The intramolecualr charge transfer (CT) complex formation and photoinduced electron-transfer reactions in aromatic donor−viologen acceptor dyad systems linked by polymethylene linkage (-(CH2) n -) have been studied. The donors and the numbers of methylene unit in the linkages are 1-naphthoxyl with n = 3, 6, 8, and 10, 2-naphthoxyl with n = 3−10 and 12, and 2-dibenzofuranoyl with n = 3, 6, 8, and 10. The formation constants of the intramolecular CT complexes (K int) were determined from the absorbance of CT absorption by using the absorptivities of the complexes determined from the intermolecular complexation between the model donor compounds, the 1-aryloxy-3-aminopropanes, and dimethyl viologen. The K int values depend little on the length of the linkage and are about 0.2 for 1-naphthol and 2-naphthol derivatives, and 0.6 for dibenzofuranoyl derivatives. The addition of β-CD disrupts the formation of the intramolecular CT complexes. The 1:1 association constants of the dyad molecules with β-CD (K CD) were estimated from the dependence of the CT absorption on the concentration of β-CD. Complexation of the dyad molecules with β-CD or methylated β-CD (Me−β-CD) also enhances the fluorescence intensity of the excited-state aromatic donors. The 1:1 complexes further associate with CD molecules resulting further enhancement of fluorescence intensity. This was attributed to the extension of the dyad molecules in the CD complexes. The electron-transfer quenching rate constants in the CD complexes formed in the presence of 150 mM Me−β-CD were calculated from fluorescence lifetime data and found to vary exponentially with the length of the linkage. The apparent β value is 0.86 Å-1 (1.09/C−C bond), regardless of the nature of donor moieties. The distance dependence of reorganization energies (λ) of the CD complexes was evaluated. Comparing the λ value with ΔG° of the reaction, it appears that the reactions stay near the top of the Marcus curve. Comparison of the effects of Me−β-CD on steady-state fluorescence intensity and excited-state lifetime indicated that through-space/through-solvent electron transfer is the predominant quenching pathway in the molecules having the linkage shorter than heptamethylene chain and the quenching rate is fast enough to show a static-like behavior.

Face selectivity in binding of methyloctyl viologen (C 1 C 8 V 2+ : 1) and adamantylmethyl viologen (AdaC 1 V 2+ : 2) with -CD and 6-O-(2-sulfonato-6-naphthyl)--CD ( -CD-NS: 4) has been investigated. Circular dichroic titration of 1 and 2 with -CD gave the binding constants and the molar ellipticities [θ] 254nm of the -CD complexes as 890 M -1 and -2000 deg cm 2 dmole -1 for 1, and as 7300 M -1 and -3600 deg cm 2 dmole -1 for 2. The [θ] values of the complexes are much less negative than -8500 deg cm 2 dmole -1 of -CD-viologen compound 3 in which the viologen is covalently bonded to the primary face of -CD. Fluorescence quenching of the naphthyl group of 4 by 1 and 2 is much more efficient than that of 6-methoxy-2-naphthalenesulfonate (MNSS), but residual fluorescence was observed even at high viologen concentration. The binding constants of the viologens with 4 and the fractions of the residual fluorescence with respect to fully monomeric 4 were determined as 4700 M -1 and 0.09 for 1 and 10 400 M -1 and 0.15 for 2. Comparing the steady-state fluorescence quenching and time-resolved fluorescence studies, it appeared that the bipyridinium moiety of 2 is predominantly on the secondary side of -CD of 4, whereas that of 1 favors the primary side, mainly due to the direct charge transfer interaction between the naphthyl and bipyridinium groups: without the interaction, it would prefer the secondary face 14 times more favorably to the primary face. The rate constants of the photoinduced electron-transfer reactions between the bipyridinium group on the secondary face and the naphthyl group are 1.9 × 10 8 s -1 for the 1/4 complex and 7.9 × 10 8 s -1 for the 2/4 complex.

A monofunctional derivative of β-cyclodextrin (β-CD) containing a 2-sulfonato-6-naphthyl group at the primary face (1: β-CD-NS) was prepared. The concentration dependences of fluorescence, circular dichroism (c.d.), and NMR spectra of 1 have been investigated and compared with the spectral changes associated with complexation of 6-methoxy-2-naphthalenesulfonate (MNSS) with β-CD. When the concentration of 1 was increased, the following were observed: the enhancement of the concentration-normalized fluorescence intensity; an increase in molar ellipticities of the negative band around 280 nm; a more pronounced exciton coupling band around 230 nm in c.d. spectra; and an upfield shift of NMR signals from aromatic protons. The concentration dependent fluorescent intensity and molar ellipticity data fitted well to the monomerdimer equilibrium of 1 with the dimerization constant of 9700 ( 2500 M -1 . The dimerization constant is much larger than the association constants of MNSS with β-CD (510 ( 40 M -1 ) and 1 with β-CD (430 ( 50 M -1 ). The naphthyl groups in the dimer do not show an excimer band in fluorescence spectra but exhibit exciton coupling bands around 230 and 330 nm. These indicate that 1 forms a head-to-head type dimer, which is stabilized by mutual inclusion of the 2-sulfonato-6-naphthyl moieties deeply into β-CD cavities of counter molecules. The naphthyl moiety of the monomer of 1 is in a similar environment to that in a 1/noctyl sulfate complex and appears to cap the primary face of β-CD loosely. Compared to the c.d. spectrum of the MNSS/β-CD complex, the spectrum of the 1/β-CD complex is similar in shape, but the rotational strengths of the c.d. bands are much greater.

[Reaction: see text]. The self-inclusion behavior and induced circular dichroism (ICD) characteristics of two beta-cyclodextrin (beta-CD) derivatives, in which a 1-methyl-4,4'-bipyridinium (viologen) group is connected by an octamethylene chain to either the primary (2(2+)) or secondary (3(2+)) side of beta-CD, and of their reduced forms, are investigated. 1H NMR studies showed that 2(2+) forms an intramolecular self-inclusion complex with K(in) = 3.1 +/- 0.4, whereas 3(2+) forms a head-to-head type of dimer with K(D) = 65 +/- 10 M(-1) at 25 degrees C. 2(2+) and 3(2+) form [2]pseudorotaxanes with alpha-CD, with the secondary side of the alpha-CD facing the viologen moiety. The ICD characteristics of mono-6-[4-(1-methyl-4-pyridinio)-1-pyridinio]-beta-CD (1(2+)), 2(2+), 3(2+), and methyloctyl viologen-beta-CD complexes were obtained for the oxidized and reduced states of the viologen units. The results indicated dimer formation for 1 degrees , and intramolecular complexation for 2*+ and 2 degrees in which the reduced viologen units are outside the beta-CD cavity. The results also indicated intramolecular complexation for 3*+ and 3 degrees, but with reduced viologen units inside the cavity. This work provides unequivocal evidence of the preference of the secondary side of cyclodextrins for viologen groups, regardless of their oxidation states, and the dependence of ICD of the viologen chromophores on their location with respect to the CD cavity.