Brighter, red-shifted Si-dyes. We examined the photophysical properties of the new Sirhodamines in aqueous buffer (Table ). As reference, we compared these properties to the known dimethylsilyl-bridged dye SiR (4). We found that the quantum yield of the new dyes was largely unperturbed by Si-modification, and most of the dyes were as bright or brighter than the parent dimethyl silyl dye SiR. For example, the diethyl silyl dye 5 is 40% brighter, primarily due to a larger extinction coefficient. Whereas simple alkyl substitution on silicon did not appreciably affect the fluorescence wavelength, the introduction of vinyl and phenyl groups caused additive red-shifts in excitation and emission (Table ). The diphenyl silyl dye 7 is the most red-shifted, while divinyl silyl dye 6 is both red-shifted and brighter than 4 dues to its higher extinction coefficient. Mixed Si-substitution with phenyl, vinyl, and methyl groups gave intermediate effects (9-11). The excitation and emission wavelengths of phenylvinylsilyl dye 11 roughly split the difference between dyes 6 and 7, and is brighter than 9, 10, and 7, but dimmer than 6. Chloropropyl silane 12 is not red-shifted, but is 50% brighter than 4 owing to its large extinction coefficient (Table ). On the other hand, incorporation of a trifluoropropyl group into the Si-bridge (13) did not red-shift the dye and lowered brightness. Dioctylsilyl dye 14 is considerably more lipophilic than the other Si-rhodamines, exhibiting weakened fluorescence in PBS (F = 0.18), a low extinction coefficient that did not follow Beer's law, and a pronounced Rayleigh scatter peak.