Extending the Light‐Harvesting Properties of Transition‐Metal Dendrimers

Abstract

We report a study of the electronic energy-transfer dynamics within the transition-metal polypyridine complex OsRu3pyr6 (Os[(dpp)Ru(bpy{pyrene})2]3(8+), where dpp=2,3-bis(2'-pyridyl)pyrazine and bpy=2,2'-bipyridine) after excitation with UV light. By using a broadband visible femtosecond probe, we are able to simultaneously detect both the energy transfer from the peripheral aromatic ligands to the Os center and the sub-picosecond energy transfer from the initially excited Ru-bpy ligand-centered state to the Os triplet metal-to-ligand charge-transfer (MLCT) state. Pyrene energy transfer occurs from both the nonrelaxed and the relaxed S(1) states on timescales of 6 and 45 ps, respectively. In both cases, the energy transfer is described by means of Förster energy transfer theory. Sub-picosecond energy transfer within the OsRu3 metal-ligand core most likely includes a direct energy transfer between the higher-lying ligand-centered states on Ru and Os, in addition to the transfer between the MLCT states. The absorption cross-sections in both the UV and the visible spectral regions are enhanced by attaching the aromatic pyrene ligands. Furthermore, energy transfer is directed only towards the Os core, which ultimately leads to an exclusive population of the Os-based triplet MLCT state, thus making the OsRu3pyr6 transition metal-polypyridine dendrimer an obvious candidate for artificial light-harvesting systems.

Aggregation-Induced Energy Transfer in a Decanuclear Os(II)/Ru(II) Polypyridine Light-Harvesting Antenna Dendrimer

“…21,22 In these latter species, the lowest-energy excited state is indeed the MLCT triplet state(s) involving the peripheral subunit(s), which collect all the light energy absorbed by the various chromophoric units of the multicomponent species by downhill energy-transfer processes on the femtosecond timescale (see schematic representation of energy-transfer pathways in 2 and 4 at the bottom of Figure 3). 25,27,31 The emission spectrum of 1, dominated by the emission of the peripheral triplet Ru-based MLCT states, also shows a component at low energies that is not present in 2 or 4. This component is assigned to the emission from the 3 MLCT state of the Os(II)-based core mainly produced by direct absorption of such a subunit.…”

“…Such ultrafast processes also involve singlet states and not only triplet states, as is the usual case for transition (heavy)-metal compounds. [25][26][27] (3) When the donor and acceptor chromophoric subunits are spatially separated by other metal-based unit(s), whose lowest-energy excited state is higher in energy than both donor and acceptor units, these interposed units behave as an insulating interface and energy transfer does not take place.…”

“…Some of us contributed to the field by designing one of the early families of light-harvesting dendrimers in which the chromophore subunits are mostly Ru(II) and Os(II) polypyridine complexes: [21][22][23][24] sub-picosecond energy transfer between nearby chromophoric units in the presence of suitable energy gradients occurred in such species. [25][26][27] Inter-dendrimer energy-transfer processes in self-assembled multidendrimer systems have never been reported, although in light-harvesting dendrimers, different energy-transfer patterns, called inter-wheel and intra-wheel energy transfer, have been described; in these, energy transfer takes place between chromophores belonging to the same branch (intra-wheel) or different branches (inter-wheel) of the dendrimers. 28 Similarly, intra-dendrimer energy-transfer processes that become active upon aggregation are also unknown.…”

Energy harvesting: a review of the interplay between structure and mechanism

“…The core is often a lanthanide ion, for reasons that will be expanded upon in the next Section, but other transition metal ions can also be employed [68]. The typical example shown in Fig.…”

Synthesis, characterization and photophysical studies of novel pyrene labeled ruthenium (II) trisbipyridine complex cored dendrimers

“…[6,12] Moreover, dendrimers containing metal ions have been developed to combine the redox properties of metals with the organization of a dendritic shell. [1][2][3][4][5][6]13,14] Transition metal polybipyridine complexes, such as ruthenium trisbipyridine complexe ([Ru(Bpy) 3 ] 2+ ), show well studied redox and photophysical properties. [7][8][9][10]15] The principal absorption bands for [Ru(Bpy) 3 ] 2+ are a ligand centered (LC) transition at 280 nm and a metal to ligand charge transfer (MLCT) transition between 450 and 480 nm.…”