Novel pyrene-perylene α-l-LNA FRET pairs described herein effectively detect assembly of 2- and 3-way branched DNA nanostructures prepared by postsynthetic microwave-assisted CuAAC click chemistry. The fluorescent signalling of assembly by internally positioned FRET pairs is achieved with low to no fluorescence background signal, remarkably low limit of target detection values and stabilization of the resulting nanostructures. © 2013 The Royal Society of Chemistry.