Comparing charge transfer tuning effects by chemical substitution and uniaxial pressure in the organic charge transfer complex tetramethoxypyrene-tetracyanoquinodimethane

摘要

In the search for novel organic charge transfer salts with variable chargetransfer degree we study the effects of two modifications to the recentlysynthesized donor-acceptor Tetramethoxypyrene (TMP)-Tetracyanoquinodimethane(TCNQ). One is of chemical nature by substituting the acceptor TCNQ moleculesby F4TCNQ molecules. The second consists in simulating the application ofuniaxial pressure along the stacking axis of the system. In order to test thechemical substitution, we have grown single crystals of TMP-F4TCNQ and analyzedits electronic structure via electronic transport measurements, ab initiodensity functional theory (DFT) calculations and UV/VIS/IR absorptionspectroscopy. This system shows an almost ideal geometrical overlap of nearlyplanar molecules alternately stacked (mixed stack) and this arrangement isechoed by a semiconductor-like transport behavior with an increasedconductivity along the stacking direction. This is in contrast to TMP-TCNQwhich shows a less pronounced anisotropy and a smaller conductivity response.Our bandstructure calculations confirm the one-dimensional behavior ofTMP-F4TCNQ with pro- nounced dispersion only along the stacking axis. Infraredmeasurements illustrating the CN vibration frequency shift in F4TCNQ suggesthowever no improvement on the degree of charge transfer in TMP-F4TCNQ withrespect to TMP-TCNQ. In both complexes about 0.1 is transferred from TMP to theacceptor. Concerning the pressure effect, our DFT calculations on designedTMP-TCNQ and TMP-F4TCNQ structures under different pressure conditions showthat application of uniaxial pressure along the stacking axis of TMP-TCNQ maybe the route to follow in order to obtain a much more pronounced chargetransfer.