1. A. T. Balaban, A. Dinculescu, G. N. Dorofeenko, G. W. Fischer, A. V. Koblik, V. V. Mezheritskii, W. Schroth, Pyry-lium Salts: Syntheses, Reactions, and Physical Properties, in Adv. Heterocycl. Chem., Ed. A. R. Katrizky, Suppl. 2, Academic Press, New York, 1982, 434 pp.
2. G. N. Dorofeenko, E. I. Sadekova, E. V. Kuznetsov, Prepar-ativnaya khimiya pirilievykh solei [The Preparative Chemistry of Pyrylium Salts], Izd. Rostov. Univ., Rostov-on-Don, 1972, 234 pp. (in Russian).
3. V. M. Vlasov, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk [Bulletin of the Siberian Branch of the Academy of Sciences of the USSR, Division of Chemical Sciences], 1971, 96 (in Russian).
4. N. A. Orlova, T. N. Gerasimova, M. A. Kudinova, A. I. Tolmachev, Zh. Org. Khim., 1990, 26, 1313 [J. Org. Chem. USSR (Engl. Transl.), 1990, 26].
5. B. K. Hofelschweiger, The Pyrylium Dyes: A New Class of Biolabels. Synthesis, Spectroscopy, and Application as Labels and in General Protein Assay, PhD Diss., Regensburg, Germany, 2005.