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.