Two-photon-induced fluorescence of cholestane

Cholestane is a fully saturated alkane with a structure analogous to cholesterol. We observed fluorescence emission from cholestane with a maximum near 235 nm when excited with picosecond laser pulses at 298 nm. The emission intensity of cholestane was found to depend on the square of the laser power, indicating the biphotonic process of two-photon excitation. The lifetime of cholestane in cyclopentane was found to be near 1.4 ns. Cholestane was found to be efficiently collisionally quenched by methanol and oxygen, as seen from decreased lifetime in the presence of these quenchers. These results suggest that cholestane or its analogs can be used as intrinsic probes in biologic systems. Two-photon excitation avoids the use of vacuum ultraviolet wavelength (130-170 nm), which requires special optics and exclusion of oxygen, and is generally incompatible with biologic samples.