Exploitation of suitably functionalized heterocyclic molecules, in the design and synthesis of Fluorescence Resonance Energy Transfer (FRET) cassettes and their corresponding dideoxynucleotide terminators culminated into efficient reagents for DNA sequencing. Additionally, these FRET cassettes/terminators, of the present invention, derived from different classes of heterocyclic systems have high potential to be used for general labelling of biological molecules to generate highly sensitized signals. Their preparation, energy transfer efficiency, and use as labels, specifically, in DNA sequencing reactions is disclosed.