Photophysical property and photostability of J-aggregate thin films of thiacyanine dyes prepared by the spin-coating method

By use of electrostatic interactions of dye molecules and poly(diallyldimethylammonium chloride) (PDDA), the spin-coating technique has been successfully applied to the preparation of stable J-aggregate thin films of thiacarbocyanine dyes on a polycarbonate or quartz plate. The J-aggregate thin films were prepared by the spin-coating of PDDA aqueous solution on dye thin films prepared on a substrate by the spin-coating of 2,2,3,3-tetrafluoro-1-propanol solution of dyes. Photophysical properties of the dye thin films and J-aggregate thin films were studied by measuring the fluorescence spectra, quantum yields, and lifetimes. Coherent size of the J-aggregates was estimated to be 3-12 by means of the absorption bandwidth (full width at half maximum) or radiative lifetime. Photostability of the J-aggregate thin films was also studied in terms of photodegradation efficiency under argon and oxygen in comparison with the dye thin films, and J-aggregate thin films were found to be more stable than the corresponding dye thin films.     

Intermolecular photoinduced electron-transfer of 1,8-naphthalimides in protic polar solvents

The intermolecular photoinduced electron transfer (PET) processes of 1,8-naphthalimide (NI) derivatives including NI-linker-phenothiazine dyads were investigated in a protic H(2)O/CH(3)CN (v/v=1:1) solvent using ns-laser flash photolysis with 355 nm-laser excitation. NI derivatives are surrounded by H(2)O in the ground state in H(2)O/CH(3)CN. The T(1)-T(n) absorption band of (3)NI* was observed at around 470 nm. The transient absorption band at around 410 nm increased concomitantly with the decay of (3)NI* in H(2)O/CH(3)CN. This implies that hydrated NI anion radical (NI*(-)) is primarily generated via the quenching of (3)NI* by NI at the diffusion control rate. This intermolecular PET did not occur in aprotic CH(3)CN. The formation and decay times of NI*(-) showed strong dependence on the concentration of NI. Then, we suggest that NI*(-) could undergo proton abstraction to give ketyl radical species of NI [NI(H)*] in H(2)O/CH(3)CN.     

Adsorption and micellization behavior of novel gluconamide-type gemini surfactants

The adsorption and micellization behavior of novel sugar-based gemini surfactants (N,N(')-dialkyl-N,N(')-digluconamide ethylenediamine, Glu(n)-2-Glu(n), where n is the hydrocarbon chain length of 8, 10 and 12) has been studied on the basis of static/dynamic surface tension, fluorescence, dynamic light scattering (DLS) and cryogenic transmission electron microscope (cryo-TEM) data. The static surface tension of the aqueous Glu(n)-2-Glu(n) solutions measured at the critical micelle concentration (cmc) is observed to be significantly lower than that of the corresponding monomeric surfactants. This suggests that the gemini surfactants, newly synthesized in the current study, are able to form a closely packed monolayer film at the air/aqueous solution interface. The greater ability in the molecular association is supported by the remarkably (approximately 100-200 times) lower cmc of the gemini surfactants compared with the corresponding monomeric ones. With a combination of the fluorescence and DLS data, a structural transformation of the Glu(n)-2-Glu(n) micelles is suggested to occur with an increase in the concentration. The cryo-TEM measurements clearly confirm the formation of worm-like micelles of Glu(12)-2-Glu(12) at the concentration well above the cmc.     

Photodecomposition profiles of beta-bond cleavage of phenylphenacyl derivatives in the higher triplet excited states during stepwise two-color two-laser flash photolysis

Photochemical properties of p-phenylphenacyl derivatives (PP-X) having C-halide, C-S, and C-O bonds in the lowest (T 1) and higher (T n ) triplet excited states were investigated in solution by using single-color and stepwise two-color two-laser flash photolysis techniques. PP-Xs (X = Br, SH, and SPh) undergo beta-bond dissociation in the lowest singlet excited states (S 1) while the C-X bonds of other PP-Xs are stable upon 266-nm laser photolysis. The T 1(pi,pi*) states of PP-X were efficiently produced during 355-nm laser photolysis of benzophenone as a triplet sensitizer. Triplet PP-Xs deactivate to the ground state without photochemical reactions. Upon 430-nm laser photolysis of the T 1 states of PP-X (X = Br, Cl, SH, SPh, OH, OMe, and OPh), decomposition of PP-X in the T n states was found. On the basis of the changes in the transient absorption, quantum yields (Phi dec) of the decomposition of PP-X in the T n states were determined, while bond dissociation energies (BDE) of the C-X bonds were calculated by computations. According to the relationship between the Phi dec and BDE values, it was shown that the decomposition of PP-X in the T n state is due to beta-cleavage of the corresponding C-X bond, and that the state energy of the reactive T n for the C-O bond cleavage differs from that for the C-halide and C-S bond cleavage. The reaction profiles of the C-X bond cleavage of PP-X in the T n states were discussed.     

Electron transfer of quinone self-assembled monolayers on a gold electrode

Dialkyl disulfide-linked naphthoquinone, (NQ-C_n-S)₂, and anthraquinone, (AQ-C_n-S)₂, derivatives with different spacer alkyl chains (C_n: n = 2, 6, 12) were synthesized and these quinone derivatives were self-assembled on a gold electrode. The formation of self-assembled monolayers (SAMs) of these derivatives on a gold electrode was confirmed by infrared reflection-absorption spectroscopy (IR-RAS). Electron transfer between the derivatives and the gold electrode was studied by cyclic voltammetry. On the cyclic voltammogram a reversible redox reaction between quinone (Q) and hydroquinone (QH₂) was clearly observed under an aqueous condition. The formal potentials for NQ and AQ derivatives were −0.48 and −0.58 V, respectively, that did not depend on the spacer length. The oxidation and reduction peak currents were strongly dependent on the spacer alkyl chain length. The redox behavior of quinone derivatives depended on the pH condition of the buffer solution. The pH dependence was in agreement with a theoretical value of E_1/2 (mV) = E′ − 59pH for 2H⁺/2e⁻ process in the pH range 3–11. In the range higher than pH 11, the value was estimated with E_1/2 (mV) = E′ − 30pH , which may correspond to H⁺/2e⁻ process. The tunneling barrier coefficients (β) for NQ and AQ SAMs were determined to be 0.12 and 0.73 per methylene group (CH₂), respectively. Comparison of the structures and the alkyl chain length of quinones derivatives on these electron transfers on the electrode is made.     

Rhodium-catalyzed silylation and intramolecular arylation of nitriles via the silicon-assisted cleavage of carbon-cyano bonds

A rhodium-catalyzed silylation reaction of carbon-cyano bonds using disilane has been developed. Under these catalytic conditions, carbon-cyano bonds in aryl, alkenyl, allyl, and benzyl cyanides bearing a variety of functional groups can be silylated. The observation of an enamine side product in the silylation of benzyl cyanides and related stoichiometric studies indicate that the carbon-cyano bond cleavage proceeds through the deinsertion of silyl isocyanide from eta(2)-iminoacyl complex B. Knowledge gained from these studies has led to the development of a new intramolecular biaryl coupling reaction in which aryl cyanides and aryl chlorides are cross-coupled.