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

Abstract

Efficient charge-transfer (CT) phosphorescence in the near-IR (NIR) spectral region is reported for four substituted Ru-(R-dipyrrinato) complexes, [Ru(bpy) 2 (R-dipy)](PF 6 ), where bpy is 2,2′-bipyridine and the substituent R is phenyl (ph), 2,4,6trimethylphenyl, 4-carboxyphenyl (HOOC-ph), or 4-pyridinyl. The experimentally determined phosphorescence efficiency, ι em(p) = k RAD(p) /(ν em(p) ) 3 (where k RAD(p) and ν em(p) are the phosphorescence rate constant and the phosphorescence frequency, respectively), of the [Ru(bpy) 2 (R-dipy)] + complexes was approximately double that of [Ru(bpy)(Am) 4 ] 2+ complexes (Am = ammine ligand) in the NIR region. Density functional theory (DFT) modeling indicated two strikingly different electronic configurations of the triplet emitting state (T e ) in the two types of complexes. The T e of [Ru(bpy) 2 (R-dipy)] + complexes shows a CT-type corresponding to the metal-to-ligand charge transfer (MLCT)-(Ru-(R-dipy)) and the ππ*-(R-dipy) moiety configurations, and the T e state in the [Ru(bpy)(Am) 4 ] 2+ complexes corresponds to an approximately MLCT excited state consisting of mostly MLCT-(Ru-bpy) with a minimal ππ*(bpy) contribution. DFT modeling also indicated that the low-energy singlet excited states in the T e geometry (S n(T) ) of the [Ru(bpy) 2 (ph-dipy)] + complex consist of numerous CT-S n(T) -type states of the Ru-dipy and Ru-bpy moieties, whereas the [Ru(bpy)(Am) 4 ] 2+ ions show quite simple MLCT-S n(T) -type states of the Ru-bpy moiety. Based on experimental observations, DFT modeling, and the plain spin−orbit coupling (SOC) principle, we conclude that the remarkably high ι em(p) amplitudes of the [Ru(bpy) 2 (R-dipy)] + complexes relative to those of [Ru(bpy)(Am) 4 ] 2+ complexes can be attributed to the relatively substantial contribution of intrinsic SOC-mediated intensity stealing from the numerous low-energy CT-type S n(T) states.

Increasing the cytotoxicity of Ru(ii) polypyridyl complexes by tuning the electron-donating ability of 1,10-phenanthroline ligands

“…Moreover, the labile Cl 4h. [62][63][64] This phenomenon also was corroborated by the fact that the novel fluorescence emission signal displayed bathochromic shifts from complex 1 to 4 with emission peaks at 709, 723, 724, and 744 nm, respectively, in agreement with the electron-donating abilities as the discussion above for the DFT calculations part. Additionally, there was a saturation effect for the Ru(II)-DNA complexes because the decreasing intensities of the emission peaks were not in a linear relationship with the increase in concentrations of Ru(II) complexes.…”

High Intrinsic Phosphorescence Efficiency and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with π-Aromatic-Rich Cyclometalated Ligands: Attributions of Spin–Orbit Coupling Perturbation and Efficient Configurational Mixing of Singlet Excited States

“…Previous work concerning the DFT modeling of the emitting 3 MLCT (T e ) states for typical Ru-bpy complexes predominantly involved MLCT-(Ru-bpy) and some ππ*-bpy components for [Ru­(bpy)­(NH 3 ) 4 ] 2+ , ,,, [Ru­(bpy)­(en) 2 ] 2+ , and [Ru­(bpy) 2 (en)] 2+ , complexes. The selected low-energy NTO plots of the triplet excited states (T n ) with excited-state energies at the T e geometry for the target [Ru­(bpy) 2 (CM)] + ions are shown in Figures S3C1C (page S34), S3C2C (page S41), and S3C3C (page S50).…”

“…A possible solution to this issue is to reconsider the perturbation principle of molecular orbitals and SOC fundamentals. The NTO plots of the low-energy S n (T) states for the [Ru­(bpy) 2 (CM)] + ions (see Figures and ) and [Ru­(bpy)­(NH 3 ) 4 ] 2+ , ,,, [Ru­(bpy)­(en) 2 ] 2+ , and [Ru­(bpy) 2 (en)] 2+ , complexes show a significant contribution from the π-ligand component in the donor SOMOs related to the configurations between MLCT and ππ*-ligand states, suggesting that the efficient ∑ f EW(S n (T) ) in eq should include the influence of the perturbation of the configuration of low-energy 1 MLCT and high-energy 1 ππ states.…”

“…Fundamentally, the ι em(p) amplitude is dependent on the efficiency of the vertical SOC configurations between the T e and the intense low-energy S n states in the T e geometry (S n (T) ), and subsequent NTO analyses are based on this logical notion. The plots of the NTOs for vertical S n (T) transitions (in the T e geometry) of the [Ru(bpy)(NH 3 ) 4 ] 2+ , 73 [Ru(bpy)(en) 2 ] 2+ , 107 [Ru(bpy) 2 (en)] 2+ , 73 , 107 and [Ru(bpy) 2 (ppy)] +73 complexes were reported in the previous studies, and the orbital compositions of the NTOs of the T e and low-energy S n (T) states of [Ru(bpy) 2 (en)] 2+ , [Ru(bpy)(en) 2 ] 2+ , and [Ru(bpy)(NH 3 ) 4 ] 2+ are shown in Table S5C (page S75) .…”

“…The DFT calculations performed in the present study used a combination of the B3PW91 functional − and the SDDall − basis set. We used several functionals to calculate the low-energy MLCT electronic absorption envelopes for [Ru­(bpy) 2 (ppy)] + , and we ultimately selected the B3PW91 functional for use in the present study; this method has been widely used to calculate absorption spectra of Ru-bpy chromophores that are analogous to the present systems. ,,,− DFT geometry optimizations for the S 0 and T 1 (T e ) states and time-dependent DFT (TDDFT) calculations for the electronic absorption spectra were performed in an acetonitrile solution simulated by the integral equation formalism polarizable continuum model (IEF-PCM) solvation model. − Harmonic vibrational frequency analyses were carried out to confirm that all of the optimized structures are minima on the potential energy surface. The electronic structures of the transitions were analyzed using the natural transition orbital (NTO) approach. , All calculations were performed using the Gaussian 09 program…”