Photophysical studies on multichromophoric cyclotriphosphazenes. Trinuclear excimer formation in hexakis(2-naphthyloxy)cyclotriphosphazene Nitin Chattopadhyay, * Basudeb Haldar, Arabinda Mallick and Saumitra Sengupta * Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India Received 14 December 2004; revised 19 February 2005; accepted 1 March 2005 Available online 16 March 2005 Abstract—Hexakis(2-naphthyloxy)cyclotriphosphazene showed an interesting emission behavior between its monomer and excimer forms, the latter nearly completely dominating in water. Encapsulation studies with b-cyclodextrin in water partially revived the monomer emission. Ó 2005 Elsevier Ltd. All rights reserved. With a view to understanding the mechanism and dynamics of natural photosynthetic reactors, 1 photo- physical studies of multichromophoric scaffolds have gained much current attention. These scaffolds are also useful for harvesting diffused sunlight for potential use in solar energy conversion and in the construction of luminescent biological and trace metal sensors. 2 Early studies on multichromophoric systems were carried out with polymers that were substituted at intervals with chromophoric units. 3 However, due to their inherent flexibility and thus a number of different coiled confor- mations, these polymers turned out to be poor mimics of the natural photosynthetic reactors. In search of bet- ter mimics, Jullien and co-workers recently studied a number of naphthyl substituted b-cyclodextrins (b- CDs) containing up to 14 chromophore units. 4 The structural rigidity of these systems held the naphthyl chromophores in spatially well-defined positions and in close proximity with each other. This led to efficient energy hopping between the naphthyl units in these mole- cules. Inspired by these results, we initiated a study on new multichromophoric scaffolds as molecular mimics of the natural photosynthetic antennae. Our initial task was to seek a conformationally rigid multitopic core that could be substituted with multiple chromophoric units. Previous studies from these labora- tories have shown that tetraphenylmethane, a centrally rigid tetrahedral scaffold, can be effectively used in this regard. 5 However, tetraphenylmethane can accommo- date only four chromophores. While searching for a higher valent core structure, we were attracted to the cyclotriphosphazenes (CTPs), which have hexavalent inorganic cores. In this paper, we describe the unusual excited state photophysical properties of hexakis(2- naphthyloxy)cyclotriphosphazene 3 arising from highly pre-organized trinuclear excimer formation. Cyclotriphosphazene (CTP) is a well known hexatopic core, which can be easily per-functionalized via substitu- tion reactions on the corresponding hexachloride (NPCl 2 ) 3 , especially with heteroatomic nucleophiles. 6 In symmetrically per-substituted CTP derivatives, the inorganic ring is nearly planar, the phosphorus atoms almost tetrahedral (sp 3 ) and the nitrogen atoms ap- proach an sp 2 geometry. 7 As a result, the geminal sub- stituents on the phosphorus atoms are oriented in a well defined spatial arrangement, one above and one below the CTP ring. Moreover, in homogeneously per- substituted CTPs, the three substituents on either side of the CTP ring are also mutually equidistant. CTP rings are photochemically inert and do not have any low en- ergy absorption band of their own. Hence, they do not interfere with the photophysical properties associated with the attached chromophores. The ease with which 0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2005.03.006 Keywords: Cyclotriphosphazene; 2-Naphthol; Excimer; b-Cyclo- dextrin. * Corresponding authors. Tel.: +91 33 473 4266; fax: +91 33 414 6266; e-mail addresses: pcnitin@yahoo.com; jusaumitra@yahoo. co.uk Tetrahedron Letters 46 (2005) 3089–3092 Tetrahedron Letters