EXCITED STATES OF 6-MEMBERED N-HETEROCYCLES . FLUORESCENCE, PHOSPHORESCENCE AND ACID-BASE EQUILIBRIA OF 5 MONO- AND DIAZAPHENANTHRENES IN LOWEST EXCITED (PI,PI) STATES

Abstract— Acid‐base equilibria of five azaphenanthrenes (1‐and 4‐aza; 1,10‐, 1,7‐, and 4,7‐diaza) in the two lowest excited (τ,τ) states have been determined by Förster's cycle. Pure electronic levels of the free bases and their conjugate acids have been located from the absorption, fluorescence and phosphorescence spectra. In diazaphenanthrenes the increase of the first pKa and the decrease of the second pKa under excitation has been observed, and an explanation is offered. In many cases the reactivity of the molecule excited to the lowest triplet state differs considerably from that in the ground state. The Pariser‐Pam‐Pople (PPP) calculations of the free bases have been performed. From the theoretical results, the changes of the net charge on nitrogen atoms under excitation and the magnitude of the S‐T splitting in the La and Lb states have been discussed and compared with experiment. Copyright © 1969, Wiley Blackwell. All rights reserved