Conjugated Chromophore Arrays with Unusually Large Hole Polaron Delocalization Lengths

We report variable temperature X-band EPR spectroscopic data for the cation radical states of meso-to-meso ethyne-bridged (porphinato)zinc(II) (PZnn) oligomers. These [PZn2−PZn7]+ 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 PZnn+ 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); PZnn+ 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 [PZn2−PZn7]+, 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

Electron Spin Relaxation of Hole and Electron Polarons in π‑Conjugated Porphyrin Arrays: Spintronic Implications

Electron spin resonance (ESR) spectroscopic line shape analysis and continuous-wave (CW) progressive microwave power saturation experiments are used to probe the relaxation behavior and the relaxation times of charged excitations (hole and electron polarons) in <i>meso</i>-to-<i>meso</i> ethyne-bridged (porphinato)­zinc­(II) oligomers (<b>PZn</b>_{<b><i>n</i></b>} compounds), which can serve as models for the relevant states generated upon spin injection. The observed ESR line shapes for the <b>PZn</b>_{<b><i>n</i></b>} hole polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b>) and electron polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>}) states evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, with solution-phase <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b> and <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>} spin–spin (<i>T</i>₂) and spin–lattice (<i>T</i>₁) relaxation times at 298 K ranging, respectively, from 40 to 230 ns and 0.2 to 2.3 μs. Notably, these very long relaxation times are preserved in thick films of these species. Because the magnitudes of spin–spin and spin–lattice relaxation times are vital metrics for spin dephasing in quantum computing or for spin-polarized transport in magnetoresistive structures, these results, coupled with the established wire-like transport behavior across metal–dithiol-<b>PZn</b>_{<b><i>n</i></b>}–metal junctions, present <i>meso</i>-to-<i>meso</i> ethyne-bridged multiporphyrin systems as leading candidates for ambient-temperature organic spintronic applications

Singlet and Triplet Excited States of Emissive, Conjugated Bis(porphyrin) Compounds Probed by Optical and EPR Spectroscopic Methods

The nature of the singlet and triplet excited states of a series of meso-to-meso ethyne-linked bis(porphyrin) compounds was probed by electronic absorption, polarized pump−probe fluorescence, electron paramagnetic resonance (EPR), electroabsorption (Stark), and transient triplet−triplet absorption spectroscopic methods. Pump−probe fluorescence anisotropy experiments show that the presence of meso-ethynyl substituents drives a reorientation of orthogonal x- and y-polarized singlet excited states in the macrocycle frame of reference with respect to that evinced for conventional free-base porphyrin chromophores. Analogous experiments in conjugated bis(porphyrin) species bis[(5,5‘,-10,20-bis[3‘ ‘,5‘ ‘-(di-tert -butyl)phenyl]porphinato)zinc(II)]ethyne, 5-[10,20-bis[3‘ ‘,5‘ ‘-(di-tert-butyl)phenyl]porphinato)zinc(II)]-5‘-[10‘,20‘-bis[3‘ ‘,5‘ ‘-(di-tert-butyl)phenyl]porphyryl]ethyne, and bis[5,5‘,-10,20-bis[3‘ ‘,5‘ ‘-(di-tert-butyl)phenyl]porphyryl]ethyne demonstrate substantial energetic splittings of the x- and y-polarized S1 states. The magnitude of this energetic gap results in the complete suppression of population exchange between excited states having orthogonal polarizations on the time scale of these measurements, and gives rise to singly degenerate emitting states polarized exclusively along the axis defined by the ethyne moiety. Stark spectroscopic experiments show that the electronically symmetric meso-to-meso ethyne-bridged bis[(porphinato)zinc(II)] complex exhibits changes in dipole moment with respect to the ground state in its respective x-polarized S2 and S1 states. The EPR spectra of the low-lying photoexcited triplet excited states of these conjugated bis(porphyrin) compounds and their ethyne-substituted porphyrinic building blocks show an evolution in the |D| and |E| ZFS parameters with augmented conjugation consistent with a progressing oblate-to-prolate spin transition that causes the direction of largest dipolar interaction to align along the vector defined by the conjugated ethyne moiety. Conjugated arrays based on meso-ethyne elaborated porphyrin and (porphinato)zinc(II) precursors thus constitute an unusual class of oligomeric porphyrin species in that once a threshold level of conjugation is reached, the optical and magnetic principal axis systems become coincident