Two-photon excited fluorescence of BF2 complexes of curcumin analogues: toward NIR-to-NIR fluorescent organic nanoparticles

Abstract

The synthesis of borondifluoride complexes of curcuminoids and one- and two-photon absorption properties of 1 and 2 are described. These compounds allow the preparation of organic nanoparticles that were characterized using DLS, TEM and AFM revealing sizes around 50–60 nm for 2. The fluorescence emission spectra of the nanoparticles are red-shifted compared to those of the dye diluted in dichloromethane solution reaching the NIR region of the spectrum. This effect stems from the occurrence of π–π interactions in the solid state, as revealed from the X-ray crystal structure analysis of 1. Those dyes exhibit a significant two-photon absorption cross-section in solution and nanoparticle suspension in water. The nanoparticles are shown to behave as NIR-to-NIR fluorescent nanomaterials that could be potentially used for studies in the biological transparency window. Two-photon brightnesses of ca. 13–14 GM are obtained for both dyes, which are amongst some of the highest values reported to date for NIR-to-NIR luminophores.