Conjugated Chromophore Arrays with Unusually Large Hole Polaron Delocalization Lengths

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Abstract
We report variable temperature X-band EPR spectroscopic data for the cation radical states of meso-to-meso ethyne-bridged (porphinato)zinc(II) (PZn n) oligomers. These [PZn 2−PZn 7]+ species span an average 18−75 Å length scale and display peak-to-peak EPR line widths (ΔBp-p) that diminish with conjugation length. Analysis of these EPR data show that PZn n + structures possess the largest hole polaron delocalization lengths yet measured; experiments carried out over a 4−298 K temperature domain demonstrate remarkably that the charge delocalization length remains invariant with temperature. These cation radical EPR data are well described by a stochastic, near barrierless, one-dimensional charge hopping model developed by Norris for N equivalent sites on a polymer chain, where the theoretical EPR line width is given by ΔBp-p(N-mer) = (1/N1/2)ΔBp-p(monomer); PZn n + oligomers are the first such systems to verify a Norris-type hole delocalization mechanism over a substantial (∼75 Å) length scale. Given the time scale of the EPR measurement, these data show that either (i) Franck−Condon effects are incapable of driving charge localization in [PZn 2−PZn 7]+, resulting in cation radical wave functions which are globally delocalized over a spatial domain that is large with respect to established benchmarks for hole-doped conjugated materials, or (ii) polaron hopping rates in these oligomers exceed 107 s-1, even at 4 K. Finally, this study demonstrates that polymeric building blocks having low magnitude inner sphere reorganization energies enable the development of electronic materials having long polaron delocalization lengths.