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Complete Charge Pooling is Prevented in Viologen‐Based Dendrimers by Self‐Protection
Author(s) -
Marchioni Filippo,
Venturi Margherita,
Ceroni Paola,
Balzani Vincenzo,
Belohradsky Martin,
Elizarov Arkadij M.,
Tseng HsianRong,
Stoddart J. Fraser
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200400747
Subject(s) - viologen , dendrimer , chemistry , photochemistry , dimer , monomer , photosensitizer , triethanolamine , electrochemistry , electron transfer , thionine , polymer chemistry , organic chemistry , polymer , electrode , analytical chemistry (journal)
Abstract We have investigated the electrochemical behavior, and chemical and photosensitized reduction of two dendrimers based on a 1,3,5‐trisubstituted benzenoid core, which contain 9 and 21 4,4′‐bipyridinium (usually called viologen) units, respectively, in their branches and are terminated with tetraarylmethane groups. For comparison purposes, the behavior of reference compounds that contain a single viologen unit have also been investigated. We have found that only part of the viologen units can be reduced in the dendrimer species. For the larger dendrimer, the number of reducible viologens (out of the 21 present) is 14 in electrochemical experiments (in MeCN), 9 on reduction with bis(benzene)chromium (in MeCN), and 13 by photoinduced electron transfer with 9‐methylanthracene as a photosensitizer and triethanolamine as a sacrificial reductant in CH 2 Cl 2 . The reduced viologen units undergo partial dimerization. The photochemical experiments have shown that only monomeric, one‐electron‐reduced viologen units are formed at the beginning of the irradiation, followed by dimer formation, until a photostationary state is reached that contains 40 % nonreduced, 33 % monomeric reduced, and 27 % reduced units associated in the dimeric form. The results suggest that, upon reduction of a fraction of the viologen units, the dendrimer structure shrinks, with the result that the bulky terminal groups protect other viologen units from being reduced.