Experimental and computational investigation of unsymmetrical cyanine dyes: Understanding torsionally responsive fluorogenic dyes

Unsymmetrical cyanine dyes are widely used in biomolecular detection due to their fluorogenic behavior, whereby fluorescence quantum yields can be very low in fluid solution but are significantly enhanced in conformationally restricted environments. Herein we describe a series of fluorinated analogues of the dye thiazole orange that exhibit improved fluorescence quantum yields and photostabilities. In addition, computational studies on these dyes revealed that twisting about the monomethine bridge beyond an interplanar angle of 60 degrees leads to a dark state that decays nonradiatively to the ground state, accounting for the observed fluorogenic behavior. The effects of position and number of fluorine substituents correlate with both observed quantum yield and calculated activation energy for twisting beyond this critical angle.