Molecular Wires Bridging Gaps between Gold Surfaces and Their Influence on SERS Intensities

Molecular wires of the oligophenyleneimine (OPI)
families were used as bridging gaps between gold flat surface and gold
nanorods, forming molecular junction systems such as (AuFlat|OPI|
AuNR). Systems with different gap sizes were synthesized from 2.2 nm
for the OPI-1p to 9.9 nm for the OPI-13p (where n is equal to the
number of phenylene groups), and the intensities of the SERS bands at
1078 cm−1 v(CS) and 1168 cm−1 β(CH) were obtained for each gap
length. Our results showed an unusual behavior for the bands 1078 cm−1
v(CS) and 1168 cm−1 β(CH) as a function of OPI (gap) size. To address
these results electromagnetic field simulations by the discrete dipole
approximation (DDA) method for the systems (AuFlat|Gap|AuNR) were
performed. Nevertheless the high SERS intensities observed for (AuFlat|
OPI|AuNR) with large gap sizes for excitation at 785 nm indicated that
there is a strong dependence on the electronic properties of the molecular
wire, which supersedes the electromagnetic contribution of the plasmonic coupling. The experimental and simulated results
indicated that both electromagnetic (dipole−image interaction and surface plasmon resonance) and molecular properties are
contributing to the SERS intensity behavior. Additionally, it has been noticed that the length of the molecular wire that resulted
in a decrease in SERS intensity is coincident with the reported length in which the transition from tunneling to hoping
conduction occurs for OPI molecular wires.