Towards the fabrication of molecular assemblies with optoelectronic properties

Abstract

Although technical improvements of electronic devices in the last decades have been remarkable, the empirical law that predicts this trend (Moore’s law) is beginning to fail. One of the alternatives to conventional silicon electronics attempting to meet society’s demands is molecular electronics, which studies the use of single molecules, or molecular assemblies as components in electronic circuits. In this approach, custom-made molecules with different structures are synthesized to perform as diodes, transistors, wires, switches... Working with molecules allows continuing downsizing devices and also exploring and applying the new phenomena that appear at the nanoscale. In this work, we describe the study of a compound containing a diketopyrrolopyrrole moiety (DPP), (Figure 1.A: Structure of the compound here studied), an electron acceptor, which bonded to donor groups forms donor-acceptor-donor complexes. Depending on the donor groups used, the resulting complex will have a specific band gap, and consequently, tunable optical properties. Moreover, these compounds exhibit a large tendency to form bidimensional aggregates, which results in highly ordered films. Once the compound was characterized in solution and at the air-water interphase, Langmuir-Blodgett (LB) and self-assembled films were prepared to study the structure of the transferred monolayers onto solid substrates. Formation of J-aggregates in the Langmuir-Blodgett films has been observed, with a distinctive shift of the DPP UV-vis peak to higher wavelengths (Figure 1.B: UV-vis spectrum in solution (red) and spectrum of a LB film (blue)). On the other hand, molecules in self-assembled films interacted differently, without forming J-aggregates. This result paves the way for the application of the obtained molecular assemblies by the LB method, with tunable optical properties, in optoelectronic devices.