Thiopyrylium, Selenopyrylium, and Telluropyrylium Salts

This chapter deals with thiopyrylium, selenopyrylium, and telluropyrylium salts. These compounds are considered the parent structures of an important class of six-membered heteroaromatic cations. It is observed that the reactivity of thiopyrylium salts resembles that of pyrylium salts with two notable differences— namely, the lesser tendency of 2H adducts formed on nucleophilic attack to undergo ring opening and the ability of sulfur to accommodate more than eight electrons in its valence shell, leading, in some cases, to the formation of thiabenzene derivatives. The chapter also describes the structural and physical properties, the methods of synthesis, the reactions, and the practical uses of these pyrylium salts. There are large number of patents and applicative studies making use of a chalcogenopyrylium salt. Most applications of chalcogenopyrylium salts exploit their photophysical and photochemical properties, and principally regard photographic and reprographic technologies. Another important application of chalcogenopyrylium salts is as polymerization and crosslinking photoinitiators, especially in the preparation of photoresists, printing plates, and photosensitive compositions for laser imaging.