Effects of Excited State−Excited State Configurational Mixing on Emission Bandshape Variations in Ruthenium−Bipyridine Complexes

Abstract

The 77 K emission spectra of 21 [Ru(L) 4bpy] ( m+ ) complexes for which the Ru/bpy metal-to-ligand-charge-transfer ( (3)MLCT) excited-state energies vary from 12 500 to 18 500 cm (-1) have vibronic contributions to their bandshapes that implicate excited-state distortions in low frequency ( lf; hnu lf < 1000 cm (-1)), largely metal-ligand vibrational modes which most likely result from configurational mixing between the (3)MLCT and a higher energy metal centered ( (3)LF) excited state. The amplitudes of the lf vibronic contributions are often comparable to, or sometimes greater than those of medium frequency ( mf; hnu mf > 1000 cm (-1)), largely bipyridine (bpy) vibrational modes, and for the [Ru(bpy) 3] (2+) and [Ru(NH 3) 4bpy] (2+) complexes they are consistent with previously reported resonance-Raman (rR) parameters. However, far smaller lf vibronic amplitudes in the rR parameters have been reported for [Os(bpy) 3 ] (2+), and this leads to a group frequency approach for interpreting the 77 K emission bandshapes of [Ru(L) 4bpy] ( m+ ) complexes with the vibronic contributions from mf vibrational modes referenced to the [Os(bpy) 3] (2+) rR parameters (OB3 model) and the envelope of lf vibronic components represented by a "progression" in an "equivalent" single vibrational mode ( lf1 model). The lf1 model is referenced to rR parameters reported for [Ru(NH 3) 4bpy] (2+). The observation of lf vibronic components indicates that the MLCT excited-state potential energy surfaces of Ru-bpy complexes are distorted by LF/MLCT excited-state/excited-state configurational mixing, but the emission spectra only probe the region near the (3)MLCT potential energy minimum, and the mixing can lead to larger distortions elsewhere with potential photochemical implications: (a) such distortions may labilize the (3)MLCT excited state; and (b) the lf vibrational modes may contribute to a temperature dependent pathway for nonradiative relaxation.

Energy Dependence of the Ruthenium(II)-Bipyridine Metal-to-Ligand-Charge-Transfer Excited State Radiative Lifetimes: Effects of ππ*(bipyridine) Mixing

“…81 As has been previously discussed, the amplitude of the dominant vibronic sideband decreases markedly as the excited state energy decreases. 20,[37][38][39] 20 The gray curves in Figure 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 The comparison in Figure 3 is of the vibronic (or Franck-Condon) amplitudes for two different conceptual models and the resulting bandshapes should not be significantly on 3 m  , as in eq 3, results in dependent on h m . However, assuming that the calculated intensities depend vibronic progressions whose amplitudes are linearly dependent on  m .…”

“…When the ground and excited state differ in geometry only in the coordinates of the k th normal mode of the ground state and assuming Gaussian component bandshapes, the emission spectrum can be represented as 24,27,39,[55][56][57] (10)…”

Effect of deprotonation on absorption and emission spectra of Ru(ii)-bpy complexes functionalized with carboxyl groups

“…4a, is more pronounced in experimental spectra of complexes 2 and 3 than in the theoretical. Since this shoulder is present even in the experimental spectrum of the complex 1 (not shown), its origin is likely attributed to the vibrational overtones 62 often seen in the linear absorbance of organic chromophores. Other broadening mechanisms, such as spin-spin and spin-orbit couplings, which are not included in our calculations, might also contribute to the linewidths of the experimental spectra.…”

Near-IR Charge-Transfer Emission at 77 K and Density Functional Theory Modeling of Ruthenium(II)-Dipyrrinato Chromophores: High Phosphorescence Efficiency of the Emitting State Related to Spin–Orbit Coupling Mediation of Intensity from Numerous Low-Energy Singlet Excited States

“…The formulation of the emission efficiency in eqs and assumes simple relaxation behavior from a single excited state, even when the transition is the sum of many vibrational modes. The observed emission vibronic-sideband intensities of typical Ru-bpy chromophores are energy dependent ,− and can be effectively modeled by density functional theory (DFT) computations of the configurational mixing of the MLCT/ππ* excited states, and this mixing is correlated with an “extra” energy dependence of k RAD . ,, Consequently, two limiting cases for the photoinduced transition related to the intensity stealing of excited states mixing have been considered in the literature: (a) when a {D, A} system has a strong spin-allowed transition localized on a D or A moiety (e.g., A → A*), it can mix with a weak donor-to-acceptor charge-transfer (DACT) excited state, {D, A} + h ν → {D + , A – }*, thereby enhancing the intensity observed for the DACT transition, and this type of mixing is based on the expression of the molecular model; ,, (b) the emission transitions in phosphorescence require significant mixing between the singlet and T e states through a SOC perturbation ,,− since a pure T e → S 0 transition is strictly spin-forbidden ( M T e ,S 0 = 0) without SOC interactions. The magnitude of the SOC element ( H SOC ) increases approximately as the fourth power of the atomic number ( Z 4 ) and is approximately 3000 times greater for Ru than for C. Furthermore, to ensure that the SOC mixing coefficient is not zero for the T e → S 0 transition, the differences in the T e and S n spin angular momenta ( S⃗ ) must be compensated by the difference in the orbital angular momenta ( L⃗ ) for a spin–orbit operator ( Ĥ SO ∝ S⃗ · L⃗ ).…”