ELECTRONIC-STRUCTURE OF ZN-CHLORIN PI-ANIONS STUDIED BY SPECTROSCOPIC AND QUANTUM-CHEMICAL METHODS

Electronic structure of neutral Zn-chlorin complexes and their pi-mono- and dianions has been considered. The a-anions were obtained by reduction of neutral chlorins with a sodium mirror in tetrahydrofuran solution. Results of spectroscopic studies have been compared with semiempirical quantum-chemical calculations. Characteristic bands in electronic absorption spectra of the uv, visible and near infrared regions are in good agreement with CNDO/S-CI calculations. Partially resolved hyperfine structure in EPR spectra of pi-radical anions was observed. Parameters of the EPR spectra obtained by using the simulation technique were compared with spin densities calculated by the INDO method. The detailed interpretation of H-1 and C-13 NMR spectra of neutral and dianion Zn-octaethylchlorins is presented. All the proton signals in pi-dianions are shifted upfield because of the influence of paramagnetic ring current. The upheld shifts in C-13 NMR spectra of the species studied have been observed too, except for two carbon atoms of the pyrrole ring being placed opposite the hydrogenated one. The C-13 peak positions correlate with the charge distribution on C atoms in neutral molecules as well as in a-dianions. (C) 1995 John Wiley & Sons, Inc.