Issue 27, 2009

Photoinduced structural modifications in multicomponent architectures containing azobenzene moieties as photoswitchable cores

Abstract

Four novel π-conjugated chromophores with an azobenzene core (1–4) have been synthesized exploiting Pd-catalysed cross-coupling reactions between ethynyl-bearing azobenzene cores and suitably-designed peripheral groups. While in molecules 2 and 3 the azobenzene core is equipped, respectively, with ethynyl and 1,3-butadiyne spacers terminated with a substituted aniline, molecule 4 is an homologue of derivative 2 in which the terminal moieties are replaced by meso-substituted Zn-porphyrins. X-Ray crystallographic studies of substituted azobenzene 2 reveal a nearly planar arrangement of the four phenyl rings and the trans configuration of the N[double bond, length as m-dash]N central unit. The UV-Vis absorption spectrum of molecule 1 in cyclohexane (CHX) is very similar to that of unsubstituted azobenzenes; upon irradiation at the maximum of the intense π–π absorption feature (360 nm), 1 undergoes transcisphotoisomerization reaching a photostationary state. The process is fully reversible both photochemically and thermally (ca. 120 min in the dark). The UV-Vis electronic absorption features of 2–4 are dramatically different compared to those of 1, but the photochemical process can still be traced and exhibits full reversibility in CHX. Also in the case of compound 4, where the photoreactive azobenzene excited states might be quenched by the low-lying porphyrin electronic levels, the photoreaction does occur. Extensive STM investigations of self-assembled monolayers (SAMs) of 2 and 3 at the solid/liquid interface were performed by means of scanning tunneling microscopy (STM) on highly oriented pyrolytic graphite (HOPG). It is evidenced that only the trans isomer can be physisorbed on the surface whereas the cis form, either produced under illumination in situ or prepared by irradiation of the solution prior to deposition (ex-situ), is never observed on the surface. The smallest azobenzene 1 and the bisporphyrin system 4 did not physisorb onto the surface because of the very small size and the bulky 3,5-di(tert-butyl)phenyl groups hindering flat adsorption on HOPG, respectively.

Graphical abstract: Photoinduced structural modifications in multicomponent architectures containing azobenzene moieties as photoswitchable cores

Supplementary files

Article information

Article type
Paper
Submitted
16 Mar 2009
Accepted
06 Apr 2009
First published
20 May 2009

J. Mater. Chem., 2009,19, 4715-4724

Photoinduced structural modifications in multicomponent architectures containing azobenzene moieties as photoswitchable cores

J. Zeitouny, C. Aurisicchio, D. Bonifazi, R. De Zorzi, S. Geremia, M. Bonini, C. Palma, P. Samorì, A. Listorti, A. Belbakra and N. Armaroli, J. Mater. Chem., 2009, 19, 4715 DOI: 10.1039/B905287A

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