Optical switching studies of an azobenzene rigidly linked to a hexa-peri-hexabenzocoronene derivative in solution and at a solid–liquid interface

Min Ai Susie Groeper Wei Zhuang Xi Dou Xinliang Feng Klaus Mllen Jrgen P. Rabe An azobenzene moiety rigidly linked to a hexa-peri-hexabenzocoronene (HBC) derivative has been switched optically between its trans- and cis-conformations in solution. Crystalline monolayers of the flat lying transconformer have been obtained at the interface between the basal plane of graphite and an organic solution. However, from the illuminated solution no cis-conformer was observed adsorbed to the interface, indicating that the resulting cis- conformer is not thermodynamically stable at this interface, possibly due to the competition with the coexisting trans-conformers. Therefore, two-dimensional honeycombs self-assembled from derivatives of three-fold symmetric polycyclic aromatic hydrocarbons were investigated, which may be employed as templates, providing the necessary space for conformational switching of an azobenzene moiety, and tune the current-voltage characteristics through the aromatic cores. As a first step towards this goal, the capability of the template to host single molecular guests was studied by filling the voids with individual coronene molecules. - PACS 81.07.Nb 81.16.Dn 68.08.-p 68.37.Ef 42.79.Ta 85.65.+h 37.70.+i Currentvoltage characteristics of hybrid molecular diodes, i.e., single molecules in nanogaps have been studied in nanoscopic metal-, break-, or STM junctions [18]. Recently, the currentvoltage characteristics through a single hexa-peri-hexabenzocoronene (HBC) moiety have been controlled via the dipole moment of covalently linked charge transfer complexes at a solidliquid interface [9]. In this case, the complex formation and consequently the switching were due to the diffusion of an electron donor in the solution towards the acceptor at the interface, which is a random and slow process. Light-induced switching of a dipole moment at the interface should allow for a better controlled switching. A suitable candidate is azobenzene, which can be switched optically and electrically from a trans- to a cisconformation [1013], accompanied by a change in dipole moment. We report here on the optical switching of an azobenzene moiety rigidly linked to an HBC derivative in solution, the immobilization of the flat lying trans-conformer in a crystalline monolayer at the interface between the basal plane of graphite and an organic solution, and its submolecularly resolved in-situ STM-imaging. Since the cis-conformer of the molecule did not adsorb in ordered monolayers, we selfassembled stable two-dimensional honeycombs from larger polycyclic aromatic hydrocarbons, which may serve as templates, providing the necessary space for conformational switching of an azobenzene moiety on the surface, and further allow tuning the currentvoltage characteristics through the aromatic cores. 2 Experimental Compounds 1 and 2 (Figs. 1 and 2) were synthesized according to procedures, which will be provided elsewhere; the synthesis of 3 has been described before [14]. Coronene has been obtained commercially (99%, Aldrich). STM experiments at the interface between almost saturated solutions in 1, 2, 4-trichlorobenzene and the basal plane of highly oriented pyrolytic graphite (HOPG) were carried out under ambient conditions [7, 15] using a home-made STM employing commercial electronics (Omicron Vakuumphysik GmbH, Taunusstein, Germany). The tips were prepared by mechanically cut Pt/Ir (80:20) wires. HOPG (grade ZYH, Advanced Ceramics, Cleveland, Ohio, U.S.A) was freshly peeled with adhesive tape prior to an experiment. STM images were obtained at negative sample bias with respective to the tip. The lattice of the underlying HOPG has been visualized during the measurements by simply changing the tunneling parameters, which allowed calibration of the piezo in the xy plane in-situ. Unit cells were averaged over several images after their correction for the piezo drift (using SPIP Scanning Probe Image Processor, Version 1.911, Image Metrology ApS, Lyngby, Denmark). The photoisomerization from trans- to cis-azobenzene within compound 1 was carried out in dilute dichloromethane solution (1.02 105 M) by irradiating the solution in-situ at room temperature with a CAMAG UV lamp (8 W) light source emitting at 366 nm. The UV spectra were recorded using a UV/Vis/NIR Perkin-Elmer Lambda 900 spectrometer. In almost saturated solutions of 1, 2, 4-trichlorobenzene, the photoisomerization was accomplished using a Shimadzu RF-5001 PC fluorescence spectrometer light source 450 nm. The UV spectra were recorded with Shimadzu UV 2102 PC UVVis spectrometer. Fig. 1 Chemical formulae of an azobenzene moiety rigidly linked to a hexa-peri-hexabenzocoronene derivative, 1, which can be switched reversibly between its trans- and cis-conformations in dichloromethane and 1, 2, 4-trichlorobenzene solution Fig. 2 Chemical formulae of 2, trizigzagHBC(Ph-esterI)3, 3, trizigzagHBC(Ph-C8,2)6, and 4, coronene 3 Optical switching in solution The azobenzene moiety rigidly linked to an HBC derivative, 1, has been optically switched between its trans- and cis-conformations in dilute dichloromethane solution with light of a wavelength of 366 nm. Figure 3a displays the optical absorption spectra as functions of illumination time up to 5 hours and the thermal back reaction time of about 15 seconds at 60C. The strong peak around 366 nm [16] is attributed to the chromophore consisting of the HBC and the alkyne group, which does not change much upon illumination. A broader peak around 450 nm decreases substantially upon illumination, while on the same time a redshifted peak appears around 550 nm, causing an isosbestic point slightly above 500 nm. We assign the initial absorption around 450 nm to the transition of the transconformation of the amine-terminated azobenzene and the absorption appearing at 552 nm to the n transition of the photoinduced cis-conformation. Thermal isomerism from the photogenerated cis- to the trans-conformer occurs after heating at 60C within seconds [10]. Obviously there is a strong electronic coupling between the HBC, the alkyne and the azobenzene, since the excitation of the former switches the latter. Since the STM experiments were carried out in a solution of 1, 2, 4-trichlorobenzene, the photoreaction of compound 1 was also investigated within this solvent. Upon illumination at 366 nm, no cis-azobenzene was formed, while irradiation at 450 nm ( transition of the transconformer) bleached the trans-band and led to the formation of the band around 552 nm, attributed to the cisconformer (Fig. 3b). Upon thermal treatment at 60C, the trans-conformer reappeared. Apparently, the switching depends on the solvent and possibly on the molecular aggregation. In both cases, while the isosbestic points indicate a clean switching between the two conformers under these conditions, in poorer solvents like alkanes, no switching is observed. We attribute this to aggregate formation, which prevents the (...truncated)