Electrical Rectification by a Molecule of Hexadecylquinolinium Tricyanoquinodimethanide
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C
N/ 1, C16 H33 Q-3CNQ
N
1 is best made from N-cetyl-lepidinium tosylate and a two-fold excess of the lithium salt of TCNQ radical anion, in dry DMSO solution with added pyridine, in 59% yield; the NMR spectrum of 1 indicates a dipolar ground state [23]. The longwavelength maximum in visible-ultraviolet spectrum of 335
Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
1 is solvatochromic [23]. The cyclic voltammogram of 1 exhibits a one-electron quasi-reversible reduction (El/ 2 = -0.513 V vs SCE) [23]. The dielectric constants of solutions of 1 in CH 2 CI2 were measured, as a function of temperature (-10°C < T < 30°C), at 1 kHz, using a coaxial capacitance cell and a Hewlett-Packard 4263B LCR meter. The extrapolation of the results to infinite dilution yields a dipole moment of 40 to 45 D (average = 43 + 8 D) for 1 in CH 2 C12 [23]. MONOLAYER PROPERTIES The H-A isotherm of 1, measured under a green safelight, shows a collapse point at 50 A2 molecule- 1 and 34 mN m- 1 (Fig. 1): this isotherm is moderately temperature-dependent [22,23] Monolayers and multilayers of I were transferred to solid substrates at 15 mm/min at a film pressure of 20 mN/m: 1 first transfers on the downstroke onto hydrophobic HOPG, and on the upstroke onto hydrophilic Al. The hexadecyl group is thought to be the end of the 35
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"2 10 ...................... 20 -.................. •••.......... 15s
.... i ...................... .. ........... " .........................
75 2..... 47
Fig. 1. Pressure-Area isotherms for 1 at 14°C and 20*C [23].
3 ................................ i........................ ....................... .". ". ......... molecule closest to HOPG, while the
3CNQ- (A-) end should be closest to Al. At 15 mm/min the LB U 5 ,-.............)..........."".........- ........................ "........................ 1 multilayers (past the first monolayer) are Z-type, while at 45 mm/min, the LB are Y-type (centromultilayers symmetric). X-ray diffraction shows 1 30 ...... ..... that the Y-type films apparently reorder 100 150 200 0 50 to Z-type some time after transfer, with Area (A9/molecule) a thickness of 23 A per monolayer: for multilayers of 1, the Z-type ordering is more stable. The approximate molecular axis is thus tilted 48 ± 5' from the normal to the monolayer [23]. LB multilayers of 1 transferred onto HOPG at 15 mm/min have orientation: (1)
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