Trends in triplet excitation delocalization in highly conjugated (porphinato)zinc(II) arrays probed by EPR spectroscopy

The lowest photoexcited metastable triplet state of a family of highly conjugated (porphinato)zinc(II) arrays in which ethyne, butadiyne, and octatetrayne units bridge the macrocycle carbon frameworks were studied by EPR spectroscopy at low temperature in solution. One series of meso-to-meso ethyne-bridged (porphinato)zinc(II) structures probes how the length of the conjugated bridge separating two (porphinato)zinc(II) units effects the nature of the triplet excited state, while a second set of compounds assesses the impact that oligomer size has upon triplet state spin distribution and orientation. Although the magnitude of the EPR |D| values decrease with increasing number of porphyrin units, the data do not suggest that the lowest triplet excitation extends beyond a single porphyrin unit. With augmented conjugation, these systems show a progressive oblate-to-prolate spin distribution transition that causes the direction of largest dipolar interaction to align along a vector defined by the highly conjugated axes of these molecules. Temperature EPR line shape studies of these Zn porphyrin arrays reveal that (1) the triplet spectrum evinces no changes up to 100 K, indicating the absence of Jahn–Teller-induced dynamics, and (2) the EPR spectra of the triplet states for all dimers, trimers, and pentamers exhibit electron spin polarization up to temperatures surpassing 100 K, suggesting spin lattice relaxation times that do not decrease significantly with temperature.