Ground and excited state electronic properties of halogenated tetraarylporphyrins. Tuning the building blocks for porphyrin-based photonic devices

The rational design of molecular photonic devices relies on the ability to select components with predictable electronic structure, excited state Life times and redox chemistry. Electronic communication in multiporphyrin array s depends critically on the relative energies and electron density distribu tions of the frontier molecular orbitals, especially the energetically clos e highest occupied molecular orbitals (a(2u) and a(1u)). To explore how the se ground and excited state properties can be modulated, we have synthesize d and characterized 40 free base (Fb), magnesium and zinc tetraarylporphyri ns. The porphyrins bear meso-substituents with the following substitution p atterns: (1) four identical substituents (phenyl, o-chlorophenyl, p-chlorop henyl, o,o'-difluorophenyl, pentafluorophenyl, mesityl); (2) one, two, thre e or four o,o'-dichlorophenyl substituents; (3) one p-ethynylphenyl group a nd three mesityl or pentafluorophenyl groups; (4) one p-ethynyl o,o'-dichlo rophenyl or p-ethynyl-o,o'-dimethylphenyl and three phenyl groups. For each neutral complex the ground state electronic properties were investigated u sing electrochemical methods and optical absorption spectroscopy. Similarly the absorption, emission, and relaxation properties of the lowest singlet excited state were probed by time-resolved absorption and fluorescence meth ods. Each oxidized complex was investigated by static absorption and Liquid and frozen solution EPR spectroscopy. The collective results of these inve stigations have provided insights into the direct (orbital overlap) and ind irect (inductive/conjugative) mechanisms by which halogenated phenyl rings influence the static and dynamic electronic properties of neutral and oxidi zed porphyrinic chromophores, Three key findings are as follows. (1) The ef fective electron-withdrawing strength of halogenated phenyl rings required to reverse the ordering of the a(2u) and a(1u) HOMOs in Mg versus Zn tetraa rylporphyrins has been elucidated. (2) Appropriate halogenation can signifi cantly increase the excited state lifetime of a Zn porphyrin relative to th e unsubstituted complex. (3) Halogenation can be used to modulate redox pot entials in a manner that complements the enhancement of other electronic pr operties. The insights gained from study of this library of porphyrins prov ide a foundation for tuning the electronic properties of monomeric porphyri ns as building blocks for multichromophoric assemblies in optoelectronics a nd other applications. Copyright (C) 1999 John Wiley & Sons, Ltd.