Switching on fluorescent emission by molecular recognition and aggregation dissociation

Abstract

A generic and effective approach to "switch on" and enhance the two-photon fluorescence (TPF) emission of quenched TPF molecules, i.e., fluorene derivatives, is reported in terms of molecular recognition with a decoupling medium. Such a medium, in this case Bombyx mori silk, can recognize TPF molecules and inhibit the aggregation of the TPF molecules. The designed TPF molecules are 2,7-bis[2-(4-nitrophenyl)ethenyl]-9,9-dibutylfluorene (4NF) and 2,7-bis[2-(4-nitrophenyl)ethenyl]-9,9-dioctylfluorene (8NF), which exhibit suppressed TPF emission owing to molecular-stacking-led aggregation in the solid form. Due to the specific recognition between -NO 2 in the quenched fluorescent molecules and -NH groups in silk fibroin molecules, the aggregated molecules of 4NF/8NF molecules are decoupled. This decoupling gives rise to a significant increase in TPF quantum yields. The mechanism is further confirmed by replacing the terminal group -NO 2 in 8NF with -CH 3 (in 2,7-bis[2-(4-methylphenyl)ethenyl]-9,9-dioctylfluorene; 8MF) to eliminate the possibility of molecular recognition. As predicted, in the case of 8MF the switching-on effect is eliminated. Completely new TPF silk fibers can additionally be applied in real-time 3D high-resolution TPF scaffold bioimaging. Luminescent silk fibers that exhibit two-photon fluorescence are prepared by combining Bombyx mori silk with fluorene derivatives. The enhancement mechanism is based on biomolecular recognition by decoupling the molecules in the special silk fibers. This work provides a promising solution for imaging of scaffolds in a noninvasive, high-resolution, 3D, and real-time manner. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.