Electronic structure of donor-spacer-acceptor molecules of potential interest for molecular electronics. IV. Geometry and device properties of P3CNQ and Q3CNQ Original Research Article

Z-β-(1-hexadecyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (C16H33-Q3CNQ) and the pyridinium analogue Z-β-(1-hexadecyl-4-pyridinium)-α-cyano-4-styryldicyanomethanide (C16H33-P3NCQ) are two very promising candidates for molecular device design. We obtain the geometry of the ground and excited state of these systems using the PM3 quantum mechanical model. The absorption spectra in vacuum and in solution are calculated using the INDO/Cl model, and compared to experimental spectra. The solvatochromic shift of the absorption spectra was calculated using a self-consistent reaction field approach. The observed bleaching of Langmuir-Blodgett films and solutions of C16H33-Q3CNQ and C16H33-P3CNQ is explained as resulting from a twisted configuration formed without barrier upon absorption into a twisted intramolecular charge transfer state. The observed rectification is explained from a ground state potential energy surface with two minima, one of which is characterized by a very large dipole moment. The relative energy of these minima is easily shifted by an electric field. The overall electron transport rate is found to be very small due to the small electronic coupling between the mono-layers of the L-B film. Thus, the electron transport through the sample is likely through defects of the L-B film. A way to increase the electronic coupling between the mono-layers is also discussed.