Article pubs.acs.org/JPCA
Photophysics of Bis-bipyridyl Nitro Complexes of Ruthenium(II) with Pyridine Ligands: Substituent Effects Sergey V. Litke Faculty of Physics, St. Petersburg State University, St. Petersburg 198904, Russia
Aleksei Yu. Ershov* Institute of Chemistry, St. Petersburg State University, St. Petersburg 198904, Russia
Thomas J. Meyer* Department of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27599-3290, United States S Supporting Information *
ABSTRACT: Emission, excitation spectra, quantum yields, and emission lifetimes are reported for the mixed ligand, bis-(2,2′-bipyridine)ruthenium(II) complexes cis-[Ru(bpy)2(L)(NO2)]+ with 4-substituted pyridines as ligands L [isonicotinamide (isnc), 4,4′-bipyridine (bipy), pyridine (py), 4-phenylpyridine (phpy), 1,2-bis(4-pyridyl)ethane (bpa), 4-picoline (mepy), 4aminopyridine (apy), and 4-dimethylaminopyridine (ampy)] in EtOH−MeOH, 4:1 (V:V), at 77 K. Radiative, kr, and nonradiative, knr, decay rate constants were determined for the series of complexes, and a linear dependence of ln knr on E00, with E00 as the 0−0 energy gap determined by emission spectral fitting, was obtained with a slope of −(3.35 ± 0.23) × 10−4 cm−1. A linear correlation of E00 for cis-[Ru(bpy)2(L)(NO2)]+ complexes and Hammett substituent constant σp is found.
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INTRODUCTION
thermal population and decay through dd states is typically unimportant.57,58 In detailed studies on chloro complexes of the type cis[Ru(bpy)2(L)Cl]+, with pyrazine and 4-substituted derivatives of pyridine (py) as ligands (L), a linear correlation between the E00 energy of the lowest-energy 3MLCT {[dπ(Ru) → π*(bpy)]} excited states and pKa(L−H+) for the free ligands was found. In this series, complexes with L as a strong electrondonating N-donor ligand have lower E00 energies than those with π-acceptor substituents. The presence of the π-donor chloride ion in the inner coordination sphere of these complexes stabilizes the dπ(Ru)5π*(bpy) 1MLCT excited states by electron donation to the dπ(Ru)5 core decreasing the energy of dπ(Ru) → π*(bpy) transitions. Complexes of the type cis-[Ru(bpy)2(L)Cl]+ have relatively low-lying 3MLCT states. They do not emit at room temperature (RT) and undergo photosubstitution with loss of the py ligands in solution.55−57 In this study, the goal was to investigate the impact on photophysical properties of replacing the weak-field Cl− ligand by strong-field NO2− or CN− ligands in the same coordination environment.59 We report absorption, emission, emission excitation spectra, quantum yields, and emission lifetimes for the complexes cis-[Ru(bpy)2(L)(NO2)]+
Polypyridyl ruthenium(II) complexes are of great interest in the study of excited-state redox and sensitization processes due to their low-lying metal-to-ligand charge transfer (MLCT) excited states.1−17 The bis(2,2′-bipyridyl)ruthenium(II) {Ru(bpy)2}core has been used as the chromophoric unit in different polynuclear complexes and in molecular assemblies for the investigation of photoinduced electron or energy transfer.1,18−20 For ruthenium(II) complexes of the type cis[Ru(bpy)2(L)X]n+, where L and X are monodentate ligands, ground- and excited-state properties can be varied systematically to a significant degree by changing the nonchromophoric ligands,1,21−24 as well as by changing the substituents on the 2,2′-bipyridine acceptor ligand.1,25−36 Complications arise in the electronic structure of these complexes from low-lying metal-centered excited states (dd), which, when populate, lead to shortened lifetimes and ligand-loss photochemistry.37−45 The resulting photochemistry has been used in complexes of the type cis-[Ru(bpy)2(L)X]n+ in synthesis and in practical applications.24,46−48 With the goal of understanding the role of dd states in deactivation pathways for the MLCT excited states in complexes of this type, we have studied systematically the electronic structure and photochemistry of cis-[Ru(bpy)2(L)X]n+ complexes in solution.49−56 We have also investigated the photophysical properties of these complexes in EtOH−MeOH glasses at low temperatures (77 K), where complications from © 2014 American Chemical Society
Received: June 6, 2014 Revised: July 22, 2014 Published: July 24, 2014 6216
dx.doi.org/10.1021/jp505619h | J. Phys. Chem. A 2014, 118, 6216−6222
The Journal of Physical Chemistry A
Article
Table 1. Ligand Structure, Hammett σp Constants,66 and Absorption Spectral Properties of cis-[Ru(bpy)2(L)(NO2)]+ Complexes in EtOH−MeOH (4:1) at 293 K
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The estimated error in the molar extinction coefficient is ≤10%.
**
Estimated from pKa = 4.82 for 4, 4′-bipyridine.67
A Dewar vessel filled with liquid nitrogen was placed near the center of the IS and shielded by a light-diffusing screen. Three samples (pure solvent, reference, and unknown) were inserted into the Dewar and illuminated independently. An intensity of diffusely scattered light (exciting and emitting) into the IS was measured simultaneously and independently by the use of two photomultipliers with crossed optical filters. Spectral sensitivities of photomultipliers, transmission of optical filter, and luminescence spectra of reference samples were taken into account for quantum yield calculations of unknown samples. Solutions of [Ru(bpy)3]Cl2 in 4:1 EtOH−MeOH were used as a reference (φ = 0.36 at 77 K1). An average error for the emission quantum yields was ∼10−12%. Emission decays were monitored following laser flash excitation by using a Spectra-Physics Model VSL-337 nitrogen laser with a DYE-120 dye attachment. The decays were monitored through an optical filter by using a Hamamatsu R928 photomultiplier. The output was recorded on a Tektronix Model TDS301B digital oscilloscope. An averaging of multiple emission-time traces was applied for improving signal-to-noise ratios. The errors for lifetime measurements were ≤3%.
[L = isonicotinamide (isnc), 4,4′-bipyridine (bipy), py, 4phenylpyridine (phpy), 1,2-bis(4-pyridyl)ethane, 4-picoline (mepy), 4-aminopyridine (apy), and 4-dimethylaminopyridine (ampy)] in 4:1 EtOH−MeOH glasses at 77 K.
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EXPERIMENTAL SECTION Materials. The complex salts cis-[Ru(bpy)2(L)(NO2)](BF4) were prepared and purified according to literature procedures.59−63 The solvents EtOH and MeOH were purchased from Vecton and Merck, respectively. Luminescence Measurements. Solutions were deoxygenated by freeze−pump−thaw cycles until the pressure of gases evolved was
