We present a new system for single-molecule detection at cryogenic temperatures, benzodiphenanthrobisanthene in a Shpolskii matrix of n-tetradecane. Most of the single-molecule lines are broader than the lifetime-limited value of 31.0 MHz, indicating the presence of spectral diffusion in the crystalline sample. For the detection of the fluorescence signal, we use a gradient index lens in a back-illumination configuration.