5-(Thien-2-yl)-2'-deoxyuridine, an isomorphic fluorescent nucleoside analogue, was incorporated into multiple positions within single stranded oligodeoxynucleotides. With minimal impact on duplex stability and overall structure, oligonucleotides containing three identical isomorphic fluorescent nucleosides in alternating or neighboring positions display enhanced, sequence-dependent on-signals for either duplex formation or dissociation.