Plasmon-Induced Light Absorption of Phthalocyanine Layer in Hybrid Nanoparticles: Enhancement Factor and Effective Spectra

Plasmon enhancement of optical absorption in phthalocyanines and related compounds is a highly effective route for substantial improvement of their photophysical properties. Herein, we report the results on the comprehensive study of optical absorption in aluminum phthalocyanine complex in spherical exciton–plasmon nanostructures based on gold nanoparticles. We synthesized hybrid nanoparticles (HNPs) composed of gold cores with an average size of 19 nm coated by phthalocyanines which formed a close-packed molecular shell having a thickness of 2–4 nm. Owing to H-type aggregation of phthalocyanines at the particle surface, the self-assembled molecular shell demonstrates optical properties typical for most of the metal-phthalocyanine thin films. On the basis of the generalized Mie theory and experimental results, we simulated effective absorption spectra of phthalocyanines in HNPs and established a great enhancement of molar absorption coefficient caused by intense plasmonic near field acting on the shell (up to 8 times at λ = 535–550 nm). The enhancement factor of molar absorption coefficient is primarily governed by a degree of spectral overlap between the plasmonic and excitonic bands that can also dramatically change the absorption line shape of the phthalocyanine. In order to overcome the limitation, we have made a theoretical assessment, which demonstrates a prospect of gold nanoshells to form a plasmonic core of HNPs.