Communication through molecular bridges: Different bridge orbital trends result in common property trends

Abstract

Common trends in communication through molecular bridges are ubiquitous in chemistry, such as the frequently observed exponential decay of conductance/electron transport and of exchange spin coupling with increasing bridge length, or the increased communication through a bridge upon closing a diarylethene photoswitch. For antiferromagnetically coupled diradicals in which two equivalent spin centers are connected by a closed-shell bridge, the molecular orbitals (MOs) whose energy splitting dominates the coupling strength are similar in shape to the MOs of the dithiolated bridges, which in turn can be used to rationalize conductance. Therefore, it appears reasonable to expect the observed common property trends to result from common orbital trends. We illustrate based on a set of model compounds that this assumption is not true, and that common property trends result from either different pairs of orbitals being involved, or from orbital energies not being the dominant contribution to property trends. For substituent effects, an effective modification of the π system can make a comparison difficult.

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. Very recently, Klein and coworkers have found that a high-spin π-diradical can play an important role in molecular junctions as a spin filter (50).…”

Enhancing the conductivity of molecular electronic devices

“…Recent work by Proppe and Herrmann on the influence of bond stretching on the transmission probability points in the same direction. 8 However, we have to face up to reality-it is very hard to control or influence in a chemical way the bond lengths in a polyene in the way we want, so that t a < t b . A better strategy to improve the conductance of a molecule might be to find a structural element that has the same effect as bond alternation on the weight of diradical resonance structure in the resonance hybrid.…”

“…A similar argument about the balance between the HOMO-LUMO gap and the orbital coefficients as determining factors for the transmission probability was recently put forward by Proppe and Herrmann. 8 The classical analogue of this phenomenon would be an increase in the area of a conducting wire that is bigger than the increase in the length of that wire,…”

“…[1][2][3][4] General strategies to increase the conductivity of molecules through structural modification have attracted less attention. [5][6][7][8] In this work, we will explore just such strategies for a class of conjugated π-electron systems, even alternant hydrocarbons.…”