Ultrafast exciton relaxation dynamics in organic nanoparticles

Abstract

Efficient conversion of light to thermal energy by non-radiative relaxation pathways is critical to the emerging medical technologies of photoacoustic imaging and photothermal therapy. Among the most powerful agents for these techniques are benzoporphyrins, which have been incorporated into biocompatible polymer nanoparticles with tunable photophysical properties. Due to their unique electronic structures, the benzoporphyrins are predicted to form excitons inside the nanoparticles; however, the effects of this excitonic coupling on non-radiative decay processes such as internal conversion and intersystem crossing are unclear. In this work, ultrafast spectroscopy is used to characterize the nanoparticle relaxation dynamics of three benzoporphyrins: a tetra-tert-butyl naphthalocyanine (H2Nc), vanadyl tetra-tert-butyl naphthalocyanine (VONc), and octabutoxy phthalocyanine (H2-OBPc). While H2-OBPc monomers fluoresce in solution, the formation of molecular excitons within H2-OBPc nanoparticles is shown to quench fluorescence such that internal conversion and intersystem crossing dominate the relaxation dynamics. A similar phenomenon is observed in the H2Nc nanoparticles, in which excitonic coupling leads to a faster rate of internal conversion with a time constant of 100 ps, and the VONc nanoparticles, in which intersystem crossing occurs with a time constant of 6 ps. The results of the study are three mechanisms by which excitonic coupling increases the rate of non-radiative relaxation and promotes efficiency in photoacoustic imaging and photothermal therapy.