- Email: [email protected]
A TDDFT study of the ruthenium(II) polyazaaromatic complex [Ru(dppz)(phen)2]2+ in solution
Chemical Physics Letters 396 (2004) 43–48 www.elsevier.com/locate/cplett
A TDDFT study of the ruthenium(II) polyazaaromatic complex [Ru(dppz)(phen)2]2+ in solution Simona Fantacci a
a,*
, Filippo De Angelis a, Antonio Sgamellotti a, Nazzareno Re
b
Istituto CNR di Scienze e Tecnologie Molecolari (ISTM), Dipartimento di Chimica, Universita` di Perugia, via Elce di Sotto 8, I-06123, Perugia, Italy b Facolta` di Farmacia, Universita` ‘‘G. DÕAnnunzio’’, via dei Vestini 13, I-66100, Chieti, Italy Received 1 July 2004; in final form 16 July 2004
Abstract DFT/TDDFT calculations were performed to investigate the structural, electronic and optical properties of the [Ru(dppz)(phen)2]2+ complex in solution. TDDFT calculations in water show two groups of metal-to-ligand charge transfer (MLCT) transitions at 450 and 415 nm whose superposition gives account of the broad absorption band experimentally characterized at 440 nm. Also, a group of almost coincident MLCT transitions partially mixed with dppz intraligand p–p* transitions centered at 380 nm is found to give rise to the narrow absorption band experimentally found at 380 nm. Our results provide insight into the hypochromic shifts experimentally characterized upon intercalation of the title complex into DNA. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction There has been increasing attention in the last decade to the design of small complexes containing spectroscopically active metal centers as probes of DNA structure and function. In particular, several ruthenium(II) polypyridyl complexes have been prepared which bind to DNA by intercalation [1–5]. Most of these Ru(II) complexes are based on byp (2,2 0 -bipyridine) and phen (1, 10-phenantroline) ligands but larger bidentate polyazaaromatic ligands have been employed in attempts to increase their binding affinity for DNA (through increased p-stacking with nucleobases) and possibly to modulate their bases sequence specificity, such as dpq (bypirido[3,2-f:2 0 ,3 0 -h]quinoxaline) and dppz (bypirido[3,2a:2 0 ,3 0 -c]phenazine) [1,2]. All these complexes are intensely colored due to a well characterized metal-to-ligand charge transfer (MLCT) transition. Such a transition is perturbed on
*
Corresponding author. Fax: +39 075 585 5606. E-mail address: [email protected] (S. Fantacci).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.101
binding to DNA which has led to the development of a family of spectroscopic probes. Extensive photophysical studies have indicated that these complexes show peculiar photoluminescence properties depending on the nature of the polyazaaromatic ligands and on the environment [1–12]. For instance, [Ru(dqp)(L)2]2+ and [Ru(dppz)(L)2]2+, L = phen, byp, complexes show no luminescence in water but brightly luminesce upon intercalation between adjacent DNA base pairs [7–9]. In spite of their increasing interest, a few theoretical studies have been performed on these Ru(II) polypyridyl complexes, mainly on complexes with the simplest byp and phen ligands, based on both semiemprirical [10–12] and DFT approaches [13–21]. Only recently, an extensive series of semiempirical calculations on several ligands and related Ru complexes has appeared in the literature [22]. In the same study, time dependent density functional theory (TDDFT) calculations in vacuo on the [Ru(dppz)(phen)2]2+ complex, which is one of the most widely investigated among the ruthenium(II) polypyridyl complexes, have been performed, but the authors focused their attention on the characterization of triplet excited states, which are indeed relevant to
44
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
the photoluminescence of the investigated complexes, describing the optical absorption spectrum at the semiempirical level [22]. It is therefore interesting to investigate the singlet excited states of the [Ru(dppz)(phen)2]2+ complex by means of TDDFT calculations in solution, thus allowing an accurate characterization and assignment of the UV– vis absorption features and the direct comparison of calculated absorption spectra with solution experimental data. In this paper we report DFT/TDDFT calculations on the [Ru(dppz)(phen)2]2+ complex in vacuo and in acetonitrile and water solvents to investigate its structural, electronic and optical properties.
2. Computational details All the calculations reported in this paper have been performed using the GAUSSIAN 03 (G03) program package [23]. Geometry optimizations were carried out, without any symmetry constraints, using both a polarized split-valence 3-21G* [24] (BS1) and a double-f LANL2DZ basis set [25] with the relative pseudopotential for the metal atom [26] (BS2). The B3LYP exchangecorrelation (XC) functional [27] was used for all the calculations. TDDFT excitation energies were computed both in vacuo and in solution; calculations in solution were performed considering the non-equilibrium version [28] of the conductor polarizable continuum model (CPCM) [29,30]. We note that in the present G03 implementation dispersion-repulsion and cavitation contributions cancel out in the evaluation of solvent shifts since they are identical for different electronic states. The effect of basis set expansion on the TDDFT excitation energies and oscillator strengths was checked considering BS1 and the larger DGDZVP [31] basis set (BS3), of double-f + polarization quality. The 20 (30) lowest spin-allowed singlet–singlet transitions, up to a wavelength of 370 (320) nm were taken into account considering BS3 (BS1).
3. Results and discussion 3.1. Molecular structure The optimized geometry of [Ru(dppz)(phen)2]2+ is reported in Fig. 1 and main optimized geometrical parameters obtained with BS1 and BS2 are compared in Table 1 with experimental data for the [Ru(dmp)2(dppz)]-(PF6)2 species, [32] (dmp = 2,9-dimethyl 1,10-phenanthroline) the closest experimentally characterized complex for which X-ray data are available. Optimized structures obtained with both basis sets show a pseudooctahedral coordination of the RuN6 core, with
Fig. 1. Optimized molecular structure of the [Ru(dppz)(phen)2]2+ complex.
Table 1 Comparison between main computed geometrical parameters (bond ˚ and angles in degrees), obtained with LANL2DZ and lengths in A 3-21g* basis sets, and X-ray data for the [Ru(dmp)2(dppz)](PF6)2 species [32]
Ru–N1 Ru–N2 Ru–N3 Ru–N4 Ru–N5 Ru–N6 Ru–N(Av.) N1–Ru–N2 N3–Ru–N4 N5–Ru–N6
Experiment
B3LYP/LANL2DZ
B3LYP/3-21g*
2.096(2) 2.109(2) 2.102(2) 2.125(2) 2.098(2) 2.094(2) 2.104 79.41(9) 78.85(9) 78.86(9)
2.106 2.107 2.106 2.108 2.107 2.107 2.107 79.4 79.4 79.2
2.112 2.113 2.113 2.112 2.115 2.115 2.113 79.3 79.3 79.1
N–Ru–N angles close to 80°. We notice that while the experimental structure shows slight differences between Ru and N distances within the phen ligands, all the Ru–N distances optimized with BS2 (BS1) show similar ˚ (2.112–2.115 A ˚ ), values in the range 2.106–2.108 A probably reflecting the absence in the optimized structure of the methyl substituents in the 2 and 9 positions of the phen ligand. However, average Ru–N distances ˚ are computed with BS2, in good agreement of 2.107 A ˚ , while BS1 prowith the experimental value of 2.104 A vides slightly longer Ru–N distances (Table 1). On the basis of the slightly better agreement of BS2 geometrical parameters with experimental values, we used the BS2 optimized structure for the results presented in the following. 3.2. Electronic structure In Fig. 2 we report a schematic representation of the energy and character of the frontier orbitals of the title complex, computed with the larger BS3 in vacuo,
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
-6.0
-2.0 vacuo
E(eV)
-7.0
-8.0
45
E(eV)
π∗dppz π∗phen+π∗dppz
-3.0
π∗dppz +π∗phen π∗phen π∗dppz+ t2g(Ru) π∗phen+ t2g(Ru) π∗phen
water
π∗phen π∗phen π∗phen+ t2g(Ru) π∗phen+π∗dppz π∗dppz +π∗phen π∗dppz
LUMO (189)
LUMO LUMO
-4.0
∆E=3.36eV
-9.0
acetonitrile
π∗dppz
∆E=3.23eV
-5.0
-6.0
-10.0
HOMO πdppz
-11.0
∆E=3.19eV
t2g(Ru) πdppz
HOMO
t2g(Ru)
HOMO (188) -7.0
πdppz πdppz
Fig. 2. Energy levels (eV) of the [Ru(dppz)(phen)2]2+ complex in vacuo and in acetonitrile and water solvents.
acetonitrile and water. Minor differences in the electronic structure are computed with BS1. Inclusion of solvation effects partially alters the composition of the frontier orbitals, and deeply affects the absolute orbital energies, which are raised by 4 eV on going from vacuo to solution, reflecting the stabilization of the doubly positively charged Ru center in polar solvents. Since the experimental spectra are recorded in solution, and we have recently shown the importance of solvation in determining the electronic structure and spectrum of Ru(II)–polypiridyl complexes, [19–21] we will discuss in detail only data in solution. In particular, both in acetonitrile and water, the first three HOMOs (orbitals 188–186) correspond to the ruthenium t2g set (an isodensity contour plot of the HOMO 2 has been reported in Fig. 3) whereas HOMO 3 (see Fig. 3) and HOMO 4 (orbitals 185 and 184) are p bonding combinations of the dppz ligand. In solution the LUMO (orbital 189) has a pure p*-dppz character (see Fig. 3), while LUMO + 1 and LUMO + 2 (orbitals 190 and 191) are a pair of mixed dppz and phen p* combinations with dominant contributions arising from dppz and phen ligands in LUMO + 1 and LUMO + 2, respectively. LUMO + 3/LUMO + 5 (192–
194) have p*-phen character, see Fig. 3, while LUMO + 6 is a higher-lying p* combination of the dppz ligand. We also notice that a small reduction of the HOMO–LUMO gap is computed with BS3 (BS1) on going from acetonitrile to water, with a decrease from 3.23 (3.24) to 3.19 (3.13) eV, respectively, as a consequence of the slightly higher destabilization of the occupied with respect to the unoccupied orbitals which follows the increase of solvent polarity. 3.3. UV–vis spectrum The experimental [Ru(dppz)(phen)2]2+ spectrum in solution shows the presence of two bands of comparable intensity, centered at approximately 440 and 380 nm, which were originally assigned as having MLCT and pp* character, respectively [7]. We report in Table 2 computed vertical excitation energies and the composition of the related transitions obtained using BS3 both in vacuo and in solution, considering those transitions characterized by an oscillator strength (f) larger than 0.05. Inclusion of solvation effects introduces relevant changes to the spectrum with respect to that computed in vacuo, see below, both in terms of absorption
46
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
Fig. 3. Isodensity surface plots (isodensity value = 0.03) of the HOMO 3 (185), HOMO 2 (186), LUMO (189) and LUMO + 3 (192) of the [Ru(dppz)(phen)2]2+ complex in water.
Table 2 Computed excitation energies (eV) and oscillator strengths (f) for the optical transitions with f > 0.05 of the [Ru(dppz)(phen)2]2+ complex both in vacuo and in solution (acetonitrile and water) in terms of single molecular orbitals excitations with percentages larger than 5% nm
f
B3LYP/DGDZVP (vacuo) 416.9 0.073 43.3%(t2g-Ru)185 ! (p*-phen + t2gRu)190; 36.6%(t2g-Ru)187 ! (p*-phen)192; 9.5%(t2g-Ru)186 ! (p*-dppz + t2g-Ru)191; 6.2%(t2g-Ru)186 ! (p*-phen)189 410.3 0.124 48.4%(t2g-Ru)186 ! (p*-dppz + t2g-Ru)191; 37.3%(t2g-Ru)187 ! (p*-phen)192 390.1 0.075 66.9%(t2g-Ru)186 ! (p*-phen)192; 10.7%(t2g-Ru)187 ! (p*-dppz + p*-phen)193 B3LYP/DGDZVP (acetonitrile) 445.2 0.122 82.5%(t2g-Ru)186 ! (p*-dppz)189; 10.0%(t2g-Ru)186 ! (p*-dppz+p*-phen)190 414.2 0.113 42.9%(t2g-Ru)186 ! (p*-phen + t2g-Ru)192; 42.6%(t2g-Ru)187 ! (p*-phen + p*-dppz)191; 5.1%(t2g-Ru)188 ! (p*-phen)194 412.8 0.085 45.9%(t2g-Ru)187 ! (p*-phen + t2gRu)192; 19.6%(t2g-Ru)188 ! (p*-phen)193; 18.5%(t2g-Ru)186 ! (p*-dppz + p*-phen)190; 5.6%(t2g-Ru)186 ! (p*-dppz)189; 5.5%(t2g-Ru)186 ! (p*-phen + p*-dppz)191 384.3 0.085 78.5%(t2g-Ru)186 ! (p*-phen)193; 5.8%(t2g-Ru)188 ! (p*-phen)194; 5.2%(p-dppz)185 ! (p*-dppz)189 382.4 0.138 60.9%(t2g-Ru)188 ! (p*-dppz)195; 27.9%(t2g-Ru)187 ! (p*-phen)193 B3LYP/DGDZVP (water) 449.2 0.071 58.2%(t2g-Ru)186 ! (p*-dppz)189; 33.6%(t2g-Ru)188 ! (p*-phen + t2g-Ru)192 414.7 0.113 43.5%(t2g-Ru)186 ! (p*-phen + t2g-Ru)192; 41.3%(t2g-Ru)187 ! (p*-phen + p*-dppz)191; 5.5%(t2g-Ru)188 ! (p*-phen)194 413.8 0.101 46.5%(t2g-Ru)187 ! (p*-phen + t2g-Ru)192; 20.1%(t2g-Ru)186 ! (p*-dppz + p*-phen)190; 13.9%(t2g-Ru)188 ! (p*-phen)193; 8.5%(t2g-Ru)186 ! (p*-phen + p*-dppz)191 384.4 0.064 47.4%(p-dppz)185 ! (p*-dppz)189; 36.3%(t2g-Ru)186 ! (p*-phen)193 383.7 0.137 54.2%(t2g-Ru)188 ! (p*-dppz)195; 36.5%(t2g-Ru)187 ! (p*-phen)193 In parentheses we report the character of the orbitals involved in the transition; orbital numbering is the same of Fig. 2.
wavelengths and band separation. A comparison of the experimental spectrum in water from [6] in the range 480–350 nm and calculated excitation energies and oscilla-
tor strengths is reported in Fig. 4. In water we find five transitions with f > 0.05 lying in the range 450–370 nm. In particular, the intense transition at 449.2 nm
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
Fig. 4. Experimental spectral shape (black line) from [6], compared to excitation energies and oscillator strengths computed with BS3 in water (red vertical lines).
(f = 0.071) has MLCT character and mainly originates from the t2g-Ru set to the p*-dppz LUMO (orbital 189) with a sizable contribution (30%) of phen p* states, see Table 2. This main transition is followed by two quasi degenerate MLCT transitions of similar intensity (f = 0.113 and 0.101) found at 414.7 and 413.8 whose final states are p* orbitals (190–192) delocalized over both the phen and dppz ligands, with larger contributions arising from the phen ligands. We therefore see that the broad experimentally characterized band at 440 nm is assigned to be a superposition of two MLCT features (respectively named Ia and Ib in Fig. 4) which differ for the character of the arriving states, Ia being dominated by transitions to the dppz ligand, Ib being composed by transitions mainly localized on the phen ligands. At shorter wavelengths, we find an intense MLCT transition 383.7 nm from the t2g-Ru set to the LUMO and higher lying p*-dppz and -phen orbitals (193,195). Interestingly, the multitransition at 384.4 nm with f = 0.064 involves transitions from the t2g-Ru HOMO2 (186) to the p*-phen LUMO + 4 (193) and from the p-dppz HOMO-3 (185) to the p*-dppz LUMO (189), see Table 2, therefore showing a mixed MLCT and intraligand dppz p ! p* character. We note that an intraligand dppz p ! p* transition at 365 nm has also been characterized by semiempirical calculations in [22]. The almost coincident transitions at 384.4 and 383.7 nm constitute the second absorption band of the experimental spectrum (II in Fig. 4), thus explaining the narrower shape of band II with respect to band I. Overall, our computed excitation energies and oscillator strengths are in good agreement with the experimental spectrum. We note that the band assignments based on our TDDFT calculations in water solution can provide an explanation for the spectral modifications of the title complex upon intercalation into DNA, showing considerable hypochromic shifts of the spectral features at
47
440 and 380, which are possibly related to the presence of p* orbitals of the intercalating dppz ligand in the arriving states of the transitions computed at 449 and 384 nm (Table 1). In acetonitrile a similar band assignment to that made in water holds, with the absorption spectrum characterized by a broad absorption band and a narrow band at shorter wavelengths. It is interesting to point out that in acetonitrile the first relevant transition is only slightly blue-shifted with respect to that in water (445.2 vs. 449.2 nm), again in agreement with the experimental data [7]. On the technical side, we note that BS1 provides a similar description of the spectra in solution as BS3; as an example, the intense transition originating the longer wavelength feature of the broad visible absorption band is found at 448.3 and 460.3 nm in acetonitrile and water, respectively, to be compared to BS3 values of 445.2 and 449.2 nm, confirming that MLCT transitions in Ru–polypyridyl complexes can be reasonably described with basis sets of rather limited dimensions [20,21]. For the sake of completeness, we note that in vacuo three closely spaced transitions are computed at 416.9, 410.3 and 390.1 nm, with f = 0.073, 0.124 and 0.075, respectively, of MLCT character, originating from t2gRu orbitals to p* combinations delocalized over both the dppz and phen ligands. The computed spectrum in vacuo seems therefore to be constituted by two closely spaced bands originated by the first two almost degenerate transitions at 415 nm and by a transition of minor intensity centered at 390 nm, and significantly differs from the solution experimental spectrum. A similar spectrum is observed with BS1.
4. Conclusions The structural, electronic and optical properties of the [Ru(dppz)(phen)2]2+ in vacuo and in solution have been investigated using a DFT/TDDFT approach. Our theoretical analysis shows that the absorption band experimentally characterized at 440 nm is composed by a superposition of two distinct MLCT features, arising from two groups of transitions computed at 450 and 415 nm, the first group of transitions having essentially t2g-Ru ! p*-dppz character, the second group showing a dominant t2g-Ru ! p*-phen character. The absorption band experimentally found at 380 nm is assigned to have a dominant MLCT character partially mixed with dppz intraligand pp* transitions, and arises from a single group of almost coincident transitions which give rise to the narrow experimentally characterized band shape. Notably, while the spectrum computed in vacuo significantly differs from the experiment, inclusion of solvation effects leads to a good agreement between experimental data and calculated
48
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
excitation energies and oscillator strengths. Also in agreement with the experiment, the spectrum computed in acetonitrile shows only a moderate blue-shift with respect to that computed in water. Our TDDFT calculations in solution provide insight into the spectral modifications of the title complex upon intercalation into DNA, which are found to be possibly related to the presence of p* orbitals of the intercalating dppz ligand in the arriving states of the absorption bands at 440 and 380 nm.
References [1] A. Pyle, J.K. Barton, in: S.J. Lippard (Ed.), Progress in Inorganic Chemistry, vol. 38, Wiley, New York, 1990, p. 413. [2] K.E. Erkkila, D.T. Odom, J.K. Barton, Chem. Rev. 99 (1999) 2777. [3] J.G. Collins, A.D. Sleeman, J.R. Aldrich-Wright, I.D. Greguric, T.W. Hambley, Inorg. Chem. 37 (1998) 3133. [4] J.G. Collins, J.R. Aldrich-Wright, I.D. Greguric, P.A. Pellegrini, Inorg. Chem. 38 (1999) 5502. [5] E. Tuite, P. Lincolin, B. Norde´n, J. Am. Chem. Soc. 119 (1997) 239. [6] P. Lincoln, A. Broo, B. Norde´n, J. Am. Chem. Soc 118 (1996) 2644. [7] E.J.C. Olson, D. Hu, A. Hormann, A. Jonkman, M.R. Arkin, E.D.A. Stemp, J.K. Barton, P.F. Barbara, J. Am. Chem. Soc. 119 (1997) 11458. [8] J.-G. Liu, B.-H. Ye, H. Li, L.-N. Ji, R.-H. Li, J.-Y. Zhou, J. Inorg. Biochem. 73 (1999) 117. [9] S. Arounaguiri, B.G. Maiya, Inorg. Chem. 38 (1999) 842. [10] A. Broo, P. Lincoln, Inorg. Chem. 36 (1997) 2544. [11] T. Yutaka, I. Mori, M. Kurihara, J. Mitzutani, K. Kubo, S. Furusho, K. Matsumura, N. Tamai, H. Nishihara, Inorg. Chem. 40 (2001) 4986.
[12] S.I. Gorelsky, E.S. Dodsworth, A.B.P. Lever, A.A. Vlcek, Coord. Chem. Rev. 174 (1998) 469. [13] C. Daul, E.J. Baerends, P. Vernooijs, Inorg. Chem. 33 (1994) 3538. [14] L.T. Zhang, J. Ko, M.J. Ondrechen, J. Am. Chem. Soc. 109 (1987) 1666. [15] G. Albano, P. Belser, C. Daul, Inorg. Chem. 40 (2001) 1408. [16] S.R. Stoyanov, J.M. Villegas, D.P. Rillema, Inorg. Chem. 41 (2002) 2941. [17] J.E. Monat, J.H. Rodriguez, J.K. McCusker, J. Phys. Chem. A 106 (2002) 7399. [18] J.F. Guillemoles, V. Barone, L. Joubert, C. Adamo, J. Phys. Chem. A 106 (2002) 11354. [19] S. Fantacci, F. De Angelis, A. Selloni, J. Am. Chem. Soc. 125 (2003) 4381. [20] F. De Angelis, S. Fantacci, A. Selloni, Chem. Phys. Lett. 389 (2004) 204. [21] S. Fantacci, F. De Angelis, J. Wang, S. Bernhard, A. Selloni, J. Am. Chem. Soc. 126 (2004) 9715. [22] G. Pourtois, D. Beljonne, C. Moucheron, S. Schumm, A. KirshDe Mesmaeker, R. Lazzaroni, J.L. Bredas, J. Am. Chem. Soc. 126 (2004) 683. [23] M.J. Frisch et al., GAUSSIAN 03, Revision A.1, Gaussian Inc., Pittsburgh PA, 2003. [24] J.S. Binkley, J.A. Pople, W.J. Hehre, J. Am. Chem. Soc. 102 (1980) 939. [25] T.H. Dunning Jr., P.J. Hay, in: H.F. Schaefer (Ed.), Modern Theoretical Chemistry, vol. 3, Plenum, New York, 1986, p. 1. [26] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270. [27] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [28] M. Cossi, V. Barone, J. Chem. Phys. 115 (2001) 4708. [29] V. Barone, M. Cossi, J. Phys. Chem. 102 (1998) 1995. [30] M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comp. Chem. 24 (2003) 669. [31] N. Godbout, D.R. Salahub, J. Andezelm, E. Wimmer, Can. J. Chem. 70 (1992) 560. [32] J.-G. Liu, Q.-L. Zhang, X.-F. Shi, L.-N. Ji, Inorg. Chem. 40 (2001) 5045.
A TDDFT study of the ruthenium(II) polyazaaromatic complex [Ru(dppz)(phen)2]2+ in solution Simona Fantacci a
a,*
, Filippo De Angelis a, Antonio Sgamellotti a, Nazzareno Re
b
Istituto CNR di Scienze e Tecnologie Molecolari (ISTM), Dipartimento di Chimica, Universita` di Perugia, via Elce di Sotto 8, I-06123, Perugia, Italy b Facolta` di Farmacia, Universita` ‘‘G. DÕAnnunzio’’, via dei Vestini 13, I-66100, Chieti, Italy Received 1 July 2004; in final form 16 July 2004
Abstract DFT/TDDFT calculations were performed to investigate the structural, electronic and optical properties of the [Ru(dppz)(phen)2]2+ complex in solution. TDDFT calculations in water show two groups of metal-to-ligand charge transfer (MLCT) transitions at 450 and 415 nm whose superposition gives account of the broad absorption band experimentally characterized at 440 nm. Also, a group of almost coincident MLCT transitions partially mixed with dppz intraligand p–p* transitions centered at 380 nm is found to give rise to the narrow absorption band experimentally found at 380 nm. Our results provide insight into the hypochromic shifts experimentally characterized upon intercalation of the title complex into DNA. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction There has been increasing attention in the last decade to the design of small complexes containing spectroscopically active metal centers as probes of DNA structure and function. In particular, several ruthenium(II) polypyridyl complexes have been prepared which bind to DNA by intercalation [1–5]. Most of these Ru(II) complexes are based on byp (2,2 0 -bipyridine) and phen (1, 10-phenantroline) ligands but larger bidentate polyazaaromatic ligands have been employed in attempts to increase their binding affinity for DNA (through increased p-stacking with nucleobases) and possibly to modulate their bases sequence specificity, such as dpq (bypirido[3,2-f:2 0 ,3 0 -h]quinoxaline) and dppz (bypirido[3,2a:2 0 ,3 0 -c]phenazine) [1,2]. All these complexes are intensely colored due to a well characterized metal-to-ligand charge transfer (MLCT) transition. Such a transition is perturbed on
*
Corresponding author. Fax: +39 075 585 5606. E-mail address: [email protected] (S. Fantacci).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.101
binding to DNA which has led to the development of a family of spectroscopic probes. Extensive photophysical studies have indicated that these complexes show peculiar photoluminescence properties depending on the nature of the polyazaaromatic ligands and on the environment [1–12]. For instance, [Ru(dqp)(L)2]2+ and [Ru(dppz)(L)2]2+, L = phen, byp, complexes show no luminescence in water but brightly luminesce upon intercalation between adjacent DNA base pairs [7–9]. In spite of their increasing interest, a few theoretical studies have been performed on these Ru(II) polypyridyl complexes, mainly on complexes with the simplest byp and phen ligands, based on both semiemprirical [10–12] and DFT approaches [13–21]. Only recently, an extensive series of semiempirical calculations on several ligands and related Ru complexes has appeared in the literature [22]. In the same study, time dependent density functional theory (TDDFT) calculations in vacuo on the [Ru(dppz)(phen)2]2+ complex, which is one of the most widely investigated among the ruthenium(II) polypyridyl complexes, have been performed, but the authors focused their attention on the characterization of triplet excited states, which are indeed relevant to
44
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
the photoluminescence of the investigated complexes, describing the optical absorption spectrum at the semiempirical level [22]. It is therefore interesting to investigate the singlet excited states of the [Ru(dppz)(phen)2]2+ complex by means of TDDFT calculations in solution, thus allowing an accurate characterization and assignment of the UV– vis absorption features and the direct comparison of calculated absorption spectra with solution experimental data. In this paper we report DFT/TDDFT calculations on the [Ru(dppz)(phen)2]2+ complex in vacuo and in acetonitrile and water solvents to investigate its structural, electronic and optical properties.
2. Computational details All the calculations reported in this paper have been performed using the GAUSSIAN 03 (G03) program package [23]. Geometry optimizations were carried out, without any symmetry constraints, using both a polarized split-valence 3-21G* [24] (BS1) and a double-f LANL2DZ basis set [25] with the relative pseudopotential for the metal atom [26] (BS2). The B3LYP exchangecorrelation (XC) functional [27] was used for all the calculations. TDDFT excitation energies were computed both in vacuo and in solution; calculations in solution were performed considering the non-equilibrium version [28] of the conductor polarizable continuum model (CPCM) [29,30]. We note that in the present G03 implementation dispersion-repulsion and cavitation contributions cancel out in the evaluation of solvent shifts since they are identical for different electronic states. The effect of basis set expansion on the TDDFT excitation energies and oscillator strengths was checked considering BS1 and the larger DGDZVP [31] basis set (BS3), of double-f + polarization quality. The 20 (30) lowest spin-allowed singlet–singlet transitions, up to a wavelength of 370 (320) nm were taken into account considering BS3 (BS1).
3. Results and discussion 3.1. Molecular structure The optimized geometry of [Ru(dppz)(phen)2]2+ is reported in Fig. 1 and main optimized geometrical parameters obtained with BS1 and BS2 are compared in Table 1 with experimental data for the [Ru(dmp)2(dppz)]-(PF6)2 species, [32] (dmp = 2,9-dimethyl 1,10-phenanthroline) the closest experimentally characterized complex for which X-ray data are available. Optimized structures obtained with both basis sets show a pseudooctahedral coordination of the RuN6 core, with
Fig. 1. Optimized molecular structure of the [Ru(dppz)(phen)2]2+ complex.
Table 1 Comparison between main computed geometrical parameters (bond ˚ and angles in degrees), obtained with LANL2DZ and lengths in A 3-21g* basis sets, and X-ray data for the [Ru(dmp)2(dppz)](PF6)2 species [32]
Ru–N1 Ru–N2 Ru–N3 Ru–N4 Ru–N5 Ru–N6 Ru–N(Av.) N1–Ru–N2 N3–Ru–N4 N5–Ru–N6
Experiment
B3LYP/LANL2DZ
B3LYP/3-21g*
2.096(2) 2.109(2) 2.102(2) 2.125(2) 2.098(2) 2.094(2) 2.104 79.41(9) 78.85(9) 78.86(9)
2.106 2.107 2.106 2.108 2.107 2.107 2.107 79.4 79.4 79.2
2.112 2.113 2.113 2.112 2.115 2.115 2.113 79.3 79.3 79.1
N–Ru–N angles close to 80°. We notice that while the experimental structure shows slight differences between Ru and N distances within the phen ligands, all the Ru–N distances optimized with BS2 (BS1) show similar ˚ (2.112–2.115 A ˚ ), values in the range 2.106–2.108 A probably reflecting the absence in the optimized structure of the methyl substituents in the 2 and 9 positions of the phen ligand. However, average Ru–N distances ˚ are computed with BS2, in good agreement of 2.107 A ˚ , while BS1 prowith the experimental value of 2.104 A vides slightly longer Ru–N distances (Table 1). On the basis of the slightly better agreement of BS2 geometrical parameters with experimental values, we used the BS2 optimized structure for the results presented in the following. 3.2. Electronic structure In Fig. 2 we report a schematic representation of the energy and character of the frontier orbitals of the title complex, computed with the larger BS3 in vacuo,
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
-6.0
-2.0 vacuo
E(eV)
-7.0
-8.0
45
E(eV)
π∗dppz π∗phen+π∗dppz
-3.0
π∗dppz +π∗phen π∗phen π∗dppz+ t2g(Ru) π∗phen+ t2g(Ru) π∗phen
water
π∗phen π∗phen π∗phen+ t2g(Ru) π∗phen+π∗dppz π∗dppz +π∗phen π∗dppz
LUMO (189)
LUMO LUMO
-4.0
∆E=3.36eV
-9.0
acetonitrile
π∗dppz
∆E=3.23eV
-5.0
-6.0
-10.0
HOMO πdppz
-11.0
∆E=3.19eV
t2g(Ru) πdppz
HOMO
t2g(Ru)
HOMO (188) -7.0
πdppz πdppz
Fig. 2. Energy levels (eV) of the [Ru(dppz)(phen)2]2+ complex in vacuo and in acetonitrile and water solvents.
acetonitrile and water. Minor differences in the electronic structure are computed with BS1. Inclusion of solvation effects partially alters the composition of the frontier orbitals, and deeply affects the absolute orbital energies, which are raised by 4 eV on going from vacuo to solution, reflecting the stabilization of the doubly positively charged Ru center in polar solvents. Since the experimental spectra are recorded in solution, and we have recently shown the importance of solvation in determining the electronic structure and spectrum of Ru(II)–polypiridyl complexes, [19–21] we will discuss in detail only data in solution. In particular, both in acetonitrile and water, the first three HOMOs (orbitals 188–186) correspond to the ruthenium t2g set (an isodensity contour plot of the HOMO 2 has been reported in Fig. 3) whereas HOMO 3 (see Fig. 3) and HOMO 4 (orbitals 185 and 184) are p bonding combinations of the dppz ligand. In solution the LUMO (orbital 189) has a pure p*-dppz character (see Fig. 3), while LUMO + 1 and LUMO + 2 (orbitals 190 and 191) are a pair of mixed dppz and phen p* combinations with dominant contributions arising from dppz and phen ligands in LUMO + 1 and LUMO + 2, respectively. LUMO + 3/LUMO + 5 (192–
194) have p*-phen character, see Fig. 3, while LUMO + 6 is a higher-lying p* combination of the dppz ligand. We also notice that a small reduction of the HOMO–LUMO gap is computed with BS3 (BS1) on going from acetonitrile to water, with a decrease from 3.23 (3.24) to 3.19 (3.13) eV, respectively, as a consequence of the slightly higher destabilization of the occupied with respect to the unoccupied orbitals which follows the increase of solvent polarity. 3.3. UV–vis spectrum The experimental [Ru(dppz)(phen)2]2+ spectrum in solution shows the presence of two bands of comparable intensity, centered at approximately 440 and 380 nm, which were originally assigned as having MLCT and pp* character, respectively [7]. We report in Table 2 computed vertical excitation energies and the composition of the related transitions obtained using BS3 both in vacuo and in solution, considering those transitions characterized by an oscillator strength (f) larger than 0.05. Inclusion of solvation effects introduces relevant changes to the spectrum with respect to that computed in vacuo, see below, both in terms of absorption
46
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
Fig. 3. Isodensity surface plots (isodensity value = 0.03) of the HOMO 3 (185), HOMO 2 (186), LUMO (189) and LUMO + 3 (192) of the [Ru(dppz)(phen)2]2+ complex in water.
Table 2 Computed excitation energies (eV) and oscillator strengths (f) for the optical transitions with f > 0.05 of the [Ru(dppz)(phen)2]2+ complex both in vacuo and in solution (acetonitrile and water) in terms of single molecular orbitals excitations with percentages larger than 5% nm
f
B3LYP/DGDZVP (vacuo) 416.9 0.073 43.3%(t2g-Ru)185 ! (p*-phen + t2gRu)190; 36.6%(t2g-Ru)187 ! (p*-phen)192; 9.5%(t2g-Ru)186 ! (p*-dppz + t2g-Ru)191; 6.2%(t2g-Ru)186 ! (p*-phen)189 410.3 0.124 48.4%(t2g-Ru)186 ! (p*-dppz + t2g-Ru)191; 37.3%(t2g-Ru)187 ! (p*-phen)192 390.1 0.075 66.9%(t2g-Ru)186 ! (p*-phen)192; 10.7%(t2g-Ru)187 ! (p*-dppz + p*-phen)193 B3LYP/DGDZVP (acetonitrile) 445.2 0.122 82.5%(t2g-Ru)186 ! (p*-dppz)189; 10.0%(t2g-Ru)186 ! (p*-dppz+p*-phen)190 414.2 0.113 42.9%(t2g-Ru)186 ! (p*-phen + t2g-Ru)192; 42.6%(t2g-Ru)187 ! (p*-phen + p*-dppz)191; 5.1%(t2g-Ru)188 ! (p*-phen)194 412.8 0.085 45.9%(t2g-Ru)187 ! (p*-phen + t2gRu)192; 19.6%(t2g-Ru)188 ! (p*-phen)193; 18.5%(t2g-Ru)186 ! (p*-dppz + p*-phen)190; 5.6%(t2g-Ru)186 ! (p*-dppz)189; 5.5%(t2g-Ru)186 ! (p*-phen + p*-dppz)191 384.3 0.085 78.5%(t2g-Ru)186 ! (p*-phen)193; 5.8%(t2g-Ru)188 ! (p*-phen)194; 5.2%(p-dppz)185 ! (p*-dppz)189 382.4 0.138 60.9%(t2g-Ru)188 ! (p*-dppz)195; 27.9%(t2g-Ru)187 ! (p*-phen)193 B3LYP/DGDZVP (water) 449.2 0.071 58.2%(t2g-Ru)186 ! (p*-dppz)189; 33.6%(t2g-Ru)188 ! (p*-phen + t2g-Ru)192 414.7 0.113 43.5%(t2g-Ru)186 ! (p*-phen + t2g-Ru)192; 41.3%(t2g-Ru)187 ! (p*-phen + p*-dppz)191; 5.5%(t2g-Ru)188 ! (p*-phen)194 413.8 0.101 46.5%(t2g-Ru)187 ! (p*-phen + t2g-Ru)192; 20.1%(t2g-Ru)186 ! (p*-dppz + p*-phen)190; 13.9%(t2g-Ru)188 ! (p*-phen)193; 8.5%(t2g-Ru)186 ! (p*-phen + p*-dppz)191 384.4 0.064 47.4%(p-dppz)185 ! (p*-dppz)189; 36.3%(t2g-Ru)186 ! (p*-phen)193 383.7 0.137 54.2%(t2g-Ru)188 ! (p*-dppz)195; 36.5%(t2g-Ru)187 ! (p*-phen)193 In parentheses we report the character of the orbitals involved in the transition; orbital numbering is the same of Fig. 2.
wavelengths and band separation. A comparison of the experimental spectrum in water from [6] in the range 480–350 nm and calculated excitation energies and oscilla-
tor strengths is reported in Fig. 4. In water we find five transitions with f > 0.05 lying in the range 450–370 nm. In particular, the intense transition at 449.2 nm
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
Fig. 4. Experimental spectral shape (black line) from [6], compared to excitation energies and oscillator strengths computed with BS3 in water (red vertical lines).
(f = 0.071) has MLCT character and mainly originates from the t2g-Ru set to the p*-dppz LUMO (orbital 189) with a sizable contribution (30%) of phen p* states, see Table 2. This main transition is followed by two quasi degenerate MLCT transitions of similar intensity (f = 0.113 and 0.101) found at 414.7 and 413.8 whose final states are p* orbitals (190–192) delocalized over both the phen and dppz ligands, with larger contributions arising from the phen ligands. We therefore see that the broad experimentally characterized band at 440 nm is assigned to be a superposition of two MLCT features (respectively named Ia and Ib in Fig. 4) which differ for the character of the arriving states, Ia being dominated by transitions to the dppz ligand, Ib being composed by transitions mainly localized on the phen ligands. At shorter wavelengths, we find an intense MLCT transition 383.7 nm from the t2g-Ru set to the LUMO and higher lying p*-dppz and -phen orbitals (193,195). Interestingly, the multitransition at 384.4 nm with f = 0.064 involves transitions from the t2g-Ru HOMO2 (186) to the p*-phen LUMO + 4 (193) and from the p-dppz HOMO-3 (185) to the p*-dppz LUMO (189), see Table 2, therefore showing a mixed MLCT and intraligand dppz p ! p* character. We note that an intraligand dppz p ! p* transition at 365 nm has also been characterized by semiempirical calculations in [22]. The almost coincident transitions at 384.4 and 383.7 nm constitute the second absorption band of the experimental spectrum (II in Fig. 4), thus explaining the narrower shape of band II with respect to band I. Overall, our computed excitation energies and oscillator strengths are in good agreement with the experimental spectrum. We note that the band assignments based on our TDDFT calculations in water solution can provide an explanation for the spectral modifications of the title complex upon intercalation into DNA, showing considerable hypochromic shifts of the spectral features at
47
440 and 380, which are possibly related to the presence of p* orbitals of the intercalating dppz ligand in the arriving states of the transitions computed at 449 and 384 nm (Table 1). In acetonitrile a similar band assignment to that made in water holds, with the absorption spectrum characterized by a broad absorption band and a narrow band at shorter wavelengths. It is interesting to point out that in acetonitrile the first relevant transition is only slightly blue-shifted with respect to that in water (445.2 vs. 449.2 nm), again in agreement with the experimental data [7]. On the technical side, we note that BS1 provides a similar description of the spectra in solution as BS3; as an example, the intense transition originating the longer wavelength feature of the broad visible absorption band is found at 448.3 and 460.3 nm in acetonitrile and water, respectively, to be compared to BS3 values of 445.2 and 449.2 nm, confirming that MLCT transitions in Ru–polypyridyl complexes can be reasonably described with basis sets of rather limited dimensions [20,21]. For the sake of completeness, we note that in vacuo three closely spaced transitions are computed at 416.9, 410.3 and 390.1 nm, with f = 0.073, 0.124 and 0.075, respectively, of MLCT character, originating from t2gRu orbitals to p* combinations delocalized over both the dppz and phen ligands. The computed spectrum in vacuo seems therefore to be constituted by two closely spaced bands originated by the first two almost degenerate transitions at 415 nm and by a transition of minor intensity centered at 390 nm, and significantly differs from the solution experimental spectrum. A similar spectrum is observed with BS1.
4. Conclusions The structural, electronic and optical properties of the [Ru(dppz)(phen)2]2+ in vacuo and in solution have been investigated using a DFT/TDDFT approach. Our theoretical analysis shows that the absorption band experimentally characterized at 440 nm is composed by a superposition of two distinct MLCT features, arising from two groups of transitions computed at 450 and 415 nm, the first group of transitions having essentially t2g-Ru ! p*-dppz character, the second group showing a dominant t2g-Ru ! p*-phen character. The absorption band experimentally found at 380 nm is assigned to have a dominant MLCT character partially mixed with dppz intraligand pp* transitions, and arises from a single group of almost coincident transitions which give rise to the narrow experimentally characterized band shape. Notably, while the spectrum computed in vacuo significantly differs from the experiment, inclusion of solvation effects leads to a good agreement between experimental data and calculated
48
S. Fantacci et al. / Chemical Physics Letters 396 (2004) 43–48
excitation energies and oscillator strengths. Also in agreement with the experiment, the spectrum computed in acetonitrile shows only a moderate blue-shift with respect to that computed in water. Our TDDFT calculations in solution provide insight into the spectral modifications of the title complex upon intercalation into DNA, which are found to be possibly related to the presence of p* orbitals of the intercalating dppz ligand in the arriving states of the absorption bands at 440 and 380 nm.
References [1] A. Pyle, J.K. Barton, in: S.J. Lippard (Ed.), Progress in Inorganic Chemistry, vol. 38, Wiley, New York, 1990, p. 413. [2] K.E. Erkkila, D.T. Odom, J.K. Barton, Chem. Rev. 99 (1999) 2777. [3] J.G. Collins, A.D. Sleeman, J.R. Aldrich-Wright, I.D. Greguric, T.W. Hambley, Inorg. Chem. 37 (1998) 3133. [4] J.G. Collins, J.R. Aldrich-Wright, I.D. Greguric, P.A. Pellegrini, Inorg. Chem. 38 (1999) 5502. [5] E. Tuite, P. Lincolin, B. Norde´n, J. Am. Chem. Soc. 119 (1997) 239. [6] P. Lincoln, A. Broo, B. Norde´n, J. Am. Chem. Soc 118 (1996) 2644. [7] E.J.C. Olson, D. Hu, A. Hormann, A. Jonkman, M.R. Arkin, E.D.A. Stemp, J.K. Barton, P.F. Barbara, J. Am. Chem. Soc. 119 (1997) 11458. [8] J.-G. Liu, B.-H. Ye, H. Li, L.-N. Ji, R.-H. Li, J.-Y. Zhou, J. Inorg. Biochem. 73 (1999) 117. [9] S. Arounaguiri, B.G. Maiya, Inorg. Chem. 38 (1999) 842. [10] A. Broo, P. Lincoln, Inorg. Chem. 36 (1997) 2544. [11] T. Yutaka, I. Mori, M. Kurihara, J. Mitzutani, K. Kubo, S. Furusho, K. Matsumura, N. Tamai, H. Nishihara, Inorg. Chem. 40 (2001) 4986.
[12] S.I. Gorelsky, E.S. Dodsworth, A.B.P. Lever, A.A. Vlcek, Coord. Chem. Rev. 174 (1998) 469. [13] C. Daul, E.J. Baerends, P. Vernooijs, Inorg. Chem. 33 (1994) 3538. [14] L.T. Zhang, J. Ko, M.J. Ondrechen, J. Am. Chem. Soc. 109 (1987) 1666. [15] G. Albano, P. Belser, C. Daul, Inorg. Chem. 40 (2001) 1408. [16] S.R. Stoyanov, J.M. Villegas, D.P. Rillema, Inorg. Chem. 41 (2002) 2941. [17] J.E. Monat, J.H. Rodriguez, J.K. McCusker, J. Phys. Chem. A 106 (2002) 7399. [18] J.F. Guillemoles, V. Barone, L. Joubert, C. Adamo, J. Phys. Chem. A 106 (2002) 11354. [19] S. Fantacci, F. De Angelis, A. Selloni, J. Am. Chem. Soc. 125 (2003) 4381. [20] F. De Angelis, S. Fantacci, A. Selloni, Chem. Phys. Lett. 389 (2004) 204. [21] S. Fantacci, F. De Angelis, J. Wang, S. Bernhard, A. Selloni, J. Am. Chem. Soc. 126 (2004) 9715. [22] G. Pourtois, D. Beljonne, C. Moucheron, S. Schumm, A. KirshDe Mesmaeker, R. Lazzaroni, J.L. Bredas, J. Am. Chem. Soc. 126 (2004) 683. [23] M.J. Frisch et al., GAUSSIAN 03, Revision A.1, Gaussian Inc., Pittsburgh PA, 2003. [24] J.S. Binkley, J.A. Pople, W.J. Hehre, J. Am. Chem. Soc. 102 (1980) 939. [25] T.H. Dunning Jr., P.J. Hay, in: H.F. Schaefer (Ed.), Modern Theoretical Chemistry, vol. 3, Plenum, New York, 1986, p. 1. [26] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270. [27] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [28] M. Cossi, V. Barone, J. Chem. Phys. 115 (2001) 4708. [29] V. Barone, M. Cossi, J. Phys. Chem. 102 (1998) 1995. [30] M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comp. Chem. 24 (2003) 669. [31] N. Godbout, D.R. Salahub, J. Andezelm, E. Wimmer, Can. J. Chem. 70 (1992) 560. [32] J.-G. Liu, Q.-L. Zhang, X.-F. Shi, L.-N. Ji, Inorg. Chem. 40 (2001) 5045.