Two self-assembled supramolecular donor-acceptor triads consisting of Al(III) porphyrin (AlPor) with axially bound naphthalenediimide (NDI) as an acceptor and tetrathiafulvalene (TTF) as a secondary donor are reported. In the triads, the NDI and TTF units are attached to Al(III) on opposite faces of the porphyrin, through covalent and coordination bonds, respectively. Fluorescence studies show that the lowest excited singlet state of the porphyrin is quenched through electron transfer to NDI and hole transfer to TTF. In dichloromethane hole transfer to TTF dominates, whereas in benzonitrile (BN) electron transfer to NDI is the main quenching pathway. In the nematic phase of the liquid crystalline solvent 4-(n-pentyl)-4'-cyanobiphenyl (5CB), a spin-polarized transient EPR spectrum that is readily assigned to the weakly coupled radical pair TTF(.+)NDI(.-) is obtained. The initial polarization pattern indicates that the charge separation occurs through the singlet channel and that singlet-triplet mixing occurs in the primary radical pair. At later time the polarization pattern inverts as a result of depopulation of the states with singlet character by recombination to the ground state. The singlet lifetime of TTF(.+)NDI(.-) is estimated to be 200-300 ns, whereas the triplet lifetime in the approximately 350 mT magnetic field of the X-band EPR spectrometer is about 10 μs. In contrast, in dichloromethane and BN the lifetime of the charge separation is <10 ns.
The coordination chemistry of the bis-salicyl-appended Schiff-base ligand L8 with Lewis acidic metal ions affords the mononuclear complex [SnĲL9)Cl 4 ] (1) and two paramagnetic dimers [CuĲL9)Ĳsal)] 2 ĲClO 4 ) 2 , (2) and [MnĲL9)Cl 2 ĲEtOH)] 2 ] (3). The X-ray crystal structures of 1-3 reveal a propensity for L8 to undergo metal catalyzed hydrolysis and cyclization to the diazepine ligand L9. Theoretical calculations on L8 and a model SnĲIV) complex reveal that coordination to a metal ion weakens the imine bonds, rendering them more susceptible to hydrolysis reactions and/or attack by nucleophiles. In contrast, reaction of L8 with FeCl 3 in the presence of base affords the partial hydrolysis product [FeĲL10) 2 ]Cl•CH 3 CN (4). Tuning the reaction conditions via the addition of a second base slows down the hydrolysis of the ligand sufficiently to afford a few crystals of the μ 2 -oxo diferric complex (μ-O)ĳFeĲL8)] 2 •2CH 3 CN (5) in which intact L8 coordinates to the FeĲIII) in a bis-bidentate manner through a deprotonated salicyl oxygen and a bis-imine nitrogen lone pair, with the nitrogen atoms of its 2,2′-bipyridine remaining uncoordinated.
The reactivity of a Schiff-base bis-imine ligand 3 is probed by X-ray diffraction studies. Its susceptibility to hydrolysis, oxidation and nucleophilic addition reactions is demonstrated by the isolation of the methanol adduct 4 and two diazapene heterocycles 5 and 6. This reactivity is also reflected in the molecular structures of two coordination complexes isolated by the reaction of 3 with MĲhfac) 2 salts, to afford ĳCuĲ5)-Ĳhfac)Ĳtfa)] (8) and ĳZnĲ6)Ĳhfac) 2 ] (9).
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