Compounds and methods for determining transmembrane potential, monitoring changes in transmembrane potential, and/or drug screening are provided. In one aspect, compounds of the invention have a structure according to the formula: E-M-A, wherein A is a fluorophore, selected from xanthenes, coumarins, cyanines, bimanes, and difluoroboradizaindacenes, charged at physiological pH; M is a molecular wire; and E is a hydrophobic moiety, wherein A and E are capable of being involved in a photo-induced, intramolecular electron transfer that quenches the fluorescence of A in response to a voltage condition. When in use, compounds of the invention are membrane-impermeant and oriented within the cell membrane such that the charged moiety localizes at the outer leaflet of the lipid bilayer and the hydrophobic moiety and molecular wire associate with the hydrophobic portion of the lipid bilayer. The rate of electron transfer, fluorescence intensity, and quenching are altered in response to changes in transmembrane potential.