Rectification between 370 and 105 K in Hexadecylquinolinium Tricyanoquinodimethanide Bo Chen and Robert M. Metzger* Laboratory for Molecular Electronics, Chemistry Department, UniVersity of Alabama, Tuscaloosa, Alabama 35487-0336 ReceiVed: January 4, 1999; In Final Form: March 11, 1999 The through-film electrical conductivity of Langmuir-Blodgett monolayer films of hexadecylquinolinium tricyanoquinodimethanide was studied as a function of temperature. The current is usually smaller at low temperature. Electrical rectification, probably due to an intramolecular electron transfer, was observed between 370 and 105 K, with a rectification ratio as large at 105 K as it is at room temperature. Introduction We recently demonstrated unimolecular electrical rectifica- tion, i.e., asymmetric electrical conductivity (or large asym- metries in the plots of direct current I versus applied potential V) through a single molecule of hexadecylquinolinium tricy- anoquinodimethanide, C 16 H 33 Q-3CNQ (1), by macroscopic means (monolayer between Al electrodes) and by nanoscopic means (scanning tunneling spectroscopy) at room temperature. 1 This molecule rectifies by a modification of the proposed Aviram-Ratner mechanism. 2 We present here the temperature dependence (370 to 80 K) of this rectification. C 16 H 33 Q-3CNQ (1) is blue in CH 3 CN solution, 3 has a dipole moment of 43 ( 8 D at infinite dilution in CH 2 Cl 2 , 1 has a second-order nonlinear optical susceptibility of 180 pm V -1 , 4 makes green Langmuir-Blodgett (LB) monolayers and multi- layers (λ max ) 570 nm), and is a rectifier in LB monolayers 1 and multilayers sandwiched between Al electrodes. 5,6 From X-ray diffraction measurements, the film thickness is 23 Å, 1 in agreement with earlier estimates by ellipsometry and surface plasmon resonance. 3 The molecule lies with its dicyanometh- ylene swallowtail (negative) end closest to the bottom Al film, but is tilted 48 ( 5° from the normal to the monolayer plane. 1 The ground state of 1 is zwitterionic, of the type D + -π-A - , where D + is the positively charged quinolinium ring, A - is the negatively tricyanoquinodimethanide moiety, and π is the double bond bridge. 1,4 From the strong negative solvatochromism of the visible spectrum of 1 and the shift between absorption and fluorescent emission, the first excited state of 1 is found to be less polar, undissociated, i.e., D 0 -π-A 0 , with an estimated dipole moment of only 8.7 D. 7 It is therefore thought that the color, the frequency doubling, and the rectification are all due to a transition from a zwitterionic state to an undissociated excited- state D + -π-A - f D 0 -π-A 0 , which is made possible because the molecule is nonplanar and the conjugation between the two ends is broken by a nonzero twist angle. 1,4,6 At room temperature the best rectifying results for the sandwich had a resistance of 3.5 MΩ and a maximum forward current of 0.4 μA, corresponding to a current density of about 0.33 electrons per molecule per second; the rectification ratio was 26:1 at (1.3 V, but this ratio decreases upon repeated cycling from positive to negative voltages. 1 The molecules, exposed to electric fields (1.3 V/23 Å ) 0.56 GV m -1 , probably reorder within the monolayer to avoid such high fields. 1 The samples for which rectification was seen had higher resistivities of 3 to 40 MΩ, while other samples, with lower resistivities (10-100 kΩ) had no rectification (symmetric IV curves). 8 By cryocooling the LB substrate while evaporating the top Al electrode, the thermal damage to the LB films was reduced. 1 Given the residual oxygen pressure in the evaporator (10 -6 Torr vacuum), thin layers (10-20 nm) of Al 2 O 3 probably form on both sides of the LB monolayer. No asymmetries in the electrical conductivity were seen when Y-type LB multilayers of arachidic acid (C 19 H 39 COOH) were deposited in a similar manner. 1 Therefore, the rectification (asymmetries in the I versus V plot) seen for 1 must not be due to the two Al | Al 2 O 3 couples, which are most likely symmetrical on both sides of the organic layer, 1 but to the organic layer. Naturally, because the C 16 H 33 alkyl termina- tion presents a smaller cross-section than the quinolinium ring, the organic film is not as compact as a simple alkyl fatty acid, e.g., arachidic acid. Therefore, some sample-to-sample vari- ability in Al 2 O 3 penetration into the monolayer is possible, thereby giving some variation in electrical conductivities. Much larger currents per molecule (0.1 to 1 nA) are measured in the STM experiments. 1 The voltage dependence 9 of through-film conductivity of fatty acids is either j ∝ V 1/2 (Sommerfeld-Bethe tunneling, Poole- Frenkel, or Schottky barrier mechanisms) 10 or j ∝ V 1/4 (Au atom migration or disclinations). 11,12 To avoid the pesky oxide problem, many efforts have been made to measure the DC and AC conductivity of LB films between noble metal electrodes. 13 Short circuits were seen very often, either because the LB monolayers physisorb onto noble metal substrates with smaller binding energies than they do onto Al | Al 2 O 3 | LB monolayer of 1 | Al 2 O 3 | Al 4447 J. Phys. Chem. B 1999, 103, 4447-4451 10.1021/jp990006e CCC: $18.00 © 1999 American Chemical Society Published on Web 05/01/1999