In Situ Spectroscopic Characterization of Rectifying Molecular Monolayers Self-Assembled on Gold

• Published in: Chemphyschem Vol. 8; no. 15; pp. 2195 - 2201
• Main Authors: Girlando, Alberto, Sissa, Cristina, Terenziani, Francesca, Painelli, Anna, Chwialkowska, Anna, Ashwell, Geoffrey J.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 22-10-2007; WILEY‐VCH Verlag; Wiley
• Subjects: charge transfer; Chemistry; Exact sciences and technology; General and physical chemistry; Langmuir blodgett films; molecular electronics; monolayers; surface chemistry; Surface physical chemistry; vibrational spectroscopy; Self assembly; Monolayer; Gold; surface chemistry; monolayers; In situ; Transition metal; Characterization; Chemistry; Molecular electronics; Molecular assembly; vibrational spectroscopy; Langmuir Blodgett layer; Charge transfer; Chromophore
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200700447

We report visible, Raman, and infrared spectra of self‐assembled monolayers (SAMs) formed by the donor‐(π‐bridge)‐acceptor chromophore, Z‐β‐[N‐(ω‐acetylthioalkyl)‐4‐quinolinium]‐α‐cyano‐4‐styryldicyanomethanide (CH3CO‐S‐CnH2n‐Q3CNQ where n=8, 10), on gold‐coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir–Blodgett (LB) monolayer of C16H33‐Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+‐π‐A−) ground state. At variance with this well‐known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well‐known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism.
In situ characterization of rectifying molecular monolayers elucidate the rectification mechanism. Visible, Raman and IR spectra of Q3CNQ self‐assembled monolayers on gold surfaces (see figure) are compared with Langmuir–Blodgett films of the same chromophore, showing that molecular polarity depends on the tilt angle of the molecule to the surface.