Designing Multifunctional Expanded Pyridiniums: Properties of Branched and Fused Head-to-Tail Bipyridiniums Je ´ro ˆ me Fortage, †,‡ Cyril Peltier, § Francesco Nastasi, | Fausto Puntoriero, | Fabien Tuye ` ras, † Sophie Griveau, ⊥ Fethi Bedioui, ⊥ Carlo Adamo, § Ilaria Ciofini,* ,§ Sebastiano Campagna,* ,| and Philippe P. Laine ´* ,† Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques (CNRS UMR-8601), UniVersite ´ Paris Descartes, 45 rue des Saints Pe `res, F-75270 Paris Cedex 06, France, Institut Parisien de Chimie Mole ´culaire (CNRS UMR-7201), Equipe Chimie Supramole ´culaire, Case 42, UniVersite ´ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France, LECIME, Laboratoire d′E ´ lectrochimie, Chimie des Interfaces et Mode ´lisation pour l′E ´ nergie (CNRS UMR-7575), E ´ cole Nationale Supe ´rieure de Chimie de Paris-Chimie ParisTech, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France, Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica and Centro InteruniVersitario per la ConVersione Chimica dell’Energia Solare, UniVersita ` di Messina, Via Sperone 31, I-98166 Messina, Italy, and Laboratoire de Pharmacologie Chimique et Ge ´ne ´tique (CNRS UMR-8151 and INSERM U-1022), UniVersite ´ Paris Descartes, E ´ cole Nationale Supe ´rieure de Chimie de Paris-Chimie ParisTech, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France Received September 25, 2010; E-mail: philippe.laine@parisdescartes.fr; campagna@unime.it; ilaria-ciofini@chimie-paristech.fr Abstract: The multifaceted potentialities of expanded pyridiniums (EPs), based on one pyridinium core bearing a 4-pyridyl or 4-pyridylium as the N-pyridinio group, are established at both experimental and theoretical levels. Two classes of head-to-tail (htt) EPs were designed, and their first representative elements were synthesized and fully characterized. The branched (B) family is made up of 2,6-diphenyl-4-aryl-1,4′- bipyridin-1-ium (or 1,1′-diium) species, denoted 1B and 2B for monocationic EPs (with aryl ) phenyl and biphenyl, respectively) and 1B Me and 2B Me for related quaternarized dicationic species. The series of fused (F) analogues comprises 9-aryl-benzo[c]benzo[1,2]quinolizino[3,4,5,6-ija][1,6]naphthyridin-15-ium species, denoted 1F and 2F, and their 2,15-diium derivatives referred to as 1F Me and 2F Me . Electrochemistry (in MeCN vs SCE) reveals that branched EPs undergo a single reversible bielectronic reduction at ca. -0.92 V for 1B/2B and -0.59 V for 1B Me /2B Me , whereas pericondensed species show two reversible monoelectronic reductions at ca. -0.83 and -1.59 V for 1F/2F and ca. -0.42 and -1.07 V for 1F Me /2F Me . Regarding electronic absorption features, all htt-EP chromophores show absorptivity in the range of ca. 1-4 × 10 4 M -1 cm -1 , with red-edge absorptions extending toward 450 and 500 nm (in MeCN) for 2B Me and 2F Me , respectively. These lowest-energy pi-pi* transitions are ascribed to intramolecular charge transfer between the electron-releasing biphenyl group and the htt-bipyridinium electron-withdrawing subsystems. EPs display room-temperature photoemission quantum yields ranging from 10% to 50%, with the exception of 1B, and branched luminophores are characterized by larger Stokes shifts (8000-10 000 cm -1 ) than fused ones. Lastly, a method to predict the efficiency of photobiscyclization of branched EPs into fused ones, based on the analysis of computed difference maps in total electron density for singlet excited states, is proposed. 1. Introduction Pyridinium and bipyridinium 1 species are among the most extensively used functional entities 2,3 because of their appealing electron-withdrawing (pull effect) 4,5 and electron-accepting (redox) 6,7 properties. By virtue of their reduction properties they are suited to artificial systems developed for solar energy conversion, both in bimolecular schemes and in supramolecular (multicomponent) arrangements in which they play the role of electron acceptors (i.e., electrophores). 8 Their mild and revers- ible reduction processes also make them choice components in supramolecular assemblies designed to behave as electron- storage systems (e.g., polyviologen dendrimers) 9,10 or molecular machines 11,12 whose organization and machinery are based on charge-transfer interactions and redox inputs, respectively. 3,13 † LCBPT-Universite ´ Paris Descartes. ‡ IPCM-Universite ´ Pierre et Marie Curie. § LECIME-ENSCP. | Universita ` di Messina. ⊥ LPCG-ENSCP-Universite ´ Paris Descartes. (1) Sliwa, W. Curr. Org. Chem. 2003, 7, 995–1048. (2) When performing a bibliometric analysis through the ISI Web of Knowledge, a striking point is the close correlation observed in the evolution over the years of the number of publications (and related citations) dealing with the following topics: “viologen” or “bipyri- dinium” and “ruthenium bipyridine”. Two remarkable inflection points are found around 1980 and 1990. They are ascribable to the launching of two related topics: “water splitting” and “artificial photosynthesis”, on the one hand, and “molecular devices” including “molecular machines”, on the other; see also ref 3. (3) Balzani, V.; Bergamini, G.; Ceroni, P. Coord. Chem. ReV. 2008, 252, 2456–2469. Published on Web 10/28/2010 10.1021/ja108668h  2010 American Chemical Society 16700 9 J. AM. CHEM. SOC. 2010, 132, 16700–16713