The Nature of Tunnel Splitting Mediated by Stacked Aromatics

Abstract

We have examined the distance and orientation dependence of the energy splitting, ΔE(π*), between the two lowest-lying unoccupied molecular orbitals of a pair of tetracyanoethylene (TCNE) molecules bridged by a stack of noncovalently bonded benzene rings; the stack length ranged from one to six benzene molecules. The distance between ring planes was fixed at 3.4 A, while the orientation of the rings and of the TCNE molecules was varied. The magnitude of the splitting energy was found to decay exponentially with increasing stack length, r; ΔE(π*) = A exp(−0.5βer), with the damping factor, βe, ranging in value from 1.1 to 1.6 A-1, as might be anticipated for instances where the “tunneling energy” lies several electron volts away from the frontier orbitals of the π-electron stack. Both ab initio and semiempirical computations predict a weak dependence of the coupling upon the orientation of the rings. The ab initio HF/3-21G calculations give βe values approximately 20% smaller than those values found in semi...