| Abstract
| - [(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 electronacceptor (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-acceptingstrength of A (by nitro-derivatization of TP+) and/or the steric hindrance about intercomponent linkage (byreplacing a phenyl spacer by a xylyl one) are changed. Electronic absorption and electrochemical behaviorrevealed that (i) chemical substitution of TP+ (i.e., TP+−NO2) has no sizable influence on P-centeredelectronic features, (ii) reduction processes located on TP+ depend on the intercomponent tilt angle.Concerning excited-state properties, photophysical investigation evidenced that phosphorescence of P isactually quenched in dyads 4 and 5 only. Ultrafast transient absorption (TA) experiments allowed attributingthe quenching in conformationally locked dyad 5 to oxidative electron transfer (ET) from the 3MLCT levelto the TP+−NO2 acceptor (kel = 1.1 × 109 s-1). For 4, geometrically unlocked, the 3MLCT state was shownto first rapidly equilibrate (reversible energy transfer; keq ≈ 2 × 109 s-1) with a ligand centered triplet statebefore undergoing CSS formation. Thus, the pivotal role of conformation in driving excited-state decaypathways is demonstrated. Also, inner P structural planarization as a relaxation mode of the 3MLCT stateshas been inferred from TA experiments.
|
