Low-Temperature Spectra and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with Cyclometalated Ancillary Ligands: The Excited State Spin–Orbit Coupling Origin of Variations in Emission Efficiencies

Abstract

The 77 K emission spectra of cyclometalated ruthenium­(II)-2,2′-bipyridine (CM-Ru-bpy) chromophores are very similar to those of related Ru-bpy complexes with am­(m)­ine or diimmine ancillary ligands, and density functional theory (DFT) modeling confirms that the lowest energy triplet metal to ligand charge transfer (3MLCT) excited states of CM-Ru-bpy and related Ru-bpy complexes have very similar electronic configurations. However, the phosphorescence decay efficiencies of CM-Ru-bpy excited states are about twice those of the conventional Ru-bpy analogues. In contrast to the similar 3MLCT excited state electronic configurations of the two classes of complexes, the CM-Ru-bpy chromophores have much broader visible region MLCT absorptions resulting from several overlapping transitions, even at 87 K. The emitting excited-state emission efficiencies depend on spin–orbit coupling (SOC) mediated intensity stealing from singlet excited states, and this work explores the relationship between the phosphorescence efficiency and visible region absorption spectra of Ru-bpy 3MLCT excited states in the weak SOC limit. The intrinsic 3MLCT emission efficiency, ιem, depends on mixing with singlet excited states whose RuIII-dπ-orbital angular momenta differ from that of the emitting state. DFT modeling of the 1MLCT excited-state electronic configurations that contribute significantly to the lowest energy absorption bands have RuIII-dπ orbitals that differ from those of their emitting 3MLCT excited states. This leads to a very close relationship between ιem and the lowest energy MLCT band absorptivities in Ru-bpy chromophores. Thus, the larger number of 1MLCT transitions that contribute to the lowest energy absorption bands accounts for the enhanced phosphorescence efficiency of Ru-bpy complexes with cyclometalated ancillary ligands.

Low-Temperature Observation of the Excited-State Decay of Ruthenium-(Mono-2,2′:6′,2″-Terpyridine) Ions with Innocent Ligands: DFT Modeling of an 3MLCT–3MC Intersystem Crossing Pathway

“…Additionally, the B3LYP, , PBE0, CAM-B3LYP, and LC- w PBE functionals were benchmarked with B3PW91 for describing the low-energy 1 MLCT electronic absorption envelopes of [Ru­(tpy) 2 ] 2+ (see Figure S3A1). It appeared that data collected using the B3PW91 functional are in better agreement with the experimental absorption peaks, being consistent with other early reports and our previous studies. ,,, Therefore, the B3PW91 functional was finally utilized to simulate the ground and excited states of the Ru-tpy complexes.…”

“…On the basis of the Ru-tpy complexes, it is possible that a 3 MC state exists that could cause emission quenching. We assume therefore that the unusually low 77 K quantum yield for complexes (c) and (e) that could exist as 3 MC excited states would reduce the value of the quantum yield at 77 K. This conclusion is consistent with our DFT calculations that have been discussed in detail in Section 4.1. As mentioned above, in the case of the Ru-tpy complexes, it is easier to control the probability of ic ( 3 MLCT → 3 MC) compared to the Ru-bpy complexes, a result that causes the Rutpy complex to have more flexibility for different applications.…”

“…This suggests that [Ru(tpy) 2 ] 2+ would have an unfavorable or nonobservable ic ( 3 MLCT → 3 MC b ) at 77 K. Furthermore, the 3 MC excited state for [Ru(tpy)(NH 3 ) 3 ] 2+ and [Ru(tpy)-(bpm)(NH 3 )] 2+ could not be theoretically located using the same computational procedure. This is in good agreement with the experimental data for both complexes with their oneexponential decay lifetime for the k obs relaxation at 77 K. Due to the strong ligand stabilization effect of the amino ligands, the interplay of 3 ■ ASSOCIATED CONTENT * sı Supporting Information…”

“…Consideration of the T e ( 3 MLCT Emitting State) and 3 MC (Emission Quenching State) States forObserving 77 K Low-Emission Quantum Yields. As demonstrated earlier in Figure3, the unusual 77 K low quantum yields with hν ave(em) and two exponential emission lifetimes for the [Ru(tpy)(AN) 3 ] 2+ complex suggested the presence of an accessible3 MLCT → 3 MC internal conversion process. As reported by Tsai et al,70 the elongation of the Ru−N coordination distance from the 3 MLCT (T e ) structure could lead to the formation of an 3 MC minimum structure.…”

Low-Temperature Observation of the Excited-State Decay of Ruthenium-(Mono-2,2′:6′,2″-Terpyridine) Ions with Innocent Ligands: DFT Modeling of an 3MLCT–3MC Intersystem Crossing Pathway

“…Additionally, the B3LYP, , PBE0, CAM-B3LYP, and LC- w PBE functionals were benchmarked with B3PW91 for describing the low-energy 1 MLCT electronic absorption envelopes of [Ru­(tpy) 2 ] 2+ (see Figure S3A1). It appeared that data collected using the B3PW91 functional are in better agreement with the experimental absorption peaks, being consistent with other early reports and our previous studies. ,,, Therefore, the B3PW91 functional was finally utilized to simulate the ground and excited states of the Ru-tpy complexes.…”

“…On the basis of the Ru-tpy complexes, it is possible that a 3 MC state exists that could cause emission quenching. We assume therefore that the unusually low 77 K quantum yield for complexes (c) and (e) that could exist as 3 MC excited states would reduce the value of the quantum yield at 77 K. This conclusion is consistent with our DFT calculations that have been discussed in detail in Section 4.1. As mentioned above, in the case of the Ru-tpy complexes, it is easier to control the probability of ic ( 3 MLCT → 3 MC) compared to the Ru-bpy complexes, a result that causes the Rutpy complex to have more flexibility for different applications.…”

“…This suggests that [Ru(tpy) 2 ] 2+ would have an unfavorable or nonobservable ic ( 3 MLCT → 3 MC b ) at 77 K. Furthermore, the 3 MC excited state for [Ru(tpy)(NH 3 ) 3 ] 2+ and [Ru(tpy)-(bpm)(NH 3 )] 2+ could not be theoretically located using the same computational procedure. This is in good agreement with the experimental data for both complexes with their oneexponential decay lifetime for the k obs relaxation at 77 K. Due to the strong ligand stabilization effect of the amino ligands, the interplay of 3 ■ ASSOCIATED CONTENT * sı Supporting Information…”

“…Consideration of the T e ( 3 MLCT Emitting State) and 3 MC (Emission Quenching State) States forObserving 77 K Low-Emission Quantum Yields. As demonstrated earlier in Figure3, the unusual 77 K low quantum yields with hν ave(em) and two exponential emission lifetimes for the [Ru(tpy)(AN) 3 ] 2+ complex suggested the presence of an accessible3 MLCT → 3 MC internal conversion process. As reported by Tsai et al,70 the elongation of the Ru−N coordination distance from the 3 MLCT (T e ) structure could lead to the formation of an 3 MC minimum structure.…”

Intrinsic 77 K Phosphorescence Characteristics and Computational Modeling of Ru(II)-(Bidentate Cyclometalated-Aromatic Ligand) Chromophores: Their Relatively Low Nonradiative Rate Constants Originating from Low Spin−Orbit Coupling Driven Vibronic Coupling Amplitudes between Emitting and Ground States

“…The E 1/2 (Ru III/II ) magnitudes of a−d in the range of 0.49−0.63 V were similar to those of A−C. 54,62 The E 1/2 (CM −1/−2 ) wave of d in this study was at −1.56 V. The E 1/2 (Ru III/II ) and E 1/2 (bpy 0/−1 ) couples of 1 and 6 are as follows:…”

Low-Temperature Observation of the Excited-State Decay of Ruthenium-(Mono-2,2′:6′,2″-Terpyridine) Ions with Innocent Ligands: DFT Modeling of an 3MLCT–3MC Intersystem Crossing Pathway

“…Additionally, the B3LYP, , PBE0, CAM-B3LYP, and LC- w PBE functionals were benchmarked with B3PW91 for describing the low-energy 1 MLCT electronic absorption envelopes of [Ru­(tpy) 2 ] 2+ (see Figure S3A1). It appeared that data collected using the B3PW91 functional are in better agreement with the experimental absorption peaks, being consistent with other early reports and our previous studies. ,,, Therefore, the B3PW91 functional was finally utilized to simulate the ground and excited states of the Ru-tpy complexes.…”

“…On the basis of the Ru-tpy complexes, it is possible that a 3 MC state exists that could cause emission quenching. We assume therefore that the unusually low 77 K quantum yield for complexes (c) and (e) that could exist as 3 MC excited states would reduce the value of the quantum yield at 77 K. This conclusion is consistent with our DFT calculations that have been discussed in detail in Section 4.1. As mentioned above, in the case of the Ru-tpy complexes, it is easier to control the probability of ic ( 3 MLCT → 3 MC) compared to the Ru-bpy complexes, a result that causes the Rutpy complex to have more flexibility for different applications.…”

“…This suggests that [Ru(tpy) 2 ] 2+ would have an unfavorable or nonobservable ic ( 3 MLCT → 3 MC b ) at 77 K. Furthermore, the 3 MC excited state for [Ru(tpy)(NH 3 ) 3 ] 2+ and [Ru(tpy)-(bpm)(NH 3 )] 2+ could not be theoretically located using the same computational procedure. This is in good agreement with the experimental data for both complexes with their oneexponential decay lifetime for the k obs relaxation at 77 K. Due to the strong ligand stabilization effect of the amino ligands, the interplay of 3 ■ ASSOCIATED CONTENT * sı Supporting Information…”

“…Consideration of the T e ( 3 MLCT Emitting State) and 3 MC (Emission Quenching State) States forObserving 77 K Low-Emission Quantum Yields. As demonstrated earlier in Figure3, the unusual 77 K low quantum yields with hν ave(em) and two exponential emission lifetimes for the [Ru(tpy)(AN) 3 ] 2+ complex suggested the presence of an accessible3 MLCT → 3 MC internal conversion process. As reported by Tsai et al,70 the elongation of the Ru−N coordination distance from the 3 MLCT (T e ) structure could lead to the formation of an 3 MC minimum structure.…”

Intrinsic 77 K Phosphorescence Characteristics and Computational Modeling of Ru(II)-(Bidentate Cyclometalated-Aromatic Ligand) Chromophores: Their Relatively Low Nonradiative Rate Constants Originating from Low Spin−Orbit Coupling Driven Vibronic Coupling Amplitudes between Emitting and Ground States

“…The E 1/2 (Ru III/II ) magnitudes of a−d in the range of 0.49−0.63 V were similar to those of A−C. 54,62 The E 1/2 (CM −1/−2 ) wave of d in this study was at −1.56 V. The E 1/2 (Ru III/II ) and E 1/2 (bpy 0/−1 ) couples of 1 and 6 are as follows:…”

Low-Temperature Observation of the Excited-State Decay of Ruthenium-(Mono-2,2′:6′,2″-Terpyridine) Ions with Innocent Ligands: DFT Modeling of an 3MLCT–3MC Intersystem Crossing Pathway

“…Additionally, the B3LYP, , PBE0, CAM-B3LYP, and LC- w PBE functionals were benchmarked with B3PW91 for describing the low-energy 1 MLCT electronic absorption envelopes of [Ru­(tpy) 2 ] 2+ (see Figure S3A1). It appeared that data collected using the B3PW91 functional are in better agreement with the experimental absorption peaks, being consistent with other early reports and our previous studies. ,,, Therefore, the B3PW91 functional was finally utilized to simulate the ground and excited states of the Ru-tpy complexes.…”

“…On the basis of the Ru-tpy complexes, it is possible that a 3 MC state exists that could cause emission quenching. We assume therefore that the unusually low 77 K quantum yield for complexes (c) and (e) that could exist as 3 MC excited states would reduce the value of the quantum yield at 77 K. This conclusion is consistent with our DFT calculations that have been discussed in detail in Section 4.1. As mentioned above, in the case of the Ru-tpy complexes, it is easier to control the probability of ic ( 3 MLCT → 3 MC) compared to the Ru-bpy complexes, a result that causes the Rutpy complex to have more flexibility for different applications.…”

“…This suggests that [Ru(tpy) 2 ] 2+ would have an unfavorable or nonobservable ic ( 3 MLCT → 3 MC b ) at 77 K. Furthermore, the 3 MC excited state for [Ru(tpy)(NH 3 ) 3 ] 2+ and [Ru(tpy)-(bpm)(NH 3 )] 2+ could not be theoretically located using the same computational procedure. This is in good agreement with the experimental data for both complexes with their oneexponential decay lifetime for the k obs relaxation at 77 K. Due to the strong ligand stabilization effect of the amino ligands, the interplay of 3 ■ ASSOCIATED CONTENT * sı Supporting Information…”

“…Consideration of the T e ( 3 MLCT Emitting State) and 3 MC (Emission Quenching State) States forObserving 77 K Low-Emission Quantum Yields. As demonstrated earlier in Figure3, the unusual 77 K low quantum yields with hν ave(em) and two exponential emission lifetimes for the [Ru(tpy)(AN) 3 ] 2+ complex suggested the presence of an accessible3 MLCT → 3 MC internal conversion process. As reported by Tsai et al,70 the elongation of the Ru−N coordination distance from the 3 MLCT (T e ) structure could lead to the formation of an 3 MC minimum structure.…”

Intrinsic 77 K Phosphorescence Characteristics and Computational Modeling of Ru(II)-(Bidentate Cyclometalated-Aromatic Ligand) Chromophores: Their Relatively Low Nonradiative Rate Constants Originating from Low Spin−Orbit Coupling Driven Vibronic Coupling Amplitudes between Emitting and Ground States

“…The E 1/2 (Ru III/II ) magnitudes of a−d in the range of 0.49−0.63 V were similar to those of A−C. 54,62 The E 1/2 (CM −1/−2 ) wave of d in this study was at −1.56 V. The E 1/2 (Ru III/II ) and E 1/2 (bpy 0/−1 ) couples of 1 and 6 are as follows:…”