and the quantum yield to 0.40, yielding a brightness of 10,400 M -1 cm -1 compared to 8,518 M -1 cm - 1 for SiR-DNA, whereas 68a exhibits little turn-on response (550 M -1 cm -1 ). Isomeric dyes modified at different ring positions on the pendant phenyl have also been shown to differ in their behavior toward biomolecules, including their interaction with DNA, HaloTag and SNAP-tag. Investigation of the reasons for this difference in Si-Bridge dyes, where there is control over which "face" of the dye is directed toward the target and/or sensor, could enable a more detailed understanding of the underlying principles for specific fluorogenic probe design.