Langmuir−Blodgett Films of Charge-Transfer Complexes between an Amphiphilic\nMonopyrrolo-TTF and TCNQ Derivatives

Abstract

Electrically active Langmuir−Blodgett (LB) films based on charge-transfer (CT) complexes between an\namphiphilic monopyrrolo-tetrathiafulvalene (MP-TTF) electron donor and different derivatives of the electron\nacceptor 7,7,8,8-tetracyano-<i>p</i>-quinodimethane (TCNQ), namely, 2,5-difluoro-TCNQ, fluoro-TCNQ, TCNQ,\ndecyl-TCNQ, 2,5-dimethyl-TCNQ, and 2-methoxy-5-ethoxy-TCNQ, have been fabricated and investigated\nsystematically. The electronic ground state of the CT complex, (MP-TTF^+δ)(TCNQs^-δ), in the LB films\nvaried from ionic (δ = 1) to neutral (δ ∼ 0) depending on the electron affinity of the TCNQ derivative used.\nCT excitation energies of the LB films were correlated against the difference between the redox potentials of\nthe MP-TTF and TCNQ derivatives, exhibiting agreement with a neutral−ionic (N−I) phase diagram proposed\nfor 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\nCT transition moment was found to be parallel to the substrate surface based on information from polarized\nUV−vis−NIR, IR transmission, and reflection−absorption spectra. After being transferred onto a mica surface\nby a single withdrawal, the surface morphologies of the films were found to be uniform or exhibited spongelike\ndomain structures of thickness about 1.8 nm.