Triplet state delocalization in a conjugated porphyrin dimer probed by transient electron paramagnetic resonance techniques.

Claudia E Tait,Patrik Neuhaus,Christiane R Timmel,Harry L Anderson

doi:10.1021/jacs.5b03249

Claudia E Tait, Patrik Neuhaus + Show 2 more

Open Access

https://doi.org/10.1021/jacs.5b03249

Copy DOI

Abstract

The delocalization of the photoexcited triplet state in a linear butadiyne-linked porphyrin dimer is investigated by time-resolved and pulse electron paramagnetic resonance (EPR) with laser excitation. The transient EPR spectra of the photoexcited triplet states of the porphyrin monomer and dimer are characterized by significantly different spin polarizations and an increase of the zero-field splitting parameter D from monomer to dimer. The proton and nitrogen hyperfine couplings, determined using electron nuclear double resonance (ENDOR) and X- and Q-band HYSCORE, are reduced to about half in the porphyrin dimer. These data unequivocally prove the delocalization of the triplet state over both porphyrin units, in contrast to the conclusions from previous studies on the triplet states of closely related porphyrin dimers. The results presented here demonstrate that the most accurate estimate of the extent of triplet state delocalization can be obtained from the hyperfine couplings, while interpretation of the zero-field splitting parameter D can lead to underestimation of the delocalization length, unless combined with quantum chemical calculations. Furthermore, orientation-selective ENDOR and HYSCORE results, in combination with the results of density functional theory (DFT) calculations, allowed determination of the orientations of the zero-field splitting tensors with respect to the molecular frame in both porphyrin monomer and dimer. The results provide evidence for a reorientation of the zero-field splitting tensor and a change in the sign of the zero-field splitting D value. The direction of maximum dipolar coupling shifts from the out-of-plane direction in the porphyrin monomer to the vector connecting the two porphyrin units in the dimer. This reorientation, leading to an alignment of the principal optical transition moment and the axis of maximum dipolar coupling, is also confirmed by magnetophotoselection experiments.

Highlights

Porphyrins, which are closely related to the chlorophyll and bacteriochlorophyll molecules found in plants and photosynthetic bacteria, have often been used as building blocks for these materials and have been designed with a wide range of different linkers and linking geometries.[9−16] Electronic π-conjugation, the fundamental phenomenon required for most applications, has been investigated in porphyrin chain systems using many different techniques, including electron paramagnetic resonance (EPR).[13,14,17−26]
Triplet state delocalization and energy transfer have been extensively investigated by EPR in the photosynthetic reaction centers and their model systems[27−37] as well as in linear arrays of π-conjugated organic molecules designed as molecular wires.[18,22,23,25,38−40] Information on the extent of triplet state delocalization can be obtained from the zero-field splitting (ZFS) interaction or from the hyperfine interaction
We focus on the characterization of the triplet state delocalization in a butadiyne-linked porphyrin dimer, P2, using information from the zero-field splitting as well as both proton and nitrogen hyperfine interactions

Summary

Introduction

Triplet state delocalization and energy transfer have been extensively investigated by EPR in the photosynthetic reaction centers and their model systems[27−37] as well as in linear arrays of π-conjugated organic molecules designed as molecular wires.[18,22,23,25,38−40] Information on the extent of triplet state delocalization can be obtained from the zero-field splitting (ZFS) interaction or from the hyperfine interaction. The zero-field splitting interaction parameters can be obtained from the EPR spectrum, whereas the hyperfine couplings can most conveniently be measured using pulse EPR techniques such as ENDOR (electron nuclear double resonance) and ESEEM (electron spin echo envelope modulation). ENDOR investigations have demonstrated triplet state delocalization over the special pair in the bacterial reaction center of Rhodobacter sphaeroides on the basis of a comparison of the ENDOR spectrum recorded on bacteriochlorophyll a in Received: March 28, 2015 Published: April 27, 2015

Results

Discussion

Conclusion