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ABSTRACT: In the present paper, we have investigated the inter- and intramolecular electron transfer processes from the higher triplet excited state (T(n)) of oligothiophenes (3T and 4T). In the case of the intermolecular systems, two-color two-laser flash photolysis using nanosecond lasers was applied to the solution including benzophenone, oligothiophene, and halogenated benzene as a photosensitizer, an electron donor, and an electron acceptor, respectively. The first laser light irradiation generated the lowest triplet excited state (T(1)) of oligothiophene via energy transfer from benzophenone. Upon the second laser light irradiation, the absorption band of the radical cation of oligothiophene appeared with the simultaneous bleaching of the absorption band of the T(1) state, indicating the electron transfer from the T(2) state of the oligothiophene to the electron acceptor. The observed electron transfer rate dependent on the free energy change was explained on the basis of the Marcus theory. The intramolecular electron transfer in the dyad molecule of oligothiophene and acceptor was investigated using the two-color two-laser flash photolysis employing femtosecond laser. Upon the second laser light irradiation, which generates the T(n) state, the kinetic trace of the absorption band of T(1) state showed the bleaching and recovery, the rate of which depends on the driving force for the charge separation from the T(2) state of the oligothiophene. This observation suggests the existence of charge separation process from the T(2) state, and the observation of the charge-separated state was difficult probably due to the low charge separation yield and fast charge recombination.
The Journal of Physical Chemistry A 10/2010; 114(40):10789-94. · 2.95 Impact Factor
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ABSTRACT: In the present paper, photoinduced processes in the dyad molecules of pentathiophene (5T) and perylene-3,4:9,10-bis(dicarboximide) (PDI) with a flexible alkyl linker (propyl or hexyl) were investigated by using femtosecond laser flash spectroscopy in various solvents. Since absorption of 5T covers the wavelength region where absorption of PDI has minima and fluorescence of 5T overlaps with absorption of PDI, combination of 5T and PDI is favorable to achieve light energy harvesting as well as efficient electron transfer. When the sample was excited at the PDI moiety of the dyad, charge separation occurred almost quantitatively even in nonpolar solvent as well as in polar solvents. When the 5T moiety of the dyad was excited, efficient energy transfer to the PDI moiety from which charge separation occurred was confirmed, indicating that 5T acts as an antenna of the charge separation system, like a photosynthesis system of a plant. On the basis of Forster and Marcus theories and the estimated energy and electron-transfer rates, it was indicated that these dyads tend to take a folded structure in all solvents investigated.
The Journal of Physical Chemistry A 10/2008; 112(41):10193-9. · 2.95 Impact Factor
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ABSTRACT: The photophysical properties of bis-1,8-naphthalimide (NI-L-NI) dyads with different linkers ( L = -C 3H 6-, -C 4H 8-, -C 6H 12-, -C 8H 16-, and -C 9H 18-) as well as the reference NI derivative (NI-C 7H 15) were investigated in CH 3CN and H 2O/CH 3CN (v/v = 1:9). The normal fluorescence peak of (1)NI*-L-NI was observed at 379 nm together with a broad emission at longer wavelength both in aprotic CH 3CN and in H 2O/CH 3CN, which is assigned to an excimer, (1)(NI-L-NI)*. The excimer emission maximum was blue-shifted with increasing length of the linker. The photoinduced electron-transfer process of NI-L-NI was also investigated in both solvents by using nanosecond-laser flash photolysis. The T 1-T n absorption band for (3)NI*-L-NI was observed around 470 nm in both solvents. In H 2O/CH 3CN, NI-L-NI is solvated with H 2O in the ground state to exist as solvated NI-L-NI. In the excited triplet state, the NI radical anion (NI (*-)) was generated via the intramolecular quenching of (3)NI*-L-NI by another NI moiety. The solvated NI (*-)-L-NI may undergo the proton abstraction process to give NI(H) (*)-L-NI, which can be confirmed by the transient absorption band at 410 nm. This band was not observed in pure aprotic CH 3CN.
The Journal of Physical Chemistry A 08/2008; 112(31):7208-13. · 2.95 Impact Factor
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ABSTRACT: TiO(2)/DNA nanoconjugates were successfully fabricated by using the catechol moiety as a binding functional group, which was confirmed by steady-state absorption and fluorescence spectroscopies. Upon UV irradiation, the photocatalytic cleavage of the TiO(2)/DNA nanoconjugates was observed at the single-molecule level by using wide-field fluorescence microscopy. The decrease in the number of conjugates, which was estimated from the luminescent spots due to semiconductor quantum dots modified at the DNA strand, was significantly inhibited by a single A/C mismatch in the DNA sequences. This result strongly suggests that the migration of holes, which are injected from the photoexcited TiO(2) into the DNA, through the DNA bases plays an important role in the photocatalytic cleavage of the conjugates. The influences of the photogenerated reactive oxygen species (ROS) on the cleavage efficiency were also examined. According to the experimental results, it was concluded that oxidation of the catechol moiety and/or the DNA damage are key reactions in this process.
Chemistry 02/2008; 14(5):1492-8. · 5.93 Impact Factor
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ABSTRACT: Electron transfer (ET) reactions from the S(1) and S(2) states of some porphyrins and phthalocyanines to the axial ligand have been investigated by means of femtosecond laser flash photolysis. As the axial ligand, which acts as an acceptor, we synthesized an asymmetric pyromellitic diimide (PI) compound that has an alkyl chain and a pyridine ring on N and N' atoms, respectively. The pyridine ring of PI can coordinate to Zn of tetrapyrrole macrocycles. The coordination was confirmed by UV-vis and (1)H NMR spectra. ET from the S(1) state of Zn tetraphenylporphyrin (ZnTPP), Zn octaethylporphyrin (ZnOEP), Zn phthalocyanine (ZnPc), and Zn naphthalocyanine (ZnNc) to PI was confirmed with transient absorption spectroscopy by observing PI*-. ET from the S1 state occurred at the rate constant of (8.6 ps)(-1) - (78 ps)(-1), and the yield was almost unity. Furthermore, ET from the S(2) state of ZnTPP and ZnPc to PI was confirmed. ET from the S(2) state of ZnPc was observed for the first time. The ET rate from the S(2) state was faster than that from the S(1) state. In the case of ZnOEP-PI and ZnNc-PI complexes, ET from the S(2) state was not observed.
The Journal of Physical Chemistry A 12/2007; 111(45):11430-6. · 2.95 Impact Factor
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ABSTRACT: The hole transfer rates in the DNA/DNA B-form duplex and DNA/2'-OMeRNA A-form duplex were measured which occurred in the time range of approximately 100 micros. The hole transfer rates in the A-form duplexes were slower and more strongly dependent on the temperature compared to those in the B-form duplexes, suggesting that the A-form is more rigid than the B-form duplex in this time scale.
Chemistry 02/2007; 13(8):2386-91. · 5.93 Impact Factor
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ABSTRACT: Airborne singlet oxygen (1O2) molecules, which are generated during the TiO2 photocatalytic reactions and diffused from the surface into air, were detected at the opposite surface using terrylenediimide (TDI) molecules at the single-molecule level. The novel 1O2 nanosensor, which has a detectable number of about 1000 1O2 molecules in 70 x 70 square micrometers, can easily detect the single 1O2 molecule at a distance of over 1000 micrometers from the place of its creation in ambient air.
Journal of the American Chemical Society 01/2007; 128(51):16430-1. · 9.91 Impact Factor
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ABSTRACT: Photoinduced electron transfer (PET) processes of 1,8-naphthalimide-linker-phenothiazine (NI-L-PTZ) dyads have been investigated using the nanosecond- and picosecond-transient absorption measurements. Two kinds of linker were introduced, i.e., polymethylene-linked dyad (NI-C8-PTZ and NI-C11-PTZ) and a poly(ethyl ether)-linked one (NI-O-PTZ). The 355 nm pulsed laser excitation of NI-C8-PTZ, NI-C11-PTZ, and NI-O-PTZ in acetonitrile produced NI radical anion (NI*-) and PTZ radical cation (PTZ*+) with the absorption bands around 420 and 520 nm, respectively, through charge transfer from PTZ to NI in the singlet excited state (NI(S1)) as well as in the triplet excited states (NI(T1)) in acetonitrile. On the other hand, the charge transfer process occurred only from NI(S1) in nonpolar solvents. The rates of charge transfer and charge recombination processes largely depended on the solvent polarity and they are affected by the length of linkers and electronic coupling through polyether linker. The PET mechanism is discussed in terms of the free energy change for the charge transfer.
The Journal of Physical Chemistry B 07/2006; 110(23):11062-8. · 3.70 Impact Factor
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ABSTRACT: Synthesis and properties of oligodeoxynucleotides (ODNs) containing terthiophene (Thp) were described. One-electron oxidation of Thp-modified ODN resulted in the formation of Thp radical cation (Thp(.+)), which remained stable in the experimental time window up to 200mus, showing that charge may be carried along DNA by Thp as Thp(.+).
Bioorganic & Medicinal Chemistry Letters 11/2005; 15(20):4547-9. · 2.55 Impact Factor
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ABSTRACT: Photoinduced intramolecular charge separation (CS) and recombination (CR) processes of the tetrathiophene-substituted benzene dyads with an amide spacer (4T-PhR, R = 4-H (1), 4-CN (2), 3,4-(CN)2 (3), 4-NO2 (4), 3,5-(NO2)2 (5)) in solvents of different polarities were investigated using various fast spectroscopies. It was revealed that the CS rates depend on the ability of the acceptor and solvent polarity. Ultrafast CS with the rate of 5 x 10(12) s(-1) was revealed for 5 in PhCN and MeCN. The ultrafast CS can be attributed to the large electronic coupling matrix element between the donor and the acceptor despite the relative long donor-acceptor distance. The existence of the state with large electron density on the spacer between 14T*-PhR and LUMO should facilitate the CS process in the present dyad system. It was also revealed that the CR rates in these dyads were rather fast because of the enhanced superexchange interaction through the amide spacer.
The Journal of Physical Chemistry B 11/2005; 109(41):19257-62. · 3.70 Impact Factor
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ABSTRACT: The formation of the pyrene (Py) dimer radical cation (Py(2)(*+)) was used to measure the kinetics of the intrastrand end-to-end contact rates of single-stranded DNAs (ssDNAs) in the 10 nanoseconds to the tens of microseconds time range. ssDNAs labeled with Py at both ends with the lengths of 3, 6, and 9 mer were synthesized, and the two-photon ionization method was employed to generate a Py(*+), which enables the measurements of the end-to-end contact rates from 10 ns. The formation rate of Py(2)(*+) depended on the length and the sequence of the ssDNAs, and about 1 order of magnitude faster rates were observed for the T-rich ssDNAs compared to those for the corresponding length of A-rich ssDNAs, showing that ssDNA made from adenines is much more rigid than that composed of thymidines. As for the T-rich ssDNAs, the formation of Py(2)(*+) attributed to the misfolded structures was also observed, which is consistent with the configurational diffusion model suggested by Ansari and co-workers.
Journal of the American Chemical Society 10/2005; 127(38):13232-7. · 9.91 Impact Factor
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ABSTRACT: Stepwise photocleavage of naphthylmethyl-oxygen (C-O) bonds of mono(substituted-methyl)naphthalenes [1- and 2-ROCH2Np, R = 4-benzoylphenyl (BP), phenyl (Ph), and methyl (CH3)] and bis(substituted-methyl)naphthalenes [1,8-(ROCH2)2Np and 1,4-(ROCH2)2Np, R = BP and Ph] was observed to give the naphthylmethyl radicals (NpCH2* or ROCH2NpCH2*) in almost 100% yield with two-step or three-step excitation by the two-color two-laser or three-color three-laser irradiation, respectively, at room temperature. The C-O bond cleavage quantum yields of 1-PhOCH2Np, 2-PhOCH2Np, 1,8-(PhOCH2)2Np, and 1,4-(PhOCH2)2Np were higher than those of 1-BPOCH2Np, 2-BPOCH2Np, 1,8-(BPOCH2)2Np, and 1,4-(BPOCH2)2Np. No C-O bond cleavage occurred from 1,8-(HOCH2)2Np and 2-CH3OCH2Np in the higher triplet excited state (T(n)). The experimental results show that the C-O bond cleavage was determined not only by the position of the substituents on Np but also by the type of the substituents. The C-O bond cleavage of 1-ROCH2Np was more efficient than that of 2-ROCH2Np. In the case of 1,8-(ROCH2)2Np and 1,4-(ROCH2)2Np (R = BP and Ph), the first C-O bond cleavage from the T(n) states occurred to give ROCH2-substituted naphthylmethyl radicals (1,8- and 1,4-ROCH2NpCH2*) when the T1 states, generated with the 308-nm first laser irradiation, were excited using the 430-nm second laser. The second C-O bond cleavage occurred when 1,8- and 1,4-ROCH2NpCH2* in the ground state [1,8- and 1,4-ROCH2NpCH2*(D0)] were excited to the excited states [1,8- and 1,4-ROCH2NpCH2*(D(n))] using the third 355-nm laser during the three-color three-laser flash photolysis at room temperature. It was revealed that acenaphthene was produced as the final product during the stepwise C-O bond cleavages of 1,8-(BPOCH2)2Np and 1,8-(PhOCH2)2Np. This is a successful example of stepwise cleavage of two equivalent C-O bonds in a molecule using the three-color three-laser photolysis method.
The Journal of Physical Chemistry A 06/2005; 109(17):3797-802. · 2.95 Impact Factor
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ABSTRACT: The TiO2 photocatalytic one-electron oxidation mechanism of aromatic sulfides with a methylene bridging group (-(CH2)n-, n=0-4) between the 4-(methylthio)phenyl chromophore and the carboxylate binding group on the surface of a TiO2 powder slurried in acetonitrile (MeCN) has been investigated by time-resolved diffuse reflectance (TDR) spectroscopy. The electronic coupling element (H(DA)) between the hole donor and acceptor, which was estimated from the spectroscopic characteristics of the charge transfer (CT) complexes of the substrates (S) and the TiO2 surface, exhibited an exponential decline with the increasing of the methylene number of S. The determined decay factor (beta) of 9 nm(-1) also supports the fact that the 4-(methylthio)phenyl chromophore is separated from the TiO2 surface. The efficiency of the one-electron oxidation of S adsorbed on the TiO2 surface, which was determined from the relationship between the amount of adsorbates and the concentration of the generated radical cations, significantly depended on the H(DA) value, but not on the oxidation potential of S determined in homogeneous solution.
Chemistry 11/2004; 10(21):5345-53. · 5.93 Impact Factor
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ABSTRACT: Properties of the higher excited triplet states (Tn) of oligo(p-phenylenevinylene)s (OPVn, n denotes the number of phenyl rings, n = 3, 4) were investigated with use of the ns−ps and ns−ns two-color two-laser flash photolysis techniques. The lowest triplet excited states of OPVn were generated through the triplet energy transfer (ENT) process by the first 355-nm laser irradiation to the triplet sensitizer. With the delay time of 4 μs after the first laser irradiation, OPVn(T1) was selectively excited with the second 532-nm laser. Bleaching of the absorption of OPVn(T1) was observed during the second ps laser irradiation, indicating the generation of OPVn(Tn). The bleaching completely recovered at a few tens of picoseconds after the second picosecond laser irradiation, because of the internal conversion from OPVn(Tn) to OPVn(T1). The lifetimes of Tn were estimated to be 35 and 30 ps for OPV3 and OPV4, respectively. In the presence of the triplet energy acceptor (Q), the quenching and recovery of the T1-absorption bands of OPVn were observed because of ENT from OPVn(Tn) to Q and Q(T1) to OPVn, respectively. On the basis of the ENT from OPVn(Tn) to a series of Q, the T2−T1 energy gaps of OPV3 and OPV4 were estimated to be 1.3 and 1.1 eV, respectively.
09/2004;
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ChemPhysChem 09/2004; 5(8):1240-2. · 3.41 Impact Factor
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Angewandte Chemie International Edition 05/2004; 43(18):2406-9. · 13.45 Impact Factor
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ABSTRACT: Charge transfer in DNA is of current interest because of the involvement of charge transfer in oxidative DNA damage and electronic molecular devices. We have investigated the charge separation process via the consecutive adenine (A)-hopping mechanism using laser flash photolysis of DNA conjugated with naphthaldiimide (NDI) as an electron acceptor and phenothiazine (PTZ) as a donor. Upon the 355-nm laser flash excitation of NDI, the charge separation and recombination process between NDI and PTZ was observed. The yields of the charge separation via the consecutive A-hopping were slightly dependent upon the number of A bases between the two chromophores, while the charge recombination rate was strongly dependent upon the distance. The charge-separated state persisted over 300 micros when NDI was separated from PTZ by eight A bases. Furthermore, the rate constant of the A-hopping process was determined to be 2 x 10(10) s(-1) from an analysis of the yield of the charge separation depending on the number of A-hopping steps.
Journal of the American Chemical Society 03/2004; 126(4):1125-9. · 9.91 Impact Factor
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ABSTRACT: The hole transfer causes the long-lived charge-separated state in DNA during the photosensitized one-electron oxidation of DNA. The combination of the transient absorption measurement and DNA damage quantification by HPLC clearly demonstrated that the yield of the DNA damage correlates well with the lifetime of the charge-separated state.
Journal of the American Chemical Society 01/2004; 125(52):16198-9. · 9.91 Impact Factor
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ABSTRACT: Transient phenomena of benzophenone (BP) in the higher triplet excited state (Tn) have been investigated by the two-colour two-laser excitation method. Triplet energy transfer from BP(Tn) to quenchers (Q) occurred within the duration of a laser pulse (5 ns) to give Q(T1) with higher triplet energy than that of BP(T1). The quantum yield of the triplet energy-transfer quenching of BP(Tn) by CCl4 was found to be 0.0023 +/- 0.0002 from the bleaching of the transient absorption of BP(T1) and the absorbed photon number. It appears that internal conversion from BP(Tn) to BP(T1) is the predominant process. The lifetimes (tauTn) of BP(Tn) and several substituted benzophenones (BPs) in the higher triplet excited state [BPs(Tn)] were estimated from the dependence of the Q concentration on the efficiency of the triplet energy-transfer quenching of BP(Tn) by Q, and found to be 110-450 ps, depending on the nature of the substituents on the BPs. The effect of the substituents on tauTn may be explained by the energy gap between the Tn and T1 states, because the main deactivation pathway for BPs(Tn) is the internal conversion process. In contrast, the substituent effect on the lifetimes of BPs(T1) cannot be explained by the energy gap law. The transient behaviour of Q(T1) depends on the properties of the quencher. Sequential triplet energy transfer from Q(T1) to BP occurred for p-dichlorobenzene and tert-butylbenzene as quenchers, while Q(T1) reacted partly with Q to form triplet excimers (3Q2*) for benzene, chlorobenzene, and o-dichlorobenzene as quenchers. When CCl4 was used as the quencher, the homolytic cleavage of a C-Cl bond of CCl4(T1) occurred to give Cl* and Cl3C* radicals.
Photochemical and Photobiological Sciences 12/2003; 2(11):1209-14. · 2.58 Impact Factor
