Linking molecular switches to platinum electrodes studied with DFT

Density functional theory (DFT) with the B3LYP exchange-correlation functional was used to study new linkages between electrodes and molecular switches (alligator-clip compounds) for molecular electronics using Pt electrodes. Starting with the commonly used molecule 3-methyl-1,2-dithiolane (MDTL), which forms a five-membered ring structure in the gas phase, we found the most stable structure of the adsorbed MDTL to be the ring-opened molecule (32.44 kcal/mol) with each S atom bound to a surface bridge position. Afterward we calculated binding energies and geometries for a variety of different compounds: S/O-based (oxathiolanes), O-based (methanol), N-based (imidazole, 1,2,3-triazole, purine, 2,4-diazapentane), and P-based molecules (methylphosphino, PCH₃, 3-methyl-1,2-diphospholane before (MDPL) and after H dissociation (H_diss-MDPL)). Among these alternative linkage molecules we find that only the P-based compounds lead to much higher binding energies than MDTL. The best compromise between strong surface attachment and mechanical stability provide the MDPL molecules. For the cis-ring-closed structure of MDPL a binding energy of 47.72 kcal/mol was calculated and even 54.88 kcal/mol for the ring-opened molecule. In the case of H-dissociative adsorption, which leads to H_diss-MDPL, both binding energies increase to 53.74 (ring-closed) and 74.99 kcal/mol (ring-opened). Thus, MDPL provides a much more stable link to the metal surface and might increase the conductance between molecular switch and electrode. In addition, the minor differences in charge and spin-density distribution between MDTL and MDPL might indicate similar properties for the attachment of the molecular switch to either of both alligator-clip compounds.