Resonance Raman spectra of n-π* singlet-triplet transition of p-benzoquinone at low concentrations

A weak visible absorption spectrum of p-benzoquinone (p-BQ) in CS2 due to n-π* singlet-triplet transition was measured. Using the resonance Raman (RR) effect in liquid-core optical fiber (LCOF), we have obtained the 514.5 nm excited RR spectra of p-benzoquinone near 1445 cm−1 and have demonstrated that the new characteristic RR band is attributed to the symmetric CO stretch (νCO) of n-π* singlet-triplet transition of p-BQ. The effect of solution concentration on the RR band was investigated at very low concentrations. The RR peak spreads toward short wavelength side with decreasing solution concentration ranging from 10−7 to 10−11 mol L−1, whereas the blue-shift isn't obvious when the concentration is, at single molecule level, lower than 10−11 mol L−1. Our result is useful for single molecule detection to some extent.