Fluorescence Properties of Phenol-Modified Zinc Phthalocyanine that Tuned by Photoinduced Intra-Molecular Electron Transfer and pH Values

Abstract

Tetra[α-(4-hydroxyphenoxy)] zinc phthalocyanine, ZnPc(α-OPhOH)(4), was synthesized and its photophysics was found to be sharply pH dependent. Dual fluorescence emission around 700 nm was observed when it is dissolved in basic solution. The fluorescence of the phthalocyanine can be sharply switched off at pH 9.1 due to the intramolecular photoinduced electron transfer (PET) in ZnPc(α-OPhONa)(4), formed by the deprotonation of ZnPc(α-OPhOH)(4). The photophysics of both ZnPc(α-OPhOH)(4) and ZnPc(α-OPhONa)(4) were studied in detail by UV-vis absorption, steady state and time-resolved fluorescence and transient absorption (TA) to reveal the fluorescence quenching mechanism. Intra-molecular PET in ZnPc(α-OPhONa)(4) from the donor, PhONa subunits, to the acceptor, ZnPc moiety, was characterized by the much smaller fluorescence quantum yield (0.003) and lifetime (<0.20 ns). PET was further evidenced by the occurrence of charge separation state (CSS) in TA spectra, i.e. the bands due to anion radical of ZnPc and phenol radical. The lifetime of the charge separation state is ca. 3 ns, the efficiency of PET is ca. 99% and the rate constant of PET is 2.3 × 10(10) s(-1).