Spectroscopic Studies of Bridge Contributions to Electronic Coupling in a Donor-Bridge-Acceptor Biradical System

Abstract

Variable temperature electronic absorption and resonance Raman spectroscopies are used to probe the excited state electronic structure of TpCum,MeZn(SQ-Ph-NN) (1) – a donor-bridge-acceptor (D-B-A) biradical complex and a ground state analog of the charge-separated excited state formed in photoinduced electron transfer reactions. Strong ferromagnetic exchange coupling that is mediated by the para-phenylene bridge stabilizes the triplet ground state of this molecule. Detailed spectroscopic and bonding calculations elucidate key bridge distortions that are involved in the SQ(π)SOMO→NN-Ph (π*)LUMO D→A charge transfer (CT) transition. We show that the primary excited state distortion that accompanies this CT is along a vibrational coordinate best described as a symmetric Ph(8a)+SQ(in-plane) linear combination and underscores the dominant role of the phenylene bridge fragment acting as an electron acceptor in the D-B-A charge transfer state. Our results show the importance of the localized phenylene bridge LUMO wavefunction in promoting (1) electron transfer in D-Ph-A systems, and (2) electron transport in biased electrode devices that employ a 1,4-phenylene linkage. We have also developed a relationship between the spin density on the acceptor, as measured via the isotropic NN nitrogen hyperfine interaction, and the strength of the D→A interaction given by the magnitude of the electronic coupling matrix element, Hab.

Heterospin biradicals provide insight into molecular conductance and rectification

“…Biradical ligand complexes SQ-P-T-NN and SQ-T-P-NN have been synthesized by standard procedures (see ESI for synthetic details † ) used for the preparation of other SQ-B-NN biradical molecules. 10 – 14 , 16 , 17 , 19 , 28 – 30 Bond line drawings and X-ray structures for SQ-P-T-NN and SQ-T-P-NN are given in Fig. 1 along with relevant SQ-B, B-B, and B-NN torsion angles.…”

“…Since both the SQ and NN are S = 1/2 spin-bearing units, the relative magnitudes of the bridge-mediated SQ-NN electronic coupling ( H SQ-B-NN ) in SQ-P-T-NN and SQ-T-P-NN can be conveniently determined at high resolution by measuring the relative magnitudes of their respective bridge-mediated SQ-NN magnetic exchange interactions, J SQ-B-NN . 13 , 14 , 31 – 34 This can be accomplished using temperature- or magnetic field-dependent spectroscopies ( e.g. , electronic absorption spectroscopy 14 , 35 or EPR, 36 ) or magnetic susceptibility measurements with the singlet–triplet exchange splitting being equal to 2 J SQ-B-NN according to the Heisenberg exchange Hamiltonian, eqn (2): where the ŝ i are the spin operators for the S = 1/2 radical spins.…”

“…This configurational mixing has been quantified in the context of a valence bond configuration interaction (VBCI) model, which has been successfully used to evaluate the electronic structure of SQ-B-NN heterospin biradical complexes. 13 , 14 The ILCT band energy can be used to approximate Δ in eqn (3) and (4), where Δ also represents the intrinsic molecular bias between SQ and NN. 15 , 17 Given the small ∼900 cm –1 energy difference in the two ILCT band maxima, we can make the approximation that Δ SQTPNN ≊ Δ SQPTNN .…”

Transferrable property relationships between magnetic exchange coupling and molecular conductance

“…Since the magnetic exchange interaction (J D/A ) is proportional to H D/A 2 via eqn (5), [32][33][34][35][36] we have initiated experiments designed to explore the correlation of J D/A between donoracceptor biradical centers that are spanned by molecular bridge units (D-B-A biradicals ¼ SQ-Bridge-NN; SQ ¼ S ¼ 1/2 semiquinone, and NN ¼ S ¼ 1/2 nitronylnitroxide radical, Fig. 1B and C) 1,[17][18][19][20]26,[35][36][37][38][39][40] with computed molecular conductance values, g mb . 1 An advantage to understanding the magnitude of g mb by correlating with experimental J D/A parameters is highlighted by the fact that the variable-temperature magnetic susceptibility measurements used to determine J D/A are performed on solid-state samples with known geometries that have been determined by X-ray crystallography.…”

Diastereoisomers of l-proline-linked trityl-nitroxide biradicals: synthesis and effect of chiral configurations on exchange interactions

“… 13 – 16 Using donor–bridge–acceptor biradical systems, the dependence of magnetic exchange interaction on the degree of π-orbital overlap and torsional rotations between different parts has been well demonstrated. 17 – 19 Comparatively, the exchange interactions in the biradicals with nonconjugated spacers are much weaker and can be through-bond and/or through-space, depending on the flexibility of the spacers. Recently, weakly coupled nitroxide biradicals have attracted intense attention as high-field DNP polarizing agents which boost the sensitivity of solid-state nuclear magnetic resonance spectroscopy.…”