Kohei Hosomizu

Substituent effects of porphyrins on structures and photophysical properties of amphiphilic porphyrin aggregates.

Substituent effects of porphyrin on the structures and photophysical properties of the J-aggregates of protonated 5-(4-alkoxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin have been examined for the first time. Selective formation of the porphyrin J-aggregate was attained when suitable length of the alkoxy group was employed for the amphiphilic porphyrin. Namely, a regular leaflike structure was observed for the J-aggregates of protonated 5-(4-octyloxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin, which was consistent with the results obtained by using the UV-visible absorption and dynamic light-scattering measurements. A bilayer structure in which the hydrophobic alkoxyl groups facing inside the bilayer are interdigitated to each other, whereas the hydrophilic porphyrin moieties are exposed outside, was proposed to explain the unique porphyrin J-aggregate. Fast energy migration and efficient quenching by defect site in the J-aggregates were suggested to rationalize the short lifetimes of the excited J-aggregates.

Photoinduced electron transfer in multilayer self-assembled structures of porphyrins and porphyrin–fullerene dyads on ITO

A new strategy for constructing well-ordered, self-assembled multilayer structures of photoactive donor–acceptor systems has been developed. In this approach indium–tin oxide (ITO) electrodes were modified with successive self-assembled monolayers of Zn porphyrins (ZnP) and free-base porphyrin–fullerene (H2P–C60) dyads to obtain oriented triad configurations. All the layers were attached with two molecular linkers to achieve an organized composition. The multilayer structures were found to enhance the photocurrent and photovoltage generation compared to monolayers of the same compounds. Particularly, combining a densely packed ZnP layer with the highly oriented H2P–C60 dyad layer resulted in 280 times higher photovoltage response than measured for a monolayer of the dyad. Furthermore, both photovoltage and photocurrent were generated in the desired direction pursued with the sample orientations and, as a result of the organization, almost ideal current–voltage curves were formed.

Efficient photocurrent generation by SnO2 electrode modified electrophoretically with composite clusters of porphyrin-modified silica microparticle and fullerene

A silica microparticle has been successfully employed as a nanoscaffold to self-organize porphyrin and C60 molecules on a nanostructured SnO2 electrode which exhibits efficient photocurrent generation.

J-aggregation of a sulfonated amphiphilic porphyrin at the air–water interface as a function of pH

π-A isotherms, ellipsometric measurements, Brewster angle microscopy (BAM) and reflection spectroscopy have been utilized to characterize the films of an amphiphilic porphyrin ((OD)(3)TPPS(3)) at the air-water interface as a function of pH. This porphyrin forms stable mono-molecular layers at such interfaces, and exhibits different J-aggregation as a function of pH. The J-aggregation of (OD)(3)TPPS(3) on neutral pH subphases is notable considering that the nitrogen atoms at the central macrocycle have a pK(a)≈4.9. The type of aggregates at neutral pH is like those detected at pH<4, because the central porphyrin ring is already protonated. However at basic pH the aggregation happens without protonation of the central ring but can be instead controlled by application of the surface pressure. At the air-water interface, (OD)(3)TPPS(3) shows two bands, a red component and a blue component, which have characteristics of non-degenerate linear oscillators being perpendicularly polarized between each other. The spectral behavior observed on subphases at different pHs is qualitatively interpreted by means of exciton coupling theory, assuming that the degenerate transitions attributed to the Soret band are split. Additionally, highly oriented molecular films of these J-aggregates were deposited onto transparent quartz slides.

Tunable Soret-Band Splitting of an Amphiphilic Porphyrin by Surface Pressure

Porphyrin aggregates, that is, Jand H-aggregates, have attracted much attention because of their peculiar structural and photophysical properties, which make them potential candidates for applications in nonlinear optics, nanometer-sized photoconductors, light-harvesting systems, and catalysis. Porphyrin aggregation can be detected through spectrum changes of the degenerated Soret band. For freebase porphyrins with D2h symmetry, the degeneracy is theoretically lost. However, the absorption is not detectably split and a sole increased bandwidth is apparent with respect to porphyrins with D4h symmetry. [2] Such splitting can be detected for densely packed porphyrins as a function of the molecular organization. Thus, the Soret band of freebase porphyrins can be described in terms of two mutually perpendicular oscillators (namely, Bx, By). Soret-band splitting of porphyrins has been described before. However, controlling this phenomenon is difficult because of the high tendency of porphyrins to autoaggregate. A tunable Soret-band splitting is described in this communication for the first time in the same experiment. A novel amphiphilic porphyrin, namely, (OD)3TPPS3 (see Figure 1)—with a selected balance between three meso-phenyl groups possessing hydrophilic sulfonato moieties and one meso-phenyl group bearing three long hydrophobic alkoxy chains—is synthesized. The used patterns are known to produce ordered array structures in porphyrins and fullerenes. Figure 1 shows the p–A isotherm of (OD)3TPPS3 as well as the ellipsotherm (dD–A isotherms) on the basic subphase (pH 11, NaOH). The monolayer stability is analyzed by means of area–time diagrams and compression–expansion cycles (see the Supporting Information). The analysis reveals the formation of a stable monomolecular layer of (OD)3TPPS3 at the air–water interface. Moreover, from the ellipsometric study (see the Supporting Information), we calculate a thickness of 3.17 nm (A= 0.9 nm), which corresponds to a monolayer of upright porphyrins with perpendicular alkyl chains at the air–water interface for such a full-packing phase. Reflection spectroscopy of monolayers can selectively detect molecules at the air–water interface because of the reflectionenhanced effect that originates from absorption. Figure 2 shows reflection spectra (DR) of monolayers of (OD)3TPPS3 on Figure 1. Molecular structure of 5,10,15-tris(4-sulfonatophenyl)-20-(3,4,5-trin-octadecyloxyphenyl)porphyrin (OD)3TPPS3. p–A isotherm (c) and ellipsotherm, dD–A (g) of a monolayer of (OD)3TPPS3 at the air–water interface with basic subphase (pH 11, NaOH).

Hydrogen bonding effect on photocurrent generation in porphyrin–fullerene photoelectrochemical devices

A hydrogen bonding effect on photocurrent generation has been evaluated successfully in a mixed film of porphyrin and fullerene with hydrogen bonding on an ITO electrode, which exhibits efficient cathodic photocurrent generation as compared to the reference system without hydrogen bonding.