Indole substituted zinc phthalocyanine: Improved photosensitizing ability and modified photooxidation mechanism

The photophysical and photochemical processes within a novel photosensitizer (PS), zinc phthalocyanine (ZnPc) modified by indole units, were explored. The properties related to photodynamic therapy of tumor (PDT) were studied by time-resolved transient UV–vis absorption spectra, steady state and time-resolved fluorescence spectra, and chemical trapping of singlet oxygen by diphenylisobenzofuran (DPBF). Intra-molecular photoinduced electron transfer (PET) within the conjugate from the indole subunits (donor A), to S1 (excited singlet state) of ZnPc moiety (acceptor D), is featured by the significant decrease of fluorescence quantum yield and lifetime of ZnPc moiety, and the occurrence of transient absorption bands of ZnPc− at 570 and 630 nm. The triplet state, on the other hand, was not quenched by indole units. The kinetics and thermodynamics of PET were analyzed quantitatively, and the quantum efficiency of PET is computed to be 38%, almost double of the emission efficiency (20%). The quantum efficiency of triplet (T1) formation is 0.50 and the quantum yield of DPBF photooxidation is 0.72. Both are larger than the expected value of 0.32. The evolution of transient absorption spectra showed that the charge separation state (ZnPc−–indole+) recombined to triplet state ZnPc(T1)–indole, which is responsible for the high yield of T1 formation. In the presence of oxygen, both T1 and ZnPc− were quenched efficiently, which forms singlet oxygen and superoxide anion, respectively. DPBF is therefore photo-oxidized by both singlet oxygen (Type II reaction, 46%) and superoxide anion radical (Type I reaction, 54%), which led to the high yield of photooxidation. This is in contrast to free ZnPc PS, in which only singlet oxygen is responsible for the photooxidation. The result suggests that the reaction mechanism is changed upon conjugation so that the importance of Type I reaction is greatly enhanced, and the indole-conjugated ZnPc is an even better PS than the free ZnPc.

► Zinc phthalocyanine (ZnPc) substituted by indole units were synthesized.
► The fluorescence quantum yield and lifetime of ZnPc moiety was significantly decreased. The transient absorption bands of ZnPc− due to photoinduced electron transfer (PET) were observed.
► The quantum efficiency of PET is 38%, almost double of the emission efficiency (20%). The quantum efficiency of triplet (T1) formation is 0.50 and the quantum yield of DPBF photo-oxidation is 0.72, both are larger than the expected value of 0.32.
► The charge separation state (ZnPc−–indole+) recombined to triplet state ZnPc(T1)–indole, which is responsible for the high yield of T1 formation.
► DPBF is photo-oxidized by both singlet oxygen (Type II reaction, 46%) and superoxide anion radical (Type I reaction, 54%), which led to the high yield of photo-oxidation. The importance of Type I reaction is greatly enhanced.