Photoacoustic signal enhancement of Al- and Si-Phthalocyanines caused by photoinduced cleavage of water-soluble axial ligand

•Al- and Si- phthalocyanine (AlPc, SiPc) derivatives bearing PEGylated axial ligands were designed and synthesized.•AlPc and SiPc released the axial PEG ligands under continuous photoirradiation to generate water-insoluble MPc derivatives.•The photoacoustic signal intensities of self-assemblies composed of PEG-grafted MPcs were increased under continuous photo-irradiation.•The mechanism of photoinduced cleavage of the water-soluble axial siloxy ligands of MPcs was revealed on the basis of both experimental and theoretical results.