Photoinduced processes within compact dyads based on triphenylpyridinium- functionalized bipyridyl complexes of ruthenium(II)

Abstract

As an alternative to conventional charge-separation functional molecular models based on long-range ET within redox cascades, a "compact approach" has been examined. To this end, spacer elements usually inserted between main redox-active units within polyad systems have been removed, allowing extended rigidity but at the expense of enhanced intercomponent electronic communication. The molecular assemblies investigated here are of the P-(θ1)-A type, where the θ1 twist angle is related to the degree of conjugation between the photosensitizer (P, of {Ru(bpy)3}2+ type) and the electron-acceptor (A). 4-N- and 4-N-,4′-N-(2,4,6-triphenylpyridinio)-2,2′-bipyridine ligands (A1-bpy and A2-bpy, re spectively) have been synthesized to give complexes with RuII, 1-bpy and 2-bpy, respectively. Combined solid-state analysis (X-ray crystallography), solution studies (1H NMR, cyclic voltammetry) and computational structural optimization allowed verifying that θ1 angle approaches 90° within 1-bpy and 2-bpy in solution. Also, anticipated existence of strong intercomponent electronic coupling has been confirmed by investigating electronic absorption properties and electrochemical behav ior of the compounds. The capability of 1-bpy and 2-bpy to undergo PET process was evaluated by carrying out their photophysical study (steady state emission and time-resolved spectroscopy at both 293 and 77 K). The conformational dependence of photoinduced processes within P-(θ1)-A systems has been established by comparing the photophysical properties of 1-bpy (and 2-bpy) with those of an affiliated species reported in the literature, 1-phen. A complementary theoretical analysis (DFT) of the change of spin density distribution within model [1-bpy(θ1)]- mono-reduced species as a function of θ1 has been undertaken and the possibility of conformationally switching emission properties of P was derived. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.