Single Molecule Conductance Measurement of Photochromic Molecules and Carotenoids

We report data for the single molecule conductance of (a) photochromic molecules in the `open' and `closed' forms and (b) carotenoid polyenes. The photochromic molecules we studied switch between an open state (that absorbs in the UV to become closed) and a closed state (that absorbs in the visible to become open) through light-induced isomerization. The molecular resistance is 526$\pm $90 M$\Omega $ in the open form and 4$\pm $1 M$\Omega $ in the closed form when attached to gold break junction electrodes via thiol linkages. Carotenoid polyenes play an essential role as `molecular wires' in photosynthesis. We measured the electrical conductance of a series of carotenoids with 5, 7, 9 and 11 double-single bond pairs. The electronic decay constant, $\beta $, is determined to be 0.224$\pm $0.036{\AA}$^{-1}$ in close agreement with the value obtained from first principles simulations (0.217$\pm $0.01 {\AA}$^{-1})$. The absolute values of the molecular conductance are within a factor three of those calculated from first-principles.