Unimolecular Electrical Rectification Down to 105 K, and Spectroscopy of Hexadecylquinolinium Tricyanoquinodimethanide

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R. M. METZGER Laboratory for Molecular Electronics, Chemistry Department, University of Alabama, Tuscaloosa, AL 354870336, USA, [email protected] ABSTRACT A Langmuir-Blodgett monolayer of γ -(n-hexadecyl)quinolinum tricyanoquinodimethanide, C16H33Q-3CNQ (1 ), sandwiched between Al electrodes, shows asymmetric DC electrical conductivity between 370 K and 105 K. The enhanced forward current is attributable to rectification of electrical current by a single molecule, and is explained by an electronic transition from a high-moment zwitterionic ground state to a low-moment undissociated first excited state. INTRODUCTION Aviram and Ratner (AR) proposed in 1974 that a single, asymmetrical, D-σ-A molecule should be a rectifier of electrical current, because the electrical conductivity in one direction through the molecule is much larger than the conductivity in the opposite direction [1]. This would be because the undissociated ground state D0-σ-A0 is close to the first zwitterionic excited state D+-σ-A-, but very far from the oppositely charged zwitterionic excited state D--σ-A+. Indeed, a good one-electron donor end D would be oxidized (relatively) easily into the cation D+, and a one-electron acceptor A would be reduced easily to the anion A-, while the oppositely charged ions D- and A+ would lie several eV above the D+ and A- states [1]. A saturated covalent bridge σ would separate the D from the A moiety. Many D-σ-A molecules that form LB films were synthesized, but, with primitive measurement techniques, rectification was not found [2-4]. Sambles and coworkers N developed better techniques, and found asymmetric currents C 16 H33 N + C in a sandwic h “Pt | LB multilayer of dodecyloxyphenyl carbamate of bromohydroxyC C ethoxytetracyanoquinodimethane | Mg | Ag” [5], but a Schottky barrier could also form in this system [6]. Ashwell N N introduced a new family of ground-state zwitterionic molecules of the type D+- -A-, such as 1 , C 16 H33 Q-3CNQ picolyltricyanoquinodimethane [7], and γ-(n-hexadecyl)quinolinum tricyanoquinodimethanide, C16H33Q-3CNQ (1 ) [8]. Z-type Langmuir-Blodgett multilayers of 1 have a large second-order nonlinear optical susceptibility χ(2)zzz = 180 pm V-l [9]. Sambles and coworkers, using dissimilar metal electrodes, found that the sandwich “Au wire | Ga/In eutectic | Pt | LB multilayer of 1 | Mg | Ag | Ga/In eutectic | Au wire “ is a multilayer rectifier [10,11]. After some preliminary work [12,13] we found electrical rectification through a monolayer of 1 by macroscopic methods in the sandwich "Au wire | Ga/In eutectic | Al pad | monolayer of 1 | Al | Ga/In eutectic | Au wire", and also by nanoscopic methods (STM) [14]; we extended the macroscopic measurements to the range 370 K to 105 K [15], and discussed the junctions with anomalous electrical behavior [16]. We established that 1 is a ground-state zwitterion of the type D+- -A-, with a large dipole moment (43 ± 8 Debyes) [14], whose first excited state has lower polarity D0- -A0 (estimated moment 3 to 9 Debyes) [17]; the two states are coup