Effect of Ligands with Extended π-System on the Photophysical Properties of Ru(II)Complexes†

Yujie Sun,† Maya El Ojaimi,‡ Richard Hammitt,‡ Randolph P. Thummel,‡,* andClaudia Turro†,*Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210 and Department of Chemistry,UniVersity of Houston, Houston, Texas 77204

ReceiVed: March 23, 2010; ReVised Manuscript ReceiVed: May 10, 2010

Density functional theory calculations were performed on a series of six ruthenium complexes possessingtridentate ligands: [Ru(tpy)2]2+ (1; tpy ) [2,2′;6′,2′′]-terpyridine), [Ru(tpy)(pydppx)]2+ (2; pydppx ) 3-(pyrid-2′-yl)-11,12-dimethyldipyrido[3,2-a: 2′,3′-c]phenazine), [Ru(pydppx)2]2+ (3), [Ru(tpy)(pydppn)]2+ (4; pydppn) 3-(pyrid-2′-yl)-4,5,9,16-tetraazadibenzo[a,c]naphthacene), [Ru(pydppn)2]2+ (5), and [Ru(tpy)(pydbn)]+ (6;pyHdbn ) 3-pyrid-2′-yl-4,9,16-triazadibenzo[a,c]naphthacene). The calculations were compared to experimentaldata, including electrochemistry and electronic absorption spectra. The theoretical results reveal that the lowest-lying singlet and triplet states in 4 and 5 are pydppn-based ππ* in character, which are remarkably differentfrom the lowest-lying metal-to-ligand charge transfer (MLCT) states in 1-3. The calculated lowest tripletstates in 4 and 5 are consistent with the 3ππ* states observed experimentally. However, although the extendedπ-system of pydbn- is similar to that of pydppn, the HOMO of 6 lies above those of 4 and 5, resulting instrikingly different spectroscopic properties. Calculations show that the lowest triplet excited state of 6 is acombination of 3MLCT and 3ππ*. This work demonstrates that the electronic structure of the tridentate ligandhas a pronounced effect on the photophysical properties of ruthenium(II) complexes and that DFT and TD-DFT methods are a useful tool that can be used to predict photophysical and redox properties of transitionmetal complexes.

Introduction

Molecular assemblies derived from ruthenium polypyridylcomplexes continue to attract intense interest in the fields ofdye-sensitized solar cells,1,2 artificial photosynthesis, andphotocatalysis,3,4 as well as potential agents in biomedicaldiagnostics and as pharmaceuticals.5,6 Their versatile useoriginates from their favorable electrochemical and photophysi-cal properties, along with the ease of structural modification.7

A considerable amount of work has focused on rutheniumcomplexes with bidentate ligands related to 2,2′-bipyridine (bpy),including the prototype complex [Ru(bpy)3]2+, which exhibitsa high emission quantum yield from the lowest-energy tripletmetal-to-ligand charge transfer (3MLCT) excited state and a longlifetime at ambient temperature.7

In contrast to the highly emissive [Ru(bpy)3]2+ (Φ ) 0.062and τ ) 820 ns in CH3CN at 298 K), [Ru(tpy)2]2+ (1; tpy )[2,2′;6′,2′′]-terpyridine, Figure 1a) exhibits very weak emission(Φ < 5 × 10-6) and a short-lived excited state (τ ) 120 ps) inCH3CN at room temperature, making it less useful for photo-chemical applications.8-10 However, ruthenium complexes withtpy-type tridentate ligands have been generating increasinginterest owing to the ease of substitution at the 4′ position ofthe central pyridine ring of the ligand, making it possible toconstruct linear supramolecular assemblies.8-10

It is well known that metal-centered ligand-field (LF) statesclose in energy to the lowest 3MLCT state in 1 result in the

efficient deactivation of the latter by the former, leading to theshort excited state lifetime and weak emission of the complex.7-10

Structural modifications, including substitution of the tpy ligandand addition of polymethylene bridges between the pyridinerings to increase the bite angle have been proposed to increasethe energy gap between the LF and 3MLCT states in thesecomplexes, thus extending the 3MLCT lifetime.8-11 In general,research involving complexes related to 1 has focused onincreasing the LF-3MLCT gap to extend the excited statelifetime.8-11 In the present work, ligands with extended π-sys-tems were designed to lower the energy of the ligand-centered(LC) 3ππ* state below that of the 3MLCT state.

The photophysical properties of a series of four newruthenium complexes, [Ru(tpy)(pydppx)]2+ (2; pydppx ) 3-(pyrid-2′-yl)-11,12-dimethyldipyrido[3,2-a:2′,3′-c]phena-zine), [Ru(pydppx)2]2+ (3), [Ru(tpy)(pydppn)]2+ (4; pydppn) 3-(pyrid-2′-yl)-4,5,9,16-tetraazadibenzo[a,c]naphthacene),and [Ru(pydppn)2]2+ (5) were recently reported by us,6b andthe molecular structures of the ligands are shown in Figure1a. Complexes 2 and 3 are weakly emissive in CH3CN at298 K, similar to [Ru(tpy)n(pydppz)2-n]2+ (n ) 0, 1; pydppz)3-(pyrid-2′-yl)dipyrido[3,2-a:2′,3′-c]phenazine, Figure 1a).It should be noted that the luminescence quantum yields ofthe pydppz and pydppx complexes are greater than that of 1and exhibit longer 3MLCT lifetimes (τ ) 1-4 ns).12 Incontrast, the excited state lifetimes of 4 and 5 are dramaticallylonger (τ ) 20-25 µs).6b Comparison of the transientabsorption spectra of 4 and 5 to that of the free pydppn ligandindicates that the lowest excited states in these complexesare pydppn-based 3ππ*, similar to that of the related complex[Ru(bpy)2(dppn)]2+ (dppn ) benzo[i]dipyrido[3,2-a:2′,3′-

† Part of the “Michael R. Wasielewski Festschrift”.* To whom correspondence should be addressed. E-mails: (C.T.)

chemistry.ohio-state.edu, (R.P.T.) thummel@uh.edu.† The Ohio State University.‡ University of Houston.

J. Phys. Chem. B 2010, 114, 14664–1467014664

10.1021/jp102613n 2010 American Chemical SocietyPublished on Web 06/14/2010

c]phenazine).13 These long-lived 3ππ* states are able tophotosensitize 1O2 with near unit quantum yield in 4 and 5,resulting in efficient DNA photocleavage under irradiationwith visible light.6,12

Theoretical work on ruthenium complexes with bidentateligands is plentiful in the literature,14-19 however, calculationson complexes with tridentate ligands related to tpy are scarce.20,21

The present work utilizes density functional theory (DFT) toexplore the electronic structures of the ground and excited statesof 1-5 and a cyclometalated complex, [Ru(tpy)(pydbn)]+ (6),in which pydbn- represents the deprotonated form of pyHdbn(pyHdbn ) 3-pyrid-2′-yl-4,9,16-triazadibenzo[a,c]naphtha-cene, Figure 1a). The calculated results for 1-6 are comparedwith experimental data, including those from electrochemistry,steady-state spectroscopy, and transient absorption. Particularly,the lowest energy triplet states of 4-6 are discussed in detail.

Experimental Section

Materials. The synthesis and characterization of complexes1-5 were previously reported.6b All reagents and solvents werepurchased from commercial sources and were used as received.2-(Pyrid-2′-yl)benzo[h]quinoline was prepared according to areported procedure.22

2-(Pyrid-2′-yl)benzo[h]quinoline-5,6-dione. To a suspensionof 2-(pyrid-2′-yl)benzo[h]quinoline (80 mg, 0.31 mmol) in aceticacid (2 mL) was added I2O5 (110 mg, 0.33 mmol). The mixturewas heated at 120 °C for 2 h, and the reddish solution wasallowed to cool to room temperature. An aqueous solution ofNa2S2O3 (10%, 20 mL) and EtOAc (20 mL) were added, andthe residue was stirred at room temperature for 1.5 h. Theorganic phase was separated and washed with water and brine,dried over MgSO4 and concentrated to generate the product asyellow flakes (56 mg, 62%). 1H NMR (DMSO-d6): δ 8.84 (d,1H, J ) 7.45 Hz), 8.75 (dd, 1H, J ) 4.45, 1.75 Hz), 8.72 (d,1H, J ) 7.4 Hz), 8.47 (dd, 2H, J ) 7.85 Hz), 8.05 (m, 2H),7.88 (td, 2H, J ) 7.4, 1.15 Hz), 7.67 (td, 1H, J ) 7.45 Hz,1.15 Hz), 7.56 (ddd, 1H, J ) 7.7, 4.6, 1.15 Hz); IR 1678 cm-1.

3-Pyrid-2′-yl-4,9,16-triazadibenzo[a,c]naphthacene (pyH-dbn). A suspension of 2,3-diaminonaphthalene (198 mg, 1.25mmol) and 2-(pyrid-2′-yl)benzo[h]quinoline-5,6-dione (88 mg,0.307 mmol) in ethanol (20 mL) was heated at reflux for 2 h.The residue was allowed to cool to room temperature, and the

precipitate was collected, washed with EtOH and acetone, anddried under vacuum to provide pyHdbn as a yellow solid (105mg, 84%).

[Ru(tpy)(pydbn)](PF6). A suspension of pyHdbn (105 mg,0.257 mmol) and Ru(tpy)Cl3 (113 mg, 0.257 mmol) in EtOH/water (3:1, 48 mL) was stirred at reflux under Ar for 22 h. Themixture was concentrated and was added dropwisely to asolution of NH4PF6 (300 mg, 1.80 mmol) in water (7 mL). Theprecipitate was filtered and washed with water. The solid waspurified by chromatography on alumina, eluting with CH2Cl2/acetone (1:1). The crude product was dissolved in a minimumamount of acetone and precipitated with ether (10 mL). Theprecipitate was collected and washed with ether, providing[Ru(tpy)(pydbn)](PF6) (112 mg, 49%) as a purple solid. 1HNMR (acetone-d6): δ 9.51 (d, 1H, J ) 8.6 Hz), 9.08 (s, 1H),9.02 (d, 1H, J ) 8.5 Hz), 8.95 (s, 1H), 8.92 (d, 2H, J ) 8.0Hz), 8.90 (d, 1H, J ) 8.2 Hz), 8.68 (d, 2H, J ) 8.0 Hz), 8.42(d, 2H, J ) 8.0, 1.15 Hz), 8.36 (m, 1H), 8.31 (m, 1H), 8.25 (t,1H, J ) 8.0 Hz), 8.08 (td, 1H, J ) 7.45, 1.7 Hz), 7.86 (d, 1H,J ) 6.3 Hz), 7.83 (t, 2H, J ) 8.0 Hz), 7.70 (m, 2H), 7.66 (d,2H, J ) 5.7 Hz), 7.30 (dd, 1H, J ) 5.7, 1.1 Hz), 7.08 (m, 2H),6.99 (t, 1H, J ) 7.45 Hz), 6.25 (dd, 1H, J ) 6.3, 1.1 Hz); MS(MALDI-TOF): m/z 742.15 [M - PF6]+.

X-ray Determination of [Ru(tpy)(pydbn)](PF6). All mea-surements were made with a Siemens SMART platformdiffractometer equipped with a 4K CCD APEX II detector. Ahemisphere of data (1271 frames at 6 cm detector distance) wascollected using a narrow-frame algorithm with scan widths of0.30% in omega and an exposure time of 40 s/frame. The datawere integrated using the Bruker-Nonius SAINT program, withthe intensities corrected for Lorentz factor, polarization, airabsorption, and absorption due to variation in the path lengththrough the detector faceplate. A psi scan absorption correctionwas applied on the basis of the entire data set. Redundantreflections were averaged. Final cell constants were refined using6162 reflections having I > 10σ(I), and these, along with otherinformation pertinent to data collection and refinement, are listedin Table S1 of the Supporting Information. The Laue symmetrywas determined to be -1, and the space group was shown tobe either P1 or P-1. The asymmetric unit consists of one cation,one anion, and one molecule of methylene chloride solvent, allin general positions. The anion was found to be disordered 70:

Figure 1. (a) Molecular structures of ligands and (b) ORTEP plot of the cation [Ru(tpy)(pydbn)]+.

Photophysical Properties of Ru(II) Complexes J. Phys. Chem. B, Vol. 114, No. 45, 2010 14665

30 over two different orientations, and this was treated byrefinement of ideal rigid bodies with occupancy factors estimatedby comparison of the isotropic displacement parameters. AnORTEP plot of the cation is shown in Figure 1b, and selectedbond lengths, bond angles, and torsion angles are collected inTable S2 of the Supporting Information.

Instrumentation. The 1H NMR spectra were recorded atroom temperature on a JEOL ECX-400 spectrometer at 400MHz. Chemical shifts are referenced to the residual solvent peakand are reported in parts per million (ppm) with J values in(0.5 Hz. Infrared spectra were measured on a Thermo-Nicolet370 FT-IR spectrometer. Mass spectra were obtained on aVoyager-DE-STR MALDI-TOF mass spectrometer. Electro-chemical studies were carried out on a CV-50W voltammetricanalyzer in a three-electrode cell with a glassy carbon workingelectrode, a Pt wire auxiliary electrode, and a saturated calomelelectrode (SCE) as the reference electrode; the latter wasseparated from the bulk solution by a glass frit. The measure-ments were conducted at a scan rate of 100 mV/s in deoxy-genated CH3CN or CH2Cl2 containing 0.1 M tetra-n-butylam-monium hexafluorophosphate as the supporting electrolyte. Theoxidation potential of ferrocene (0.42 V vs SCE) was measuredunder identical conditions and was used as the internal reference.Steady-state absorption spectra were recorded on a HP diodearray spectrometer (HP8453) with HP8453 Win System soft-ware. Corrected steady-state emission spectra were measuredon an SPEX Fluoromax-2 spectrometer with a 90° opticalgeometry equipped with a 150 W Xe arc lamp as the source.Transient absorption spectra were collected using a home-builtinstrument described in detail previously.23

Computational Methods. All calculations were performedwith the Gaussian 0324 program package employing the DFTmethod with Becke’s three-parameter hybrid functional andLee-Yang-Parr’s gradient corrected correlation functional(B3LYP).25 The Stuttgart/Dresden (SDD) basis set and effectivecore potential were used for the Ru atom,26 and the 6-31G*basis set was applied for H, C, and N.27 The geometries of thesinglet ground states and the lowest energy triplet states of 1-6were optimized in CH3CN using the conductive polarizablecontinuum model (CPCM). The local minimum on eachpotential energy surface was confirmed by frequency analysis.Time-dependent DFT calculations produced the singlet andtriplet excited states of each complex starting from the optimizedgeometry of the corresponding singlet ground state, using theCPCM method with CH3CN as the solvent. The dipole momentof each complex was calculated using the correspondingoptimized geometries of the ground states and the lowest tripletstates. The calculated absorption spectra, electronic transitioncontributions, and electron density difference maps were gener-ated by GaussSum 1.0.28

Results and Discussion

Ground State Geometry, Orbital Analysis, and Electro-chemistry. Figure 1b displays the crystal structure of 6, andvarious crystallographic parameters are listed in Tables S1 andS2. A comparison of the experimental crystal structure and thatfrom the calculated minimized ground state of 6 is provided inTable S3, highlighting the similarities in the bond lengths andangles between the two methods. Table S4 lists selectedgeometric features of the optimized singlet ground state of 1,which are consistent with the reported crystal structure and theresults of other theoretical calculations.20,21,29 The calculatedgeometric parameters for 2-5 are also included in the Sup-porting Information, which parallel those of 1 and 6 (Table S5).

The calculated energy diagram of the frontier molecularorbitals of 1-6 is shown in Figure 2. Complexes 1-3 werereported to display an oxidation potential in the range of +1.31to +1.38 V vs SCE, assigned to the Ru(III/II) couple.6b Thesevalues are similar to those of the related [Ru(tpy)n(pydppz)2-n]2+

(n ) 0, 1) complexes.12 Table S6 shows that the HOMO,HOMO-1, and HOMO-2 of 1-3 are localized on the Ru(d)orbitals, similar to those of [Ru(tpy)n(pydppz)2-n]2+ (n ) 0, 1).12

With the further extended π-delocalization of the pydppn ligand,4 exhibits a mainly pydppn-centered HOMO with a smallcontribution from the Ru(d) orbitals, which lies very close inenergy (0.16 eV) to the Ru(d)-centered HOMO-1. Similarly,the Ru(d)-based HOMO-2 in 5 also lies at slightly lower energy(0.30 eV) than its degenerate HOMO and HOMO-1, which areprimarily localized on pydppn with a small contribution fromthe Ru(d) orbitals. Selected occupied molecular orbitals of 4-6are shown in Figure 3.

The cyclic and square-wave voltammograms of the freepydppn ligand in deoxygenated CH2Cl2 show an irreversibleoxidation wave at +1.57 V vs SCE (Figure S2). This oxidationpotential is significantly lower than those of other ligands,showing that pydppn is easier to oxidize than pydppz andpydppx.6,7,12 It is necessary to mention that DFT calculationspreviously predicted a dppn-based (dppn ) 4,5,9,16-tetraazadi-benzo[a,c]naphthacene) HOMO for dppn-containing Ru(II)complexes using DFT methods, in which the dppn ligandpossesses the same extended π-system as pydppn.13,30 It wasalso reported that the dppn ligand in [Ru(bpy)2(dppn)]2+ canbe oxidized by O2 under photolysis conditions, while theoxidation state of the Ru(II) center remained unchanged.31

Substitution of pydppn with the cyclometalating pydbn-

ligand in 6 alters the electron density and energy of themolecular orbitals significantly. The HOMO of 6 is primarilylocalized on the Ru(d) orbitals, but with a large contributionfrom pydbn- (Figure 3). Furthermore, the HOMO of 6 lies athigher energy than those of 1-5 by 0.70-0.98 eV. Indeed, asshown in Figure S3, the cyclic and square-wave voltammogramsof 6 in CH3CN exhibit a reversible oxidation peak at +0.67 Vvs SCE attributed to the Ru(III/II) couple, shifted to morenegative potentials as compared with those of 1-5 (0.64-0.74V). This shift matches the calculations, and the value isconsistent with reported oxidation potentials of Ru(II) coordi-nated by a cyclometalating ligand.32 The HOMO-1 and HO-MO-2 of 6 are also primarily Ru(d) in character, with somecontribution from the ligands (Table S6). It should be noted,however, that the HOMO-3 of 6 is centered mainly on the distal

Figure 2. MO diagrams comparing the relative energies of the frontierorbitals of 1-6 (red indicates Ru(d) orbital and blue indicates LUMO).

14666 J. Phys. Chem. B, Vol. 114, No. 45, 2010 Sun et al.

portion of the pydbn- ligand, with only a small contributionfrom the Ru(d) orbitals. This orbital lies at a slightly higherenergy than the HOMOs of 1-5 (Figure 2).

As expected, the calculated LUMOs of 1-6 are localizedon the ligands (Table S6), typical of ruthenium polypyridylcomplexes.7 The LUMOs of the heteroleptic complexes 2,4, and 6 are localized on the ligand with the extendedπ-system, pydppx, pydppn, and pydbn-, respectively, whereasthose of the homoleptic complexes 1, 3, and 5 have electrondensity distributed over the two ligands. The LUMOs of 4and 5 exhibit greater electron density on the portion ofpydppn distal to the metal, whereas those of 2 and 3 show arelatively high density on the proximal part of the pydppxligand (Table S6). The differences in the energies of theLUMOs of 2-6 relative to that of 1 correspond well to thedifference in the experimental first reduction potentials, E1/2

red

(Table 1). The E1/2red value measured for 1, -1.21 V vs SCE,

shifts to -0.99 and -0.96 V vs SCE in 2 and 3, respectively,both of which possess the pydppx ligand with an extendedaromatic structure. The calculated LUMOs of 2 and 3 lie0.38 eV below that of 1, which qualitatively matches theexperimental trend in electrochemical shift (Table 1). Thelargest aromatic ligand of the series, pydppn in 4 and 5,results in E1/2

red values of -0.70 and -0.66 V vs SCE, at 0.51and 0.55 V more positive potentials than that of 1, respec-tively. The energy differences of the calculated LUMOs of4 and 5 compared with that of 1, 0.51 and 0.62 eV, are alsoconsistent with the experimental differences (Table 1). Asshown in Figure S3, three reversible reduction potentials at-0.96, -1.43, and -1.75 V vs SCE were recorded for 6 inCH3CN. The calculated gap between the LUMOs of 1 and 6is 0.21 eV, consistent with the 0.26 V difference betweenthe first reduction potentials of these complexes. Complex 6is easier to oxidize than 1-5; this shift can be attributed tothe additional electron density on the metal center uponcyclometalation by pydbn-.32 Selected MOs of 1-6 areshown in the Supporting Information (Table S6).

Singlet Excited States and Absorption Spectra. TD-DFTcalculations were performed starting from the optimizedgeometries of the ground states of 1-6 in CH3CN, resultingin the vertical singlet electronic transitions (λ > 300 nm andf > 0.01) listed in Tables S7-S12. A comparison betweenthe calculated singlet electronic transitions and the experi-mental absorption spectra of 1-3 shows a qualitative matchbetween them, with MLCT bands predicted in the 400-500nm range.6b The intense peaks of 1-3 in the ultraviolet (uv)region can be assigned as predominantly ligand-centered, inagreement with the known spectral profiles of tpy andpydppx.6b

Different from 1-3, the lowest singlet electronic transitionsof 4 and 5 are pydppn-based 1ππ* at 545 nm (f ) 0.0317)and 552 nm (f ) 0.0585), respectively (Tables S10 and S11).In the 450-500 nm range, the calculated results of 4 and 5reproduce the broad MLCT band, showing strong transitionsat 446 nm (f ) 0.2295) and 448 nm (f ) 0.4984),respectively, blue-shifted ∼30 nm as compared with theexperimental maxima at 473 and 479 nm.6b Overestimationof the absorption maxima of ruthenium polypyridyl com-plexes of similar magnitude with the TD-DFT method hasbeen previously reported.33 However, the pydppn-based 1ππ*transitions at 432 nm (f ) 0.0643) and 402 nm (f ) 0.0001)are comparable to the LC band at 414 nm in the experimentalabsorption spectrum of 4.6b Similarly, an intense pydppn-based transition is predicted for 5 at 411 nm (f ) 0.2048),corresponding to the absorption feature of pydppn at 415nm.6b In the ultraviolet region (300-400 nm), the calculatedabsorption spectra of 4 and 5 also reflect the experimentalspectra, including intensities, where the pydppn-centered1ππ* transitions are more intense in 5 than in 4 (Tables S10and S11).6b The calculated and experimental absorptionspectra of 4 are compared in Figure 4a.

Due to the introduction of the cyclometalating ligandpydbn-, the experimental and calculated absorption spectraof 6 are dramatically different from those of 1-5. Theabsorption spectrum of 6 in CH3CN shows a broad band witha maximum at 499 nm (ε ) 14.3 × 103 M-1 cm-1) and ashoulder at 577 nm (ε ) 9.6 × 103 M-1cm-1) (Figure 4b),which is qualitatively reproduced by the calculations (TableS12 and Figure 4b). The calculated singlet excited statesexhibit several transitions at 660 nm (f ) 0.0609), 496 nm(f ) 0.0635), and 470 nm (f ) 0.2819), with the lowest-energy peak blue-shifted by 29 nm relative to the experi-mental absorption maximum (499 nm), similar to those of4-5. The lowest-energy MLCT state corresponds to atransition where the electron is localized on pydbn-. At higherenergy, several LC bands are observed for 6 at 419 nm (ε )17.7 × 103 M-1 cm-1), 396 nm (ε ) 21.5 × 103 M-1 cm-1),

Figure 3. Selected occupied molecular orbitals (a) 4, (b) 5, and (c) 6 (isovalue ) 0.04).

TABLE 1: First Reduction Potentials and CalculatedLUMO Energies of 1-6

complexes E1/2red/Va ∆Eexp/Vb ELUMO/eVc ∆ELUMO/eVd

1 -1.21 -2.592 -0.99 0.22 -2.97 0.383 -0.96 0.25 -2.97 0.384 -0.70 0.51 -3.10 0.515 -0.66 0.55 -3.21 0.626 -0.95e 0.26 -2.80 0.21

a The first reduction potential of 1-5 from ref 6d; in CH3CN vsSCE. b Difference of the first reduction potentials (E1/2

red) of 2-6relative to that of 1. c Calculated LUMO energy. d ∆ELUMO )ELUMO(1) - ELUMO(2-6). e This work, in CH3CN vs SCE.

Photophysical Properties of Ru(II) Complexes J. Phys. Chem. B, Vol. 114, No. 45, 2010 14667

347 nm (ε ) 51.3 × 103 M-1 cm-1), 313 nm (ε ) 65.3 ×103 M-1 cm-1), and 298 nm (ε ) 60.4 × 103 M-1 cm-1),which are also qualitatively reflected by the calculated singlettransitions.

Triplet States. To further elucidate the identity of thelowest excited states of 4 and 5, the lowest-energy tripletstate and higher-energy triplet excited states were alsocalculated using TD-DFT. As shown in Table 2, the lowest

triplet excited state of 4 in CH3CN, T1, corresponds to aHOMO f LUMO vertical transition at 888 nm (1.40 eV),consistent with the estimated 3ππ* energy of ∼1.50 eV fromthe transient absorption quenching experiments with variousorganic chromophores.6b The electron density difference map(EDDM) of the lowest triplet excited state of 4 clearlyillustrates the pydppn-localized 3ππ*, showing the promotionof the electron density from the terminal naphthalene portionof the ligand to the region of pydppn proximal to the metal(Figure 5). As expected and also listed in Table 2, 5 exhibitsbehavior similar to that of 4, with two nearly degeneratecalculated pydppn-based 3ππ* transitions at 896 nm (1.38eV).6b The EDDMs of these two states of 5 are also includedin Figure 5. In addition, transient absorption spectra of 4and 5 have been reported and compared with that of the freeligand pydppn, from which lifetimes of 20 and 24 µs wereobtained for 4 and 5, respectively.6b These values aresignificantly longer than those of 2 and 3 (1-4 ns) whoselowest triplet excited states are 3MLCT in character.6b

Investigation of the calculated geometries of the lowestenergy singlet (S) and triplet (T) states of 2-5 can also beused to gain insight into the identity of the lowest energytriplet state. As listed in Table S5, the calculated Ru-N bondlengths between the optimized geometries of the S and Tstates of 2 display a relatively large deviation, with the largestincrease of Ru-N2 bond length from 2.011 Å (S) to 2.050Å (T). However, only a small change is calculated for S andT of 4 and 5, with differences in Ru-N bond lengths of<0.005 Å. It is evident from Table S5 that the change in thebond angles (N-Ru-N′) between the S and T states is alsogreater for 2 and 3 (0.2-3.2°) than for 4 and 5 (<0.2°). Thedifferences in the geometry of the optimized S and T statessurrounding the Ru(II) coordination sphere further supportsthe assignment of the lowest energy triplet state of 2 and 3as 3MLCT, whereas in the LC 3ππ* states of 4 and 5, theruthenium center experiences little geometrical distortion.20

As listed in Table S13, the dipole moment of 4 decreasesfrom S to T by -6.72 D, also supporting the assignment of apydppn-based 3ππ* lowest-energy triplet excited state in thecomplex, since a 3MLCT excited state would result in anincrease in the dipole moment relative to the ground state.Similarly, a decrease in the dipole moment from S to T wasalso calculated for 5 (Table S13). In contrast, an increase indipole moment from S to T was calculated for 1-3 (+2.80 to+6.87 D), consistent with the 3MLCT character of the lowesttriplet excited states in these complexes (Table S13).

Figure 4. Absorption spectra and calculated singlet electronic transi-tions of (a) 4 and (b) 6 in CH3CN.

TABLE 2: Orbital Contributions of the Lowest TripletExcited States of 4-6 in CH3CNa

complex λcalc/nm major transitions (contribution)

4 888.2H f L (0.770), H f L+1 (-0.131),

H f L+5 (-0.144), H f L+9 (0.131)5 896.5 H-1 f L (0.267), H-1 f L+1 (0.480),

H f L (0.485), H f L+1 (0.242)896.0 H-1 f L (0.480), H-1 f L+1 (-0.250),

H f L (-0.259), H f L+1 (0.485)6 840.9 H-3 f L (0.702), H-2 f L (0.278)b

a H ) HOMO, L ) LUMO. b Additional transitions H-9 f L(0.10794), H-3 f L+2 (-0.18208), H-3 f L+4 (-0.10608), H-3f L+9 (0.11050), H-2 f L+2 (-0.12265), H f L (0.15933).

Figure 5. Electron density difference maps (EDDMs) of the lowest triplet excited states of 4-6 (yellow indicates an increase in electron densityand red indicates a decrease, isovalue ) 0.001).

14668 J. Phys. Chem. B, Vol. 114, No. 45, 2010 Sun et al.

Unlike the weak 3MLCT emission in 1-3 and long-lived3ππ* states in 4-5, no luminescence or long-lived LC statewas observed for 6 at room temperature in CH3CN. Asmentioned above, the lowest singlet excited state of 6 iscalculated to be 1MLCT, whereas its lowest triplet excited stateat 840.9 nm (1.47 eV) exhibits contributions from both 3MLCTand 3ππ* transitions, as shown in Figure 5 and Table 2. Theabsence of both luminescence and long-lived LC 3ππ* state in6 demonstrates that the introduction of a negatively chargedcyclometalating ring in the ligation sphere of 6 dramaticallychanges its photophysical properties relative to the relatedcomplexes 4 and 5, which possess ligands with similar extendedπ-systems.

Jablonski diagrams of complexes 2-6 are shown in Figure6, where the energies of the 1MLCT, 1ππ*, 3MLCT, and 3ππ*states of 2-5 are derived from the corresponding absorption,emission spectra, and transient absorption quenching experi-ments. The lifetimes of the lowest triplet excited states of 2-5are obtained from transient absorption spectra.6b The energiesof the 1MLCT and 1ππ* of 6 were estimated from the absorptionmaxima at 499 and 415 nm, respectively, and the energies ofthe triplet excited states were derived from the TD-DFTcalculations.

Conclusions

This work focuses on the use of DFT and TD-DFT methodsof the singlet and the lowest triplet states of 1-6 to gaininsight into the observed differences in the photophysicalproperties of these complexes. The oxidation potential of 6exhibits a negative shift of 0.3 V relative to those of 1-5,which is reflected by the energy of the calculated HOMO ofeach complex in the series. The energies of the ligand-centered LUMOs of 1-6 are also consistent with themeasured differences in the electrochemical data as the firstreduction potential. In contrast to 1-3, whose lowest tripletexcited states were experimentally assigned as 3MLCT, 4 and5 possess pydppn-based 3ππ* states as their lowest excitedstates. These findings are supported by the calculation of theorbital contributions to the lowest energy triplet excited stateof each complex, as well as the geometric changes aroundthe coordination sphere of the metal and the dipole momentchanges between their optimized singlet and triplet geom-etries. In addition, the calculated energies of the 3ππ* excitedstates of 4 and 5 are similar to those obtained experimentally.Due to the additional electron density on the Ru centerprovided by the cyclometalating ring of pydbn- in 6, the lackof luminescence and long-lived LC 3ππ* states distinguishesthis complex from 1-5.

Acknowledgment. C.T. and Y.S. thank the NationalScience Foundation (CHE-0911354) and the Ohio Super-computer Center for their generous support. R.P.T., M.E.O.,and R.H. thank the National Science Foundation (CHE-0714751 and CHE-0911354), the Robert A. Welch Founda-tion (E-621), and Dr. James Korp for assistance with theX-ray determination.

Supporting Information Available: Crystal parameters,calculated geometric features, selected molecular orbitals,electrochemistry, singlet electronic transitions, and calculateddipole moments. These materials are available free of chargevia the Internet at http://pubs.acs.org.

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