Abstract
The photoexcited triplet states of a series of linear and cyclic butadiyne-linked porphyrin oligomers were investigated by transient Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR). The spatial delocalization of the triplet state wave function in systems with different numbers of porphyrin units and different geometries was analyzed in terms of zero-field splitting parameters and proton hyperfine couplings. Even though no significant change in the zero-field splitting parameters (D and E) is observed for linear oligomers with two to six porphyrin units, the spin polarization of the transient EPR spectra is particularly sensitive to the number of porphyrin units, implying a change of the mechanism of intersystem crossing. Analysis of the proton hyperfine couplings in linear oligomers with more than two porphyrin units, in combination with density functional theory calculations, indicates that the spin density is localized mainly on two to three porphyrin units rather than being distributed evenly over the whole π-system. The sensitivity of the zero-field splitting parameters to changes in geometry was investigated by comparing free linear oligomers with oligomers bound to a hexapyridyl template. Significant changes in the zero-field splitting parameter D were observed, while the proton hyperfine couplings show no change in the extent of triplet state delocalization. The triplet state of the cyclic porphyrin hexamer has a much decreased zero-field splitting parameter D and much smaller proton hyperfine couplings with respect to the monomeric unit, indicating complete delocalization over six porphyrin units in this symmetric system. This surprising result provides the first evidence for extensive triplet state delocalization in an artificial supramolecular assembly of porphyrins.
Highlights
Nanoscale organic materials, such as π-conjugated oligomers, are of considerable interest in the fields of molecular electronics,[1−5] photonics,[6,7] and spintronics.[8,9] Understanding of the factors determining exciton delocalization, as well as charge and spin transport, is of fundamental importance for the design and further development of supramolecular systems with properties tailored to specific applications
Conjugated porphyrin oligomers have been extensively investigated using a range of different linkers to create different two- and three-dimensional supramolecular structures with varying optical and electronic properties.[6,17−23] The delocalization of unpaired electrons in these systems can be investigated by Electron Paramagnetic Resonance (EPR) in radicals generated by chemical oxidation or reduction and in triplet states obtained by photoexcitation.[6,21,24−32]
Comparison of the populations of the singly occupied molecular orbitals (SOMOs) on the different porphyrin units for the linear oligomers and the oligomers bound to a template shows that there is a larger overlap for the linear systems with respect to the bent ones (Figures S3 and S4, Supporting Information), leading to larger D-values, as observed experimentally. These results show that caution must be exerted in the interpretation of zero-field splitting (ZFS) D-values in terms of triplet state delocalization in molecular-wire-type systems with extensive conjugation between the monomeric units, as changes in geometry can cause significant changes in the magnitude of D, which could be wrongly interpreted in terms of increased or decreased triplet state delocalization
Summary
Nanoscale organic materials, such as π-conjugated oligomers, are of considerable interest in the fields of molecular electronics,[1−5] photonics,[6,7] and spintronics.[8,9] Understanding of the factors determining exciton delocalization, as well as charge and spin transport, is of fundamental importance for the design and further development of supramolecular systems with properties tailored to specific applications. Comparison of the populations of the SOMOs on the different porphyrin units for the linear oligomers and the oligomers bound to a template shows that there is a larger overlap for the linear systems with respect to the bent ones (Figures S3 and S4, Supporting Information), leading to larger D-values, as observed experimentally These results show that caution must be exerted in the interpretation of ZFS D-values in terms of triplet state delocalization in molecular-wire-type systems with extensive conjugation between the monomeric units, as changes in geometry can cause significant changes in the magnitude of D, which could be wrongly interpreted in terms of increased or decreased triplet state delocalization. The ZFS parameters and relative sublevel populations determined by simulation are reported in Table
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