A New Heptanuclear Dendritic Ruthenium(II ) Complex Featuring Photoinduced Energy Transfer Across High‐Energy Subunits
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The fact that processes 3 and 3′ of Figure 4 that is the reduction of the inner bridges take place simultaneously could appear anomalous. In fact the process is the reduction of (identical) ligands connected to the same metal center and it is known[4]that such types of processes are commonly split in Ru(II) polypyridine complexes because of ligand–ligand interactions mediated by the shared metal center. However it has already been reported that the reduction of outer ligands in multicomponent metal complexes similar to1destabilizes the LUMO of the inner bridges substantially increasing their energy gap with the metal‐centered orbitals.[14]Since ligand–ligand interactions are largely governed by such energy gaps [12–14]coalescence of the inner bridge reduction of1is justified.
The charge‐separated state is almost isoergonic with the luminescent excited state of1 which is in its turn equivalent to the luminescent level of2. Luminescence lifetimes of2and1have not been measured since they are too short to be determined with our apparatus when considering the emission range and the weak intensity (time limit for obtaining reliable results in these conditions: >10 ns). However from time‐resolved transient absorption spectroscopy (see later in the main text) the lifetime of the luminescent level is close to 1 ns and dominated by nonradiative transitions. Since the electron transfer which should deactivate the charge‐separated state directly to the ground state can be considered a nonradiative process involving different subunits of the “supermolecule”1 it should be slower than the analogous process deactivating the luminescent level (worse electronic factors) and therefore even slower than 1 ns and cannot compete with the electron transfer yielding the excited state of the RuCl2(μ‐2 3‐dpp)2moiety.
Both exoergonic electron hopping[20]and energy transfer between directly connected subunits of the type present in1[7 8]are known to take place in the femtosecond timescale in ruthenium(II) polypyridine complexes. In the present discussion only triplet states are considered to be present after the laser pulse (in our case 0.6 ps) since intersystem‐crossing (ISC) processes in Ru(II) polypyridine complexes are reported to be faster than 200 fs [7 8 20–22] except in particular cases.[23]
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Some evidence for this ultrafast energy‐transfer process is found in kinetic traces in the 500–540‐nm range showing transient bleaching that recovers in 450 fs.
Thermally accessible metal‐centered states might also play some role in such processes.
Actually the 560‐nm decay is not single exponential. A shorter component of 0.35 ps can be attributed as in the model compound5 to vibrational cooling.
This finding implies an ultrafast charge‐separation process ET(1) which occurrs within 0.6 ps.