Exploring the Spatial Distribution and Transport Behavior of Charge Carriers in a Single Titania Nanowire

One-dimensional nanostructures of metal oxide semiconductors have both potential and demonstrated applications for use in light waveguides, photodetectors, solar energy conversion, photocatalysis, etc. We investigated the transport and reaction dynamics of the photogenerated charge carriers in individual titania nanowires using single-particle photoluminescence (PL) spectroscopy. Examination of the spectral and kinetic characteristics revealed that the photoluminescence bands originating from defects in the bulk and/or on the surface appeared in the visible region with numerous photon bursts by photoirradiation using a 405-nm laser under an Ar atmosphere. From the single-molecule kinetic analysis of the bursts, it was found that the quenching reaction of trapped electrons by molecular oxygen follows a Langmuir−Hinshelwood mechanism. In addition, a novel spectroscopic method, i.e., single-molecule spectroelectrochemistry, was utilized to explore the nature of the defect states inherent in the wires. The spatially resolved PL imaging techniques thus enable us to ascertain the location of the luminescent active sites that are related to the heterogeneously distributed defects and to present experimental evidence of the long-distance transport of charge carriers in the wire. Consequently, this study provides a great opportunity to understand the role of defects in the behavior of charge carriers in TiO2 nanomaterials with various morphologies

Exploring the Spatial Distribution and Transport Behavior of Charge Carriers in a Single Titania Nanowire

One-dimensional nanostructures of metal oxide semiconductors have both potential and demonstrated applications for use in light waveguides, photodetectors, solar energy conversion, photocatalysis, etc. We investigated the transport and reaction dynamics of the photogenerated charge carriers in individual titania nanowires using single-particle photoluminescence (PL) spectroscopy. Examination of the spectral and kinetic characteristics revealed that the photoluminescence bands originating from defects in the bulk and/or on the surface appeared in the visible region with numerous photon bursts by photoirradiation using a 405-nm laser under an Ar atmosphere. From the single-molecule kinetic analysis of the bursts, it was found that the quenching reaction of trapped electrons by molecular oxygen follows a Langmuir−Hinshelwood mechanism. In addition, a novel spectroscopic method, i.e., single-molecule spectroelectrochemistry, was utilized to explore the nature of the defect states inherent in the wires. The spatially resolved PL imaging techniques thus enable us to ascertain the location of the luminescent active sites that are related to the heterogeneously distributed defects and to present experimental evidence of the long-distance transport of charge carriers in the wire. Consequently, this study provides a great opportunity to understand the role of defects in the behavior of charge carriers in TiO2 nanomaterials with various morphologies

Exploring the Spatial Distribution and Transport Behavior of Charge Carriers in a Single Titania Nanowire

One-dimensional nanostructures of metal oxide semiconductors have both potential and demonstrated applications for use in light waveguides, photodetectors, solar energy conversion, photocatalysis, etc. We investigated the transport and reaction dynamics of the photogenerated charge carriers in individual titania nanowires using single-particle photoluminescence (PL) spectroscopy. Examination of the spectral and kinetic characteristics revealed that the photoluminescence bands originating from defects in the bulk and/or on the surface appeared in the visible region with numerous photon bursts by photoirradiation using a 405-nm laser under an Ar atmosphere. From the single-molecule kinetic analysis of the bursts, it was found that the quenching reaction of trapped electrons by molecular oxygen follows a Langmuir−Hinshelwood mechanism. In addition, a novel spectroscopic method, i.e., single-molecule spectroelectrochemistry, was utilized to explore the nature of the defect states inherent in the wires. The spatially resolved PL imaging techniques thus enable us to ascertain the location of the luminescent active sites that are related to the heterogeneously distributed defects and to present experimental evidence of the long-distance transport of charge carriers in the wire. Consequently, this study provides a great opportunity to understand the role of defects in the behavior of charge carriers in TiO2 nanomaterials with various morphologies

Control of A Double Helix DNA Assembly by Use of Cross-Linked Oligonucleotides

Disulfide cross-linked oligonucleotides for connecting two DNA double helixes have been designed, synthesized, and characterized. Employing these cross-linked oligonucleotides, two double helixes can be arranged side by side, and the orientations can be controlled both in parallel and antiparallel ways by addition of a specific complementary DNA strand

Protein Recording Material: Photorecord/Erasable Protein Array Using a UV-Eliminative Linker

Protein patterning on solid surfaces is a topic of significant importance in the fields of biosensors, diagnostic assays, cell adhesion technologies, and biochip microarrays. In this letter, we have established a novel, rapid method for the fabrication of a “protein recording material”, which enables us to spatiotemporally regulate the recording, reading, and erasing of a fluorescent protein array as information by a photochemical technique. A photolinker that we synthesized here was used to control the protein array spatiotemporally. The recording process was almost completed after 1 min of photoirradiation to read a clear pattern consisting of a specific protein−ligand complex with high spatiotemporal resolution. The erasing of the protein array was then achieved by photoirradiation onto the entire patterned surface

Diastereochemically Controlled Porphyrin Dimer Formation on a DNA Duplex Scaffold

DNA-porphyrin conjugates were designed and synthesized for the preparation of the conformationally controlled porphyrin dimer structures constructed on a d(GCGTATACGC)2. Porphyrin derivatives were introduced to the central TATpA sequence where p represents the phosphoramidate for the attachment of the free-base porphyrin (FbP) and zinc-coordinated porphyrin (ZnP), which allows contact of the two porphyrins in the minor groove. The porphyrin dimers were characterized using CD, UV−vis, steady-state, and time-resolved fluorescence spectroscopies, indicating that the porphyrins form face-to-face conformations. Also the co-facial conformation was confirmed by comparison with spectra of the non-self-complementary duplex containing one porphyrin moiety. Introduction of zinc into porphyrin moiety destabilized the duplex formation. Two diastereomers showed different thermal stabilities and affected the conformations of porphyrin dimers. The temperature-dependent assembly and the conformational change of the porphyrin dimer on the duplex DNA were observed in the UV−vis spectra, indicating that the dynamic movement of the porphyrin dimer occurs on the duplex. The results indicate that the porphyrin dimers of DNA-FbP conjugates are overlapped clockwise and are located in the minor groove of the usual B-form DNA backbone. The interaction and conformation of two porphyrin moieties are controlled by the following three factors:  (1) temperature change during and after formation of the duplex porphyrins at lower temperature; (2) diastereochemistry of the phosphoramidates where porphyrins are connected via a linker; and (3) zinc ion coordination that destabilizes the interaction of porphyrins as well duplex formation

Formation of Dimer Radical Anions of Aromatic Acetylenes during Pulse Radiolysis and γ-Radiolysis

Dimerization of the radical anions of aromatic acetylenes (A•-) such as diphenylacetylene and its derivatives with substituents on the benzene ring (1•-), 1,4-diphenyl-1,3-butadiyne (2•-), and intramolecular dimer model compounds having two diphenylacetylene chromophores linked by several methylene chains (3•-) has been studied with pulse radiolysis of A in solutions at room temperature and γ-radiolysis in rigid matrices of A at 77 K. The transient absorptions of A•- decayed with the formation of new bands assignable to the dimer radical anions of A•- and A. Because the decay and formation depend on the concentration of A, the bimolecular rate constants of kb = 7.3 × 106 to 6.6 × 107 M-1 s-1 were estimated for the intermolecular dimerization at room temperature. The spectral changes were also observed upon warming of 77 K rigid matrices of A•-. It is suggested that A•- dimerizes with A through the formation of one C−C bond between two sp carbons, giving σ-type dimer radical anions (σ-A2•-) with a diene-type structure. Absorption spectra similar to those of 1•- were initially observed in 3•- generated by the radiolyses but changed to those assignable to the intramolecular dimer radical anions similar to σ-12•-. The yield of the dimer radical anions of 3•- with a tetramethylene chain was the largest among the dimer radical anions with several methylene chains

Properties of Excited Radical Cations of Substituted Oligothiophenes

Excited-state properties of radical cations of substituted oligothiophenes (nT•+, n denotes the number of thiophene rings, n = 3, 4, 5) in solution were investigated by using various laser flash photolysis techniques including two-color two-laser flash photolysis. nT•+ generated by photoinduced electron transfer to p-chloranil or resonant two-photon ionization (RTPI) by using the first 355-nm ns laser irradiation was selectively excited with the second picosecond laser (532 nm). Bleaching of the absorption of nT•+ together with growth of a new absorption was observed during the second laser irradiation, indicating the generation of nT•+ in the excited state (nT•+*). The D1 state lifetime was estimated to be 34 ± 4, 24 ± 2, and 18 ± 1 ps for 3T•+, 4T•+, and 5T•+, respectively. In the presence of hole acceptor (Q), bleaching of nT•+ and growth of Q•+ were observed upon selective excitation of nT•+ during the nanosecond−nanosecond two-color two-laser flash photolysis, indicating the hole transfer from nT•+(D1) to Q. Recovery of nT•+ was also observed together with decay of Q•+ because of regeneration of nT•+ by hole transfer from Q•+ to nT at the diffusion-limiting rate. It was suggested that the hole transfer rate (kHT) from nT•+(D1) to Q depended on the free-energy change for hole transfer (−ΔG = 1.41−0.46 eV). The estimated kHT faster than the diffusion-limiting rate can be explained by the contribution of the static quenching for the excited species in the presence of high concentration of Q (0.1−1.0 M)

Site-Selective Bimodal Absorption and Emission of Distonic Radical Cation

An acyclic 1,4-distonic dimer radical cation (DAE2•+) was generated from the dimerization of 1,1-bis(4-methoxyphenyl)ethylene radical cation (DAE•+) with the neutral molecule (DAE) in solution. The absorption spectrum of DAE2•+ shows bimodal absorption bands with peaks at 350 and 500 nm corresponding to the 1,1-bis(4-methoxyphenyl)ethyl radical (An2C•CH3) and 1,1-bis(4-methoxyphenyl)ethyl cation (An2C+CH3), respectively. Therefore, DAE2•+ in the ground state has the spin and positive charge localized on the 1- and 4-positions, respectively. The bimodal characteristic emissions by the site-selective excitation of radical and cation sites of DAE2•+ were observed at 77 K, showing that the excitation energy is localized on the radical or cation site of DAE2•+ in the excited state. The interaction between radical and cation sites of DAE2•+ in the ground and excited states are discussed on the basis of the steady-state spectroscopic and transient absorption measurements, as well as theoretical calculations

Self-Assembly of Polydeoxyadenylic Acid Studied at the Single-Molecule Level

The investigation on the self-assembly of polydeoxyadenylic acid (poly(dA)) is highly important to fully understand its biological function and for its application in the field of nanotechnology. Using the fluorescence resonance energy transfer (FRET) technique, we report investigations for the self-assembly of adenine oligomers induced by pH and coralyne binding at the single-molecule level and in the bulk phase. Results presented here show that A-motif 1 (Alexa488-5′-(dA)20-3′-Cy5-5′-(dA)20-3′-Alexa488) forms the wire-type duplex at acidic pH, whereas the same conformation of A-motif 2 (Alexa488-5′-(dA)20-3′-Cy5-3′-(dA)20-5′-Alexa488) is induced by coralyne binding at neutral pH. These results indicate that poly(dA) at acidic pH forms a right-handed helical duplex with parallel-mannered chains, whereas the coralyne–poly(dA) binding induces a stable antiparallel duplex. Furthermore, we found that the antiparallel duplex of poly(dA) formed by coralyne binding has a rather extended and less twisted structure as compared to the parallel duplex of poly(dA) formed at acidic pH. On the other hand, from dilution experiments, we found that the parallel duplex formed at acidic pH is converted to “S-form”, which has the single-stranded structure with short intramolecular double-stranded regions formed by intramolecular A:A base pairing, while the A-motif–coralyne assembly is dissociated into single strands below a certain concentration. The formation of S-form with a short intramolecular double-stranded region formed at acidic pH and very low concentration is confirmed by the quantitative analysis of FCS curve to measure the hydrodynamic radius of a molecule