Theoretical Study on the Electronic Excitations of a Porphyrin

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Theoretical Study on the Electronic Excitations of a Porphyrin-Polypyridyl Ruthenium(II) Photosensitizer Gloria I. Cardenas-Jiron,*,† Cristina A. Barboza,†,‡ Ramon Lopez,*,§ and M. Isabel Menendez§ †

Laboratorio de Química Teorica, Departamento de Ciencias del Ambiente, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Casilla 40, Correo 33, Santiago, Chile ‡ Departamento de Ciencias Químicas, Facultad de Ecología y Recursos Naturales, Universidad Andres Bello (UNAB), Republica 275, Santiago, Chile § Departamento de Química Física y Analítica, Facultad de Química, Universidad de Oviedo, C/Julian Clavería 8, 33006 Oviedo, Asturias, Spain

bS Supporting Information ABSTRACT: In this work, we investigated the UVvis spectra of the [Ru(bipy)2(MPyTPP)Cl]+ (MPyTPP = 5-pyridyl-15, 20,25-triphenylporphyrin) complex and its related species [Ru(bipy)2(py)Cl]+ and MPyTPP, by using time-dependent density functional theory and a set of functionals (B3LYP, M05, MPWB1K, and PBE0) in chloroform with the basis set 6-31++G(d,p) for nonmetal atoms and the pseudopotential LANL2DZ for Ru. Practically no geometrical changes are observed in the Ru environment when py ligand is replaced by MPyTPP. This replacement favors the electronic redistribution from bipy ligands to Ru, and from the metal to MPyTPP ligand, as indicated by NBO analysis. We found that M05 functional predicts very well the UVvis spectra, as it shows a low deviation with respect to the experimental data, with a maximum error of 0.19 eV (11 nm). M05 theoretical electronic spectrum of [Ru(bipy)2(MPyTPP)Cl]+ complex indicates that the presence of the Ru complex does not alter Q porphyrin bands, while charge transfer bands from Ru to bipy and porphyrin ligands mixes up in the region close to the porphyrin Soret band. Theoretical analysis allows the decomposition of this broad experimental band into specific ones identifying the Soret band and new metal to ligand charge transfers toward porphyrin at 425 and 478 nm, which were not possible in none of the moieties MPyTPP and [Ru(bipy)2(Py)Cl]+ complex. In the UV region, the most intense intraligand band of bipy ligands becomes slightly blue-shifted both in the experimental and in the theoretical spectrum of [Ru(bipy)2(MPyTPP)Cl]+ complex compared to that in [Ru(bipy)2(py)Cl]+ complex. Some of the bands of [Ru(bipy)2(MPyTPP)Cl]+ showed in this theoretical study may have practical applications. That is the case for the band at 478 nm, with potential use in PDT, and those more energetic at 348 and 329 nm, which could help in the cleavage mechanism of DNA performed by this ruthenium complex.

’ INTRODUCTION The wide diversity of the coordination and organometallic chemistry of ruthenium has motivated numerous studies with the aim of finding ruthenium complexes with anticancer activity.111 This research area has significantly increased in recent years since the complexes [ImH][trans-RuCl4(DMSO)Im] (NAMI-A; Im = imidazole, DMSO = dimethyl sulfoxide) and [IndH][trans-RuCl4In2] (KP1019; Ind = Indazole) have successfully completed the clinical trials of the first phase as antimetastatic drugs.3,4,1214 Although gene products and cellular transduction pathways are the targets of NAMI-A and KP1019 drugs,15 many other ruthenium-based anticancer compounds interact specifically with the classical target, DNA. Particularly, Ru(II) complexes with polypyridyl ligands have been extensively investigated as potential DNA binding agents as well as being implicated in the r 2011 American Chemical Society

oxidative damage of DNA.6,10,1623 Besides this, the absorption in the visible region of the electromagnetic spectrum of these Ru(II) complexes has been used for photodynamic therapy (PDT) of cancer.2426 To enhance their properties as suitable photosensitizers for PDT, it has recently been synthesized Ru(II) polypyridyl moieties (e. g., [Ru(bipy)2Cl]+; bipy =2,20 -bipyridine) covalently linked to other well-known photosensitizers like porphyrins.2732 Specifically, these conjugated complexes have provided the added water solubility and the rise of the quantum efficiency of the photochemical process. In UVvis spectroscopy, on one hand, Ru(II) polypyridyl complexes present a metal Received: March 13, 2011 Revised: July 23, 2011 Published: September 13, 2011 11988

dx.doi.org/10.1021/jp202377d | J. Phys. Chem. A 2011, 115, 11988–11997

The Journal of Physical Chemistry A (M) to ligand (L) charge transfer (MLCT) and intraligand (IL) bands, which have been shown to lead to the formation of reactive oxygen species (ROS) resulting in efficient cleavage of Scheme 1. Atom Numbering in the [Ru(bipy)2(MPyTPP)Cl]+ (MPyTPP = 5-Pyridyl-15,20,25-triphenylporphyrin) Complex

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DNA.33 On the other hand, porphyrins are characterized by the absorption Q-band, which leads to singlet molecular oxygen (1O2) through the formation of a triplet state of the porphyrin. The superposition of the bands and mixing of excitations such as M f bipy and M f L (L = porphyrin) often makes the spectral assignment difficult,20,24 and therefore, quantum-chemical computations can be of help in the interpretation of these electronic absorption spectra, which is very important to explain and further predict the photochemical behavior of these compounds. In this work we investigate the electronic excitations in the UVvis region of the [Ru(bipy)2(MPyTPP)Cl]+ (MPyTPP = 5-pyridyl-15,20,25-triphenylporphyrin) complex29 (see Scheme 1) by using time-dependent density functional theory (TD-DFT)34,35 methodology in conjunction with the basis set 6-31++G(d,p)36 for nonmetal atoms and the pseudopotential LANL2DZ for Ru.3739 In addition, we analyze how the structural and electronic factors affect this electronic spectra through the comparison of the electronic excitations in [Ru(bipy)2(MPyTPP)Cl]+, [Ru(bipy)2(py)Cl]+ and MPyTPP species.

Figure 1. B3LYP/6-31++G(d,p) (LANL2DZ for Ru) optimized geometries corresponding to (a) [Ru(bipy)2(MPyTPP)Cl]+, (b) [Ru(bipy)2(Py)Cl]+, and (c) MPyTPP. Two geometrical views are shown for [Ru(bipy)2(MPyTPP)Cl]+ and MPyTPP. Some relevant distances (angstrom) and angles (degrees) are displayed. 11989

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The Journal of Physical Chemistry A

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Figure 2. Electronic absorption spectra of MPyTPP calculated with the functionals B3LYP, M05, MPWB1K, and PBE0 along with the basis set 6-31++G(d,p) (LANL2DZ).

’ COMPUTATIONAL DETAILS Much effort has been devoted to theoretically investigate electronic absorption spectra of regular porphyrins and several derivates. These computational investigations have included calculations ranging from semiempirical,40,41 single and multireference (MR) configuration interaction (CI),4250 multiconfiguration second-order perturbation theory (CASPT2),5154 and TDDFT.42,4749,55,56 Excitation energies have also been calculated

Figure 3. Electronic absorption spectra of [Ru(bipy)2(py)Cl]+ complex calculated with the functionals B3LYP, M05, MPWB1K, and PBE0 and the basis set 6-31++G(d,p) (LANL2DZ). 11990



Figure 4. Electronic absorption spectra of [Ru(bipy)2(MPyTPP)Cl]+ complex calculated with the functionals B3LYP, M05, MPWB1K, and PBE0 and the basis set 6-31++G(d,p) (LANL2DZ).

at the similarity transformed equation-of-motion coupled cluster and perturbation theoretical levels (STEOM-CC46,57 and STEOMPT43,49). The less expensive TD-DFT has been shown to be in better agreement with experiment than the more expensive ab initio CI, CASPT2, or STEOM-CC calculations.43,49 Concerning polypyridyl ruthenium complexes, TD-DFT is the most

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widely used methodology to study their electronic absorption spectra.5863 However, to the best of our knowledge, the only theoretical study devoted to porphyrin-polypyridyl Ru(II) complexes has centered its attention on the ground-state geometric structures, molecular orbital energies and components, etc. using ab initio (HartreeFock, HF) and DFT methods, but not on the excited state properties.29 With all of this in mind, we calculated the absorption spectra as vertical excitations from the optimized ground-state geometries of the complexes [Ru(bipy)2(MPyTPP)Cl]+, [Ru(bipy)2(py)Cl]+ and MPyTPP within the TD-DFT framework as implemented in the Gaussian 03 series of programs.64 These ground-state structures were optimized applying the standard Schlegel’s algorithm65 and the DFT-B3LYP6668 method in conjunction with the basis set 6-31++G(d,p) for the nonmetal atoms and LANL2DZ for Ru. The B3LYP functional combines the Becke’s three-parameter nonlocal hybrid exchange potential with the nonlocal correlation functional of Lee, Yang, and Parr. The nature of the stationary points was verified by analytical calculations of harmonic vibrational frequencies. On the basis of our previous theoretical work on the spectroscopic properties of porphyrin derivates,69 the absorption spectra were calculated by using the hybrid density functionals B3LYP, M05,70,71 MPWB1K,72,73 and PBE0.74,75 The hybrid functionals B3LYP and PBE0 have 20% and 25% of HF exchange energy, respectively, while for M05 and MPWB1K hybrid-meta functionals this percentage grows to 28% and 44%, respectively. Solvent effects were taken into account in the calculations on the absorption spectra of the systems investigated in this work using a conductor-like polarizable continuum model (C-PCM)7679 with a dielectric constant of 4.9 to simulate chloroform as the solvent experimentally used for Ru(bipy)2(MPyTPP)Cl]+ and MPyTPP.29,30,80 Although for [Ru(bipy)2(py)Cl]+ experimental spectra were done in acetonitrile and dichloromethane,81,82 we used chloroform in all of the calculations for the sake of homogeneity. As it will be seen, theoretical results in this solvent are close to experimental ones. The inclusion of solvent effects in the calculations has been previously shown to be necessary in describing the absorption spectra both of porphyrin derivatives and polypyridyl ruthenium complexes.83,84 Finally, the natural bond orbital (NBO)85 approach was also used for interpretation purposes using the NBO 5.0 package. Natural bond orbitals are orbitals localized on one or two atomic centers, that describe molecular bonding in a manner similar to a Lewis electron pair structure, and they correspond to an orthonormal set of localized orbitals of maximum occupancy. In an idealized Lewis structure context, NBOs form a set of Lewis-type orbitals (formally filled) paired with a set of non-Lewis-type orbitals (formally empty). The former one includes a one-center core (CR), a valence lone pair (LP), and two-center bonds (BD), while unoccupied valence nonbonding (LP*), extra-valence-shell Rydberg (RY*), and valence antibonding (BD*) orbitals are included in the latter set. All possible interactions between filled Lewis-type NBOs (donor) and empty non-Lewis-type NBOs (acceptor) were analyzed on both [Ru(bipy)2(py)Cl]+ and [Ru(bipy)2(MPyTPP)Cl]+ complexes, and quantified in terms of their corresponding energy (E(2)) calculated by second-order perturbation theory. E(2) values provide the delocalization degree of the electronic density produced between a NBO orbital acting as a donor fragment, and another NBO orbital acting as an acceptor fragment. 11991



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Table 1. Theoretical and Experimental Excitation Energies (eV) (Wavelengths (nm) in Parentheses) Calculated in Chloroform Using Four Functionals and the LANL2DZ/6-31++G(d,p) Basis Set for MPyTPP and [Ru(bipy)2(py)Cl]+ Complexa B3LYP

M05

MPWB1K

PBE0

character

experimental

MPyTPP 2.16(574)

2.13 (583)

2.19 (565)

2.05 (605)

0.027 [3%]

0.019 [1%]

0.011 [4%]

0.041 [3%]

2.30 (539)

2.25 (552)

2.36 (526)

2.17 (572)

0.028 [2%]

0.024 [0.0%]

0.014 [5%]

0.040 [4%]

3.04 (408)

3.05 (407)

3.19 (389)

2.85 (435)

1.722 [2%]

1.816 [3%]

1.945 [7%]

1.109 [-4%]

Q

1.92 (646) ε=0.40b 2.10 (590) ε=0.60b

Q

2.25 (550) ε=0.70b 2.40 (516) ε=1.70b

Soret

2.97 (418) ε=25.0b

[Ru(bipy)2(py)Cl]+ 2.67 (464) 0.068 [7%, 8%]

2.59 (479) 0.097 [4%, 5%]

2.86 (434) 0.004 [13%, 14% ]

2.17(571) 0.083 [14%, 13%]

MLCT

2.48 (500) ε=0.80c 2.46 (505)d

3.65 (339)

3.54 (350)

3.50 (354)

2.97 (418)

MLCT

3.50 (354) ε=0.97c

0.118 [4%, 5%]

0.119 [0%, 2%]

0.122 [0%, 1%]

0.109 [18%,- 17%]

4.37 (284)

4.33 (286)

4.54 (273)

4.08 (304)

0.693 [2%]

0.628 [1%]

0.324 [6%]

0.254 [5%]

5.72 (217)

3.46 (358)d IL

4.28 (290)ε= 4.54c

IL

5.04 (246) ε=2.08c

0.096 [13%] a

The character of the calculated excitation band is included. Oscillator strengths (f) in italic and relative errors in square brackets. b Ref 80 in chloroform. c Ref 82 in acetonitrile. d Ref 81. Visible spectrum in dichloromethane. (ε 104 L mol1 cm1).

’ RESULTS AND DISCUSSION Ground-State Geometrical Structures. Our first task was to determine the optimized ground-state geometry of the [Ru(bipy)2(MPyTPP)Cl]+ complex at the B3LYP/6-31++G(d,p) (LANL2DZ for Ru) level of theory. For comparison purposes we also optimized the ground-state molecular structure of [Ru(bipy)2(py)Cl]+ and MPyTPP. Table S1 in the Supporting Information collects the corresponding optimized Cartesian coordinates and electronic energies. As it can be seen in Figure 1, both Ru complexes have a distorted octahedral geometry, where each bidentate bipy ligand presents a N atom in axial position and the other, in equatorial position, while the remaining equatorial positions are occupied by Cl and the N atom (N29 in Scheme 1) of py or MPyTPP (the equatorial plane is defined by Ru, Cl, and N29). According to our theoretical results, the replacement of the pyridine ligand at [Ru(bipy)2(py)Cl]+ by MPyTPP provokes a very slight variation of the geometrical parameters implying the linkage between Ru and its ligands. The major changes in bond distances take place in the plane defined by Ru, Cl and N29. Concerning the effect of replacing the pyridine ring at MPyTPP by [Ru(bipy)2(py)Cl]+ on the geometrical data of the core atoms of porphyrin, the most significant changes were found for the orientation of the porphyrin meso aryl substituents with respect to the molecular plane defined by the porphyrin core atoms (see Figure 1). Natural Bond Orbital (NBO) Analysis. A NBO analysis of the [Ru(bipy)2(py)Cl]+ and [Ru(bipy)2(MPyTPP)Cl]+ structures calculated at the B3LYP/6-31++G(d,p) (LANL2DZ for Ru) level of theory also corroborates the geometrical similarity found for the coordination environment of Ru at these complexes. As it can be seen in Table S2 in the Supporting Information, for both complexes, the most significant donoracceptor interactions are of the type LP N(py) (donor) f LP* Ru (acceptor) and LP N(bipy) (donor) f LP* (Ru) (acceptor) whose E(2) values are in the range of 140169 kcal/mol. Weaker interactions for the reverse

electron delocalization corresponding to LP (Ru) (donor) f π* (or σ*) N(py) (acceptor) and to LP (Ru) (donor) f π* (or σ*) N(bipy) (acceptor) are found. This means that the both ruthenium complexes are stabilized by the charge transfer from the nitrogenated ligands to the metal center with a very weak backdonation. Besides this, it is noteworthy that, in general, the replacement of the py ligand at [Ru(bipy)2(py)Cl]+ by MPyTPP provokes a slight decrease of the stabilizing character of the interactions LP(Npy)fLP*(Ru), LP(Nbipy)fLP*(Ru), σ(Npy-C)f LP*(Ru), σ(Nbipy-C)fLP*(Ru), π(Npy-C)fLP*(Ru), and LP(Ru)f π*(Npy-C) while the ones π(Nbipy-C)fLP*(Ru), LP(Ru)f σ*(Npy-C), and LP(Ru)f π*(Npy-C) present a small increase. The remaining donoracceptor interactions practically do no change due to the presence of the porphyrin moiety at the Ru complex. Therefore, all of these data indicate that the replacement above-mentioned favors an electronic redistribution from bipy ligands to Ru, and from the metal to MPyTPP ligand. Electronic Absorption Spectra. Figures 2, 3, and 4 displays the electronic absorption spectra of porphyrin MPyTPP, and the complexes [Ru(bipy)2(py)Cl]+ and [Ru(bipy)2(MPyTPP)Cl]+, respectively, calculated in chloroform solution with the four density functionals B3LYP, M05, MPWB1K, and PBE0. Table 1 collects the most significant excitation energies, wavelengths, λ, and oscillator strengths calculated for the moieties MPyTPP and [Ru(bipy)2(py)Cl]+, as well as their experimental electronic spectra. Table 2 shows this information for [Ru(bipy)2(MPyTPP)Cl]+ along with the molecular orbitals involved in each electronic transition obtained with M05 functional. Concerning porphyrin moiety, the experimental electronic spectrum in chloroform shows four low intensity Q bands in the range of 646 to 516 nm, corresponding to a π f π* transition, and a very intense Soret band at 418 nm.80 The four functionals locate only two Q bands. This result has been previously reported for the theoretical spectra of many porphyrin systems calculated with different functionals, and it can be explained by the near degeneration of the frontier molecular orbitals involved in these electronic 11992



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Table 2. Theoretical and Experimental Excitation Energies (eV) (Wavelengths (nm) in Parentheses) Calculated in Chloroform Using M05 Functional and the LANL2DZ/6-31++G(d,p) Basis Set for [Ru(bipy)2(MpTPP)Cl]+ Complexa M05

character

2.09 (592) 0.078 H-2 f L H-2 f L+1

Q 0.18 0.17

H-1 f L+2

0.07

HfL

0.34

H f L+1

0.24 Q 0.25

H-2 f L+1

0.15

HfL H f L+1

0.30 0.31 MLCT 0.11

H-3 f L+3

0.10

H-1 f L

0.72

H-1 f L+5

0.03

H-1fL+11

0.03

HfL 2.92 (425) 0.682

0.02 2.99 (415) 1.021

3.03 (409) 0.2826

3.05 (407) 0.4718

3.06 (405) 0.5897

H-4 f L

0.69

H-3 f L+1

0.21

H-4 f L

0.06

H-4 f L

H-4 f L

H-4 f L+5

0.04

H-2 f L

0.39

H-3 f L+1

0.78

H-4 f L+1

0.13

H-4f L+3

0.11

H-3 f L

0.03

H-1 f L+1

0.08

H-2 f L

0.05

H-4 f L+12

0.25

H-4fL+11

0.06

H-2 f L+1

0.17

H f L+1

0.24

H-2 f L+1

0.07

H-3 f L+1

0.27

H-4f L+12

0.13

HfL

0.06

H f L+3

0.08

H f L+3

0.04

H-3fL+11

0.10

H-3 f L+1

0.14

H-2 f L

0.07

H-2 f L+1

0.32

H-2 f L+1 H-1 f L+1

0.33 0.72

H-1 f L+12 HfL

0.28 0.07

HfL

0.10

H f L+3

0.25

Hf L+12

0.09

H f L+5

0.06

0.19

3.19 (388) 0.033 H-4f L+1

Soret + MLCT

2.95 (421.0) ε =11.95

MLCT

3.34 (371) sh

0.24

1.00

3.56 (348) 0.296

3.77 (329) 0.1157

H-8 f L

0.07

H-8 f L+1

0.08

H-8 f L+1 H-7 f L

0.03 0.33

H-7 f L H-7 f L+1

0.27 0.36

H-7 f L+1

0.10

H-2 f L+4

0.05

H-6 f L

0.20

H-2 f L+5

0.25

H-5 f L

0.26

H-5 f L+1

0.11

H f L+5

0.12

ILP

4.44 (279) 0.648

a

2.24 (554.5) ε =0.56 2.41(516.0)ε=1.03

2.60 (478) 0.120 H-4 f L+3

1.92 (647.5) ε = 0.02* 2.11(589.5)ε=0.29

2.21 (562) 0.109 H-2 f L

experimental

ILbipy

H-16 f L+2 H-15 f L+2

028 0.04

H-15 f L+3

0.56

H-14 f L+3

0.05

H-13 f L+3

0.03

H-4 f L+10

0.05

4.63 (268.0) ε =2.56

The character of the calculated excitation band is included. Oscillator strengths (f) in italic. ε 104 L mol1 cm1. sh: shoulder.

transitions (HOMO, HOMO1) and (LUMO, LUMO+1). Besides, the four functionals used in this work provide good description of the strong Soret band, being B3LYP and M05 those rendering the smallest relative errors in excitation energies.

The experimental electronic spectrum of the [Ru(bipy)2(py)Cl]+ complex in acetonitrile82 presents two low intensity bands at 500 and 354 nm. In this region the MLCT toward bipy and py ligands are present. Further, two intense bands, at 290 nm 11993



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Figure 5. Molecular orbitals and their energy (a.u.) involved in the most significant electron transitions of the [Ru(bipy)2(MPyTPP)Cl]+ complex electronic spectrum obtained in chloroform at the M05/6-31++G(d,p) (LANL2DZ) theory level.

(absorption coefficient, ε = 4.54 104 M1 cm1) and 246 nm (ε = 2.08 104 M1 cm1), for bipy f bipy intraligand transfers are also displayed in the experimental spectrum. B3LYP and M05 spectra in chloroform render the best agreement with these data, both for the wavelengths of the bands and for their relative intensity, although M05 seems to describe better this last aspect. It is interesting to note that the intensities of the theoretical bands for [Ru(bipy)2(py)Cl]+ complex are smaller than those calculated for porphyrin MPyTPP. MPWB1K places all of the

bands at shorter wavelengths and does not reproduce their relative intensities, and PBE0 gives an abnormally large weight to the bands found at long wavelengths. The experimental spectrum of [Ru(bipy)2(MPyTPP)Cl]+ complex in chloroform29,30 shows absorptions in two regions, in the visible (λ > 400 nm), with five bands at 647.5, 589.5, 554.5, 516.0, 421.0 nm, and in the UV region (λ < 400 nm), with two bands at 371.0 and 268.0 nm (see experimental absorption coefficients at Table 2). Supporting Information collects 11994


The Journal of Physical Chemistry A excitation energies, wavelengths, oscillator strengths, and relative errors in electronic energy for the theoretical spectra of [Ru(bipy)2(MPyTPP)Cl]+ complex calculated with the four functionals considered in this work. The theoretical spectra drawn in Figure 4 indicate that B3LYP and PBE0 are unable to reproduce these two absorption regions, so the assignment of their theoretical bands to the experimental ones must be necessarily confusing. As M05 has performed the best description of the species MPyTPP, [Ru(bipy)2(py)Cl]+, we shall analyze in detail the spectra of [Ru(bipy)2(MPyTPP)Cl]+ obtained with this functional and we shall compare this spectrum with those above-described for MPyTPP, [Ru(bipy)2(py)Cl]+. For a better understanding of the nature of the electronic transitions, we shall analyze the molecular orbitals (MOs) involved in each of them, which are displayed in Figure 5. One of the main uses of theoretical calculations is their capability to decompose an experimental band into specific transitions. Thus, in the visible region the five experimental bands of [Ru(bipy)2(MPyTPP)Cl]+ complex are split in as many as eight theoretical significant ones: two at 592 and 562 nm (see Table 3) with low intensity and mainly involving HOMO-2, HOMO, LUMO, and LUMO+1 orbitals. As can be seen in Figure 5, these orbitals mainly belong to the porphyrin moiety, so this long wavelength excitations can be assigned to the Q bands of the isolated porphyrin, which seem not to be much affected by the presence of the ruthenium complex; the remaining six transitions locate in the range of 478 to 405 nm and can be classified into three groups. On the one hand, those at 478, 425, and 409 nm involve a transfer from HOMO-1, HOMO-3 or HOMO-4 occupied molecular orbitals, all of them extended over Ru metal and one of the bipy ligands, to LUMO or LUMO+1 empty molecular orbitals, mainly spread through porphyrin moiety, so they can be considered MLCT toward porphyrin ligand. It is interesting to remark how the theoretical spectrum uncovers the existence of a band at 478 nm which could be useful in PDT treatment of cancer diseases, as it has higher penetration depth in tumor tissues than the Q ones with an acceptable energy value. The second group contains only a band at 415 nm, which is the most intense and involves molecular orbitals almost completely centered at the porphyrin, such as HOMO-2 and LUMO, so can be assigned to the Soret band. Finally, the transitions at 407 and 405 nm are complex combinations of transitions involving transfers to both porphyrin and bipy ligands. Concerning the UV region of [Ru(bipy)2(MPyTPP)Cl]+ spectrum four bands have been theoretically found to justify two experimental ones. The band at 388 nm is assigned to MLCT, as it is produced by one transition from HOMO-4 (Ru and bipy ligand) to LUMO+1 (porphyrin). The low oscillator strength of this band (0.033) shows coherence with the MLCT experimental band found in this region (371 nm) that corresponds to a shoulder peak.30 Compounds similar to the ruthenium-porphyrin complex here considered also show a shoulder peak (MLCT) in their experimental spectra, such as those bearing flourine phenyl (360 nm),31 dimethoxy phenyl (360 nm), and methoxy and hidroxy phenyl (370 nm) substituents in meso positions of porphyrin.86 Theoretical spectrum also includes two bands at 348 and 329 nm that are assigned to IL transitions involving the porphyrin moiety HOMO7, LUMO, and LUMO +1 molecular orbitals. The band at 279 nm (4.44 eV) is the most energetic one and clearly corresponds to the IL transition between MOs belonging to bipy ligand. Theoretical spectrum of [Ru(bipy)2(MpTPP)Cl]+ complex agrees with experimental

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one, as both display this last band blue-shifted with respect to the IL one observed for [Ru(bipy)2(py)Cl]+ complex. This fact explains why this complex could be useful to generate the energy necessary for the formation of the reactive oxygen species (ROS), which have been shown to produce an efficient cleavage of DNA. On the other hand, it is known that low-lying TD-DFT excitation energies associated with long-range charge transfer (IL) are sometimes problematic to be described by standard xc functionals producing errors of several eV.87 In our case, we found that the long-range charge transfer band (4.44 eV) predicted with M05 functional is underestimated by 0.19 eV (11 nm) with respect to the experimental one (4.63 eV) which represents an error of 5%, that indicates a reasonable theoretical result.

’ CONCLUSIONS We have studied the electronic excitations of the [Ru(bipy)2(MPyTPP)Cl]+ (MPyTPP = 5-pyridyl-15,20,25-triphenylporphyrin) complex and of its related species [Ru(bipy)2(py)Cl]+ and MPyTPP, by using TD-DFT methodology. Practically no geometrical changes are observed in the Ru environment when py ligand is replaced by MPyTPP. This replacement favors the electronic redistribution from bipy ligands to Ru, and from the metal to MPyTPP ligand, as indicated by NBO analysis. A set of functionals (B3LYP, M05, MPWB1K, and PBE0) was used for the calculation of the UVvis absorption spectra at the solution phase (chloroform) calculated with the basis set 6-31++G(d,p) for nonmetal atoms and the pseudopotential LANL2DZ for ruthenium. As it was previously reported for porphyrin derivative systems in absence of Ru,69 we found that M05 is the best functional to predict the UVvis spectra of the three molecular systems above-mentioned, and in particular in [Ru(bipy)2(MPyTPP)Cl]+ complex, it produces a small deviation with respect to the experimental data of 0.19 eV. M05 theoretical electronic spectrum of [Ru(bipy)2(MPyTPP)Cl]+ complex displays the Q porphyrin bands practically at the same wavelength and with the same low intensity as in the isolated MPyTPP moiety, so the presence of the Ru complex does not alter them. Charge transfer bands from Ru to bipy and porphyrin ligands intercross in the region close to the porphyrin Soret band (at 421 nm in the experimental spectrum). The theoretical spectrum and the analysis of the shape of the MOs involved in each transition allows the decomposition of this broad experimental band into specific ones identifying the Soret band and making clear the presence of new metal to ligand charge transfers toward porphyrin at 478 and 425 nm which were not possible in none of the moieties MPyTPP and [Ru(bipy)2(Py)Cl]+ complex. Although more energetic than Q bands, these new ones could still be useful in the photodynamic treatment of cancer diseases. In the UV region the most intense intraligand band of bipy ligands becomes blue-shifted by about 7 nm in the theoretical spectrum of [Ru(bipy)2(MPyTPP)Cl]+ complex compared to that in [Ru(bipy)2(Py)Cl]+ complex, indicating that the presence of the porphyrin ligand slightly affects it. ’ ASSOCIATED CONTENT

bS

Supporting Information. Table S1, B3LYP/6-31++G(d,p) optimized Cartesian coordinates and electronic energies of the species [Ru(bipy)2(MPyTPP)Cl]+, [Ru(bipy)2(py)Cl]+, MPyTPP)Cl; Table S2, Second-order perturbation energies, E(2), for the most significant “donoracceptor” (bond-antibond)

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The Journal of Physical Chemistry A interactions in the NBO basis for [Ru(bipy)2(py)Cl]+ and [Ru(bipy)2(MPyTPP)Cl]+; Table S3, Excitation energies, wavelengths, oscillator strengths and relative errors in electronic energy for the spectra of [Ru(bipy)2(MPyTPP)Cl]+ complex calculated with the four functionals considered in this work. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (G.I.C.-J.); [email protected] (R.L.).

’ ACKNOWLEDGMENT The authors thank MEC (SPAIN, CTQ2010-18231) and FONDECYT No. 1090700 (CONICYT/CHILE) for financial support. G.I.C.-J. thanks to DICYT/USACH Complementary Support for computational time provided. ’ REFERENCES (1) Alessio, E.; Mestroni, G.; Bergamo, A.; Sava, G. Met. Ions Biol. Syst. 2004, 42, 323–351. (2) Ang, W. H.; Dyson, P. J. Eur. J. Inorg. Chem. 2006, No. 20, 4003–4018. (3) Bergamo, A.; Sava, G. Dalton Trans. 2007, No. 13, 1267–1272. (4) Bratsos, I.; Jedner, S.; Gianferrara, T.; Alessio, E. Chimia 2007, 61, 692–697. (5) Bruijnincx, P. C. A.; Sadler, P. J. Curr. Opin. Chem. Biol. 2008, 12, 197–206. (6) Clarke, M. J. Coord. Chem. Rev. 2003, 236, 209–233. (7) Dougan, S. J.; Sadler, P. J. Chimia 2007, 61, 704–715. (8) Dyson, P. J. Chimia 2007, 61, 698–703. (9) Heffeter, P.; Jungwirth, U.; Jakupec, M. A.; Hartinger, C. G.; Galanski, M.; Elbling, L.; Micksche, E.; Keppler, B. K.; Berger, W. Drug Resist. Updates 2008, 11, 1–16. (10) Levina, A.; Mitra, A.; Lay, P. A. Metallomics 2009, 1, 458–470. (11) Reitner, E.; Arion, V. B.; Keppler, B. K.; Pombeiro, A. J. L. Inorg. Chim. Acta 2008, 361, 1569–1583. (12) Hartinger, C. G.; Jakupec, M. A.; Zorbas-Seifried, S.; Groessl, M.; Egger, A.; Berger, W.; Zorbas, H.; Dyson, P. J.; Keppler, B. K. Chem. Biodiversity 2008, 5, 2140–2155. (13) Lentz, F.; Drescher, A.; Lindauer, A.; Henke, M.; Hilger, R. A.; Hartinger, C. G.; Scheulen, M. E.; Dittrich, C.; Keppler, B. K.; Jaehde, U. Anti-Cancer Drugs 2009, 20, 97–103. (14) Sava, G.; Capozzi, I.; Clerici, K.; Gagliardi, G.; Alessio, E.; Mestroni, G. Clin. Exp. Metastasis 1998, 16, 371–379. (15) Dyson, P. J.; Sava, G. Dalton Trans. 2006, No. 16, 1929–1933. (16) Armistead, P. M.; Thorp, H. H. Anal. Chem. 2001, 73, 558–564. (17) Carter, P. J.; Cheng, C. C.; Thorp, H. H. J. Am. Chem. Soc. 1998, 120, 632–642. (18) Deng, H.; Li, J.; Zheng, K. C.; Yang, Y.; Chao, H.; Ji, L. N. Inorg. Chim. Acta 2005, 358, 3430–3440. (19) Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. Rev. 1999, 99, 2777–2796. (20) Liu, J.; Mei, W. J.; Lin, L. J.; Zheng, K. C.; Chao, H.; Yun, F. C.; Ji, L. N. Inorg. Chim. Acta 2004, 357, 285–293. (21) Vialas, C.; Claparols, C.; Pratviel, G.; Meunier, B. J. Am. Chem. Soc. 2000, 122, 2157–2167. (22) Vos, J. G.; Kelly, J. M. Dalton Trans. 2006, No. 41, 4869–4883. (23) Yang, I. V.; Thorp, H. H. Inorg. Chem. 2000, 39, 4969–4976. (24) Adelt, M.; Devenney, M.; Meyer, T. J.; Thompson, D. W.; Treadway, J. A. Inorg. Chem. 1998, 37, 2616–2617. (25) Sizova, O. V.; Ershov, A. Y.; Ivanova, N. V.; Shashko, A. D.; Kuteikina-Teplyakova, A. V. Russ. J. Coord. Chem. 2003, 29, 530–536.

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Theoretical Study on the Electronic Excitations of a Porphyrin

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