Fig. 2. (a) General molecular structures of the investigated dye families and commercial names of the tested fluorophores. Full dye structures are given in Fig. S1–S2.† (b) Molar decadic absorption coefficient of the investigated fluorophores at their peak absorption wavelength (as provided by the manufacturer), (c) measured fluorescence quantum yield in H2O, and (d) measured S1 excited-state lifetime in H2O as a function of their S0–S1 energy gap. (e) Fluorescence enhancement of the investigated dyes in D2O with respect to H2O and (f) in MeOH-d4 with respect to MeOH, measured as the ratio of the excited-state lifetime in the deuterated and in the protonated solvent. Error bars reflect the estimated uncertainty arising from the measurement of the excited-state lifetimes. (g) Brightness (ε·Φfl) distribution of the investigated dyes in H2O (hollow circles) and D2O (filled circles). The grey dashed line underlines the general trend for the maximal observed brightness in H2O, the black dashed line the trend in D2O; both lines merely serve as a guide to the eye. In (b–g), every dye is represented by a full circle at the wavelength corresponding to its S0–S1 energy gap.
