Photochemical &Photobiological Sciences

PAPER

Cite this: Photochem. Photobiol. Sci.,2014, 13, 272

Received 15th September 2013,Accepted 5th November 2013

DOI: 10.1039/c3pp50327e

www.rsc.org/pps

New cyclometallated Ru(II) complex for potentialapplication in photochemotherapy?†

Bryan A. Albani,a Bruno Peña,b Kim R. Dunbar*b and Claudia Turro*a

In an effort to create a molecule that absorbs further into the optimum window for photochemotherapy

(PCT), the new cyclometallated complex [Ru(biq)2(phpy)](PF6) (1, biq = 2,2’-biquinoline, phpy− = deproto-

nated 2-phenylpyridine) was synthesized, characterized and compared to the known photoactive

complexes [Ru(biq)2(bpy)](PF6)2 (2, bpy = 2,2’-bipyridine) and [Ru(biq)2(phen)](PF6)2 (3, phen = 1,10-phe-

nanthroline), both of which undergo exchange of one biq ligand when irradiated with red light in co-

ordinating solvents. Excited state ligand dissociation in 2 and 3 is believed to be related to the steric

hindrance afforded by the presence of two coordinated biq ligands. The ligand exchange quantum yield

of 2 is ∼2-fold greater than that of 3, which was shown to be cytotoxic when irradiated with visible light.

Cyclometallation results in a red shift of the MLCT absorption maximum of 1 by ∼100 nm relative to those

of 2 and 3, but, although 1 exhibits a distorted octahedral geometry, photoinduced ligand exchange does

not occur. DFT calculations were used to aid in our understanding of the lack of photochemistry of 1

which is explained by the destabilization of the eg(σ*) orbitals upon cyclometallation.

Introduction

Ruthenium(II) polypyridyl complexes with extended aromaticligands have been shown to interact with DNA as chemothera-peutic agents and molecular light switches through intercala-tion and electrostatic interactions.1–4 Recent studies indicatethat certain Ru(II) complexes have the ability to undergo photo-induced ligand exchange forming covalent bonds with DNA ina manner akin to cisplatin, such that these lesions may resultin cell death. Unlike traditional photodynamic therapy (PDT)agents that rely on the generation of singlet oxygen for action,these photo-cisplatin analogs achieve cell death via mecha-nisms that are independent of oxygen; in order to differentiatethe two methods, the latter is referred to as photochemotherapy(PCT).5–7

PCT involving transition metal complexes has generallyfocused on the exchange of monodentate ligands uponirradiation.5,8,9 The photoinduced ligand exchange of biden-tate ligands bound to ruthenium(II), however, is well documen-ted for sterically strained complexes including those withligands such as 2,2′-biquinoline (biq).10 For example, the

photoinduced exchange of a biq ligand in [Ru(biq)2(bpy)]2+

(bpy = 2,2′-bipyridine) in CH3CN results in the formation ofthe intermediate cis-[Ru(biq)(bpy)(CH3CN)2]

2+, which can beused in the synthesis of tris-heteroleptic Ru(II) complexes ofthe type [Ru(biq)(phen)(L)]2+ in the presence of a varietyof bidentate ligands, L.10 It was not until decades later that[Ru(biq)(phen)2]

2+ and [Ru(biq)2(phen)]2+ (phen = 1,10-phenan-

throline) were shown to exhibit cytotoxicity upon irradiationwith visible light, while being relatively non-toxic undersimilar conditions in the dark.11 Both Ru(II) complexesundergo ligand dissociation in water following the absorptionof visible light to generate the corresponding bis-aqua com-plexes; the latter species covalently bind to DNA in vitro andthese adducts are believed to result in cell death.6,11

Photoinduced ligand exchange occurs in complexes with3LF (ligand field) dd states that are thermally accessible fromthe lower-lying energy 3MLCT (metal-to-ligand charge transfer)state(s).12–15 The thermal population of the metal centered 3LFstate results in electron density on the eg-type orbitals withRu–L(σ*) character, thus resulting in ligand dissociation.12–15

The exchange of bidentate ligands, however, is unusualbecause both bonds must be cleaved upon MLCT excitation.Bulky biq ligands that sterically strain the conventional octa-hedral geometry in Ru(II) complexes are believed to lower theenergy of the 3LF state relative to the 3MLCT state, thus result-ing in enhanced photochemistry.6,11

The maximum of the MLCT absorption of [Ru(biq)2-(phen)]2+ located at 550 nm in H2O is outside the optimal exci-tation range for PCT which is 600–850 nm.11 Therefore,

†Electronic supplementary information (ESI) available: Additional crystal struc-ture and crystallographic data, calculations, 1H NMR data, emission spectra. SeeDOI: 10.1039/c3pp50327e

aDepartment of Chemistry and Biochemistry, The Ohio State University, Columbus,

OH, USA. E-mail: turro.1@osu.edubDepartment of Chemistry, Texas A&M University, College Station, TX, USA.

E-mail: dunbar@mail.chem.tamu.edu

272 | Photochem. Photobiol. Sci., 2014, 13, 272–280 This journal is © The Royal Society of Chemistry and Owner Societies 2014

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article OnlineView Journal | View Issue

biquinoline complexes that are capable of undergoing photo-chemical ligand exchange, but with lower energy absorption,are desirable. Cyclometallated Ru(II) complexes have been uti-lized as light harvesters in solar energy conversion schemesbecause their MLCT absorption bands are broader and atlower energies relative to the diimine analogs.16–18 The presentwork focuses on the synthesis and characterization of [Ru-(biq)2(phpy)](PF6) (1) (phpy− = deprotonated 2-phenylpyri-dine). The photophysical properties and photochemistry of 1were investigated and compared to those of [Ru(biq)2(bpy)]-(PF6)2 (2) and [Ru(biq)2(phen)](PF6)2 (3). A schematic represen-tation of the molecular structures of 1–3 is displayed in Fig. 1.

Experimental sectionMaterials

The starting materials RuCl3·3H2O (Sigma-Aldrich), 2,2′-biqui-noline (Acros, TCI), 2,2′-bipyridine (Sigma-Aldrich), 1,10-phe-nanthroline (Sigma-Aldrich), and ascorbic acid (Sigma-Aldrich)were used as received without further purification. Ethanol(Decon Laboratories Inc. 200 proof) was used as received forthe synthesis of 2 and 3. For the preparation of 1, ethanol(KORTEP, 200 proof) was dried over Mg/I2 and was distilledprior to use. Standard Schlenk-line techniques (N2 atmos-phere) were used to maintain anaerobic conditions duringpreparation of the compounds. All solvents used for chromato-graphy (EMD Chemicals) were used as received without furtherpurification. Analytical Thin Layer Chromatography (TLC) wasperformed on aluminum-backed sheets coated with aluminumoxide neutral 60 F254 adsorbent (0.20 mm thickness, EMDChemicals). Flash chromatography was carried out withalumina (activated, basic, Brockmann I) from Sigma-Aldrich.The compounds [Ru(phpy)(CH3CN)4](PF6),

19 Ru(biq)2Cl2,20

2,21 and 321 were prepared following reported procedures.

Instrumentation

The 1H NMR spectrum of 1 was measured on a Varian Inova500 MHz spectrometer whereas those of 2 and 3 were recordedon a Bruker 400 MHz DPX ultrashield system. Electrosprayionization (ESI) mass spectrometry data were collected on a

Bruker micrOTOF instrument with methanol as the eluent.Electronic absorption spectral measurements were carried outusing a Hewlett Packard 8453 diode array spectrometer andelectrochemical studies were performed on a BAS CV-50Wvoltammetric analyzer. Emission spectra were obtained on aHoriba Fluormax-4 spectrometer for 2 and 3, and near IR emis-sion in 1 was obtained on a home built instrument utilizing agermanium detector. Photolysis and quantum yield experi-ments were carried out using a 150 W Xe short arc lamp(USHIO) in an upright Milliarc lamp housing unit (PTI)powered by a LPS-220 power supply (PTI) equipped with aLPS-221 igniter (PTI). The desired wavelength range wasattained using band-pass filters (Thorlabs, fwhm ∼10 nm) or3 mm thick (2 mm for 610 nm) long-pass filters (CVI Melles-Griot). Gel imaging was performed with a Bio Rad Gel Doc2000 transilluminator.

[Ru(biq)2(phpy)](PF6) (1)

The ligand 2,2′-biquinoline (100 mg, 0.39 mmol) was added toa yellow suspension of [Ru(phpy)(CH3CN)4][PF6] (0.10 g,0.18 mmol) in ethanol (15 mL), and the mixture was refluxedfor 5 h. The resulting dark green solution was reduced todryness, and the residue was purified by column chromato-graphy (basic Al2O3, CH3CN–CH2Cl2, gradient from 0% to 25%CH3CN). The first green band was collected and reduced todryness. The solid residue was dissolved with CH2Cl2 (10 mL)and hexanes (8 mL) was added slowly. The green precipitatewas collected by filtration and washed with CH2Cl2–hexanes1 : 1 (3 × 20 mL). Yield: 0.089 g (53%). 1H NMR (500 MHz,CD3CN): δ 8.93 (d, 1H, 3J = 8.5 Hz), 8.84 (d, 1H, 3J = 8.5 Hz),8.78 (d, 1H, 3J = 9.0 Hz), 8.70 (d, 1H, 3J = 8.5 Hz), 8.64 (d, 1H,3J = 9.0 Hz), 8.60 (d, 1H, 3J = 9.0 Hz), 8.28 (d, 1H, 3J = 8.5 Hz),8.24 (d, 1H, 3J = 9.0 Hz), 8.04 (dd, 1H, 3J = 8.0 Hz, 4J = 1.5 Hz),7.96 (dd, 1H, 3J = 8.0 Hz, 4J = 1.5 Hz), 7.73 (dd, 1H, 3J = 8.0 Hz,4J = 1.5 Hz), 7.62 (d, 2H, 3J = 8.5 Hz), 7.43–7.33 (m, 4H), 7.31(ddd, 1H, 3J = 8.0 Hz, 3J = 6.5 Hz, 4J = 1.0 Hz), 7.27 (m, 2H),7.22 (ddd, 1H, 3J = 8.0 Hz, 3J = 6.5 Hz, 4J = 1.0 Hz), 7.13 (d, 1H,3J = 8.0 Hz), 7.08 (d, 1H, 3J = 9.0 Hz), 6.94 (m, 2H), 6.88 (ddd,1H, 3J = 8.5 Hz, 3J = 7.0 Hz, 4J = 1.5 Hz), 6.85–6.79 (m, 3H), 6.66(ddd, 1H, 3J = 8.5 Hz, 3J = 7.0 Hz, 4J = 1.5 Hz), 6.31 (ddd, 1H,

Fig. 1 Schematic representations of the molecular structures of 1–3.

Photochemical & Photobiological Sciences Paper

This journal is © The Royal Society of Chemistry and Owner Societies 2014 Photochem. Photobiol. Sci., 2014, 13, 272–280 | 273

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article Online

3J = 7.5 Hz, 3J = 7.5 Hz, 4J = 1.5 Hz), 6.25 (dd, 1H, 3J = 8.0 Hz,4J = 1.0 Hz). Elem. anal. calcd for [Ru(biq)2(phpy)](PF6): C, 61.8%;N, 7.67%; H, 3.54%. Found: C, 61.4%; N, 7.61%; H, 3.66%.

Methods1H NMR spectroscopy was performed in (CD3)2CO (acetone-d6)or CD3CN and all resonances were referenced to the residualprotonated solvent peak. Cyclic voltammetry experiments wereperformed in a three-electrode cell with a Pt working electrode,a Pt wire auxiliary electrode, and a saturated Ag/AgCl referenceelectrode. The samples were dissolved in distilled CH3CNcontaining 0.1 M tetrabutylammonium hexafluorophosphateas the supporting electrolyte, and bubbled with N2 for10 minutes prior to each measurement. The cyclic voltammetrydata were recorded at a scan rate of 100 mV s−1 with ferrocenebeing added to the sample after each measurement to serveas an internal standard (+0.40 V vs. SCE in CH3CN).

22 Thechloride salt of each complex was used for experimentsperformed in H2O, which were obtained using an AmberliteIRA-410 ion exchange resin, prepared by soaking the powder ina 1 M HCl solution at 50 °C for 3 days, with methanol as theeluent. Emission spectra were measured at both 298 K and77 K in CH3CN in a 1 × 1 cm quartz cuvette using an excitationwavelength corresponding to the maximum of the MLCTabsorption for 2 and 3, and 405 nm for 1. Elemental Analysiswas performed by Atlantic Microlab Inc.

The quantum yields (Ф) for photoinduced ligand exchangeof the biq ligand in H2O were measured for complexes 2 and 3with 550 nm and 600 nm irradiation wavelengths using theappropriate band-pass filters.23 The moles of complex reactedwere quantitated using electronic absorption spectroscopy bymonitoring the decrease in MLCT absorption maximum ofeach complex as a function of irradiation time (moles reactedper s) at early irradiation times, and Reinecke’s salt was usedas an actinometer to determine the intensity (Einstein s−1) ofthe Xe arc lamp at the desired wavelength.23,24

Dark green single crystals of 1 were grown by slow diffusionof hexanes into a dichloromethane solution of the compoundin a fine tube. X-ray data were collected at 291 K on a BrukerAPEX II CCD X-ray diffractometer equipped with a graphitemonochromated MoKα radiation source (λ = 0.71073 Å). Thedata sets were integrated with the Bruker SAINT softwarepackage.25 The absorption correction (SADABS)26 was based onfitting a function to the empirical transmission surface assampled by multiple equivalent measurements. Solution andrefinement of the crystal structures was carried out using theSHELX27 suite of programs as implemented in X-SEED.28 Thestructure was solved by direct methods with all non-hydrogenatoms being refined with anisotropic displacement parametersusing a full-matrix least-squares technique on F2. Hydrogenatoms were fixed to parent atoms and refined using the ridingmodel.

The supercoiled pUC18 (Bayou Biolabs) used in the gel elec-trophoresis mobility shift assays was purified using a standardQIAprep® Spin Miniprep Kit (QIAGEN). The DNA was boundto a miniprep spin column, washed with 500 μL PB buffer,

with 750 μL PE buffer, and extracted from the column withwarm water. The purified DNA was then linearized using aQIAquick® GelExtraction Kit (QIAGEN). For the linearization,55 μL of water, 25 μL of pUC18, 10 μL of React 4 Buffer (Invitro-gen), and 10 μL Sma1 enzyme (Invitrogen) were added to asmall Eppendorf tube and heated at 30 °C for 1 h and theenzyme was then deactivated by incubating the sample at65 °C for 10 minutes. After this procedure, QG buffer (300 μL)and isopropanol (100 μL) were added to the mixture, wasadded to a spin column and washed with 750 μL of PE buffer,and was then extracted from the column with 40 μL of water.The agarose gel electrophoresis was carried out using 1× TBEbuffer (pH = 8.28) at room temperature for one hour at 95 Vpowered by an EC 105 voltmeter produced by E-C ApparatusCorporation. Gels were then stained in ethidium bromide solu-tions (0.5 μg mL−1) for 30 minutes then soaked in water for30 minutes.

Calculations were performed with density functional theory(DFT) using the Gaussian 09 program.29 The B3LYP30–32 func-tional along with the 6-31G* basis set for H, C, and N33 andthe SDD energy consistent pseudopotentials were used forRu.34 Optimization of full geometries was carried out with therespective programs and orbital analysis was performed inGaussview version 3.09.35 Following optimization of the mole-cular structures, frequency analysis was performed to ensurethe existence of local minima on the potential energy surfaces.Electronic absorption singlet-to-singlet transitions were calcu-lated using time-dependent DFT (TD-DFT) methods with thepolarizable continuum model (PCM) that mimicked thesolvation effect of CH3CN in Gaussian 09.36

Results and discussionX-ray crystal structure of 1

The molecular structure of 1 is depicted in Fig. 2 withcrystallographic data being compiled in Tables 1 and S1†

Fig. 2 Thermal ellipsoid plot of the [PF6]− salt of complex 1 (ellipsoids

drawn at 50% probability).

Paper Photochemical & Photobiological Sciences

274 | Photochem. Photobiol. Sci., 2014, 13, 272–280 This journal is © The Royal Society of Chemistry and Owner Societies 2014

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article Online

(CCDC 961134). Compound 1 crystallizes in the monoclinicspace group P21/n and there are two interstitial dichloro-methane molecules in the asymmetric unit. The coordinationsphere of the metal center consists of five nitrogen atoms andone carbon atom in a distorted octahedral environment. TheRu1–C1 bond length of 2.095(4) Å in 1 is longer than the corres-ponding bond distances in [Ru(bpy)2(phpy)]

+, 2.044(1) Å,37 andin [Ru(phen)2(phpy)]

+, 2.036(7) Å,38 which may be attributed tothe steric repulsion between the biq ligands. Three of the Ru–Nbond lengths with the biq ligands, Ru1–N2, Ru1–N4 andRu1–N5, are similar (∼2.09 Å) and within the range of thosefound in the closely related compound 3 and [Ru(phen)2(biq)]

2+,2.079(2) to 2.0112(3) Å.11 In contrast, the Ru1–N3biq bondlocated trans to the C donor atom of phpy− is the longest,2.148(3) Å, reflecting the strong trans influence of the phenylring of phpy−. The angle between adjacent biq ligands,N3–Ru1–N4 is 98.3(1)° which is larger than the angles formedbetween each biq and phpy−, viz., N1–Ru1–N3 and C1–Ru1–N4,which are 90.8(1) and 92.4(2)°, respectively (Table 1). The moreobtuse N3–Ru1–N4 angle is likely a consequence of stericrepulsion between the two bulky biq ligands. The biq ligandtrans to the C1 atom of phpy− is twisted about the C–C bond,N2–C20–C21–N3, −9.9(6)°, whereas such a distortion is notobserved in the other biq ligand, N4–C38–C39–N5, 0.9(6)°(Table 1). In addition, the quinoline moieties of both biqligands are bent by ∼15° out of the plane formed with themetal center, a distortion that was also observed in 3.11

Electronic absorption, emission, and electrochemistry

The absorption profiles of 2 and 3 are in good agreementwith previously published data (Fig. 3).21 Complex 2exhibits maxima at 549 nm (ε = 6600 M−1 cm−1), 482 nm (ε =4800 M−1 cm−1), and 407 nm (ε = 2800 M−1 cm−1) in CH3CN,and those for 3 are observed at 552 nm (ε = 9600 M−1 cm−1),480 nm (ε = 7100 M−1 cm−1), and 409 nm (ε = 3900 M−1 cm−1)in the same solvent. The transitions at ∼410 nm are assignedas Ru(t2g)→L(π*) (L = bpy, phen) 1MLCT in 2 and 3, respect-ively, whereas those at ∼480 nm and ∼550 nm are attributed toRu(t2g)→biq(π*) transitions.21 Three 1MLCT absorption peaks

are also observed in 1, but are red-shifted as compared to thecorresponding bands in 2 and 3 (Fig. 3a), with Ru(t2g)→phpy−(π*)absorption maximum at 455 nm (ε = 1700 M−1 cm−1), andbands associated with Ru(t2g)→biq(π*) transitions at 545 (ε =2200 M−1 cm−1) and 640 nm (ε = 5200 M−1 cm−1). The redshift in 1 is attributed to an increase in energy of the highestoccupied molecular orbital (HOMO) resulting from cyclometal-lation, since the energy of the biq(π*) orbitals is expected toremain relatively unchanged.16,39 The carbon to metal bondprovides significant ligand character to the HOMO, which istypically nearly solely metal in character in Ru(II) polypyridylcomplexes.16,39 Emission in the near-IR spectral region isobserved from complex 1 in CH3CN with maxima at 1030 nmand 980 nm at 298 K and 77 K, respectively (λexc = 405 nm)shown in Fig. 3b, which is significantly red shifted from thatof 2 and 3 with maxima at 748 nm and 747 nm at room temp-erature, respectively, and at 740 nm and 735 at 77 K, respect-ively, in agreement with published results.21

Cyclic voltammetry reveals quasi-reversible metal-centeredoxidation events, E1/2(Ru

3+/2+), at +1.44 V vs. SCE for 2 and 3 inCH3CN, which is typical of Ru(II) polypyridyl complexes.40,41

Table 1 Selected bond distance, angles, and dihedral angles for thecation [Ru(biq)2(phpy)]

+ (1)

Bond lengths (Å) Bond angles (°)

Ru1–C1 2.095(4) C1–Ru1–N1 78.8(2)Ru1–N1 2.087(4) N2–Ru1–N3 76.8(1)Ru1–N2 2.092(3) N4–Ru1–N5 76.7(1)Ru1–N3 2.148(3) N1–Ru1–N3 90.8(1)Ru1–N4 2.091(4) N3–Ru1–N4 98.3(1)Ru1–N5 2.096(3) C1–Ru1–N4 92.4(2)

Dihedral angles (°)

N2–C20–C21–N3 −9.9(6)C19–C20–C21–C22 −11.9(8)N4–C38–C39–N5 −0.9(6)C37–C38–C39–C40 0.9(8)N2–Ru1–N3–C29 −165.1(4)N4–Ru1–N5–C47 166.2(4)

Fig. 3 (a) Electronic absorption spectra for 1–3 in CH3CN and (b) nor-malized emission spectra of 1 and 2 at 77 K in CH3CN.

Photochemical & Photobiological Sciences Paper

This journal is © The Royal Society of Chemistry and Owner Societies 2014 Photochem. Photobiol. Sci., 2014, 13, 272–280 | 275

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article Online

In the case of the cyclometallated complex, 1, the observed Ru(III/II)couple is observed at a less positive potential, with E1/2(Ru

3+/2+) =+0.65 V vs. SCE. This cathodic shift in oxidation potential istypical of cyclometallated complexes relative to bpy or phenand results from the increased electron density on the metalprovided by the covalent bonding of the phpy− ligand.39 Theincreased electron density on the metal also increases theπ-backbonding to the pyridyl rings of the biq ligands whichleads to a cathodic shift in the sequential ligand centeredreductions for 1 relative to those of 2 and 3.39 Quasi-reversiblereduction waves, assigned as reduction of the biq ligands, areobserved with E1/2(Ru

2+/+) = −1.06 and −1.31 vs. SCE for 1 inCH3CN, whereas the analogous reduction events of the biqligands occur at −0.80 V and −1.03 V vs. SCE for 2 and at −0.80 Vand −1.05 V vs. SCE for 3 in the same solvent. A thirdreduction process is observed for 2 and 3 in CH3CN with thevalues E1/2(Ru

2+/+) = −1.59 V and E1/2(Ru2+/+) = −1.56 V vs. SCE,

respectively, assigned to the reduction of the bpy and phenligands. The reduction of the phpy− ligand in 1 is not observedand must occur outside the spectral analysis window for CH3CN.

Photochemistry

The photoreactivity of 1–3 was evaluated by monitoring thechanges to the electronic absorption spectrum of eachcomplex as a function of irradiation time. As expected basedon prior work, the irradiation of 2 and 3 in H2O and CH3CNwith visible light (λirr ≥ 530 nm or λirr ≥ 630 nm) results inexchange of one biq ligand with two molecules of coordinat-ing solvent, generating [Ru(biq)(L)(H2O)2]

2+ and [Ru(biq)(L)-(CH3CN)2]

2+ (L = bpy, phen), respectively (Fig. 4 and S6†), butthe complexes are not reactive when kept in the dark undersimilar experimental conditions (Fig. S4 and S5†).10,11 In con-trast, complex 1 is not photochemically active in CH3CN (λirr ≥530 nm, Fig. S7†) or H2O based on absorption changesor when monitored by mass spectrometry as a function ofirradiation time (λirr ≥ 530 nm).

The quantum yield for the exchange of the biq ligand in 2in H2O with λirr = 550 nm and λirr = 600 nm irradiation weremeasured to be 0.068(2) and 0.053(3), respectively.These values are factors of 3.4 and 2.2 greater than thosemeasured for 3, Φ550 = 0.020(3) and Φ600 = 0.024(2) at eachwavelength, respectively. A similar trend was observed betweenthe sterically hindered complexes [Ru(bpy)2(dmbpy)]2+ and[Ru(bpy)2(dmdpq)]2+ (dmbpy = 6,6′-dimethyl-2,2′-bipyridine,dmdpq = 7,10-dimethyl-pyrazino[2,3-f ]-1,10-phenanthroline)in which the exchange of the dmdpq ligand possessing fusedaromatic rings occurred less efficiently than the analogousdmbpy complex,6 suggesting that less rigid bidentate ligandsenhance the ligand exchange.

Calculations

DFT and TD-DFT calculations were performed to gain a betterunderstanding of the electrochemistry, photophysical pro-perties, and photochemistry observed for the three complexes.The DFT calculations reveal metal-based HOMO, HOMO−1,and HOMO−2 levels representing the dxy, dxz, and dyz orbitals

for complexes 1–3. These HOMOs are calculated at nearly iden-tical energies for 2 and 3, but that of 1 is destabilized by ∼0.6 eVand exhibits significant phpy− ligand character and elec-tron density on the metal–carbon bond (Fig. 5). The HOMO of2 was arbitrarily set at 0.0 eV. The differences in the calculatedenergies of the HOMOs agree with the cathodic shift in theexperimental oxidation potential of 0.75 V between 1 and 2 or3. The additional ease in oxidation by ∼0.15 V may be relatedto the overall +1 charge of 1 as compared to +2 of 2 and 3which renders removal of an electron from the former morefavorable. The LUMO and LUMO+1 orbitals of 1–3 are calcu-lated to be localized on the biq ligands and to lie at similarenergies in the three complexes. Although 1 is more difficultto reduce than 2 and 3 by ∼0.2 V, this difference may also berelated to the overall charge of the complexes.

TD-DFT calculations reveal that the lowest vertical singletexcited states of 1, 2, and 3 possess significant contributions,∼88% for 1 and ∼97% for 2 and 3, from HOMO→LUMO tran-sitions, but low oscillator strengths, with maxima at 840 nm( f = 0.0013), 586 nm ( f = 0.0003), and 587 nm ( f = 0.0004),respectively (Tables S2–S4†). More intense absorption bandswith f > 0.01 are predicted at 686 nm ( f = 0.027) for 1, at523 nm ( f = 0.076) for 2, and at 527 nm ( f = 0.086) for 3calculated to possess 83%, 93%, and 96% contributionfrom HOMO−1→LUMO transitions, respectively. These calcu-lated absorption maxima are similar to the experimentalvalues, 640 nm, 549 nm, and 552 nm, for 1, 2, and 3,respectively.

Fig. 4 Electronic absorption spectral changes of (a) 2 (40 μM) withincreasing irradiation times, tirr, 0, 1, 2, 3, 5, 10, 15, 20, 30, 45, 60, 90,and 180 min and (b) 3 (15 μM) at tirr = 0, 2, 5, 10, 20, 30, and 60 min inH2O (λirr ≥ 630 nm).

Paper Photochemical & Photobiological Sciences

276 | Photochem. Photobiol. Sci., 2014, 13, 272–280 This journal is © The Royal Society of Chemistry and Owner Societies 2014

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article Online

In order to understand the lack of photodissociation of thebiq ligand in 1 as compared to 2 and 3, the orbitals andtransitions involved in the process must be considered. Asstated previously, ligand dissociation is believed to occur viathermal population of the 3LF states from the low-lying 3MLCTstate(s).12–15 Experimental data, calculations, and previouswork all indicate that the HOMO in 1 lies at a higher energythan those of 2 and 3, but that the energies of the LUMOsremain relatively unchanged among the three complexes.16 Itis expected that cyclometallation results in a larger gapbetween the 3MLCT state and 3LF states because the eg(σ*)orbitals are destabilized due to the increased covalent inter-action provided by the Ru–C bond, significantly raising theenergy of the eg orbitals with Ru–L(σ*) character.16 The eg(σ*)orbitals in 2 are calculated as the LUMO+9 (dz2) and LUMO+10(dx2−y2), are relatively close in energy (ΔE = 0.26 eV). In contrast,the eg(σ*) orbitals in 1 are calculated to be LUMO+10 (dz2) andLUMO+17 (dx2−y2) with an energy difference of 3.22 eV.

The relative energies of the two eg(σ*) orbitals is related tothe energies of the 3LF states involving the population of eachd-orbital, dz2 and dx2−y2. In order for photodissociation of thebidentate ligand to take place, both Ru–N(biq) bonds must bebroken. Because the 3LF state associated with the dx2−y2 orbitallies at a very high energy in 1, it is not likely to be accessibleupon irradiation with visible light. It is proposed that,although one Ru–N(biq) bond may break upon irradiation, itquickly re-coordinates to the metal. This explanation is alsoconsistent with the observed exchange of only one CH3CN in

the related cyclometallated complex [Ru(phpy)(bpy)(CH3CN)2]+.42

Upon irradiation, one bond is weakened due to population ofthe lower lying energy 3LF state resulting in ligand dissociationand solvent coordination, but the remaining CH3CN does notexchange with extended photolysis because the higher-lying3LF state is not populated.

Gel mobility assays

Gel electrophoresis mobility shift assays were carried out inorder to compare the photoinduced DNA binding of 2 and 3. Itis well documented that cisplatin thermally binds to linearizedDNA and reduces its migration through an agarose gel in aconcentration-dependent manner.43 The same pattern isobserved for 2 and 3 upon irradiation with low energy light,but not in the dark (Fig. 6). In Fig. 6, lanes 1 and 8 contain 1 kbDNA ladder, lanes 2–7 were loaded with 50 μM pUC18 DNA,and lanes 3–6 contain increasing concentrations of 2 or 3. InFig. 6a and 6c, the samples in lanes 3–6 were irradiated for15 minutes with λirr ≥ 630 nm light prior to loading. It isevident in Fig. 6a that, as the concentration of 2 is increased,the DNA mobility decreases, whereas no shift in mobility isobserved when the samples are incubated in the dark for20 min under similar experimental conditions (Fig. 6b). Theseresults are indicative of covalent binding of 2 to DNA onlyupon irradiation. Fig. 6c and 6d display the results for complex3 irradiated under the same conditions as 2 (Fig. 6a) and inthe dark, respectively. The reduced effect of 3 as compared to 2on the DNA mobility can be attributed to the ∼2-fold lowerФ600 value of the former, such that a smaller amount of thephotoproduct that binds to DNA is formed. Increased DNAbinding was observed for 3 when the irradiation times weredoubled, but it nevertheless remained lower than the effectobserved for 2 (Fig. S8†).

Conclusions

The new cyclometallated complex [Ru(biq)2(phpy)](PF6) (1) wassynthesized and characterized by various methods includingX-ray crystallography. The photophysical electrochemical andphotochemical properties of 1 were compared to those ofknown photoactive complexes [Ru(biq)2(bpy)](PF6)2 (2) and[Ru(biq)2(phen)](PF6)2 (3). Complexes 2 and 3 undergoexchange of one of the biq ligands when irradiated with λirr ≥630 nm light in water to generate the corresponding complexes[Ru(biq)2(bpy)(H2O)2](PF6)2 and [Ru(biq)2(phen)(H2O)2](PF6)2.The resulting bis-aqua photoproducts covalently bind tolinearized ds-DNA. The quantum yield for ligand exchange for 2,Φ600 = 0.053(3), was measured to be 2.2-fold greater than thatdetermined for 3, Φ600 = 0.024(2), at the same irradiation wave-length, 600 nm, thereby lending support to the hypothesis thatless rigid ancillary ligands lead to more efficient biq ligandexchange. The differences in ligand exchange quantum yieldsof 2 and 3 were correlated to the DNA binding ability of thecomplexes.

Fig. 5 Calculated (a) MO diagrams showing the frontier orbitals and (b)electron densities of the HOMOs (isovalue = 0.04) of 1 and 2.

Photochemical & Photobiological Sciences Paper

This journal is © The Royal Society of Chemistry and Owner Societies 2014 Photochem. Photobiol. Sci., 2014, 13, 272–280 | 277

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article Online

Since the compounds [Ru(bpy)3](PF6)2 and [Ru(phen)3]-(PF6)2 are stable upon irradiation relative to the biq complexes2 and 3, steric bulk was thought to be a key criteria for photo-induced bidentate ligand exchange. Although the crystal struc-ture of 1 reveals an elongated Ru–N(biq) bond, the complexdoes not display photoinduced ligand substitution in coordi-nating solvents under similar conditions as those used for 2and 3. The difference in reactivity of the cyclometallatedcomplex 1 is ascribed to increased energy of the metal-cen-tered 3LF states resulting from the bonding of the strongσ-donor phpy− ligand, a finding supported by DFT calculations.Complexes 2 and 3 are good candidates as PCT agents owingto their covalent DNA binding upon irradiation with lightin the photodynamic window. Although the absorptionmaximum of 1 is red-shifted relative to those of 2 and 3, thelack of photochemistry of the former compound indicates thatthe use of cyclometallated Ru(II) complexes may not be a goodstrategy for the design of new PCT agents.

Acknowledgements

K.R.D. and C.T. thank the National Science Foundation forpartial support of this work (CHE-1213646) and the OhioSupercomputer Center.

References

1 (a) H. Song, J. T. Kaiser and J. K. Barton, Crystal structureof Δ-[Ru(bpy)2dppz]2+ bound to mismatched DNA revealsside-by-side metalloinsertion and intercalation, Nat. Chem.,2012, 4, 615–620; (b) A. E. Friedman, J. Chambron,J. Sauvage, N. J. Turro and J. K. Barton, A molecular lightswitch for DNA: [Ru(bpy)2dppz]

2+, J. Am. Chem. Soc., 1990,

112, 4960–4962; (c) R. Hartshorn and J. K. Barton, Noveldipyridophenazine complexes of ruthenium(II): exploringluminescent reporters of DNA, J. Am. Chem. Soc., 1992, 114,5915–5925.

2 (a) J. D. Aguirre, A. M. Angeles-Boza, A. Chouai, C. Turro,J. Pellois and K. R. Dunbar, Anticancer activity of heterolepticdiimine complexes of dirhodium: A study of intercalatingproperties, hydrophobicity and in cellulo activity, DaltonTrans., 2009, 48, 10806–10812; (b) Y. Sun, D. A. Luttermanand C. Turro, Role of electronic structure on DNA light-switch behavior of Ru(II) intercalators, Inorg. Chem., 2008,47, 6427–6434; (c) Y. Liu, R. Hammitt, D. A. Lutterman,R. P. Thummel and C. Turro, Marked differences in light-switch behavior of Ru(II) complexes possessing a tridentateDNA intercalating ligand, Inorg. Chem., 2007, 46, 6011–6021.

3 P. K. Yata, M. Shilpa, P. Nagababu, M. R. Reddy, L. R. Kotha,N. M. Gabra and S. J. Satyanarayana, Study of DNA lightswitch Ru(II) complexes: synthesis, characterization, photo-cleavage and antimicrobial activity, J. Fluoresc., 2012, 22,835–847.

4 V. Brabec and O. Novakova, DNA binding mode of ruthe-nium complexes and relationship to tumor cell toxicity,Drug Resist. Updat., 2006, 9, 111–122.

5 (a) R. N. Garner, J. C. Gallucci, K. R. Dunbar and C. Turro,[Ru(bpy)2(5-cyanouracil)2]

2+ as a potential light activateddual action therapeutic agent, Inorg. Chem., 2011, 50, 9213–9215; (b) T. Respondek, R. N. Garner, M. K. Herroon,I. Podgorski, C. Turro and J. J. Kodanko, Light activation ofa cysteine protease inhibitor: Caging of a peptidomimeticnitrile with RuII(bpy)2, J. Am. Chem. Soc., 2011, 133, 17164–17167; (c) T. N. Singh and C. Turro, Photoinitiated DNAbinding by cis-[Ru(bpy)2(NH3)2]

2+, Inorg. Chem., 2004, 43,7260–7262; (d) M. A. Sgambellone, A. David, R. N. Garner,K. R. Dunbar and C. Turro, Cellular toxicity induced by the

Fig. 6 Imaged ethidium bromide-stained agarose gels of 50 μM linearized pUC18 plasmid (10 mM phosphate buffer, pH = 7.8) in the presence ofvarious concentrations of complex: lanes 1 and 8, 1 kb DNA molecular weight standard; lanes 2 and 7, linearized plasmid alone; lanes 3–6, 25, 50,75, 100 μM of 2 (a) irradiated (λirr ≥ 630 nm, 15 min), (b) incubated in the dark (20 min, 298 K) and 3 (c) irradiated (λirr ≥ 630 nm, 15 min), and (d) incu-bated in the dark (20 min, 298 K).

Paper Photochemical & Photobiological Sciences

278 | Photochem. Photobiol. Sci., 2014, 13, 272–280 This journal is © The Royal Society of Chemistry and Owner Societies 2014

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article Online

photorelease of a caged bioactive molecule: Design of apotential dual-action Ru(II) complex, J. Am. Chem. Soc.,2013, 135, 11274–11282.

6 B. S. Howerton, D. K. Heidary and E. C. Glazer, Strainedruthenium complexes are potent light-activated anticanceragents, J. Am. Chem. Soc., 2012, 134, 8324–8327.

7 S. L. H. Higgins, A. J. Tucker, B. S. J. Winkel andK. J. Brewer, Metal to ligand charge transfer induced DNAphotobinding in a Ru(II)–Pt(II) supramolecule using redlight in the therapeutic window: a new mechanism for DNAmodification, Chem. Commun., 2012, 48, 67–69.

8 A. E. Pierri, A. Pallaoro, G. Wu and P. C. Ford, A lumine-scent and biocompatible photoCORM, J. Am. Chem. Soc.,2012, 134, 18197–18200.

9 P. C. Ford, Photochemical delivery of nitric oxide, Biol.Chem. Ther. Appl. Nitric Oxide, 2013, 34, 56–64.

10 A. von Zelewsky and G. Gremaud, Ruthenium(II) complexeswith three different diimine ligands, Helv. Chim. Acta,1988, 71, 1108–1115.

11 E. Wachter, D. K. Heidary, B. S. Howerton, S. Parkin andE. C. Glazer, Light-activated ruthenium complexes photo-bind DNA and are cytotoxic in the photodynamic therapywindow, Chem. Commun., 2012, 48, 9649–9451.

12 (a) G. Malouf and P. C. Ford, Photochemical reaction path-ways of ruthenium(II) complexes. Evidence regarding thereactive excited state(s) from metal-to-ligand charge trans-fer excitation of pentaamine(pyridine)ruthenium(2+) andrelated complexes, J. Am. Chem. Soc., 1974, 96, 601–603;(b) G. Malouf and P. C. Ford, Photochemistry of theruthenium(II) ammine complexes, Ru(NH3)5(py-X)

2+. Vari-ation of systemic parameters to modify photochemicalreactivities, J. Am. Chem. Soc., 1977, 99, 7213–7221;(c) V. A. Durante and P. C. Ford, Flash photolysis studiesof ruthenium(II)-ammine complexes. 1. Transient inter-mediates in the photolysis of Ru(NH3)5(py-X)

2+ and theirrelationship to photosubstitution pathways, Inorg. Chem.,1979, 18, 588–593.

13 (a) E. Tfouni, Photochemical reactions of ammineruthe-nium(II) complexes, Coord. Chem. Rev., 2000, 196, 281–305;(b) M. S. Martinez, Photochemical reactions of pentaammine-(cyanopyridine)ruthenium(II) complexes, [Ru(NH3)5L]

2+,J. Photochem. Photobiol., A, 1999, 122, 103–108; (c) L. A. Pavanin,Z. N. da Rocha, E. Giesbrecht and E. Tfouni, Photoaqua-tion of cis-bis(azine)tetraammineruthenium(II) complexes,cis-Ru(NH3)4(L)(L′)

n+1, Inorg. Chem., 1991, 30, 2185–2190.

14 (a) B. P. Sullivan, D. J. Salmon and T. J. Meyer, Mixed phos-phine 2,2′-bipyridine complexes of ruthenium, Inorg.Chem., 1978, 17, 3334–3341; (b) B. Durham, J. L. Walsh,C. L. Carter and T. J. Meyer, Synthetic applications ofphotosubstitution reactions of poly(pyridyl) complexes ofruthenium(II), Inorg. Chem., 1980, 19, 860–865; (c) J. V. Casparand T. J. Meyer, Photochemistry of MLCT excited states.Effect of nonchromophoric ligand variations on photo-physical properties in the series cis-(bpy)2L2

2+, Inorg.Chem., 1983, 22, 2444–2453.

15 (a) B. Durham, J. V. Caspar, J. K. Nagle and T. J. Meyer,Photochemistry of tris(2,2′-bipyridine)ruthenium(2+) ion,J. Am. Chem. Soc., 1982, 104, 4803–4810; (b) G. H. Allen,R. P. White, D. P. Rillema and T. J. Meyer, Synthetic controlof excited-state properties. Tris-chelate complexes contain-ing the ligands 2,2′-bipyridine, and 2,2′-bipyrimidine,J. Am. Chem. Soc., 1984, 106, 2613–2620; (c) D. P. Rillema,D. G. Taghdiri, D. S. Jones, C. D. Keller, L. A. Worl,T. J. Meyer and H. Levy, A. Structure and redox and photo-physical properties of a series of ruthenium heterocyclesbased on the ligand 2,3-bis(2-pyridyl)quinoxaline, Inorg.Chem., 1987, 26, 578–585.

16 M. Maestri, V. Balzani, C. Deuschel-Cornioley and A. VonZelewsky, Photochemistry and luminescence of cyclo-metallated complexes, Adv. Photochem., 1992, 17, 1–68.

17 J. Xie, C. Li, Q. Zhou, W. Wang, Y. Hou, B. Zhang andX. Wang, Large improvement in the catalytic activity due tosmall changes in the diimine ligands: New mechanisticinsight into the dirhodium(II,II) complex-based photo-catalytic H2 production, Inorg. Chem., 2012, 51, 6376–6384.

18 K. C. D. Robson, B. D. Koivisto, A. Yella, B. Sporinova,M. K. Nazeeruddin, T. Baumgartner, M. Grätzel andC. P. Berlinguette, Design and development of functiona-lized cyclometalated ruthenium chromophores for light-harvesting applications, Inorg. Chem., 2011, 50, 5494–5508.

19 S. Fernandez, M. Pfeffer, V. Ritleng and C. Sirlin, Aneffective route to cycloruthenated N-ligands under mildconditions, Organometallics, 1999, 18, 2390–2394.

20 B. A. Albani, C. B. Durr and C. Turro, Selective photo-induced ligand exchange in a new tris-heteroleptic Ru(II)complex, J. Phys. Chem. A, 2013, DOI: 10.1021/jp4085684.

21 D. M. Klassen, Excited states of mixed ligand complexes ofruthenium(II) with 2-(2′pyridyl)quinoline and 2,2′-biquino-line, Chem. Phys. Lett., 1982, 93, 383–386.

22 J. Bolger, A. Gourdon, E. Ishow and J. P. Launay,Mononuclear and binuclear tetrapyrido[3,2-a:2′-3′-c:3″,2″-h:2′′′,3′′′-j]phenazine (tpphz) ruthenium and osmium com-plexes, Inorg. Chem., 1996, 35, 2937–2944.

23 M. Montalti, A. Credi, L. Prodi and M. T. Gandolfi, Chemi-cal actinometry, Handbook of Photochemistry, Taylor &Francis Group, Boca Raton, Florida, 3rd edn, 2006,pp. 601–616.

24 E. A. Wegner and A. W. Adamson, Photochemistry ofcomplex ions. III. Absolute quantum yields for the photo-lysis of some aqueous chromium(III) complexes. Chemicalactinometry in long wavelength visible region, J. Am. Chem.Soc., 1966, 88, 394–402.

25 SMART and SAINT, Siemens Analytical X-ray InstrumentsInc., Madison, WI, 1996.

26 G. M. Sheldrick, SADABS, University of Gottingen, Gottin-gen, Germany, 1996.

27 G. M. Sheldrick, A short history of SHELX, Acta Crystallogr.,Sect. A: Fundam. Crystallogr., 2008, 64, 112–122.

28 L. J. Barbour, X-seed: A software tool for supramolecularcrystallography, J. Supramol. Chem., 2001, 1, 189–191.

Photochemical & Photobiological Sciences Paper

This journal is © The Royal Society of Chemistry and Owner Societies 2014 Photochem. Photobiol. Sci., 2014, 13, 272–280 | 279

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article Online

29 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaus-sian 09, Revision A.1, Gaussian, Inc., Wallingford, CT, 2009.

30 A. D. Becke, Density-functional exchange-energy approxi-mation with correct asymptotic behavior, Phys. Rev. A,1988, 38, 3098–3100.

31 A. D. Becke, Density-functional thermochemistry. III. Therole of exact exchange, J. Chem. Phys., 1993, 98, 5648–5652.

32 C. Lee, W. Yang and R. G. Parr, Development of theColle-Salvetti correlation-energy formula into a functionalof electron density, Phys. Rev. B: Condens. Matter Mater.Phys., 1988, 37, 785–789.

33 W. J. Hehre, L. Radom, P. V. Chleyer and J. A. Pople,Ab initio Molecular Orbital Theory, John Wiley & Sons,New York, 1986.

34 D. Andrae, U. Haussermann, M. Dolg, H. Stoll andH. Preuss, Energy-adjusted ab initio pseudopotentials forthe second and third row transition elements, Theor. Chim.Acta, 1990, 77, 123–141.

35 R. Dennington II, T. Keith and J. Millam, GaussView 3,Semichem, Inc., Shawnee Mission, KS, 2007.

36 S. Fantacci, F. D. Angelis and A. J. Selloni, Absorption spec-trum and solvatochromism of the [Ru(4,4′-COOH-2,2′-bpy)2(NCS)2] molecular dye by time dependent densityfunctional theory, J. Am. Chem. Soc., 2003, 125, 4381–4387.

37 M. Brissard, M. Gruselle, B. Malézieux, R. Thouvenot,C. Guyard-Duhayon and O. Convert, An anionic {[MnCo-(ox)3]

−}n network with appropriate cavities for the enantio-selective recognition and resolution of the hexacoordinatedmonocation [Ru(bpy)2(ppy)]

+ (bpy = bipyridine, ppy =phenylpyridine), Eur. J. Inorg. Chem., 2001, 7, 1745–1751.

38 A. D. Ryabov, V. S. Sukharev, L. Alexandrova, R. L. Lagadecand M. Pfeffer, New synthesis and new bio-application ofcyclometalated ruthenium(II) complexes for fast mediatedelectron transfer with peroxidase and glucose oxidase,Inorg. Chem., 2001, 40, 6529–6532.

39 B. Peña, N. A. Leed, K. R. Dunbar and C. Turro, Excitedstate dynamics of two new Ru(II) cyclometallated dyes:Relation to cells for solar energy conversion and compari-son to conventional systems, J. Phys. Chem. C, 2012, 116,22186–22195.

40 A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belserand A. Von Zelewsky, Ru(II) polypyridine complexes: photo-physics, photochemistry, electrochemistry, and chemilumi-nescence, Coord. Chem. Rev., 1988, 84, 85–277.

41 F. Bolletta and M. Vitale, Electrochemiluminescencequantum yield of some Ru(II) polypyridine complexes,Inorg. Chim. Acta, 1990, 175, 127–131.

42 A. M. Palmer, B. Peña, R. B. Sears, O. Chen, M. El Ojaimi,R. P. Thummel, K. R. Dunbar and C. Turro, Cytotoxicity ofcyclometallated ruthenium complexes: the role of ligandexchange on the activity, Philos. Trans. R. Soc. London, Ser.A, 2013, 371, 20120135.

43 (a) E. R. Jamieson and S. J. Lippard, Structure, recognition,and processing of cisplatin-DNA adducts, Chem. Rev., 1999,99, 2467–2498; (b) K. S. Lovejoy and S. J. Lippard, Metalanticancer compounds, Dalton Trans., 2009, 10651–10659.

Paper Photochemical & Photobiological Sciences

280 | Photochem. Photobiol. Sci., 2014, 13, 272–280 This journal is © The Royal Society of Chemistry and Owner Societies 2014

Publ

ishe

d on

13

Nov

embe

r 20

13. D

ownl

oade

d by

Tex

as A

& M

Uni

vers

ity o

n 23

/09/

2014

13:

42:1

1.

View Article Online