The chalcogenapyrylium dyes can be treated as model systems whose properties can be designed by proper choice of substituents. The use of different chalcogen atoms and substituents in the dye chromophore allows control of the wavelength of absorption maxima, redox properties, fluorescence yields, and hydrolytic stability.
Michael R. Detty,
"Rational Design Of Properties In Chalcogenapyrylium Dyes.", Proc. SPIE 0847, New Directions in Photodynamic Therapy, (19 February 1988); doi: 10.1117/12.942692; https://doi.org/10.1117/12.942692