Conformationally Gated Photoinduced Processes within PhotosensitizerAcceptor Dyads Based on Osmium(II) Complexes with Triarylpyridinio-Functionalized Terpyridyl Ligands:  Insights from Experimental Study

Abstract

[(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.

Kinetic pathway for interfacial electron transfer from a semiconductor to a molecule

“…Given the homologous nature of these molecules and the parity of the TPA •+/0 reduction potentials, it is clear that the pathway for electrons includes transfer through the bridge orbitals. The use of methyl substituents that sterically prevent planarization and lower electronic couplings in molecular donor-bridge-acceptor compounds has been exploited previously in molecular energy transfer 38 , thermal electron transfer 39 and light-driven electron transfer [40][41][42][43] . This is the first example at an interface and is important for controlling electron transfer at illuminated semiconductor interfaces that does not necessitate the loss of free energy or rely on distance.…”

Efficient access to conjugated 4,4′-bipyridinium oligomers using the Zincke reaction: synthesis, spectroscopic and electrochemical properties

“…29 In addition, there have been several reports of the synthesis of socalled "extended viologens" whereby two pyridinium residues are separated by increasing numbers of aromatic rings. [30][31][32][33][34][35][36][37][38][39][40] These have rapidly found widespread application in the construction of supramolecular complexes. [41][42][43][44][45][46] There are also examples of (di)phenyl viologen structures in which the conjugation length has been extended by the addition of an aromatic residue to the N-termini of the viologen.…”

Coupling one electron photoprocesses to multielectron catalysts: Towards a photoelectrocatalytic system

“…The literature for the preparation of 4 0 -p-aminophenyl-2,2 0 :6 0 ,2 00 -terpyridine is vast and divergent [23,[45][46][47][48][49][50][51]. During synthesis of the perylene terpyridine species, an optimized synthesis was developed.…”