Molecular dynamics of a grid-mounted molecular dipolar rotor in a rotating electric field

Abstract

Classical molecular dynamics is applied to the rotation of a dipolar molecular rotor mounted on a square grid and driven by rotating electric field E(ν) at T ≃ 150 K. The rotor is a complex of Re with two substituted o-phenanthrolines, one positively and one negatively charged, attached to an axial position of Rh in a [2]staffanedicarboxylate grid through 2-(3-cyanobicyclo[1.1.1]pent-1-yl)malonic dialdehyde. Four regimes are characterized by a, the average lag per turn: (i) synchronous (a < 1/e) at E(ν) = |E(ν)| > E_c(ν) [E_c(ν) is the critical field strength], (ii) asynchronous (1/e < a < 1) at E_c(ν) > E(ν) > E_bo(ν) > kT/μ, [E_bo(ν) is the break-off field strength], (iii) random driven (a ≃ 1) at E_bo(ν) > E(ν) > kT/μ, and (iv) random thermal (a ≃ 1) at kT/μ > E(ν). A fifth regime, (v) strongly hindered, W > kT, Eμ, (W is the rotational barrier), has not been examined. We find E_bo(ν)/kVcm⁻¹ ≃ (kT/μ)/kVcm⁻¹ + 0.13(ν/GHz)^1.9 and E_c(ν)/kVcm⁻¹ ≃ (2.3kT/μ)/kVcm⁻¹ + 0.87(ν/GHz)^1.6. For ν > 40 GHz, the rotor behaves as a macroscopic body with a friction constant proportional to frequency, η/eVps ≃ 1.14 ν/THz, and for ν < 20 GHz, it exhibits a uniquely molecular behavior.