Steady-state spectral properties of dendrimer supermolecule as a light harvesting system Dipankar Rana, Gautam Gangopadhyay * S.N.Bose National Center For Basic Sciences, JD Block, Sector-III, Salt Lake City, Calcutta-700098, India Received 3 October 2000; in ®nal form 21 November 2000 Abstract We have considered the dendrimer supermolecule as comprised of two-level systems as monomer units on the nodes of a Cayley tree. Each unit undergoes dissipation and driving by an external laser ®eld along with nearest neighbour interaction among the monomer units. We have studied the steady-state absorption spectra. The eects of Cayley tree geometry on the spectral properties are investigated through the intensity and width of the spectra. The origin of the enhanced energy transfer mechanism of the extended dendrimer in comparison to the compact one is explained. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Dendrimers [1±9] are nanometer-sized macromolecules with repeating units arranged in a hierarchical self-similar fashion around a core to give highly branched tree-like structure (see Fig. 1). They are capable of displaying unusual chemical, transport and optical properties. Phenylacetylene dendrimer is a special case. Two kinds of phenylacetylene dendrimers have been studied extensively. (A) Compact dendrimers: Perfectly symmetric molecules with each of three legs composed of many independent diphenyl acetylene (DPA) units; branching occurs at the meta position of each benzene ring. (B) Extended dendrimers: Molecules with unequal legs which are composed of linear chains of DPA units with increasing length towards the center of the molecule. Both kinds of dendrimers show the common feature of monotonic increase in intensity of absorbance with the size of the dendrimer molecule since the number of units increases exponentially with an increase in generation number [3]. For the extended dendrimers a shift of the lowest energy peak is observed with an increase in generation number. With this observation Kopelmann et al. [3] have conjectured that a superimposed energy funnel from the periphery to the core is inbuilt in the extended dendrimer. This makes the extended dendrimer a good candidate for an arti®cial energy antenna [4]. 9 February 2001 Chemical Physics Letters 334 (2001) 314±324 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +91-33-335-3477. E-mail addresses: dipankar@boson.bose.res.in (D. Rana), gautam@boson.bose.res.in (G. Gangopadhyay). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 4 2 1 - 4