One Carbon Matters: The Origin and Reversal of Odd-Even Effects in Molecular Diodes with Self-Assembled Monolayers of Ferrocenyl-Alkanethiolates

We investigated the origin Of odd-even effects in molecular diodes: based oh self-assembled mOnolayers,-(SAMs) of ferrocenyl--terminated n-alkanethiolates S(CH2) Fc -7-4.1 44i! with n = 6-15 on Ag-or Au surfates contacted with EGalii top electrodes: These S.iVVIs have different MS-C bond angles of 1800 when M = Ag and 104 degrees when M = Au,causing,-a multitude,of odd-even effects in the performance of the,diode&-, By changing th.e M-S-C bond angles and using several characterization techniques, we were able to,syStern-atically identify and rationalize odc11-even effects in the leCtronic structure of the device. Changing n from 6 to 15 resulted- in an odd-even effect in the tilt angle of the Pc units (a), which, in turn, caused odd-even effects in the surfaCe dipole, ',work function, and HOMO onset -,(HOMO = highest occupied molecular orbital). 'These odd-even effects -Caused an -odd-even modulation of the funneling Current across the -diode in the on state (the current that flows across,the junctions when the idicide allows the current to pass through) -The current that flows across the diodes in their off state,(the leakage current) -also followed an odd-even effect that was related- to an odd-even effect hi the packing energy: =SAMs with small tilt angles of the ferrocenyl units ez (with the Pc units standing up) pack better than SAMs with large a values (with the Fc Units in a parallel orientation with the plane of the electrode). All these odd-even effects were completely and consistently re-Versed when the Ag electrodes were replaced with Au electrodes proving they are induced by the M-S-C bond angle.