Aggregation-enhanced two-photon absorption and up-converted fluorescence of quadrupolar 1,4-bis(cyanostyryl)benzene derivatives showing solvatochromic fluorescence

Abstract

This paper reports synthesis and semiempirical modeling of 1,4-bis(cyanostyryl)benzene (CSB)-based quadrupolar isomeric molecules (α- and β-CSB-TPs), designed to produce enhancement in fluorescence quantum yield and two-photon absorption cross-sections for the nanoaggregate form. Fluorescence yield together with high two-photon optical properties of the isomers have been fine-tuned by moving the cyano group from α to β position, which results in longer absorption-fluorescence wavelengths, and higher one- and two-photon absorptivities for the β-CSB-TP. In nonpolar toluene solution, both isomers exhibit strong one- and two-photon induced fluorescence. Both isomers followed a trend of strong solvatochromisms which was gradually on increasing solvent polarity. Aqueous dispersions of nanoparticles with diameters of ca. 150 nm have been prepared by aggregation of each isomer. The quenched fluorescence in polar media was greatly intensified followed by a 21 fold increase in TPA cross-sections. This helped achieve intense up-converted fluorescence by two-photon absorption of excited β-CSB-TP organic nanoparticles. Coaggregation-enhanced tunable fluorescence has also been demonstrated as a possible application of the isomeric α- and β-CSB-TP mixture.