EXCITED STATES OF SIX‐MEMBERED N‐HETEROCYCLES. FLUORESCENCE, PHOSPHORESCENCE AND ACID‐BASE EQUILIBRIA OF FIVE MONO‐ AND DIAZA‐PHENANTHRENES IN THE LOWEST EXCITED (τ,τ) STATES

Abstract

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.

Fluorescence enhancement of quinolines by protonation

“…9,[16][17][18] Benzo[h]quinoline and acridine (benzo[b] quinoline), belonging to a group of polycyclic aromatic Nheterocycles, have been discovered to be particularly interesting since they possess different pK a values in the ground and excited state. [19][20][21][22] However, there has been no studies that are based on comparing different acids and the effects of the resulting counter-anion. The previous research in this area has been scarce and focused on studying the emission of these compounds with a single base and, to the best of our knowledge, there are no reports of studies using single crystal X-ray diffraction to study the structural effects of protonation on these N-heterocycles.…”

Spectroscopy and Photophysics of Monoazaphenanthrenes III. Luminescence of Phenanthridine and 7,8-benzoquinoline in Crystalline State

“…Although these molecules and other members of this family are strong fluorescing agent in protic solvents (serving as fluorescent probes in biological studies) and are of recognized importance in chemistry, life sciences and other related fields, their luminescence and photophysics were studied rather occasionally [3][4][5][6][7][8], as compared to diaza-or triaza-substituted derivatives of polycyclic hydrocarbons.…”

The Scientific Work of Theodor Förster: A Brief Sketch of his Life and Personality

“…Grabowska and her group have verified this for a wide range of aza heterocycles. [11,12] Grabowski and Rubaszewska have extended the original concept to the Generalized Fçrster Cycle which combines thermodynamic and non-thermodynamic interrelations between proton transfer, electron transfer, and electronic excitation. [13] At the time when the Fçrster Cycle was discovered, protolytic reactions were still considered as unmeasurably fast.…”