Fluorescent and triplet state photoactive J-type phthalocyanine nano assemblies: controlled formation and photosensitizing properties

Cofacial J type nano aggregates, which are both fluorescent and triplet state photoactive, were formed in chloroform via multiple O -> Zn2+ coordination bonds between molecules of ZnPc(alpha-OPh)(4), i.e. tetra(alpha-phenoxy) zinc phthalocyanine. Non photoactive H aggregates, on the other hand, were observed in DMF-water mixtures. The formation, structure and bonding of the J aggregates were examined by TEM, electronic absorption and fluorescence methods. Photophysical studies revealed that the J-aggregates have narrowed absorption and emission bands with peak wavelengths at 740 and 751 nm, which are 50 nm and 43 nm red shifted from the values of the monomers, respectively. The fluorescence decay of the J aggregates is monoexponential with a long lifetime of 2.41 ns, in contrast to J aggregates of other dyes that usually exhibit much shorter, while widely distributed, lifetimes in the ps scale due to the loosely formed structures. The J aggregates emit fluorescence with an efficiency as high as 50% of its monomer. The laser flash photolysis study revealed that both the triplet state and anion species were generated upon photo excitation. The triplet state was formed in a quantum yield of 0.20 and its lifetime is 11 mu s, which makes it sufficiently effective to transfer triplet energy and produce singlet oxygen with a yield of 0.23. The J aggregates represent a new category of nano assemblies that have the potential to be utilized in a variety of optoelectronic devices and photodynamic therapy of tumors.