An N,N 0 -diaryl urea based conjugated polymer model system Holly L. Ricks, Linda S. Shimizu, Mark D. Smith, Uwe H. F. Bunz and Ken D. Shimizu * Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, SC 29208, USA Received 19 February 2004; accepted 25 February 2004 Abstract—An N,N 0 -diphenyl urea was designed as a model system for aggregation phenomenon in poly(phenyleneethynylenes) (PPEs). The unmethylated N,N 0 -diphenyl urea adopts an open, unfolded conformation in which the two diphenyl acetylene flu- orophores are far enough away, mimicking the unaggregated state. Dimethylation forces the aromatic surfaces together into p–p contact, mimicking the aggregated state of PPEs. Analogous to bulk PPEs, this model system shows dramatic differences in quantum yield between the folded and unfolded states, with the unfolded urea having greater than 400-fold higher fluorescence quantum yield than its folded equivalent. Ó 2004 Published by Elsevier Ltd. Polymer morphology and supramolecular ordering can have a dramatic effect on optical properties and thus, can either improve or diminish the utility of a polymer for a particular application. 1 For example, poly(phenyl- eneethynylenes) (PPEs) have a quantum yield in solution approaching unity. On the other hand, solid state de- vices based on thin films of PPEs have dramatically lower quantum yields. 2 These differences presumably arise from the differences in the properties between the aggregated and unaggregated PPEs. A better under- standing of the emission properties of polymer aggre- gates could, therefore, lead to improvements in PPE based devices. However, the study of the aggregated state of conjugated polymers is difficult because the observed emission behavior of the bulk material arises from a combination of different aggregated and unag- gregated structures. One solution has been to design model systems that fold the conjugated polymer onto itself, quantitatively mimicking intramolecular interac- tions of the aggregated state. Typically, two conjugated polymer chains are attached to a rigid U-shaped tem- plate, which forces the two chains into a parallel p–p stacked conformation (Fig. 1a). Examples of appropri- ate U-shaped templates including: 1,8-anthracene, 1,8- naphthalene, para-phenylcyclophanes, meso-porphyrins, ethane, and ortho-benzene. 3 We report, herein, a new PPE model system based on an N,N 0 -diphenyl urea template that can serve as both a model of the aggregated and unaggregated states. Comparison of the respective emission properties of the cis,cis and trans,trans conformers (Fig. 1b) can then be used to help gauge the effects of aggregation behavior on the emission properties of PPEs. Control of the diphenyl urea conformation was effectively exerted by alkylation. Unmethylated N,N 0 -diphenyl urea exists in the trans, trans conformations in which the phenyl groups are in a divergent ÔunfoldedÕ conformation. 4;5b Dimethylation reverses the conformational preferences as the sterically larger methyl groups occupy the trans-position adjacent to the carbonyl oxygen. This forces the two phenyl rings together into the folded cis,cis conformation. Azumaya and co-workers have demonstrated that folded cis,cis ureas are effective U-shaped templates for the study of p-stacking interactions. 5c Bradley Smith has utilized the ability to control the cis,cis to trans,trans equilibrium as a supramolecular switch. Lewis et al. has also used the Keywords: Urea; Conjugated polymers; Fluorescence. * Corresponding author. Tel.: +1-803-777-6523; fax: +1-803-777-9521; e-mail addresses: shimizu@mail.chem.sc.edu; shimizu@psc.sc.edu (a) n n (b) O N N H H O N N Me Me R R R R (trans- / trans-) (cis- / cis-) Template Figure 1. Representation of a model system for conjugated polymer aggregation based on a rigid U-shaped template. 0040-4039/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.tetlet.2004.02.126 Tetrahedron Letters 45 (2004) 3229–3232 Tetrahedron Letters