ELECTRONIC, SPECTRAL, AND ELECTROCHEMICAL PROPERTIES OF (TPPBRX)ZN WHERE TPPBRX IS THE DIANION OF BETA-BROMINATED-PYRROLE TETRAPHENYLPORPHYRIN AND X VARIES FROM 0 TO 8

The electronic, spectral, and electrochemical characterization of (mes o-tetraphenylporphyrinato)zinc (II) complexes bearing between 0 and 8 bromo substituents at the beta-pyrrole positions is reported. The inve stigated compounds are represented as (TPPBrx)Zn wh ere TPPBrx is the dianion of brominated 5,10,15,20-tetraphenylporphyrin and x varies bet ween 0 and 8. Each porphyrin undergoes four well-defined one-electron transfer reactions to yield porphyrin pi-cation radicals and dications upon oxidation and porphyrin pi-anion radicals and dianions upon redu ction. Half-wave potentials for the first reduction of (TPPBrx)Zn can be described by a single linear free energy relationship, and plots of E-1/2 versus the number of Br groups on the complex show a linear cor relation with a positive slope of 63 mV per Br group. This is not the case for the other three electron transfer processes of the compounds where plots of E-1/2 versus the number of Br groups show distinctly di fferent linear correlations for derivatives with 0-4 Br groups and tho se with 4-8 Br groups. The effect of increasing number of Br groups on the spectral and electrochemical properties of the neutral complexes was examined over the whole series of compounds, and these experimenta l results are compared to results of theoretical calculations by semie mpirical molecular orbital AM1 methods using configurational interacti ons (CI) over the four Gouterman frontier pi-orbitals. The dihedral an gle containing the four porphyrin macrocycle ring nitrogens is propose d as a measure of porphyrin ring nonplanarity, and this value increase s with increasing number of Br substituents on (TPPBrx)Zn. Results of the AM1-CI = 4 calculations indicate that the spectrally determined HO MO-LUMO gap, i.e., the energy corresponding to the low-energy absorpti on band, varies in a nonlinear fashion with increasing number of Br su bstituents on the macrocycle and this is due to both the electronic ef fect of the substituents and the macrocycle nonplanarity. The HOMO-LUM O gaps theoretically calculated by AM1-CI = 4 methods thus parallel va lues which are experimentally obtained by electrochemistry or spectros copy. The lack of well-defined Linear free energy relationships for al l processes except for the first reduction can be explained on the bas is of electronic effects caused by the halogen substituents and nonpla nar macrocyclic distortions induced by steric interactions among the p eripheral substituents. In the case of porphyrin dication formation, t he redox potentials are virtually independent of the bromo substituent s.