Treatment of 1 with cyclohexyl dichlorosilane yielded the expected Si-rhodamine dye 8 (Scheme 1). However, the reactions with the corresponding five and four-membered ring analogs yielded many side products. We were able to obtain the cyclopentyl analog 42 in good yield through an alternate synthetic pathway (Scheme 2), but the four-membered ring analog could not be isolated by either route. The silacyclopentyl analog 42 is red-shifted compared to 4 and 8, consistent with a strain-induced LUMO-lowering effect (Table ). However, we found that 42 is chemically unstable in solution, degrading from a blue-colored near-IR dye to a red-colored red fluorescent dye along with other side-products. LC/MS and HRMS indicated an increase in mass from 425 to 457, consistent with the net addition of CH3OH (Figure ). Our operating hypothesis is that this involves ring opening of the silacyclopentane (Figure ). Chemical instability has previously been noted for spirocyclic siloles, due in part to this same strain-induced LUMO-lowering effect, particularly for the four-membered rings. Introduction of sensors and functional handles. A general strategy used to develop fluorescent sensors is photoinduced electron transfer (PeT). Such sensors have typically been incorporated into the pendant phenyl group of rhodamines (Figure ). We investigated whether a PeT sensor could be introduced directly onto the Si atom of a Si-rhodamine. Dye 17 (Scheme 1) was anticipated to exhibit quenched dye fluorescence at physiological pH, but become brightly fluorescent at acidic pH when the aryl amine becomes protonated. The quantum yield in PBS at pH 7.4 is <0.02 (Table ), consistent with PeT quenching when the aryl amine is not protonated. Conversely, the quantum yield in pH 3 acetate buffer is 0.31, consistent with relief of PeT quenching when the aryl amine is protonated (Table ).