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Switching on Fluorescent Emission by Molecular Recognition and Aggregation Dissociation

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 –NO2 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 –NO2 in 8NF with –CH3 (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.