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We prepared perylene dications 1(2+) and 2(2+) by using "capped" perylene derivatives, and for the first time, successfully obtained single crystals of a perylene dication 1(2+) that enabled us to perform its structural analysis. We realized that the substituted aryl groups on perylene control the positions of positive charges, thus the remaining electronic system satisfies Clar's sextet rule toward the highest number of localized sextets. Experimental and theoretical evidence proved that Clar's aromatic π-sextet rule could be applied even for the dicationic perylenes in a very simple way.

This work demonstrates a selection criteria that determines whether molecular assembly occurs through a one-step or stepwise manner in ligand-bridged dinuclear zinc(II) (Zn(2+)) complex formation, which is associated with the π stacking of building blocks. The building blocks of carbazole ligands (L(1) and L(4)) that contain two imidazole moieties at the 3,6-positions form 4:2 complexes (i.e., [L]4-(Zn(2+))2) at a molar ratio of 0.50 ([Zn(2+)]/[L]0 = 0.50), thereby providing π stacking between the carbazole ligands. At the molar ratio of 0.67 ([Zn(2+)]/[L]0 = 0.67), the 4:2 complexes change to 3:2 complexes (i.e., [L]3-(Zn(2+))2) with no π-stacked carbazole unit. In contrast, when the imidazole groups in L(1) are replaced with benzoimidazole groups (L(3)), L(3) also yields the 4:2 complex [(L(3))4-(Zn(2+))2] at a molar ratio of 0.50. However, there is no structural transition from (L(3))4-(Zn(2+))2 to other complex species above a molar ratio of 0.50. Similarly, when two imidazole groups are introduced into the carbazole ring at 2,7-positions (L(5)), L(5) also gives the 4:2 complex [(L(5))4-(Zn(2+))2] that shows no structural transition to other complex species at a higher molar ratio.

This work demonstrates sign reversal of large circularly polarized luminescence (CPL) signal based on the hinge-like twisting motion of a bidentate ligand, 3,3-bis(diphenylphosphoryl)-2,2-bipyridine (BIPYPO), in a cis-trans isomerization of chiral europium(III) complexes. X-ray diffraction analysis revealed that twisting motion of BIPYPO provides s-cis and s-trans geometries of a chiral Eu(III) complex containing either tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (D-1) or tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphorate] (D-2). The s-cis Eu(III) complexes show eight-coordinate geometry around the Eu(III) ion, in which the chelate between the phosphoryl oxygen and the Eu(III) ion forces the s-cis geometry of BIPYPO. In contrast, the phosphorus-nitrogen interaction provides a conformational lock for the s-trans geometry of the BIPYPO ligand, inducing a quasi-seven-coordinate Eu(III) complex. The difference in coordination geometry causes the sign change of the CPL signals between the s-cis and s-trans isomers, whereby the s-cis and s-trans isomers of Eu(III) complexes exhibit the positive and negative CPL signals, respectively, for the (5) D0 →(7) F1 transition. The proportion of the s-trans-D-1 against s-cis-D-1 increases upon changing the solvent from [D3 ]acetonitrile to [D6 ]acetone, inducing a sign change of the CPL signals. The complexes D-1 and D-2 show a biexponential decay with two different lifetimes, suggesting two emitting species, that is, the s-cis and s-trans isomers of Eu(III) complexes. In both cases, the proportions of the longer lifetime components (τ1 ) decrease and instead the shorter lifetime components (τ2 ) increase upon changing the solvent from [D3 ]acetonitrile to [D6 ]acetone.

A partially hydrophobic carbazole ligand ((Im(+))2Cz: 2,2'-(9-ethyl-9 H-carbazole-3,6-diyl)bis(ethyne-2,1-diyl)bis(1,3-dimethyl-1H-imidazol-3-ium)) adopts two different binding states (binding states I and II) in its interactions with calf-thymus (ct-) DNA. Two distinct binding states were identified by biphasic UV/Vis and circular dichroism (CD) spectral changes during the titration of DNA into the carbazole ligand. At low concentrations of ct-DNA, (Im(+))2Cz binds to nearly every part of ct-DNA (binding state I). By contrast, an increased concentration of ct-DNA results in a switch in the DNA-binding state, so that the ligands are bound per five DNA base pairs. Similarly, a monocationic carbazole ligand (Im(+)Cz: 2-((6-bromo-9-ethyl-9 H-carbazol-3-yl)ethynyl)-1,3-dimethyl-1H-imidazol-3-ium) also shows biphasic UV/Vis spectral changes during the titration of ct-DNA into Im(+)Cz, which suggests two different binding states of the Im(+)Cz ligand with ct-DNA. The stepwise equilibrium of the ligand-DNA-complex formation is capable of switching the thermal stability of ct-DNA, as well as the enzymatic activity of deoxyribonuclease (DNase I). In binding state I, the (Im(+))2Cz ligands interact with nearly every base pair in ct-DNA and stabilize the double-helix structure, which results in a larger increase in the melting temperature of the ct-DNA than that observed with binding state II. On the other hand, the (Im(+))2Cz ligand significantly reduces the enzymatic activity of DNase I in binding state I, although the enzymatic activity is recovered once the binding state of the ligand-DNA complex is changed to binding state II. The (Im(+))2Cz ligand was also employed as a binder for G-quadruplex DNA. In contrast to the stepwise complex formation between (Im(+))2Cz and ct-DNA, (Im(+))2Cz shows a monotonous UV/Vis spectral response during the titration of G-quadruplex DNA into (Im(+))2Cz, which suggests a single binding state for (Im(+))2Cz with G-quadruplex DNA.

The accelerating effect of Sc(3+) on the electron-transfer (ET) reduction of the p-benzoquinone derivative 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) by 10,10'-dimethyl-9,9'-biacridine ((AcrH)(2)) at 233 K changes to a decelerating effect with increasing reaction temperature; the observed second-order rate constant k(et) decreases with increasing Sc(3+) concentration at high concentrations of Sc(3+) at 298 K. At 263 K the k(et) value remains constant with increasing Sc(3+) concentration. Such a remarkable difference with regard to dependence of k(et) on [Sc(3+)] between low and high temperatures results from the difference in relative activity of two ET pathways that depend on temperature, one of which affords 1:1 complex TolSQ*(-)-Sc(3+), and the other 1:2 complex TolSQ*(-)-(Sc(3+))(2) with additional binding of Sc(3+) to TolSQ*(-)-Sc(3+). The formation of TolSQ*(-)-Sc(3+) and TolSQ*(-)-(Sc(3+))(2) complexes was confirmed by EPR spectroscopy in the ET reduction of TolSQ in the presence of low and high concentrations of Sc(3+), respectively. The effects of metal ions on other ET reactions of quinones to afford 1:1 and 1:2 complexes between semiquinone radical anions and metal ions are also reported. The ET pathway affording the 1:2 complexes has smaller activation enthalpies DeltaH( not equal) and more negative activation entropies DeltaS( not equal) because of stronger binding of metal ions and more restricted geometries of the ET transition states as compared with the ET pathway to afford the 1:1 complexes.