LT-STM study of self-organization of b-carotene

All-trans b-carotene molecules have been deposited at room temperature on Cu (1 1 1) and subsequently studied by STM at liquid nitrogen. They form a self-organized multilayer structure. The STM images allow to unambiguously identify single molecules and to resolve important aspects of their internal structure like its curved backbone polyene chain, its attached methyl groups and the b-ionone rings. We suggest that the molecules lose their center of symmetry, the molecule being flexible enough to allow the formation of large three-dimensional periodic extensions. 2003 Elsevier Science B.V. All rights reserved. The potential of STM for molecular imaging has been demonstrated in recent years [1,2]. In this direction, studies on objects of larger sizes in the 1–10 nm range are currently performed. By this way, the capability of Scanning Probe techniques to deal with more complex systems has been demonstrated. Moreover both imaging and manipulation by the STM tip became possible [3,4]. A good review on the basic and applied interest of this research can be found in [5]. The present work is the first LT-STM study of b-carotene, performed under well-controlled surface science conditions. At the same time the system is a molecular multilayer structure, where we expect to retain some of the properties which are necessary to understand the important biological activity of this and other related molecules like vitamin A [6]. From chemical evidence, C40H56 (alltrans b-carotene), is a planar, all-trans polyene chain characterized by a set of conjugated double bonds with attached methyl groups, terminated with two b-ionone carbon rings also with attached methyl groups and symmetric with respect to its center [7]. Taylor first studied single crystals of b-carotene [8] and the crystal structure was solved by Sterling [9]. The crystals of b-carotene are monoclinic, spatial group P21=c with the unit-cell dimensions determined. A new X-ray determination reported in [10] and confirmed in [11] is at variance with the precedent one. In particular the b angle changes from 105 to 94 . b-carotene belongs to a larger family, the carotenoids, one of the most known pigments, naturally present in the Chemical Physics Letters 369 (2003) 240–247 www.elsevier.com/locate/cplett * Corresponding author. Permanent address: Departamento de Fisica de la Materia Condensada, C-III, Universidad Aut onoma de Madrid, E-28049-Madrid, Spain. Fax: +34-91-