Electronic decay constant of carotenoid polyenes from single-molecule measurements

The conductance of carotenoid polyenes chemically bound at each end to gold contacts has been measured for single molecules containing 5, 7, 9, and 11 carbon-carbon double bonds in conjugation. The electronic decay constant, β, is determined to be 0.22 ± 0.04 Å^-1, in close agreement with the value obtained from first principles simulations (0.22 ± 0.01 Å^-1). The absolute values of the molecular conductance are within a factor of 3 of those calculated from first principles. The small value of β demonstrates that conductivity drops off only slowly with chain length, confirming that carotenoid conjugated chains are relatively good molecular "wires".