Steric Influence on the Excited-State Lifetimes of Ruthenium Complexes with Bipyridyl−Alkanylene−Pyridyl Ligands

Abstract

The structural effect on the metal-to-ligand charge transfer (MLCT) excited-state lifetime has been investigated in bis-tridentate Ru(II)-polypyridyl complexes based on the terpyridine-like ligands [6-(2,2'-bipyridyl)](2-pyridyl)methane ( 1) and 2-[6-(2,2'-bipyridyl)]-2-(2-pyridyl)propane ( 2). A homoleptic ([Ru( 2) 2] (2+)) and a heteroleptic complex ([Ru(ttpy)( 2)] (2+)) based on the new ligand 2 have been prepared and their photophysical and structural properties studied experimentally and theoretically and compared to the results for the previously reported [Ru( 1) 2] (2+). The excited-state lifetime of the homoleptic Ru (II) complex with the isopropylene-bridged ligand 2 was found to be 50 times shorter than that of the corresponding homoleptic Ru (II) complex of ligand 1, containing a methylene bridge. A comparison of the ground-state geometries of the two homoleptic complexes shows that steric interactions involving the isopropylene bridges make the coordination to the central Ru (II) ion less octahedral in [Ru( 2) 2] (2+) than in [Ru( 1) 2] (2+). Calculations indicate that the structural differences in these complexes influence their ligand field splittings as well as the relative stabilities of the triplet metal-to-ligand charge transfer ( (3)MLCT) and metal-centered ( (3)MC) excited states. The large difference in measured excited-state lifetimes for the two homoleptic Ru (II) complexes is attributed to a strong influence of steric interactions on the ligand field strength, which in turn affects the activation barriers for thermal conversion from (3)MLCT states to short-lived (3)MC states.

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“…Examples are shown in Fig. 26 [176][177][178][179]. The photophysical properties of several homoleptic bis-tridentate Ru(II) complexes (along with those of Ru(bpy) 3 2+ for comparison) are summarized in Table 3.…”

“…Indeed, Ru(bqp) 2 2+ has a longer lifetime than even Ru(bpy) 3 2+ in fluid solution. These longer lifetimes can be ascribed to increasing the energy of the 3 MC state, thus raising the activation barrier for decay via population of this state [178,179] (see Fig. 21).…”

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“…Herein, the proof of concept for tuning the energy gap between 3 MLCT/pp* and 3 MC states is conducted by using complexes 1 b and 4 b. In this approach, the higher-lying 3 MC state was calculated according to the methodology illustrated in the work of Persson et al [20] Details of the calculation procedures are described in the Experimental Section. The result shown in Figure 8 MC!S 0 radiationless pathway is expected to be efficient, accounting for its weak emissive nature (i.e., relatively low quantum yield (0.0067) and short observed lifetime (t obs % 6.3 ms)) compared with that of 4 b (quantum yield (0.49), and t obs % 40.7 ms, see Table 1), and vice versa for 1 a and 4 a (see Table 1).…”

“…[42] The electronic configurations of 3 MC dd states were calculated following the literature methodology. [20] The 3 MLCT state geometry was obtained by performing geometry optimization along the triplet state potentialenergy surface (PES) by using the X-ray structural data as the initial geometry. As for the 3 MC state, because the electron densities are mainly distributed on the central metal atom, the chelating interaction between metal and ligands must be rather weak.…”

“…As for the 3 MC state, because the electron densities are mainly distributed on the central metal atom, the chelating interaction between metal and ligands must be rather weak. We thus performed geometry optimization of the 3 MC state following the methodology illustrated in the work of Persson et al [20] The method starting with a distorted geometry, for which the metal-ligand bonds are largely elongated, such that its associated energy is expected to be far away from the global minimum along the PES. Based on this methodology, the optimization is then able to fall into the presumably shallow local minimum associated with the 3 MC dd excited state.…”