Photoinduced color change of naphthalene diimides (NDIs) bearing alkylamine moieties has been observed in the solid state. The color change is attributed to the generation of a NDI radical-anion species, which may be formed through a photoinduced electron-transfer process from the alkylamine moiety to the NDI. The photosensitivity of NDIs is highly dependent on the structures of the alkylamine moieties. Crystallographic analysis, kinetic analysis, UV/Vis/NIR spectroscopic measurements, and analysis of the photoproduct suggested that a radical anion was formed through an irreversible process initiated by proton abstraction between an amine radical cation and the neutral amine moiety. The radical anions formed stacks including mixed-valence stacks and radical-anion stacks, as shown by the broad absorption bands in near-IR spectra. These photosensitive NDIs also showed crystal bending upon photoirradiation, which may be associated with a change in the intermolecular distance of the NDI stacks by the formation of monomeric radical anions, mixed-valence stacks, and radical-anion stacks.

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In ion-pair extractive separation of metal cations using a complexation reagent and a counter anion, use of a higher selective complexation reagent is one of the most important factors to realize their mutual separation. 1 Therefore, development of new selective complexation reagent and investigations about the effect of steric structure around donor atoms in the reagent on the ion recognition ability (extraction selectivity) are very important for improving the mutual separation. 2 Earlier, we studied the nature of neutral di-Schiff base ligands having two 2-pyridyl pendant arms, N,N′-bis(2-pyridylmethylidene)-1,2-diiminoethane (BPIE) [3][4][5][6] and its analogues, as complexation reagents for the ion-pair extraction of divalent metal cations into nitrobenzene with picrate anion. 7 These ligands acted as bidentate ones using two imine-N donor atoms in the extraction system, and 2-pyridyl pendant arms acted mainly to raise the hydrophobicity of the ligand and the extractability of the complexes. Furthermore, it was found that the extraction selectivity and extractability can be controlled by introduction of substituents near the imine-N donor atoms. In these extraction systems, in addition, relatively high extraction selectivity for Cu 2+ was obtained. In this research, we investigated the effects of pendant arms in the Schiff base ligands and of distance between the two imine-N donor atoms on the extractability and the ion-pair extraction selectivity; BPIE was used as a model compound. Two BPIE analogues, N,N′-bis(2-quinolylmethylidene)-1,2-diiminoethane (BQIE) having 2-quinolyl groups as pendant arms and N,N′-bis(2-pyridylmethylidene)-1,3-diimino-2,2-dimethylpropane (BPMP) 8 (see Fig. 1) having relatively long distance between two imine-N atoms, were used for the investigation. The pendant arms affected the lipophilicity of ligand to nitrobenzene, whereas the distance between the imine-N atoms was a factor in controlling the extraction selectivity. Consequently, the relative selectivity of Cu 2+ in BPMP-NaPic system was higher than that of BPIE-NaPic system due to the suitable distance for complexation of Cu 2+ . From these viewpoints, it was found that another BPIE analogue, N,N′- bis(2-quinolylmethylidene)-1,3-diimino-2,2-dimethylpropane (BQMP) (see Fig. 1), can be used for selective extraction of Cu 2+ from other divalent cations. Experimental ApparatusA Hitachi Model Z-6100 polarized Zeeman atomic absorption spectrophotometer was used for the determination of the concentration of a metal in aqueous solution. A Shimadzu Model UV-160 UV/VIS spectrophotometer with 1.0 cm quartz cell was used for the measurement of absorption spectra. A Horiba Model F-12 pH meter equipped with a Horiba 9610-10D combined glass electrode was used to determine the pH values. ReagentsBPMP was synthesized as follows: To a 2-propanol solution of 2-pyridinecarboxaldehyde (6.4 g, 0.06 mol) was added slowly 2,2-dimethyl-1,3-propanediamine (3.1 g, 0.03 mol) in 2-propanol. After the mixture was stirred gently for 24 h and the 2-propano...

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