Koichi Nozaki

Syntheses and Phosphorescent Properties of Blue Emissive Iridium Complexes with Tridentate Pyrazolyl Ligands

Novel neutral mixed-ligand Ir(N=C=N)(N=C)X complexes (N=C=N = 1,3-bis(3-methylpyrazolyl)benzene (bpzb), 1,5-dimethyl-2,4-bis(3-methylpyrazolyl)benzene (dmbpzb), and 1,5-difluoro-2,4-bis(3-methylpyrazolyl)benzene (dfbpzb); N=C = 2-phenyl pyridine (ppy); and X = Cl or CN) have been synthesized and characterized. An X-ray single-crystal structure of the complex Ir(dmbpzb)(ppy)Cl shows that the nitrogen atom in the ppy ligand occupied the trans position to the carbon atom in the tridentate N=C=N ligand of dmbpzb with the Ir-C bond length of 1.94(1) A, whereas the coordinating carbon atom occupied the trans position of chlorine. Electrochemical data show that the complexes exhibit an oxidation Ir(III/IV) process in the potential range of +0.5 approximately 0.9 V and two irreversible reductions at approximately -2.6 and -3.0 V against Fc (0)/Fc (+), respectively. All of the Ir(III) complexes do not emit phosphorescence at room temperature, although strong phosphorescence is exhibited at 77 K with the 0-0 transition centered at around 450 nm and lifetimes of 3-14 mus. DFT calculations indicate that the HOMOs are mainly localized on iridium 5dpi and chlorine ppi*, whereas the LUMOs are mainly from the ppy ligand pi* orbitals. The phosphorescence originates from a (3)LC state mixed with the (3)MLCT and (3)XLCT ones. Temperature-dependent lifetime measurements of Ir(dfbpzb)(ppy)Cl reveal the existence of a thermal deactivation process with a low activation energy (1720 cm (-1)) and very high frequency factor (2.3 x 10 (13) s (-1)). An unrestricted density functional theory indicates that the dd state, in which both the Ir-N (pyrazolyl) bond lengths increase considerably, exists almost at the same energy as that for the phosphorescent state. A thorough analysis based on the potential energy surfaces for the T 1 and S 0 states allows us to determine the reaction pathway responsible for this thermal deactivation. The calculated activation energies of 1600 approximately 1800 cm (-1) are in excellent agreement with the observed values.

Synthesis and characterization of luminescent zinc(II) and cadmium(II) complexes with N,S-chelating Schiff base ligands.

The reactions of zinc(II) acetate with a variety of 2-substituted benzothiazolines afforded tetrahedral mononuclear complexes with a N 2S 2 donor set, [Zn(RPhC(H) NC 6H 4 S) 2]. The obtained zinc(II) complexes can be divided into three groups based on the characteristics of the absorption spectra; Group 1 (R = 2,4,6-triMe ( 1), 2,6-diCl ( 2)) showing an intense band at 250-300 nm and a weak band at 400-450 nm, Group 2 (R = 4-Cl ( 3), H ( 4), 4-Et ( 5), 4-OMe ( 6)) showing two intense bands at 250-300 nm and a weak band at 400-450 nm, and Group 3 (R = 4-NMe 2 ( 7), 4-NEt 2 ( 8)) showing an intense band at 250-300 nm and two very intense bands at 350-450 nm. The Group 2 and Group 3 complexes exhibited a strong emission on irradiating with ultraviolet light while the Group 1 complexes were not emissive at room temperature. However, all the zinc(II) complexes were luminescent in CH 2Cl 2/toluene glass at 77 K, and their emission peak energies were found to correlate with the Hammett constant of the substituent at para position of a pendent phenyl ring in each complex. Similar reactions of cadmium(II) acetate with 2-substituted benzothiazolines were also carried out to synthesize corresponding cadmium(II) complexes. While [Cd(RPhC(H) NC 6H 4 S) 2] (R = 2,4,6-triMe ( 9)) with bulky substituents at ortho positions of a pendent phenyl ring had a tetrahedral mononuclear structure, other cadmium(II) complexes [Cd 2(RPhC(H) NC 6H 4 S) 4] (R = 4-Et ( 10), 4-OMe ( 11), 4-NMe 2 ( 12)) possessed S-bridged dinuclear structures. These cadmium(II) complexes, which are assumed to have a mononuclear structure in solution, showed photophysical properties similar to those of the corresponding zinc(II) complexes.

A Doubly Alkynylpyrene‐Threaded [4]Rotaxane That Exhibits Strong Circularly Polarized Luminescence from the Spatially Restricted Excimer

The Sonogashira coupling of γ-CD-encapsulated alkynylpyrenes with terphenyl-type stopper molecules gave a doubly alkynylpyrene-threaded [4]rotaxane. The rotaxane showed only excimer emission, with a high fluorescence quantum yield of Φf =0.37, arising from the spatially restricted excimer within the cavity of the γ-CD. The excimer emission suffered little from self-quenching up to a concentration of 1.5×10(-5)  M and was circularly polarized with a high glum  value of -1.5×10(-2) . The strong circularly polarized luminescence may result from the two stacked pyrenes existing in the rotaxane in an asymmetrically twisted manner.

Highly efficient photoproduction of charge-separated states in donor-acceptor-linked bis(acetylide) platinum complexes.

We report a highly efficient charge separation system, D-Pt-A, where D (triphenylamine) and A (naphthalenediimide) are bonded to the Pt moiety through highly twisted phenylene ethynylene linkages. The quantum yields for the formation of the charge-separated state were determined to be nearly unity. The lifetimes of D(+)-Pt-A(-) were approximately 1 micros at room temperature and much longer at low temperature. The spin-correlated radical ion pair was directly observed by means of time-resolved EPR spectroscopy.

Synthesis and proton-coupled redox properties of mononuclear or asymmetric dinuclear complexes of ruthenium, rhodium and/or osmium containing 2,2′-bis(2-pyridyl)-6,6′-bibenzimidazole

New mononuclear and heterodinuclear complexes, [ML2(H2L1)]2+ and [L′2M(H2L1)M′L″2]4+[M, M′= Ru, Rh and/or Os; L′ or L″= 2,2′-bipyridine (bipy), 1,10-phenanthroline or 4,4′-dimethylbipyridine], containing the dinucleating ligand 2,2′-bis(2-pyridyl)-6,6′-bibenzimidazole (H2L1) have been prepared. The metal-to-ligand charge-transfer bands are almost unaltered when changing from the mono- to di-nuclear complexes, indicating that the bridging H2L1 ligand has slightly lower π* orbital energy than that of bipy. The bridging H2L1 ligand acts as a σ/π-donor ligand. Both the absorption spectra and the oxidation potentials of the complexes are strongly dependent on the solution pH, which determines the NH deprotonation of the co-ordinated ligand H2L1. The mononuclear complexes of Ru and Os act not only as basic acids but also as diacidic bases, while the heterodinuclear complexes essentially act as dibasic acids. The proton-coupled redox reaction was demonstrated by plots of E½vs. pH (Pourbaix diagrams). The pka values of the complexes reflect on both the type of metals and their oxidation states, MII and MIII. The introduction of asymmetry in the dinuclear complexes containing H2L1 can provide not only a potential difference between the two metal sites but also a preferential protonation (or deprotonation) site.

Long-lived charge separated states from distance fixed triads consisting of zinc porphyrin, free-base porphyrin, and pyromellitimide

Abstract Photoexcitation of distance restricted triads consisting of zinc porphyrin (ZnP), free-base porphyrin (H 2 P), and pyromellitimide (Im) at 532 nm in THF at room temperature led to long-lived charge separated states (ZnP) + -H 2 P-(Im) − with a lifetime of 0.16–80 μs via electron transfer from the 1 (H 2 P)* to the Im followed by electron transfer from the ZnP to the (H 2 P) + . The lifetime of (ZnP) + -H 2 P-(Im) − state is dependent upon distances between the charges and the structures of linkage between the ZnP and H 2 P.

Non-radiative processes of excited CT states of Ru(II) and Pt(II) compounds and excited d-d states of Rh(III) compounds in the solid state and at higher temperatures

Abstract Radiative and non-radiative processes of excited CT states of Ru(II) and Pt(II) compounds and excited d-d states of Rh(III) compounds in the solid state and at higher temperatures are examined. A rapid non-radiative transition of a metal-to-ligand charge transfer excited state to the ground state does not occur over a wide temperature range (77–300 K) for [Ru(bpy) 3 ](PF 6 ) 2 and [Pt 2 (tpy) 2 (guanidinato)](ClO 4 ) 3 . Spectral simulations of the d-d emissions of Rh(III) compounds in the range 77–536 K are carried out by assuming a simple configuration coordinate. The large Huang-Rhys factors indicate that the lowest and second lowest d-d excited states of Rh(III) compounds are strongly coupled to the excited vibrational state of the ground state. The frequency factors of non-radiative decay are 10 11 –10 12 s −1 for the excited triplet state.

Photoinduced electron transfer of N-[(3- and 4-diarylamino)phenyl]-1,8-naphthalimide dyads: orbital-orthogonal approach in a short-linked D-A system.

The photoinduced electron transfer of a series of meta- and para-linked triphenylamine-naphthalimide dyads, N-{3- and 4-[bis(4-R-substituted phenyl)amino]phenyl}-1,8-naphthalimide, 1m,p (R = H), 2m,p (R = Me), 3m,p (R = OMe), and 4m,p (R = NMe2) was investigated in toluene and DMF. The singlet charge-transfer (CT) states were observed in all cases. The decay rates were found to be faster in DMF (tau = 6.5 ps to 100 ps) than those in toluene (tau = 190 ps to 7 ns). The long-lived triplet CT states were observed in toluene for 3 (ca. 10% contribution, tau = 670 ns for 3m, 240 ns for 3p). No long-lived species were detected in DMF. The decay rates were somewhat faster in the para-isomers than in the meta-isomers in most cases. The photolysis of 5 (p-phenylene extended analogue of 3, R = OMe) gave a singlet CT state and a locally excited triplet state on the naphthalimide chromophore.

Proton-induced tuning of metal-metal communication in rack-type dinuclear Ru complexes containing benzimidazolyl moieties.

Ru complexes bearing a bis-tridentate benzimidazolyl ligand have been synthesized. The dinuclear ones act as a bibasic acid with pK(a1)=4.36 and pK(a2)=5.90. The protonated form of the dinuclear complex exhibited two one-electron oxidations at +0.91 and +1.02 V versus the ferrocenium/ferrocene (Fc/Fc(+)) couple (the potential difference (ΔE)=0.11 V), but the di-deprotonated form showed two waves at +0.50 and +0.58 V versus Fc/Fc(+) (ΔE=0.08 V). Since the potential difference between two waves reflects the strength of the metal-metal interaction, the deprotonation of the benzimidazole moieties in the complexes weakened the Ru-Ru communication. The degree of electronic coupling between two metal centers, estimated from the intervalence charge transfer (IVCT) band, was greater for the protonated form. DFT calculations for the protonated and deprotonated forms of the dinuclear complex suggest that the Ru(II)-L(H(2)) π* interaction plays a key role in the Ru-Ru interaction.

Kinetic study on the dimerization reaction of 9-methoxyanthracene cation radical by means of fast scan cyclic voltammetry

Abstract The instrumentation of a fast scan cyclic voltammetric (FSCV) system with a maximum scan rate of 300 kV s and its application to the kinetic studies of very short-lived electrogenerated intermediates are presented. The time resolution of the system was inspected by cyclic voltammetry and chronoamperometry, and was found to be better than 1 μs at an ultramicrodisk electrode (UMDE) of micrometre size. The mechanism and kinetics of the very rapid anodic dimerization of 9-methoxyanthracene (9MeOA) are discussed in further detail for the first time by means of FSCV. The reductive wave of 9MeOA cation radical was detected successfully, although the reversible cyclic voltammogram could not be observed even at a scan rate as high as 300 kV s . It was concluded, from the dependences of the anodic peak potentials on both the scan rate and the substrate concentration, that the dimerization proceeded via coupling of 9MeOA cation radical with the substrate. The dimerization rate constant was estimated to be (3.5± 0.5) × 108 M−1 s−1 from the results of digital simulation analysis of the observed fast scan cyclic voltammograms.