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

The lowest photoexcited metastable tripler state of a family of highly conj ugated (porphinato)zinc(II) arrays in which ethyne, butadiyne, and octatetr ayne units bridge the macrocycle carbon frameworks were studied by EPR spec troscopy at low temperature in solution. One series of meso-to-meso ethyne- bridged (porphinato)zinc(II) structures probes how the length of the conjug ated bridge separating two (porphinato)zinc(II) units effects the nature of the tripler excited state, while a second set of compounds assesses the: i mpact that oligomer size has upon triplet state spin distribution and orien tation. Although the magnitude of the EPR /D/ values decrease with increasi ng number of porphyrin units, the data do not suggest that the lowest tripl et excitation extends beyond a single porphyrin unit. With augmented conjug ation, these systems show a progressive oblate-to-prolate spin distribution transition that causes the direction of largest dipolar interaction to ali gn along a vector defined by the highly conjugated axes of these molecules. Temperature EPR line shape studies of these Zn porphyrin arrays reveal tha t (1) the triplet spectrum evinces no changes up to 100 K, indicating the a bsence of Jahn-Teller-induced dynamics, and (2) the EPR spectra of the trip let states for all dimers, trimers, and pentamers exhibit electron spin pol arization up to temperatures surpassing 100 K, suggesting spin lattice rela xation times that do not decrease significantly with temperature. (C) 2001 Elsevier Science B.V. All rights reserved.