Langmuir-Blodgett films of charge-transfer complexes between an amphiphilic monopyrrolo-TTF and TCNQ derivatives

Electrically active Langmuir-Blodgett (LB) films based on charge-transfer (CT) complexes between an amphiphilic monopyrrolo-tetrathiafulvalene (MP-TTF) electron donor and different derivatives of the electron acceptor 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), namely, 2,5-difluoro-TCNQ, fluoro-TCNQ, TCNQ, decyl-TCNQ, 2,5-dimethyl-TCNQ, and 2-methoxy-5-ethoxy-TCNQ, have been fabricated and investigated systematically. The electronic ground state of the CT complex, (MP-TTF+delta)(TCNQs(-delta)), in the LB films varied from ionic (delta = 1) to neutral (delta similar to 0) depending on the electron affinity of the TCNQ derivative used. CT excitation energies of the LB films were correlated against the difference between the redox potentials of the MP-TTF and TCNQ derivatives, exhibiting agreement with a neutral-ionic (N-I) phase diagram proposed for mixed-stack CT complexes. Of the six kinds of LB films, the (MP-TTF+0.3)(decyl-TCNQ(-0.3)) and (MP-TTF (+0.6))(TCNQ(-0.6)) complexes were located closest to the N-I phase boundary. In all of the LB films, the CT transition moment was found to be parallel to the substrate surface based on information from polarized UV-vis-NIR, IR transmission, and reflection-absorption spectra. After being transferred onto a mica surface by a single withdrawal, the surface morphologies of the films were found to be uniform or exhibited spongelike domain structures of thickness about 1.8 nm.