Substituent effects on the structure–property relationship of unsymmetrical methyloxy and methoxycarbonyl phthalocyanines: DFT and TDDFT theoretical studies

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were carried out to simulate the molecular and electronic structures together with the electronic absorption spectra of a series of peripheral methyloxy/methoxycarbonyl substituented phthalocyanines M[Pc(β-OMe)2n(β-COOMe)8−2n] (M = 2H, Zn; n = 0, 1, 2, 3, and 4). Fragment charge distribution and electrostatic potential analysis indicate that the presence of electron-withdrawing and -donation groups leads to the redistribution of charges and obvious polarization effects to the unsymmetrical phthalocyanine series. Peripheral methyloxy/methoxycarbonyl groups introduced onto phthalocyanine ring were revealed to destroy the degeneracy of LUMOs, resulting in significant Q-band splitting for the unsymmetrical phthalocyanine compounds. In addition, metal-free and zinc phthalocyanine compounds display similar electronic structures and absorptions due to the almost none contribution of the zinc atom or inner hydrogens to the frontier molecular orbitals. The microscopic mechanism of the UV–Vis spectra has been clarified on the basis of multi-band photon-induced electron transference. These theoretical studies would be helpful for the molecular design of novel unsymmetrical phthalocyanines.

Graphical abstractThe structure–property relationships of a series of peripheral methyloxy/methoxycarbonyl substituented phthalocyanines are calculated using DFT and TDDFT theory. The molecular and electronic structures, fragment charge distribution and electrostatic potentials, and photon-induced electron transference and microscopic mechanism of UV–Vis spectra are studied. The substituent effects change with the number and position of the substituents.Figure optionsDownload full-size imageDownload high-quality image (175 K)Download as PowerPoint slideHighlights► The presence of peripheral electron-withdrawing and donation groups results in the charge redistribution and obvious polarization effects. ► Metal-free and zinc phthalocyanine compounds display similar electronic structures and absorptions. ► The unsymmetrical phthalocyanines display significant Q-band splitting. ► The substituent effects change with the number and position of the substituents.