Benzene ‘dimers’ in polystyrene films Sudeshna Chattopadhyay, Alokmay Datta * Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India Received 13 February 2004; in final form 22 March 2004 Available online 25 May 2004 Abstract Benzene ‘dimers’ formed by excitation transfer interaction (J ), in weakly disordered aggregates, have been observed in atactic polystyrene films (200–2500  A thick) spin-coated on fused quartz, through presence of a doublet in the main vibronic M 1 -band in the 1 A 1g ! 1 B 2u optical singlet band. Only V-shaped ‘dimers’ were found with dihedral angle (a) varying from 90° to 107° with no apparent dependence on film thickness. A model for the ‘dimer’ consistent with observed J ðaÞ yields the value of the transition moment and provides the ‘dimer’ geometry. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Supramolecular aggregates are important means to extend electron delocalization and this extended delo- calization maybe used for exciton transport [1]. The simplest supramolecular aggregate is of course the so- called ‘physical dimer’ [2], a pair of molecules strongly correlated by non-bonded, short-range interactions. Molecular benzene is known to form such ‘dimers’ in condensed phases [3,4], and in jet-cooled beams [5]. Quantum mechanical calculations predict ‘slipped parallel’, ‘T-shaped’ [6] and ‘V-shaped’ [7] configura- tions for these dimers. In benzene crystals both the ‘T-shaped’ and the ‘slipped parallel’ forms exist as neighbouring rings, as observed from crystallography [3,4], whereas in jet-cooled beams the dominant di- meric configuration is the ‘V-shaped’ form [5]. In polystyrene, benzene rings are coupled through the backbone chain but are otherwise free to interact and form aggregates within a broad range of configura- tions. Presence of ‘physical dimers’ of benzene is thus a distinct possibility in polystyrene, even in absence of correlations such as tacticity, imposed by the polymer backbone on the rings. In the confined geometry of films molecular configurations may differ from the equilibrium configuration [8] and this may prefer such ‘dimer formation’. Also, due to the ring-segregating interactions of particular film/substrate interfaces [9], ‘dimer formation’ may be enhanced. Again, due to the absence of conjugation paths along the polymer backbone, the p-electrons in the benzene rings are localized and it is known that the optical spectra of polystyrene is determined by excitons localized on these rings [10]. Thus any delocalization effect in the optical spectra of polystyrene is related to correlations among the rings and an analysis of these effects would lead to an understanding of these correlations. In this communication we present results of trans- mission ultraviolet (UV) spectroscopy around the first singlet optical transition in films of atactic polystyrene spin-coated on fused quartz substrates. We have studied films of thickness varying from 200 to 2500  A and have analyzed the 1 A 1g ! 1 B 2u transition band using the theory of coupled two-level systems [11] to interpret the main vibronic M 1 band. We have de- tected clear evidence of weakly disordered aggregates of V-shaped benzene dimers formed by the resonant transfer interaction, and have extracted a geometry of the ‘dimer’ that shows no clear dependence on poly- styrene film thickness. * Corresponding author. Fax: +91-33-2337-4637. E-mail address: alok@surf.saha.ernet.in (A. Datta). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.04.107 Chemical Physics Letters 391 (2004) 216–219 www.elsevier.com/locate/cplett