Abstract
The extent of triplet state delocalization is investigated in rigid linear zinc porphyrin oligomers as a function of interporphyrin bonding characteristics, specifically in meso-meso singly linked and β,meso,β fused structures, using electron paramagnetic resonance techniques. The results are compared with those of earlier measurements on porphyrin oligomers with alkyne linkers exhibiting different preferred conformations. It is shown that dihedral angles near 90° between the porphyrin planes in directly meso-to-meso linked porphyrin oligomers lead to localization of the photoexcited triplet state on a single porphyrin unit, whereas previous work demonstrated even delocalization over two units in meso-to-meso ethyne or butadiyne-bridged oligomers, where the preferred dihedral angles amount to roughly 30° and 0°, respectively. The triplet states of fused porphyrin oligomers (i.e., porphyrin tapes) exhibit extended conjugation and even delocalization over more than two porphyrin macrocycles, in contrast to meso-to-meso ethyne or butadiyne-bridged oligomers, where the spin density distribution in molecules composed of more than two porphyrin units is not evenly spread across the oligomer chain.
Highlights
The interporphyrin dihedral angles have previously been shown to amount to roughly 90° with very little conformational flexibility.[28−30] The results obtained for the singly linked oligomers will be contrasted with those obtained for their fused counterparts, where two additional covalent bonds in βposition introduce additional π-conjugated connections and force the individual porphyrin macrocycles into a planar, tapelike structure
The influence of different bridges on triplet state delocalization in linear zinc porphyrin oligomers was explored by electron paramagnetic resonance (EPR) spectroscopy
The results obtained on porphyrins linked by a direct meso-to-meso bond are consistent with previous studies on ethyne and butadiyne-bridged oligomers[18,21,22,27] and clearly indicate that the triplet state localizes on a single porphyrin unit as the dihedral angle between adjacent porphyrin planes approaches 90°
Summary
Triplet excitons are important intermediates in many processes of biological or technological relevance.[1−5] The kinetics of their formation and their properties have a major influence on the performance of devices relying on singlet or triplet exciton fission or fusion, such as organic semiconductors.[6,7] Triplet formation is a common loss mechanism in organic solar cells.[8−10] the extent of delocalization of the photoexcited triplet state has recently been shown to impact the performance of organic photovoltaic devices and organic light emitting diodes.[11,12]. Dimer, and trimer will be referred to as P1H, P2H, and P3H, respectively In these oligomers, the interporphyrin dihedral angles have previously been shown to amount to roughly 90° with very little conformational flexibility.[28−30] The results obtained for the singly linked oligomers will be contrasted with those obtained for their fused counterparts, where two additional covalent bonds in βposition introduce additional π-conjugated connections and force the individual porphyrin macrocycles into a planar, tapelike structure
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