Pj. Angiolillo et al., Trends in triplet excitation delocalization in highly conjugated (porphinato)zinc(II) arrays probed by EPR spectroscopy, SYNTH METAL, 116(1-3), 2001, pp. 247-253
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
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