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

The L-1(a) and L-1(b) classification of electronically excited states of cata-condensed hydrocarbons proposed by Platt in 1949 (Platt, J. R. J. Chem. Phys. 1949, 17, 484) is challenged by investigating a series of N-heteronaphthalenes and comparison of their low-lying pi pi* excited states to those of naphthalene. The breakdown of Platt's classification scheme for N-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 L-1(w) (weakly correlated) and L-1(s) (strongly correlated) is proposed. Furthermore, fundamental guidelines for their property modulation through N-atom substitution patterns are discussed.