Intercalation effect of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimetane having strong electron affinity in self-assembled monolayers composed of charge transfer complex prepared by coadsorption and layer-by-layer adsorption methods

Charge transfer (CT) complex self-assembled monolayers (SAMs) on a gold substrate are prepared using layer-by-layer adsorption and coadsorption methods with mercapto-methyl-tetrathiafulvalene (TTF-CH2SH) and strong acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), in comparison with TTF-CH2SH/TCNQ SAMs consisting of moderate acceptor TCNQ. The layer-by-layer method yields TTF-CH2SH/TCNQF4 SAMs with the coexistence of neutral TCNQF4 and anionic TCNQF4-, where the TCNQF4- anions are laid on the TTF layer with the TTF-CH2SH molecules standing perpendicular to the gold substrate. The neutral TCNQF4 molecules are stacked above the TCNQF4- anion layer. This is in contrast to the fact that TCNQ having intermediate strength of acceptor character does not yield any CT SAMs when using the layer-by-layer technique. In the coadsorption method, TTF-CH2SH/TCNQF4 SAMs are formed, where all the TCNQF4 molecules are completely reduced as TCNQF4- anions, similar to those in bulk TTF-TCNQF4 crystals. Taking into account that TTF-CH2SH/TCNQ SAMs have the same fractional value (0.6) of the degree of charge transfer to that of bulk TTF-TCNQ crystal, the coadsorption technique can reproduce the electronic structure of the bulk CT complex in the 2D SAMs. The coadsorbed SAMs have an intercalation structure, where acceptor molecules are intercalated into the interstitials of TTF-CH2SH/Au units with the molecular axes of both acceptor and donor molecules parallel to each other. Such a donor/acceptor molecular arrangement can provide a favorable situation in the charge transfer between the two ingredients, resulting in the similar electronic structure to that of bulk CT crystals.