The singular coincidence of fluorescence spectra of the anionic and cationic species formed by the respective deprotonated and protonated pyrido-pyrrolo bases

Study of the chemically produced anionic and cationic species of the hetero cyclics 7-azaindole (7-AI) and 1-H-pyrrolo[3,2-h]quinoline (1-HPQ) in 298 K solution spectroscopy displays a striking coincidence of the anion/cation spectra for each. A hybrid density functional calculation revealed that, in spite of a very large electrostatic skeletal perturbation for the respecti ve anion and cation pair, the lowest S-0 --> S-1 transition energy was larg ely unperturbed. Generalization to other anion/cation pairs suggests the fo llowing theorem: In a diheterocyclic aromatic molecule with proton-acceptor and proton-donor heteroatom sites, a coincidence or near coincidence of th e coa corresponding cation and anion fluorescence bands will be manifested as a result of(+) and (-) electrostatic skeletal perturbations on the intac t pi-electron system. An analysis of the detailed electronic mechanistic st eps involved in the sequential laser pulse (1) excitation and (2) photoioni zation steps indicate that an observed (7-AI)(H+) mass-119 cation observed in femtosecond time-of-flight mass spectrometry is produced in the invasive photoionization step and is not a likely intermediate in the biprotonic ph ototransfer in the 7-AI H-bonded dimer. (C) 2000 John Wiley & Sons, Inc.