Low symmetrical phthalocyanine analogues substituted with three crown ether voids and their cation-induced supermolecules

Unsymmetrical phthalocyanine analogues with three 15-crown-5 ether voids at the 3,4-positions (MtNIL, MtBZ, MtNAP) (Mt = Zn, Cu) have been synthesized and characterized. Their dimerization is induced by addition of some cations, particularly Rb2+ and K+. Cofacial dimer formation in the presence of these cations proceeds in a two-step three-stage process, as indicated by absorption and emission spectroscopy. These cofacial species have a highly specific C-2v eclipsed configuration providing well-defined dimeric species for spectroscopic analysis. The ESR spectra of the cation-induced dimeric copper derivatives show axial symmetry and may be analyzed in terms of interplanar separation of 4.2 Angstrom. CuNAP alone forms a cofacial dimer even in the presence of Na+ or Cs+, and the estimated interplanar distances are 4.1-4.2 Angstrom without depending on the size of cations. The H-1 NMR spectra of zinc dimers are consistent with a cofacial configuration. Magnetic circular dichroism (MCD) spectra of C-2v type monomers can be interpreted as the superimposition of Faraday B-terms. Upper excited state (Soret, S-2) emission is observed for all zinc mononuclear species, and the quantum yield of S-1 emission is smaller than that of zinc phthalocyanine containing four crown units (ZnCRPc), suggesting that the lowering of molecular symmetry produces a decrease in quantum yield. Fluorescence decay of S-1 and S-2 emissions can be analyzed by mono- and biexponential fits, respectively. The zero field splitting (zfs) parameters, D, of the excited triplet (T-1) states of the monomers estimated from time-resolved EPR (TREPR) technique decrease in the order ZnNIL, ZnBZ, and ZnNAP, qualitatively indicating delocalization of excited pi-electrons over additionally fused benzo and naphtho rings. A remarkable decrease in D value on dimerization is interpreted as an indication of delocalization of pi-electrons over two macrocycles. Molecular orbital (MO) calculations within the framework of the Pariser-Parr-Pople (PPP) approximation succeed to reproduce the splitting, intensity, and the relative position of the Q absorption band of unsymmetrical monomers.