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-TPH(3)(+))](3+) (2), [(ttpy)Os(tpy-xy-TPH(3)(+))](3+) (3), [(ttpy)Os(tpy-ph-TPH(2)(NO(2))(+))](3+) (4), and [(ttpy)Os(tpy-xy-TPH(2)(NO(2))(+))](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(+)-NO(2)) 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 (3)MLCT level to the TP(+)-NO(2) acceptor (k(el) = 1.1 x 10(9) s(-)(1)). For 4, geometrically unlocked, the (3)MLCT state was shown to first rapidly equilibrate (reversible energy transfer; k(eq) approximately 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 (3)MLCT states has been inferred from TA experiments.
Lainé
Philippe P
PP
Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques, CNRS UMR-8601, Université René Descartes, 45 rue des Saints Pères, F-75270 Paris Cedex 06, France. philippe.laine@univ-paris5.fr
Bedioui
Fethi
F
Loiseau
Frédérique
F
Chiorboli
Claudio
C
Campagna
Sebastiano
S
D004583:Electrons; D004735:Energy Transfer; D006841:Hydrocarbons, Aromatic / Q000737:chemistry*; D008024:Ligands; D049449:Luminescence; D008968:Molecular Conformation; D009589:Nitrogen Oxides / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis*; D009992:Osmium / Q000737:chemistry*; D010777:Photochemistry; D017319:Photosensitizing Agents / Q000138:chemical synthesis*; D011726:Pyridinium Compounds / Q000737:chemistry*; D012428:Ruthenium / Q000737:chemistry; D013057:Spectrum Analysis; D014992:Xylenes / Q000737:chemistry
D006841:Hydrocarbons, Aromatic; D008024:Ligands; D009589:Nitrogen Oxides; D009942:Organometallic Compounds; D017319:Photosensitizing Agents; D011726:Pyridinium Compounds; D014992:Xylenes; D009992:Osmium; D012428:Ruthenium