Quantifying the exchange coupling in linear copper porphyrin oligomers

Abstract

Linear π-conjugated porphyrin oligomers are of significant current interest due to their potential applications as molecular wires. In this study we investigate electronic communication in linear butadiyne-linked copper porphyrin oligomers by electron paramagnetic resonance (EPR) spectroscopy via measurement of the exchange interaction, J, between the copper(ii) centers. The contributions of dipolar and exchange interactions to the frozen solution continuous wave (cw) EPR spectra of the compounds with two or more copper porphyrin units were explicitly accounted for in numerical simulations using a spin Hamiltonian approach. It is demonstrated that a complete numerical simulation of the powder spectrum of a large spin system with a Hamiltonian dimension of 26 244 and beyond can be made feasible by simulating the spectra in the time domain. The exchange coupling in the Cu dimer (CuCu distance 1.35 nm) is of the order of tens of MHz (Ĥ = -2JS·S) and is strongly modulated by low-energy molecular motions such as twisting of the molecule.

ELDOR-detected NMR beyond hyperfine couplings: a case study with Cu(ii)-porphyrin dimers

“…By including more parameters in the simulation, such as the nitrogen couplings and the exchange coupling, the fit could most likely be improved. 43 However, the rather featureless character of the spectrum makes it difficult to assign quantitatively and confidently precise values to the different interactions, as all of these additional parameters only contribute to fielddependent line broadening in the present case. Note that similar, butadiyne-linked, compounds, 43 exhibit an exchange coupling of about 50 MHz.…”

“…43 However, the rather featureless character of the spectrum makes it difficult to assign quantitatively and confidently precise values to the different interactions, as all of these additional parameters only contribute to fielddependent line broadening in the present case. Note that similar, butadiyne-linked, compounds, 43 exhibit an exchange coupling of about 50 MHz. But if the two porphyrin planes of CuP2 are perpendicular to each other, consistent with X-ray and DFT studies of similar compounds, the exchange coupling is expected to be close to zero.…”

“…While there are many nuclei with nuclear spin I 4 0, we focus on the copper nuclei and the unpaired electrons, which contribute the dominating interactions. 43 The Hamiltonian that describes a system of two electrons S 1 = S 2 = 1/2 and two nuclei I 1 = I 2 = 3/2, with zero hyperfine coupling between S 1 and I 2 as well as between S 2 and I 1 , reads in linear frequency unitŝ…”

Improving the accuracy of Cu(ii)–nitroxide RIDME in the presence of orientation correlation in water-soluble Cu(ii)–nitroxide rulers

“…In recent work it was demonstrated that such measurements reveal otherwise inaccessible information on the electronic structure of the systems. 16,25 Secondly, complexes of Cu(II) ions are attracting attention in bio-EPR spectroscopy, [26][27][28][29] both as native metal centres in proteins, 23,26,[30][31][32][33][34][35][36][37][38] and as spin labels for orthogonal pulse dipolar spectroscopy. 39,40 The potential of the latter approach has been recognised previously and is reflected in a growing interest in Cu(II)-based spin labelling strategies.…”

Accessing distributions of exchange and dipolar couplings in stiff molecular rulers with Cu(ii) centres

“…[24][25][26][27] Significant progress in performing and analysing Cu(II)-based PDS experiments has been achieved over the past few years. [28][29][30][31][32][33] Exchange interactions are often observed for Cu(II) ions that are bound to conjugated p-systems, [34][35][36][37][38][39] or in strongly coupled systems, such as molecular magnets. 7,40,41 The exchange coupling may give additional information on the system of interest, yet, the dipolar and exchange couplings can become difficult to disentangle.…”