•Binary amorphous solid IIa were rapidly formed from mixed solution of phenoxyl 2M and its dimer 2D.•The stoichiometric ratio of 2M and 2D in IIa was 9:91.•The formation of IIa could be due to the good solubilities of 2M and 2D and high packing efficiencies.Although molecular amorphous materials represent an important area of research in solid-state chemistry, studies pertaining to these systems have been restricted almost exclusively to amorphous solids based on a single molecule. In this study, we found that, while the 2,4,6-bis(4-tert-butylphenyl)phenoxyl radical (2M) and its dimer (2D) did not give single-component amorphous solids, they rapidly formed the corresponding binary amorphous solid IIa following their condensation from benzene, dichloromethane, chloroform, and ethyl acetate solutions. The formation of IIa could be attributed to the good solubilities of 2M and 2D in these solvents and the high packing efficiencies of these amorphous solids. IIa was also obtained when crystals of 2D (IIb) were ground together. The solid-state formation of IIa would not only involve the locational exchange of 2M and 2D, but would also involve chemical exchanges.Download high-res image (76KB)Download full-size image

Three isomeric difuran analogs of phenanthrene, including benzo[1,2-b;6,5-b′]difuran (3O), benzo[1,2-b;4,3-b′]difuran (4O) and benzo[1,2-b;3,4-b′]difuran (5O) have prepared and their properties investigated systematically. Compared with benzo[1,2-b;4,5-b′]difuran (1O) and benzo[1,2-b;5,4-b′]difuran (2O), there were pronounced differences in the fluorescence quantum yields of 3O–5O. Consideration of the absorption and fluorescence spectra, cyclic voltammograms and B3LYP/6-31G(d,p) calculations revealed that 4O had the highest electron-donating/accepting characteristics of all of the compounds prepared in this study. The lower electron-donating/accepting properties of 5O were attributed to the shorter chain length of it π-conjugated system. The unexpectedly high electron-donating and low electron-accepting properties of 3O were attributed to changes in the radical cationic and anionic states, respectively. The energy levels of the highest occupied and lowest unoccupied molecular orbitals of the thiophene and selenophene analogs of 3O–5O have also been calculated, and their relative energies explained in a similar manner.

Statistical studies using the Cambridge Structural Database have revealed that there are several elongated phenoxide C–O bonds. They are characterized by the formation of 3-fold (or occasionally 2-fold) hydrogen bonds to the phenoxide oxygen atoms, and their mean bond length extends up to 1.320 Å, which is quite different from the theoretically predicted carbon–oxygen bond length of C6H5O– (1.26 Å). Elongated phenoxide C–O bonds associated with the formation of 3-fold hydrogen bonds were also observed in the X-ray structures of proton-transfer complexes (2X–O–)(TEAH+)s derived from 5′-X-substituted 5,5′′-dimethyl-1,1′:3′,1′′-terphenyl-2,2′,2′′-triols (2X–OHs, where X = NO2, CN, COOCH3, Cl, F, H, and CH3) and triethylamine (TEA). By comparing the X-ray structures, C–O bond elongation was found to be only slightly affected by an electron-withdrawing substituent at the para position (X). This along with strong bathochromic shifts of N–H(···O–) and O–H(···O–) stretching vibrations in the IR spectra indicates that the elongated C–O bonds in (2X–O–)(TEAH+)s essentially have single-bond character. This is further confirmed by molecular orbital calculations on a model complex, showing that the negatively charged phenoxide oxygen atom is no longer conjugated to the central benzene ring, and the NICS values of the three benzene rings are virtually identical. However, C–O bond elongation in (2X–O–)(TEAH+)s was considerably influenced by a change in the hydrogen-bond geometry. This also suggests that hydrogen bonds significantly affect phenoxide C–O bond elongation.

Electronic absorption/emission spectra, absolute fluorescence quantum yields, and oxidation potentials of isomeric benzo[1,2-b:4,5-b′]difurans (1a) and benzo[1,2-b:5,4-b′]difurans (2a) along with their α,α′-di-n-butyl (1b and 2b) and bis(3,5-dihexyloxyphenyl) derivatives (1c and 2c) were studied. The longest wavelength absorption maxima were very close between 1a and 2a and between 1b and 2b; however, the maximum absorption of 1c was significantly red-shifted compared to that of 2c, due to cross-conjugation in the latter. Unlike related compounds, the fluorescence quantum yields of syn (1a−c) and anti (2a−c) isomers were virtually identical. On the other hand, the oxidation potentials of the syn isomers were significantly lower than those of the anti isomers. Molecular orbital calculations revealed that this is likely to be characteristic of benzodifurans, because HOMO energy levels of the [1,2-b:4,5-b′] and [1,2-b:5,4-b′] isomers were estimated to be virtually identical in the other benzodichalcogenophenes.