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 Å. CuNAP alone forms a cofacial dimer even in the presence of Na+ or Cs+, and the estimated interplanar distances are 4.1-4.2 Å without depending on the size of cations. The 1H 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, S2) emission is observed for all zinc mononuclear species, and the quantum yield of S1 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 S1 and S2 emissions can be analyzed by mono- and biexponential fits, respectively. The zero field splitting (zfs) parameters, D, of the excited triplet (T1) states of the monomers estimated from time-resolved EPR (TREPR) technique decrease in the order ZnNIL, ZnBZ, and ZnNAP, qualitatively indicating delocalization of excited π-electrons over additionally fused benzo and naphtho rings. A remarkable decrease in D value on dimerization is interpreted as an indication of delocalization of π-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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry