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. © 2014 the Partner Organisations.