Electron-Hole Correlation as Unambiguous and Universal Classification for the Nature of Low-Lying ππ* States of Nitrogen Heterocycles.

The and classification of electronically excited states of cata-condensed hydrocarbons proposed by Platt in 1949 ( Platt , J. R. 1949 , 17 , 484 ) is challenged by investigating a series of -heteronaphthalenes and comparison of their low-lying ππ* excited states to those of naphthalene. The breakdown of Platt's classification scheme for -heterocycles is highlighted, and a reliable and versatile alternative using exciton analyses is presented. The strength of electron-hole correlation turns out to be the most reliable distinguishing feature, and thus, an alternative nomenclature of ( correlated) and ( correlated) is proposed. Furthermore, fundamental guidelines for their property modulation through -atom substitution patterns are discussed.