DOI: 10.1002/chem.200800773 Polyviologen Dendrimers as Hosts and Charge-Storing Devices CØlia M. Ronconi, [b, d] J. Fraser Stoddart,* [a] Vincenzo Balzani, [c] Massimo Baroncini, [c] Paola Ceroni, [c] Carlo Giansante, [c] and Margherita Venturi* [c] Introduction Dendrimers are repetitively branched, yet constitutionally well-defined, macromolecules exhibiting high monodispersi- ty. [1] These nanoscopic compounds can contain selected chemical units in predetermined sites of their structure (core, branches, periphery), mutual interactions of which can lead to unusual, sometimes unpredictable, physicochem- ical properties that result in a wide range of potential appli- cations. [2] Dendrimers containing photoactive and/or electro- active moieties are currently attracting much attention, since they can be designed to perform useful functions, such as light harvesting, [3] charge pooling, [4,5,6] ion sensing with signal amplification, [7] and molecular recognition. [8] Dendrimers B9 18 + and B21 42 + (Scheme 1), discussed in this paper, are based on a 1,3,5-trisubstituted benzenoid core and contain 9 and 21 4,4’-bipyridinium (viologen) units, respectively, in their branches, and 6 and 12 aryloxy groups, respectively, at their peripheries. Viologen is a well-known electroactive species that undergoes two successive one-elec- tron, reversible reduction processes [9,10] and exhibits peculiar spectroscopic features in both its dicationic and radical-cat- ionic forms. Viologen units are also known to give strong donor–acceptor complexes with electron-donating species. [10] We report 1) the synthesis of dendrimers B9 18 + and B21 42 + as their hexafluorophosphate salts, 2) their interac- tion with the eosin dianion (Ey 2 , Scheme 1), and 3) their electrochemical and photosensitized reduction. For compari- son purposes, we also investigated the properties of mono- viologen compounds 1,1’-dibenzyl-4,4’-bipyridinium (dbV 2 + ) and 1,1’-dioctyl-4,4’-bipyridinium (doV 2 + ), as well as den- dron B2 4 + , which contains only two viologen units (Scheme 1). Abstract: Two dendrimers were de- signed and synthesized that contain a 1,3,5-trisubstituted benzenoid core and incorporate 9 and 21 viologen (4,4’-bi- pyridinium) units in their branches in addition to hydrophilic (aryloxy) termi- nal groups. For comparison purposes, model compounds containing one and two viologen units were also studied. These polycationic dendrimers form strong host–guest complexes with the dianionic form of the red dye eosin in dilute CH 2 Cl 2 solutions. Titration ex- periments, based on fluorescence meas- urements, showed that each viologen unit in the dendritic structures becomes associated with an eosin dianion. Elec- trochemical (in MeCN) and photosen- sitization (in CH 2 Cl 2 ) experiments re- vealed that only a fraction of the violo- gen units present in the dendritic struc- tures can be reduced. This fraction cor- responds to the number of viologen units present in the outer shells of the dendrimers. The reasons for incomplete charge pooling are discussed. Compari- son with the behavior of polyviologen dendrimers that are terminated with bulky tetraarylmethane groups and were studied previously enabled the role played by the terminal groups in the redox and hosting properties to be elucidated. Keywords: dendrimers · electro- chemistry · host–guest systems · photosensitization · viologens [a] Prof. J. F. Stoddart Department of Chemistry, Northwestern University 2145 Sheridan Road, Evanston, IL 6028 (USA) Fax: (+ 1)847-491-7713 E-mail: stoddart@northwestern.edu [b] Prof. C. M. Ronconi Department of Chemistry and Biochemistry University of California, Los Angeles 405 Hilgard Avenue, Los Angeles, CA90095 (USA) [c] Prof. V. Balzani, M. Baroncini, Prof. P. Ceroni, C. Giansante, Prof. M. Venturi Dipartimento di Chimica “G. Ciamician” Università di Bologna, Via Selmi 2, 40126 Bologna (Italy) Fax: (+ 39)051-209-9456 E-mail: margherita.venturi@unibo.it [d] Prof. C. M. Ronconi Present address: Instituto de Quimica Universidade Federal do Rio de Janeiro Centro de Tecnologia, Bloco A, 68 Andar, sala 637 Cid. Universitµria, Rio de Janeiro, RJ CEP 21.941.590 (Brasil) Chem. Eur. J. 2008, 14, 8365 – 8373 # 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 8365 FULL PAPER