Distance Dependence of Intrahelix Ru^II* to Os^II Polypyridyl Excited-State Energy Transfer in Oligoproline Assemblies

Abstract

Energy transfer between the metal-to-ligand charge transfer (MLCT) excited states of [Pra [M(II)(bpy)2(4-Me-4'(-N(H)CO)bpy)](PF6)2 units ([Pra(M(II)bpy2(mbpy)](2+): M(II) = Ru(II) or Os(II), bpy = 2,2'-bipyridine, mbpy = 4'-methyl-2,2'-bipyridine-4-carboxamido, Pra = 4-M(II)-L-proline) linked covalently to oligoproline assemblies in room temperature acetonitrile occurs on the picosecond-nanosecond time scale and has been time-resolved by transient emission measurements. Three derivatized oligoprolines, [CH3-CO-Pro6-Pra[Os(II)(bpy)2(mbpy)](2+)-Pro2-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro2-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro6-Glu-NH2](6+) (ORR-2, Pro = L-proline and Glu = glutamic acid); [CH3-CO-Pro6-Pra[Os(II)(bpy)2(mbpy)](2+)-Pro3-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro3-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro6-Glu-NH2](6+) (ORR-3); and CH3-CO-Pro6-Pra[Os(II)(bpy)2(mbpy)](2+)-Pro5-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro5-Pra[Ru(II)(bpy)2(mbpy)](2+)Pro6-Glu2-NH2](6+) (ORR-5), were prepared by using solid-phase peptide synthesis. Given the helical nature of the resulting assemblies and the nature of the synthesis, composition, length, and loading pattern are precisely controlled in the assemblies. In acetonitrile, they adopt a proline I helical secondary structure, confirmed by circular dichroism, in which the appended chromophores are ordered in well-defined orientations and internuclear separation distances although helix formation for ORR-2 is incomplete. Quantitative comparison of oligoproline ground-state absorption and steady-state emission spectra to those for the constituents, [Boc-Pra[M(II)(bpy)2(mbpy)](2+)-OH](PF6)2 (Boc = N(α)-(1,1-dimethylethoxycarbonyl), shows that following Ru(II) light absorption, Ru(II)* undergoes facile energy transfer resulting in sensitization of Os(II). Sensitization efficiencies are 93% for ORR-2, 77% for ORR-3, and 73% for ORR-5. Picosecond-resolved emission measurements reveal complex, coupled dynamics that arise from excited-state decay and kinetically competitive -Ru(II)*-Ru(II)- → -Ru(II)-Ru(II)*- energy transfer migration/exchange and downhill -Ru(II)*-Os(II) → -Ru(II)-Os(II)* energy transfer. These processes were modeled simultaneously to extract rate constants for Ru(II)* → Ru(II) energy-transfer migration, k(Ru*-Ru), and Ru(II)* → Os(II) energy transfer, k(Ru*-Os). For ORR-2, k(Ru*-Ru) = 2.9 × 10(7) s(-1) and k(Ru*-Os) = 3.4 × 10(8) s(-1). For ORR-3, k(Ru*-Ru) = 1.2 × 10(7) s(-1) and k(Ru*-Os) = 1.3 × 10(8) s(-1). For ORR-5, k(Ru*-Ru) = 3.6 × 10(6) s(-1) and k(Ru*-Os) = 5.8 × 10(7) s(-1), all in acetonitrile at 22 °C. The data were analyzed by assuming Dexter energy transfer with the Franck-Condon factors arising from intramolecular structural and medium changes evaluated by use of an emission spectral fitting procedure. Fits of the data to the Dexter mechanism were consistent with the predicted distance dependence of energy transfer.

Supramolecular Luminescent Sensors

“…The distance dependence of the donor and acceptor in energy transfer schemes depends largely on the operative energy transfer mechanism. Photoinduced electron transfer in particular requires close proximity between the donor and acceptor in order for effective orbital overlap to occur . Of note, the exploitation of such strong distance dependence to determine key structural information about a variety of experimental systems has also been reported. , In some cases, the charge transfer complex undergoes a nonradiative transition back to the ground state and no emission profile is seen.…”

“…This exchange of electrons, rather than the transfer of one single electron, is what differentiates this mechanism from PET. In contrast to FRET, Dexter energy transfer requires close-range interactions between the donor and acceptor to enable direct orbital overlap between the interacting moieties, and the efficiency of such energy transfer decreases dramatically as the distance between the donor and acceptor increases. ,, Collisions of the donor and acceptor molecules induce direct interactions of the wavefunctions of the molecules, which is necessary for the electron exchange to occur and which requires close intermolecular distances . In order to ensure close proximity between the molecules and induce Dexter interactions, donor and acceptor molecules can be held in fixed media such as frozen solvents or polymeric scaffolds .…”

Excimer Formation in a Confined Space: Photophysics of Ladderphanes with Tetraarylethylene Linkers

“…[3] Relative orientation and distance among these photophysically active modules would influence the electronic delocalization and thus bulk materials properties. [9] The interactions between chromophores would be more efficient if they could be better aligned along the polymeric backbone. This consideration has led to substantial efforts in the design and syntheses of chromophore arrays within supramolecular structures that may provide a confined spatial environment.…”

Controlling the Orientation of Pendants in Two-Dimensional Comb-Like Polymers by Varying Stiffness of Polymeric Backbones

“…Intrachain exciton and electron migrations between chromophores appended to a polymeric backbone − or associated with a supramolecular scaffold , have offered useful models to understand various natural light harvesting processes. These photophysical routes are known to be sensitive to distance and orientation of chromophores . Flexible polymeric framework, such as polystyrene, , poly(methyl methacrylate) polypeptides, and polymers derived from dibenzofulvenes are frequently used.…”

“…These photophysical routes are known to be sensitive to distance and orientation of chromophores . Flexible polymeric framework, such as polystyrene, , poly(methyl methacrylate) polypeptides, and polymers derived from dibenzofulvenes are frequently used. Conformational disorder arising from rotatory single bonds may lead to irregular microstructures on the backbone.…”