Akito Ishida

Surface plasmon excitation of porphyrin self-assembly monolayers on an Au surface

Surface plasmon (SP) enhanced fluorescence spectroscopy has been employed to elucidate the molecular orientation, molecular exchange process, and photoelectrochemical properties of photofunctional self-assembly monolayers (SAMs) for the first time. A 48 nm Au film on a BK-7 right-angle prism was treated with a porphyrin disulfide ((-S(CH2)10CONH-Por)2, Por = p-(tris(p-tolyl)porphyrinyl)phenyl) (1) giving a porphyrin SAM (2). SP excitation of 2 using p-polarized 425 nm light at an incident angle of 55° gave a strongly p-polarized fluorescence of the porphyrin. This indicates selective excitation of the porphyrins having transition moments effectively overlap with the SP field vectors and the suppressed molecular motions in 2. The Q bands in the fluorescence excitation spectrum were considerably enhanced attributable to the field enhancement of SP. The molecular exchange process of a decanethiol SAM with 1 has been elucidated based on changes in the fluorescence properties. The photoelectrochemical measurement using 632.8 nm light gave a much more intense photocurrent (18.6 µA cm-2) than that provided by conventional direct photoexcitation (2.7 µA cm-2). Moreover, the photocurrent was observed using near-IR light while the energy is considerably lower than the lowest singlet excited state of the porphyrin.

Photocurrent generation of a porphyrin self-assembly monolayer on a gold film electrode by surface plasmon excitation using near-infrared light

Abstract Photocurrent was generated by irradiation of a porphyrin self-assembly monolayer on a gold film electrode with near-infrared (IR) as well as visible light under the surface plasmon (SP) resonance conditions. The action spectrum by the SP excitation demonstrated existence of discrete active bands in the near-IR region together with the Soret and Q -bands. These bands may be attributable to the absorption of the aggregates and the singlet–triplet transition of the porphyrin. The strongly enhanced electromagnetic field of SP in the near-IR is considered to undergo excitation of very weak discrete absorption bands which cannot be observed by the conventional absorption measurement.

Stilbene Dimer Radical Cations in the Radiolyses of Stilbenes and 1,2,3,4-Tetraphenylcyclobutanes

The reaction of the stilbene radical cation formed by pulse radiolysis or γ-radiolyses is explained based on neutralization as well as the formation of a π-type stilbene dimer radical cation (π-St2+•), converting to the σ-type St2+• (σ-St2+•). The r-1,c-2,t-3,t-4-tetraphenylcyclobutane radical cation generated in a rigid matrix at 77 K which converted to σ-St2+• upon warming. Both r-1,c-2,t-3,t-4- and r-1,t-2,c-3,t-4-tetraphenylcyclobutane radical cations underwent photochemical cycloreversion to π-St2+• upon irradiation at wavelengths longer than 390 nm at 77 K, and converted to σ-St2+• upon warming. It is suggested that π-St2+• has overlapping arrangements of π-electrons, while σ-St2+• has radical and cation centers on the 1- and 4-positions of the C4 linkage.The reaction of the stilbene radical cation formed by pulse radiolysis or γ-radiolyses is explained based on neutralization as well as the formation of a π-type stilbene dimer radical cation (π-St2+•), converting to the σ-type St2+• (σ-St2+•). The r-1,c-2,t-3,t-4-tetraphenylcyclobutane radical cation generated in a rigid matrix at 77 K which converted to σ-St2+• upon warming. Both r-1,c-2,t-3,t-4- and r-1,t-2,c-3,t-4-tetraphenylcyclobutane radical cations underwent photochemical cycloreversion to π-St2+• upon irradiation at wavelengths longer than 390 nm at 77 K, and converted to σ-St2+• upon warming. It is suggested that π-St2+• has overlapping arrangements of π-electrons, while σ-St2+• has radical and cation centers on the 1- and 4-positions of the C4 linkage.

Surface plasmon enhanced fluorescence measurement on flat and constructed gold surfaces

The fluorescence properties of porphyrin self-assembly monolayers (SAMs) and FITC labeled polylysine (PL) on a gold surface under surface plasmon (SP) excitation and the effect of the surface structure were studied for the effective application of SP excitation to fluorescence analysis on a gold surface. The spacer between the chromophores and the gold surface effectively mitigates the fluorescence quenching by gold, which is an unavoidable disadvantage of fluorescence analysis on a gold surface. The decay of the SP field intensity by the spacer was negligible and the chromophores were effectively excited to give enhanced fluorescence. SP excitation provided considerable advantages over the conventional direct photoirradiation. The forbidden Q-bands were clearly identified in the fluorescence excitation spectrum of the porphyrin SAM under SP excitation, but identification by direct photoirradiation was impossible. The background scattered excitation light was effectively eliminated by the SP excitation, which was particularly effective for fluorescein iosthiocyanate labeled PL having a small Stokes shift. The advantages of SP excitation were remarkably enhanced by a two-axis grating structure of the gold surface. Moreover, direct excitation of the porphyrin SAM prepared on the gold-coated two-axis grating and gold microdots on Si showed enhanced fluorescence similarly to the SP excitation, suggesting induction of the edge and/or particle plasmons.

Selective two-electron reduction of C60 by10-methyl-9,10-dihydroacridine via photoinduced electrontransfer

The selective two-electron reduction of C 60 to 1,2-dihydro[60]fullerene is attained via photoinduced electron transfer from 10-methyl-9,10-dihydroacridine to the triplet excited state of C 60 in the presence of trifluoroacetic acid in benzonitrile under irradiation of visible light.

Synthesis of 2,′:5′,2″-terpyridine and 2,2′:5′,2″:5″,2‴-quaterpyridine and their photocatalysis of the reduction of water

2,2′:5′,2″-Terpyridine (OPy-3) and 2,2′:5′,2″:5″,2‴-quaterpyridine (OPy-4) are prepared by Ni°complex-catalysed polycondensation of 2,5-dichloropyridine in the presence of an excess of 2chloropyridine. These two compounds show more efficient photocatalysis of H2 evolution by the reduction of water than p-terphenyl and p-quaterphenyl in the presence of triethylamine as an electron donor, and RuCl3 or K2PtCl6 as a source of Ruo or Pto colloids which function as an electron mediator. The primary photochemical processes of OPy-n(n= 3 or 4) and the successive reactions have been investigated by γ-radiolysis, pulse radiolysis and laser flash photolysis. The OPy-n molecules are converted to their anion radicals through reductive quenching of the excited states. A mechanism for the photocatalysis of hydrogen evolution is proposed in which the anion radicals supply electrons to protons on the metal colloids. The anion radicals undergo protonation in competition with electron transfer, resulting in the loss of photocatalytic activity.

Mechanistic studies of the one-electron oxidation of water to hydroxyl radicals photosensitized by perfluorinated p-terphenyl

Photo-oxidation of water in a system consisting of perfluorinated p-terphenyl (F-OPP-3) as a photosensitizer, O2 as an oxidative quencher and benzene as a hydroxyl radical scavenger is mechanistically studied by laser flash photolysis and γ-ray and pulse radiolysis. The radical cation of F-OPP-3 (F-OPP-3˙+) is formed via the oxidative quenching of its excited singlet state by O2. The decay of F-OPP-3˙+ observed in transient spectroscopy is enhanced by the addition of water to the system, indicating that F-OPP-3˙+ plays a crucial role in the one-electron oxidation of water.

Photoluminescence lifetime of GaP/AlP superlattices grown by gas‐source molecular‐beam epitaxy

The photoluminescence lifetimes of GaP/AlP short‐period superlattices (GaP)11/(AlP)3 and (GaP)9/(AlP)5, grown on GaP(001) by gas‐source molecular‐beam epitaxy are studied by a time‐correlated single photon counting method at 4.2 and 298 K. Two components of the photoluminescence lifetime are found for each sample. The fast component is about 2 ns accompanied by a slow component of about 20 ns. Discussion of the possible explanations of the two components includes the co‐existence of direct‐ and indirect‐band‐gap transitions in the superlattice, which was fabricated from the indirect‐band‐gap constituents.

Surface plasmon excitation of a porphyrin covalently linked to a gold surface

Fluorescence of a porphyrin covalently linked to a gold surface is observed with surface plasmon excitation using p-polarized light to show higher excitation efficiency for the Q-bands than the Soret band.

A novel CO2 photoreduction system consisting of phenazine as a photosensitizer and cobalt cyclam as a CO2 scavenger

Abstract Photoreduction of CO 2 to formate (HCO 2 − ) can be achieved by the phenazine-catalyzed system consisting of cobalt cyclam complex (Co-cyclam, cyclam = 1,4,8,11-tetraazacyclotetradecane) as an electron mediator and triethylamine as an electron donor. Flash photolysis revealed that the catalytic system should involve electron transfer from the photoformed radical anion of phenazine (P •− to Co III cyclam and hydrogen transfer from phenazinyl radical (P •− ) to intermediary Co II cyclam. The resulting cobalt hydride complex, Co-cyclam(H), provides formate through Co III formate complex formed by CO 2 insertion.