TCNQ molecular semiconductor of the Cu(II)TAAB macrocycle: Optical and electrical properties.

The present study reports the doping of a semiconducting molecular material through the formation of hydrogen bonds between the macrocycle Cu(II)(TAAB) and the electronic acceptor TCNQ. According to density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) analysis, the doped compound has the shape of a distorted square pyramid, with four nitrogen atoms in the equatorial position and the apical oxygen atom from the water ligands. These water molecules can generate strong hydrogen bonds with TCNQ and the TAAB metallic complex. Thin films of copper molecular material were obtained through high vacuum evaporation and were structurally characterized by IR spectroscopy, EPR and scanning electron microscopy (SEM). Additionally, the absorption coefficient (α) and photon energy (hν) were calculated from UV-vis spectroscopy and used to determine the optical activation energy in each film, from which its semiconducting behavior was established. An important aspect to consider is that the presence of hydrogen bonds is essential to establish the semiconducting nature of these species; this chemical behavior, as well as the resulting electronic mobility, have been studied by DFT theoretical calculations, which reinforce the experimental conclusion of a relationship between Cu(II)TAAB and TCNQ moieties generated by a weak bond. Finally, I-V characteristics have been obtained from a glass/ITO/doped molecular semiconductor/Ag device using Ag and ITO electrodes. Results for the copper-based device show that, at low voltages, the conduction process is of an ohmic nature while, at higher voltages, space-charge-limited current (SCLC) is found. It is highly probable that the doping effect in TCNQ favors electronic transport due to the formation of conduction channels caused by dopant-favored anisotropy.

Electrical and optical properties of copper and nickel molecular materials with tetrabenzo [b,f,j,n] [1,5,9,13] tetraazacyclohexadecine thin films grown by the vacuum thermal evaporation technique.

Semiconducting molecular-material thin-films of tetrabenzo (b,f,j,n) [1,5,9,13] tetraazacyclohexadecine copper(II) and nickel(II) bisanthraflavates have been prepared by using vacuum thermal evaporation on Corning glass substrates and crystalline silicon wafers. The films thus obtained were characterized by infrared spectroscopy (FTIR), atomic force microscopy (AFM), ultraviolet-visible (UV-vis) spectroscopy and ellipsometry. IR spectroscopy showed that the molecular-material thin-films exhibit the same intra-molecular bonds as the original compounds, which suggests that the thermal evaporation process does not significantly alter their bonds. The optical band-gap values calculated from the absorption coefficient may be related to non-direct electronic interband transitions. The effect of temperature on conductivity was also measured in these samples. It was found that the temperature-dependent electric current is always higher for the nickel-based material and suggests a semiconductor-like behavior with conductivities in the order of 10(-8)Omega(-1)cm(-1).