.1-0.2, and lifetime of the lowest singlet excited state of 2-5 ns. The a-helical structure of the native-like peptide is retained upon conjugation with a synthetic bacteriochlorin, as judged by single-reflection infrared studies. Static and time-resolved optical studies of the oligomeric biohybrid architectures in aqueous detergent solution reveal efficient ($90%) excitation energy transfer from the attached bacteriochlorin to the native-like bacteriochlorophyll a sites. The biohybrid light-harvesting architectures thus exploit the self-constituting features of the natural systems yet enable versatile incorporation of members from a palette of synthetic chromophores, thereby opening the door to a wide variety of studies in artificial photosynthesis.

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.