Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(para-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes

Abstract

The preparation and characterization of three new donor-bridge-acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo(para-phenylene), β = 0.39 Å(-1) and oligo(2,5-thiophene), β = 0.22 Å(-1). This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo(para-phenylene) and oligo(2,5-thiophene) are due primarily to the D-B energy gap, Δ, rather than bridge-bridge electronic couplings, H(BB). This is supported by the fact that the H(BB) values extracted from the experimental data for oligo(para-phenylene) (H(BB) = 11,400 cm(-1)) and oligo(2,5-thiophene) (12,300 cm(-1)) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern H(DA) and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges.

Close relation between quantum interference in molecular conductance and diradical existence

“…This was partially explained by the orbital interaction between the parent molecule and the attached atoms. This correlation has also been noted by others in a variety of systems (12)(13)(14)(15)(16)(17)(18). The generality is intriguing and begs for an explanation.…”

“…Furthermore, recent combined theoretical and experimental studies for donor-bridge-acceptor (D-B-A) diradical systems by Kirk, Shultz, and coworkers (12,13) and for π-conjugated molecules bridged between two nitronyl nitroxide radicals by Matsuda and coworkers (14-16) have shown similar trends among magnetic coupling, electron transfer, and molecular conductance, with respect to the exponential attenuation as well as the dihedral-angle dependence. Herrmann and coworkers (17,18) have explored the relationship between molecular conductance and magnetic coupling, but also its limit.…”

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…Thus, the J SQ-NN value was estimated from the temperature dependence of the magnetic susceptibility of the relevant ground-state biradicals. [110] The magnitude of the J SQ-NN exchange interaction wasd irectly addressedfrom the low-temperature MCD. The MCD intensity depended on the J SQ-bpy throught he mixing of the wavefunction with the coefficient l. Figure 11 bs hows the excited doublet net spin populations of the NN, SQ, and bpy centers (green, blue, and red, respectively) calculateda ccording to the Heitler-London approachu sing the J data.…”

Radical Cation π‐Dimers of Conjugated Oligomers as Molecular Wires: An Analysis Based on Nitronyl Nitroxide Spin Labels

“…[42] Recently,t he similarity between exchange interactions of biradicals including NN unit(s) ande lectron transport through molecular wires has been pointed out. [43][44][45][46][47] This intimater elationship was deduced from the almosti dentical exponential attenuation via electron tunneling [48] in both the exchange interaction and molecular conduction throughm olecular wires. Electron spin resonance (ESR) in the solution phase is ultrasensitivef or observing exchange interactions in NN biradicals.…”