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 3MLCT level to the TP+-NO2 acceptor (kel ) 1.1 109 s-1). For 4, geometrically unlocked, the 3MLCT state was shown to first rapidly equilibrate (reversible energy transfer; keq 2 109 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 3MLCT states has been inferred from TA experiments.