Noboru Kitamura

Spectroscopic and electrochemical studies on ruthenium(II) complexes containing diazadiimine ligands

Abstract Newly prepared ruthenium(II) complexes, RuL 3 , containing 3,3′-bipyridazine, 6,6′-dimethyl-4,4′-bipyrimidine, and 4-methyl-2-(2′-pyridyl)pyrimidine as ligands (L) were studied by absorption and emission spectroscopy as well as by cyclic voltammetry. The metal-to-ligand charge-transfer absorption spectra and the photocatalytic ability of these RuL 3 complexes were discussed on the basis of the redox properties of L and RuL 3 .

The role of intramolecular association in the electrochemical reduction of viologen dimers and trimers

Abstract The electrochemical reduction of several viologen dimers (D) and trimers (T) was studied by absorption spectroscopy and differential pulse polarography in N , N -dimethylformamide. The viologen dimer and trimer linked by a propane chain (D3 and T3) exhibited efficient intramolecular association between two viologen radial cations, while the association in other viologens having longer alkyl spacer chains was less efficient. For D3 and T3, reduction of the second viologen group proceeded at a more positive potential compared with that of the first one, indicating that the association of the radical cations facilitated the reduction processes. For the viologens which showed inefficient intramolecular association, the four (for dimers) and six (for trimers) formal potentials were explained by taking statistical factors into account. The effects of the chemical structure of the viologens on both their spectroscopic and electrochemical behavior were also discussed.

Temperature dependence of photoinduced electron-transfer reactions in tris(2,2′-bipyridine)ruthenium(II) complex—organic quencher systems

Abstract To obtain information on the rate-determining factors for photoinduced electron-transfer reactions, thermodynamic parameters were determined for Ru(bpy) 2+ 3 —organic quencher—acetonitrile systems. We found large difference in thermodynamic parameters between systems containing electron donors and acceptors as quenchers.

Triboluminescence in N-alkyl and N-alkyl-3-substituted carbazole crystals

Abstract The triboluminescence (TL) of crystals of four carbazole derivatives was investigated by means of a multichannel plate/photodiode array detector system. 3,9-diethylcarbazole and N-isopropyl-3-vinylcarbazole were demonstrated for the first time to be triboluminescent. The TL active carbazole crystals belong to polar space groups, while the non-TL active crystals belong to non-polar ones. This suggests that TL activity in carbazole derivatives is closely related to crystal structure, in particular, to the space group polarity and thus to the generation of piezo- or pyro- electric charges.

Fluorescence spectra, fluorescence quantum yields and fluorescence lifetimes of polymers having trans-1,2-dicarbazolylyclobutane or carbazolyl group

Abstract Quantum yields, lifetimes and shapes of fluorescence from polymers containing the trans-1,2-dicarbazolylcyclobutane (DCZB) or carbazolyl structures were studied in N,N-dimethylformamide. No sandwich-type excimer formation was observed for DCZB polymers. The so-called second excimer observed in poly(9-vinylcarbazole) might also be produced in poly(9-ethyl-3-vinylcarbazole).

Excited state relaxation of Ru(bpy)32+ at low temperature. Time evolution of the emission quantum yield

Abstract Decay of emission from the metal-to-ligand charge transfer (MLCT) excited state of Ru(bpy)32+ (bpy=2,2′-bipyridine) was studied by single photon counting as well as by nanosecond time-resolved emission spectroscopy in 4:1 ethanol-methanol. While the MLCT emission decay of *Ru (bpy)32+ could be fitted by a double exponential function at 125 K, the decay profile was strongly dependent on the monitoring wavelength. This dependence, and a rise component detected for the emission at the lower energy edge of the spectrum, were interpreted on the basis of a time-dependent shift of the emission spectrum.

Problems of back electron transfer in electron transfer sensitization

Abstract The problems of back electron transfer in electron transfer sensitization were discussed and methods of preventing back electron transfer to improve the quantum efficiency of charge separation in homogeneous systems were proposed. Although complete quenching of an excited sensitizer by an electron donor or acceptor is not difficult, chemical yields of oxidized or reduced species are in general low. The importance of the coulombic effect was demonstrated for benzophenone/leuco crystal violet, pyrene/methylviologen (MV 2+ )/ethylenediaminetetraacetic acid and phenothiazine/viologen analogue systems. Then the discussion was extended to the quenching of Ru(bpy) 3 2+* (bpy ≡ 2,2′-bipyridine) by organic acceptors and donors in acetonitrile. The magnitude of the quenching constant k q as a function of exoergicity could be explainable by the Rehm—Weller equation in both cases: Δ H ‡ is apparently negative for oxidative quenching (RuL 3 2+* + A → RuL 3 3+ + − ; in contrast, for reductive quenching (RuL 3 2+* + D → RuL 3 + + D + ) Δ H ‡ is normal and control k q . A detailed kinetic mechanistic study leads to the conclusion that the attraction between RuL 3 3+ and A − in oxidative quenching brings about back electron transfer to the excited state, and the charge separation yield is found is found to be lower than that for reductive quenching as would be expected. By modulation of the ligand structure from 2,2′-bipyridine to 2,2′-bipyrazine and other ligands, the redox properties of RuL 3 2+* can be modified. Thus, the photo-reaction of the RuL 3 2+ /MV 2+ /triethanolamine (TEOA) system proceeds via reductive quenching when L is 2,2-bipyrazine or some other ligands (RuL 3 2+* + TEOA → RuL 3 + ; RuL 3 + + MV 2+ → RuL 3 2+ + MV + . The quantum yield of MV + formation approaches 100% by tris(2,2′-bipyrazine)-ruthenium(II) whereas the conventional Ru(bpy) 2 2+ sensitizer reacting via the oxidative quenching mechanism (RuL 3 2+* + MV 2+ → RuL 3 3+ + MV + ; RuL 3 3+ + TEOA → RuL 3 2+ ) gives a quantum yield of MV + formation of only about 20%.

Versatile setup for pressure and temperature‐controlled time‐resolved emission spectroscopy

A versatile instrumental setup for time‐resolved emission spectroscopy under variable pressure (1 atm–11 GPa) and variable temperature (77–400 K) was described. This setup, consisting of a diamond (or sapphire) anvil cell, was successfully interphased to a conventional fluorescence spectrometer, a gated diode‐array system with nanosecond laser‐pulse excitation, and a photon counting system excited with a cw YAG/dye laser system. The form of the sample for measurements can be either solid or liquid. Furthermore, the anvil cell can be used for IR spectroscopy by means of a FTIR spectrometer as well as for qualitative absorption spectroscopy. The performance was checked with a number of examples such as pressure and temperature effect on the time‐dependent shift of emission from Ru(bpy)2+3 in viscous solution, pressure effect on a solid binuclear complex of platinum, and fluorescence behaviors of several organic molecules. Scope and limitation of this novel setup were discussed.

Effects of bulky substituents on intramolecular exciplex formation in 1-(1-pyrenyl)-3-N,N-dimethylaminophenylpropane derivatives

Abstract Intramolecular exciplex formation in 1-(1-pyrenyl)-3-N,N-dimethylaminophenylpropane ( I ) and its derivatives with one ( II ) and two ( III ) ester substituent(s) at the 2-position of the propane linkage has been studied in isooctane. Introduction of the two bulky ester groups (i.e. III ) is characterized by a large decrease in the quantum yield of exciplex formation as well as by the appearance of two rise components of exciplex emission (4.2 and 24.1 ns at 35°C). Extremely slow rise time and low quantum yield of exciplex emission in III are explained by steric hindrance of the bulky ester groups for the structural change from (tt) to (tg ± ) and from (tg ± ) to (g ± g ∓ ) conformers in the excited singlet state.

Pressure- and temperature-controlled time-resolved emission spectroscopy applied to coordination compounds

Abstract Pressure effects on emission spectra of [Pt 2 (P 2 O 5 H 2 ) 4 ] 4- (Pt 2 4- ) crystals and Ru(bpy) 3 2+ solution (RuB 3 2+ , in ethanol-methanol) were studied. In Pt 2 4- crystals, reduction of both intra- and intermolecular Pt-Pt distances under high pressure resulted in lower energy shifts of fluorescence and phosphorescence, emergence of eximer-like emission, as well as in efficient T-T annihalation. In RuB 3 2+ solution, pressure induced high energy shift and sharpening of the emission were observed at 110 - 200 K, and were interpreted by viscosity dependent solvent relaxation.