Temperature dependence of excitation energy transport in a benzene branching molecular system Mahinda I. Ranasinghe a , Prescott Murphy a , Zhikuan Lu b , Songping D. Huang b , Robert J. Twieg b , Theodore Goodson III a, * a Department of Chemistry, Wayne State University, Room 63, Detroit, MI 48202, USA b Department of Chemistry, Kent State University, Kent, OH 44242, USA Received 22 July 2003; in final form 18 November 2003 Published online: 6 December 2003 Abstract The dynamics of excitation energy transport in a benzene centered branched molecule are presented. This system contains pyridine distrylbenzene (pyridine-DSB) chromophores attached to the benzene center giving the 1,3,5-tris((E)-2-{4-[(E)-2-pyridin-4- ylvinyl]phenyl}vinyl)-2,4,6-trimethylbenzene (T155) multi-chromophore system. Excitation energy transport was investigated by ultra-fast time resolved fluorescence anisotropy measurements. A depolarization time of 800 fs was observed at room temperature and the anisotropy decay time increased at lower temperatures while a new fast decay component appeared. The anisotropy decay residual value was found to decrease systematically with decreasing temperature. From consideration of both steady-state and time- resolved results it is found that the benzene branching center has excitations localized on pyridine-DSB chromophores at room temperature. The appearance of fast anisotropy decay component in addition to slow (1 ps) component at low temperature suggests that energy transfer dynamics approaches the intermediate (crossover) characteristics of incoherent and coherent regimes. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction The optical excitations and excitation energy transfer in branched dendritic macromolecular architectures has been an important subject of recent investigations [1–5]. It has been found that the strength of interactions amongst chromophore units held together in close proximity plays a key role in determining the mode of energy transfer dynamics in dendritic molecules [6]. Therefore, the effect of the branching center in deter- mining the strength of interactions amongst chromo- phores is a key issue in probing the energy transfer dynamics in dendrimers. The understanding of energy transfer dynamics in branched macromolecules is ex- tremely important as such branched macromolecules have potential applications in optical emitting devices [7], non-linear optical applications [8], solar energy harvesting antenna [9] and medicinal applications [10] such as drug delivery systems. There are significant number of both experimental and theoretical [3,6,11,12] reports published addressing the issue of the mode of energy transport in dendrimer (and dendritic core) sys- tems at room temperature. For example, a nitrogen centered triphenylamine and DSB system [3,13,14], a carbon centered DSB system, and an adamantane cen- tered DSB system [6] as well as a phosphorous centered DSB system [15] have all been investigated in this regard. In these investigations it was found that the ni- trogen atom acts as a very efficient electronic coupler- giving rise to strong intersegmental interactions in the dendritic branched systems. To understand the energy transfer dynamics completely it is important to investi- gate the various mechanistic processes at various tem- peratures. As there are only a few reports published thus far on the excitation energy transfer at low temperatures [13,16,17], it is not clear how the energy transfer dynamics might be affected in the absence of strong phonon contributions. Chemical Physics Letters 383 (2004) 411–417 www.elsevier.com/locate/cplett * Corresponding author. Fax: +3135778822. E-mail address: tgoodson@chem.wayne.edu (T. Goodson III). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.051