Conformationally gated photoinduced processes within photosensitizer-acceptor dyads based on Osmium(II) complexes with triarylpyridinio-functionalized terpyridyl ligands:: Insights from experimental study

[(ttpy) Os( tpy-ph-TPH3+)](3+) ( 2), [(ttpy) Os( tpy-xy-TPH3+)](3+) ( 3), [(ttpy) Os( tpy-ph-TPH2( NO2)(+))](3+) ( 4), and [(ttpy) Os( tpy-xy-TPH2( NO2)(+))](3+) ( 5) are a series of dyads made of an Os( II) bis-tpy complex ( tpy) 2,2': 6', 2"-terpyridine) as the photosensitizer ( P) and 2,4,6-triarylpyridinium group ( TP+) as the electron acceptor ( A). These dyads were designed to form charge-separated states ( CSS) upon light excitation. Together with analogous Ru( II) complexes ( 7-10), they have been synthesized and fully characterized. We describe herein how intramolecular photoinduced processes are affected when the electron-accepting strength of A ( by nitro-derivatization of TP+) and/or the steric hindrance about intercomponent linkage ( by replacing a phenyl spacer by a xylyl one) are changed. Electronic absorption and electrochemical behavior revealed that ( i) chemical substitution of TP+ ( i. e., TP+- NO2) has no sizable influence on P-centered electronic features, ( ii) reduction processes located on TP+ depend on the intercomponent tilt angle. Concerning excited-state properties, photophysical investigation evidenced that phosphorescence of P is actually quenched in dyads 4 and 5 only. Ultrafast transient absorption ( TA) experiments allowed attributing the quenching in conformationally locked dyad 5 to oxidative electron transfer ( ET) from the (MLCT)-M-3 level to the TP+-NO2 acceptor ( k(el) = 1.1 x 10(9) s(-1)). For 4, geometrically unlocked, the (MLCT)-M-3 state was shown to first rapidly equilibrate ( reversible energy transfer; k(eq) approximate to 2 x 10(9) s(-1)) with a ligand centered triplet state before undergoing CSS formation. Thus, the pivotal role of conformation in driving excited-state decay pathways is demonstrated. Also, inner P structural planarization as a relaxation mode of the (MLCT)-M-3 states has been inferred from TA experiments.