Derivatives of the [Ru(bipy)(CN)4]2– chromophore with pendant pyridyl-based binding sites: synthesis, pH dependent-luminescence, and time-resolved infrared spectroscopic studies

Abstract

Reaction of K 4 [Ru(CN) 6 ] with 2,2Ј:4Ј,4Љ-terpyridine (L 1 ) or 2,2Ј:3Ј,2Љ:6Љ,2ٞ-quaterpyridine (L 2 ) in acidic aqueous methanol affords the complexes K 2 [Ru(L 1 )(CN) 4 ] and K 2 [Ru(L 2 )(CN) 4 ] respectively, both containing the {Ru(bipy)(CN) 4 } 2Ϫ chromophore but with pendant pyridyl or bipyridyl units, respectively. Time-resolved IR analysis of K 2 [Ru(CN) 4 (L 2 )] in MeCN-D 2 O showed that the most intense CN stretching vibration shifted to higher energy by ca. 50 cm Ϫ1 after laser excitation, consistent with formation of a Ru()/(L 2 ) ؒϪ MLCT excited state for which the lifetime measurement (38 ± 5 ns, measured by TRIR) agrees reasonably well with the value measured by luminescence methods (30 ± 2 ns). Study of the pH dependence of the absorption and emission spectra of the two complexes revealed the presence of two different effects arising from protonation of the pendant pyridyl/bipyridyl site (which occurs with pK a ≈ 3.1 in each case) and protonation of the cyanide ligands (which occurs with pK a ≈ 2 in each case). For K 2 [Ru(L 1 )(CN) 4 ], protonation of the pendant pyridyl unit results in the 1 MLCT excited state being lowered in energy by ca. 1000 cm Ϫ1 , whereas at lower pH values (2.5-1), protonation of the cyanide ligands raises the 1 MLCT excited state energy by over 2000 cm Ϫ1 . For K 2 [Ru(L 2 )(CN) 4 ] in contrast, protonation of the pendant bipyridyl unit has no detectable effect on the 1 MLCT energy, as the pendant site is electronically decoupled from the complex core by a substantial twist between the free and coordinated bipy components of L 2 ; protonation of the cyanides at lower pH values however destabilises the 1 MLCT excited state. Protonation of the pendant pyridyl sites results in complete (for [Ru(L 1 )(CN) 4 ] 2Ϫ ) or near-complete (for [Ru(L 2 )(CN) 4 ] 2Ϫ ) quenching of the luminescence; possible reasons for this behaviour are discussed. The crystal structures of the two related complexes [Ru( t Bu 2 bipy) 2 (L 1 )][PF 6 ] 2 and [Cl 2 Pt(µ-L 2 )Ru(bipy) 2 ][PF 6 ] 2 are also described to illustrate arguments about the conformations of L 1 and L 2 . DALTON

Photophysical and Structural Properties of Cyanoruthenate Complexes of Hexaazatriphenylene

“…Figure a shows the transient IR difference spectra in the ν(CN) region of Na 2 [ 1 ] in D 2 O with different time delays between pump and probe. The results are in agreement with what we have observed for other mononuclear complexes of the [Ru(diimine)(CN) 4 ] 2- type. ,, The strongest ground-state ν(CN) band at ca. 2066 cm -1 is bleached, and a weak transient band to higher energy (ca.…”

Visible-Light-Driven Photoreactions of [(bpy)2Ru(II)L]Cl2 in Aqueous Solutions (bpy = Bipyridine, L = 1,2-Bis(4-(4‘-methyl)-2,2‘-bipyridyl) Ethene)

“…Addition of the proton is expected to occur at the position with highest electron density of radical anion, i.e., the nitrogen atoms. Quenching of the excited state by protonation has been observed for Ru polypyridyl complexes containing ligands with protonable sites. ,, The product I formed in reaction 2 as a result of protonation and electron localization is a radical with structure shown in Scheme (I). The Stern−Volmer plot in Figure b indicates that emission quenching is indeed occurring with pH, but the nonlinear nature of the plot suggests that other reactions are also occurring.…”

“…However, their photochemical properties in aqueous solutions have not been explored. Another important feature of Ru complexes containing protonable ligands is their acid−base reactions in the excited state. − Charge localization on the ligand in the MLCT state can lead to pH-dependent luminescence properties.…”

Quaterpyridines as Scaffolds for Functional Metallosupramolecular Materials

“…An additional feature that makes some of these systems highly appropriate for further studies regarding the effective tuning of their photooptical properties is that of solvatochromism, and in this respect, the [Ru­(CN) 4 ] 2– molecular building block still represents a relatively unexplored field with respect to quaterpyridines. , …”