Solvent effects on the absorption and fluorescence spectra of Cu(II)-phthalocyanine and DFT calculations

The absorption and fluorescence spectra of Cu(II)-phthalocyanine were recorded in different solvents of varying types. The data fitted well to Lippert-Mataga equation thus confirming the dependence of orientation polarizability on the absorbance and emission values. The spectroscopic data also fitted well to the Kamlet-Abboud-Taft model. Density functional calculations were used to assign the absorption in the visible region to a π-π* transition between the HOMO-1 and LUMO of the complex. The visible region transition occurs between orbitals largely based on the phthalocyanine ligand, with no significant Cu character. The fluorescence emission of the molecule was effectively quenched in the presence of tetrachlorobenzoquinone (TCQ). The fluorescence quenching data shows that both static and dynamic quenching are operative in the present system. This may be due to the aggregation of the phthalocyanine complex in solution which arises from π-π stacking of its wide planar aromatic structure. The fluorescence quantum yields suggest that in the excited state the molecule interacts more with an aprotic solvent, while the ground state is preferentially stabilized in the presence of a protic solvent.