Consistent with the difference of a single vinylene unit in the polymethine chain, the absorption/emission maxima wavelengths for heptamethine ICG are 100 nm longer than the maxima peaks for ICB and the molar absorption coefficient (ϵ) is larger. The fluorescence quantum yield for an aqueous solution of ICB (fF = 5.0 %) is higher than ICG (fF = 2.9 %) which matches the relative difference in electronic excited state lifetimes obtained using timecorrelated single photon counting (see Figure ). Defining brightness as, ϵ x fF, we find that a low concentration solution of ICG is 30% brighter than an equivalent concentration of ICB.