Ground and Excited State Properties of New Porphyrin Based Dyads

Article pubs.acs.org/JPCA

Ground and Excited State Properties of New Porphyrin Based Dyads: A Combined Theoretical and Experimental Study. Fabien Lachaud,† Christophe Jeandon,‡ Marc Beley,† Romain Ruppert,‡ Philippe C. Gros,† Antonio Monari,*,§ and Xavier Assfeld§ †

Groupe SOR and §Equipe CBT, SRSMC, Université de Lorraine Nancy et CNRS, Faculté des Sciences, Boulevard des Aiguillettes, F-54506 Vandoeuvre-Lès-Nancy, France ‡ Laboratoire CLAC, UMR 7177 du CNRS, Université de Strasbourg, 1 rue Blaise Pascal, BP 296 R8, 67008 Strasbourg Cedex, France S Supporting Information *

ABSTRACT: The properties of the ground and excited states of several porphyrins appended with external chelates coordinated to rutheniumbisbipyridine units are reported. The important modification of the absorption spectrum upon coordination with the ruthenium complex showed that a significant electronic communication between the two subunits was present in the ground state. Experimental results were compared with quantum chemistry calculations performed at density functional theory and time-dependent density functional theory level. The influence of the exchange-correlation functional on the quality of the computed absorption spectrum is shown, and the better behavior of hybrid functionals over long-range corrected ones was rationalized. The excited states topology analysis, performed using natural transition orbitals, gave a more evident confirmation of the communication between the subunits and showed that these new compounds can be promising as dyes in dyesensitized solar cells.

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INTRODUCTION Porphyrins are well-known and recognized for their unique photochemical and redox properties. First of all, let us think about the central role played by chlorophyll in the photosynthetic process1,2 where it acts as a very efficient antenna to absorb solar light and therefore to start all the complex mechanisms ultimately leading to glucose synthesis. Metallo-porphyrins and to a less extent phthalocyanines show therefore a very wide range of applications going from phototherapy,3,4 where their ability in promoting the production of singlet oxygen is exploited, to photovoltaic application5,6 and molecular electronics,7,8 where one takes advantage of the intense absorption spectrum and of their redox properties. Indeed porphyrins show a very peculiar UV/ vis spectrum striking for the extremely high value of the molar extinction coefficient.9−11 In particular two bands are quite characteristic of the porphyrin electronic spectra: the Q bands appearing in the red and infrared region of the electromagnetic spectrum, and the extremely intense Soret band between 400 and 500 nm. Moreover, similar classes of discotic like materials, in particular phthalocyanines, are known to form liquid crystal columnar arrangements and to show a remarkable electron mobility.12,13 Among photosensitizers, another class of compounds have emerged with a leading role in photoactive processes: coordination compounds and in particular ruthenium poly bipyridine complexes.14−17 The latter are generally octahedral coordination compounds exhibiting particular photophysical and redox properties that © 2012 American Chemical Society

are strongly influenced by the environment. Most importantly, ruthenium poly bipyridinyl complexes are known to have a wide absorption band, that can be easily extended toward the red region by an opportune tuning of the coordinated ligands. Apart for their important light-harvesting abilities, Ru complexes present also important emission properties, and interactions with biological macromolecules, like DNA, have emerged as a promising research field.18−20 Let us cite, for instance, the notable light-switching effect,18 that has opened the way to their use in biology and medicine for phototherapy or imaging purposes.21 Moreover their absorption spectrum combined with the optimum redox potential of the Ru(III)/ Ru(II) couple has pushed the way to their use in photovoltaic devices.22,23 Indeed up to now some of the most efficient dyesensitized solar cells (DSSC) always include Ru complexes as photosensitizer, and many experimental24−30 and theoretical investigations were performed in this field.29−33 Their use as switches34,35 or molecular engines should also be cited.36,37 In the past years a fascinating research topic has emerged aiming at the coordination of other metals to the porphyrin (or phthalocyanine) periphery38−42 to build supramolecular arrangements able to promote synergetic electronic effects between the two centers. When looking at the properties of porphyrin and of Ru polybipyridine and poliazyne compounds Received: July 27, 2012 Revised: October 21, 2012 Published: October 22, 2012 10736

dx.doi.org/10.1021/jp307452d | J. Phys. Chem. A 2012, 116, 10736−10744

The Journal of Physical Chemistry A

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states, respectively, because they combine a moderate computational cost and a good to excellent accuracy.46 Nevertheless the computation, and in particular the study of excited states, for such systems is not a trivial task. Let us cite, for instance, the role played by charge-transfer excitations, precisely the ones responsible for the two-center communication, that require a well balanced choice of the correlation-exchange functional used.47 The performance of different functionals, and eventually the reason for their failure should, therefore, be carefully investigated if one wants to develop a reasonable and general computational protocol. Our computational results will also be compared with experimental data, obtained for the two parent compounds, namely, C2 and C5 (Ni metal with oxygen and sulfur bridge, respectively) and the corresponding porphyrins P2 and P5. The nature of the excited state wave function will also be analyzed showing the important antenna effect induced by the porphyrin unit. All the complexes (C1−C6) will therefore behave like molecular dyads, and we will highlight the possible use of such systems as more efficient dyes in DSSC based devices.

the possibility to built such arrangement with these two building blocks appears as particularly intriguing to get additive electronic effects from each partner. It has to be underlined that normally the latter aggregates are realized by covalently binding of a coordinative azine to the porphyrin and then make it react with the opportune Ru complex.40−42 This path even if straightforward can bring more synthetic difficulties and thus become problematic. Recently some of us have synthesized porphyrins and metallo-porphyrins bearing conjugated chelating sites at their periphery43 (See Scheme 1). The two external heteroatoms Scheme 1. General Formula of (metallo)-Poprphyrins

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COMPUTATIONAL DETAILS All quantum chemistry calculations have been performed using Gaussian09 B.01 code.48 Geometry for all the systems, porphyrin and dyads, has been fully optimized at DFT level. For geometry optimization the double-ζ quality basis set LANL2DZ49 has been used throughout. This basis uses a pseudopotential on the metal atoms and has already proved efficient in the computation of Ru organo-metallic complexes properties.20,31,50 UV/vis spectra have been calculated as vertical excitation from previously obtained ground state minimum (Franck−Condon principle) at the TD-DFT level; in all cases 35 excited states have been computed. Solvatochromic effects have been taken into account by using a PCM model51 for acetonitrile. The effects of the functional and of the basis set have been analyzed. In particular B3LYP52 and PBE053 hybrid functionals have been considered, as well as long-range corrected CAM-B3LYP54 and wBD97XD55 functionals. Some tests have also been performed to check the quality of the basis using LANL2TZ56 plus polarization basis for all the atoms, and LANL2DZ for metals and 6-31+G(d,p)57 for lighter atoms. The two more extended basis both gave results absolutely comparable with LANL2DZ ones, especially in the most interesting part of the spectrum, that is, corresponding to the Q- and Soret-band. The shifting in transition wavelengths has been always less than 10 nm. For these reasons in the following we will present LANL2DZ results, only. To simulate the vibrational structure and better reproduce experimental results computed vertical transitions have been convoluted with Gaussians functions with fixed halflength-width of 0.25 eV. The excited state nature has been analyzed using the Natural Transition Orbitals (NTO)58−60 formalism. We note that NTOs, obtained by a singular value decomposition of the transition density matrix, represent the best basis to describe the excited state. In particular, in contrast with the Kohn−Sham representation, where a combination of transition from occupied to virtual orbitals, sometimes characterized by similar weights, are necessary, only one, or in the worst case two, occupied/virtual orbital couples are sufficient to catch all the physics of the process using NTOs. In particular the occupied orbital represents the orbital from where the electron is depleted during the transition (i.e., a sort of “hole” density),

(N,O or N,S) can therefore be used to coordinate metal ions.44 For instance porphyrin-porphyrin dimers have been reported, and communication between the metal centers has been underlined.45 In this work we want to investigate the possibility to build a Ru-porphyrin dyad (see Scheme 2), exploiting the Scheme 2. General Formula of Supramolecular Dyads

coordinating capabilities of the porphyrin itself. In particular we want to underline the possibilities to exploit the supramolecular systems using porphyrin groups as ancillary ligands and light antennae to harvest light, in the framework of a possible use in photovoltaic DSSC applications. We are particularly interested in examining the extension of the metal−metal communication and the role played by the different bridging ligands, in particular considering oxygen and sulfur atoms as bridges. Moreover we will examine the influence of the porphyrin coordinating metal (Zn and Ni, as well as the metal free system being considered) in tuning the spectral properties of the systems. Metal−metal communication can be evidenced by different phenomena, in particular by the UV/vis spectrum and by the difference of redox properties between the isolated subunits and the final supramolecular systems. In this contribution, we will describe a systematic computational study on the complexes bearing different metal and different bridges (see Scheme 2 for nomenclature) compared to the parent porphyrin compounds (Scheme 1). In these systems, Density Functional Theory (DFT) and its Time Dependent extension (TD-DFT) represent the method of choice to treat ground and excited 10737



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RESULTS AND DISCUSSION Ground State Structure. The optimized structure of one of the porphyrins and the dyads are presented in Figure 1. One

while the virtual orbital represent the orbital into which electron density is accumulated in the excited state. NTOs have been obtained by a suitable postprocessing of Gaussian files realized with Nancy-EX59,60 a GPL code written in our laboratory and freely distributed (see http://sourceforge.net/ projects/nancyex). Synthesis. Synthesis of P5. A solution of nickelenaminoketone (200 mg, 0.28 mmol) and Lawesson’s reagent (152 mg, 2.66 mmol) in benzene (100 mL) was heated under reflux until consumption of all starting material (approximatively 1.5 h). After evaporation of the solvent, the residue was chromatographed on a silica gel column (eluent: cyclohexane/dichloromethane). After crystallization from dichloromethane/methanol, the green nickelenaminothioketone was isolated in 93% yield (190 mg, 0.261 mmol). 1H NMR (400 MHz, CDCl3) δ = 12.27 (broad d, J = 2.4 Hz, 1H), 9.16 (d, J = 5.2 Hz, 1H), 9.11 (dd, J ∼ 8 and 1 Hz, 1H,), 8.67 (d, J = 5.2 Hz, 1H), 8.41 (d, J = 5.2 Hz, 1H), 8.36 (d, J = 5.2 Hz, 1H), 8.28 (d, J = 5.2 Hz, 1H), 8.23 (d, J = 5.2 Hz, 1H), 8.10 (dd, J ∼ 8 and 1 Hz, 1H), 7.80− 7.93 (m, Ph, 9H), 7.74 (ddd, J ∼ 8, 8, and 1 Hz, 1H), 7.61− 7.67 (m, Ph, 6H), 7.54 (ddd, J ∼ 8, 8, and 1 Hz, 1H), 6.12 (broad d, J = 2.4 Hz, 1H). UV−vis (CH2Cl2): λmax (nm) (ε (103 M−1 cm−1))= 385 nm (ε = 58.0), 425 (65.0), 492 (90.5), 685 (23.3). Anal. Calcd for C45H27N5NiS.CH3OH: C 72.65; H 4.11; N 9.21. Found: C 72.92; H 3.86; N 9.23. Synthesis of C2. P2 (7.1 mg, 0.01 mmol) and Ru(DMSO)4Cl2 (4.8 mg, 0.01 mmol) were dissolved in DMF (5 mL). The solution was then irradiated in the microwave oven (200W, 160 °C) for 20 min. After the solution was allowed to cool down to room temperature, DMF (5 mL) and 2,2′-bipyridine (3.1 mg, 0.02 mmol) were added. The obtained solution was again irradiated in the microwave oven (200W, 160 °C, 20 min). To this solution was added a saturated solution of potassium hexafluorophosphate in water. The precipitate was washed with CH2Cl2. Afterward, the solid was dissolved in acetone and poured on a silica gel column. Unreacted P2 was first eluted using acetone. C2 was next obtained by using an acetone/H2O/sat.KNO3 (10/1/1) mixture as eluent. After half of the eluent was removed under vacuum, a saturated solution of potassium hexafluorophosphate in water was added. After 12 h, the dark red precipitate was filtered, washed with water and dried under vacuum yielding C2 (3.2 mg, 22%) as a brown solid.1H NMR-(400 MHz, Acetonitrile d6), δ (ppm):. 9.12 (bs, 1H), 8.96 (s, 1H), 8.83 (d, J = 5.2 Hz, 1H), 8.70 (d, J = 5.2 Hz, 1H), 8.55−8.40 (m, 5H), 8,31 (d, J = 8.6 Hz, 1H), 8.19 (dd, J = 8.0 and 3.9 Hz, 2H), 8.10 (dd, J = 8.2 and 1.2 Hz, 1H), 8.07−8.0 (m, 4H), 8.0−7.93 (m, 3H), 7.92−7.82 (m, 4H), 7.79 (t, J = 7.1 Hz, 2H), 7.74 (d, J = 7.1 Hz, 1H), 7.72−7.60 (m, 5H), 7.50−7.33 (m, 5H), 7.29 (brs, 1H), 7.24 (t, J = 6.8 Hz, 1H), 7.22−7.16 (m, 2H), 7.10 (t, J = 6.8 Hz, 1H). HRMS (ESI): calcd for C65H42N9ONiRu m/z = 1124.191, Found: 1124.184 (M+). UV−vis (CH3CN), λmax (nm) (ε (103 M−1 cm−1))= 292 (39.1), 435 (35.3), 478, 23.1), 529 (22.2), 763 (4.8). Synthesis of C5. The procedure was repeated using P5 (7.2 mg, 0.01 mmol) yielding C5 (2.3 mg, 18%). We were unable to obtain an attributable NMR spectrum of this compound. HRMS (ESI): calcd for C65H42N9SNiRu m/z = 1140.168, Found: 1140.165 (M+). UV−vis (CH3CN), λmax (nm) (ε (103 M−1 cm−1)) = 292 (46.5), 405 (35.5), 430 (37.8), 457 (33.5), 510 (30.4), 680 (13.9).

Figure 1. Optimized geometry for the P2 (left) and C2 (right) system.

can easily see that, as expected, the metal ion inside the porphyrin presents a roughly square planar coordination, while ruthenium keeps its octahedral coordination sphere, with the porphyrin bridge acting as a bidentate ligand. However, it has to be noted that the planar structure of the porphyrin is slightly perturbed because of the relative hindrance induced by the peripheral substituents that tends to assume a sort of helicoidal arrangement to minimize steric repulsion. The small size of the nickel(II) ion is also known to induce a ruffled deformation to the porphyrin structure. The same arrangement is kept in the dyad structure, and this geometry appears able to induce communication between the two subunits via π-conjugation. The structure of the sulfur based compounds (P4-P6 and C4C6) does not present significant difference with the oxygenbased ones. As far as the influence of the metal is involved in Figure 2 we report the different optimized structures for the oxygen based dyads. One can see that some noticeable differences can be underlined, even if the general characteristics of the structure are conserved. In particular if the size of the porphyrin inner rings appears quite stable with the three structures almost perfectly matching, the peripheral substituents experience a more important deviation from planarity, with the metal free porphyrin being the more planar and the Zn one the more deformed. Correspondingly to the modification of the conformation of the porphyrin external ligands, the bipyridyl groups coordinating to the ruthenium ion also rearrange to keep the coordination of ruthenium as close as possible to the octahedral one. It is noticeable to underline that the distance between ruthenium and the bridging atoms (N and O or S), as well as the angle between them remain constant among the different classes of compounds, the only obvious difference being the longer Ru−S bond with respect to the Ru−O one (2.46 vs 2.10 Å); indeed the rather short metal-heteroatom bond suggests a strong coordination and hence can be thought to favor an extended communication between the subunits. Notice also that the angle between Ru and the two bridging atoms remains close to the ideal value of 90°, the larger deviation experienced by the C3 (Zn based) complex can be ascribed to the more pronounced loss of planarity of the constituent porphyrin that will induce a more pronounced adjustment of the octahedral coordination sphere. UV/vis Spectra. The experimental UV/vis spectra recorded in acetonitrile solution are presented in Figure 3 for the porphyrin and the dyads, respectively. The porphyrins clearly show the presence of the well-known Q-bands occurring at high wavelenghts (650 and 700 nm for P2 and P5 respectively), 10738



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Figure 2. Superposed optimized geometries for C2, C3, and C4. Right top view, left lateral view.

Figure 3. Experimental UV/vis spectra for P2 and P5 (left) and C2 and C5 (right) in acetonitrile. Wavelengths in nm.

while the intense Soret band is also clearly identified at about 450 nm for the oxygen-bridged P2 and at about 490−500 nm for P5. It is, therefore, evident that the presence of the sulfur induces a significant red-shift of the principal bands. One should also notice the very high molar extinction coefficients typical of porphyrin systems. Upon coordination with ruthenium bis-bipyridinyl moiety the UV/vis spectra appears strongly altered, both for C2 and C5 compounds. In particular, if the absolute intensities are lowered, the Soret band is widely enlarged, especially in the case of C5, with an absorption that almost covers all the spectral width between 350 and 700 nm. It is also noteworthy that many less resolved bands are appearing. In particular, in the high wavelengths region one can notice the appearance of the typical metal to ligand charge transfer (MLCT) absorption pattern involving electron transfer from Ru to the bipyridine ligand. All these factors clearly indicate a strong participation and a considerable effect induced by the ruthenium moiety. If the antenna effect appears confirmed with an absorption band covering almost all the visible region with a reasonable intensity, quantum chemistry can help to correctly assign the transitions and to confirm the communication taking place within the subunits, an essential feature that can be exploited in photovoltaic applications. Moreover, computational studies will allow us to systematically take into account the influence of different metals coordinating the porphyrin on the spectroscopic features, as well as the behavior of the metal free porphyrin moiety. In Tables 1 and 2 we report the most intense vertical transitions computed at the TD-DFT level with different functionals (B3LYP, PBE0, CAM-B3LYP, and wBD97XD) and LANL2DZ basis for the porphyrins (P2 and P5) and for the

Table 1. Principal TD-DFT Vertical Transitions Wavelengths (nm) for P2 and P5a B3LYP P2

P5

a

609.02 477.12 456.11 452.06 433.60 381.25 367.38 359.50 349.99 309.87 659.38 483.52 469.14 459.32 400.19 390.61 389.85 382.05 378.88 375.46

(0.21) (0.10) (0.32) (0.57) (0.42) (0.20) (0.28) (0.19) (0.11) (0.11) (0.22) (0.27) (0.25) (0.36) (0.16) (0.43) (0.19) (0.21) (0.24) (0.24)

PBE0 592.60 454.93 440.52 434.85 413.46 365.75 352.11 344.90 335.93 295.74 640.15 465.79 451.09 436.30 385.33 375.45 366.16 359.00 327.69

(0.21) (0.19) (0.41) (0.80) (0.29) (0.26) (0.22) (0.17) (0.13) (0.17) (0.23) (0.29) (0.63) (0.13) (0.25) (0.46) (0.57) (0.09) (0.11)

CAM-B3LYP 586.24 411.58 407.95 392.07 362.28 329.42 310.74 298.97

(0.14) (1.11) (0.81) (0.37) (0.11) (0.30) (0.12) (0.13)

612.41 585.21 428.67 416.47 352.77 343.60 338.20 328.08 324.03

(0.21) (0.11) (0.45) (1.32) (0.54) (0.10) (0.15) (0.15) (0.25)

wB97XD 589.33 563.20 407.00 402.11 388.69 362.80 331.37 323.15 307.29 294.94 610.60 586.54 421.91 408.69 347.25 343.88 340.74 338.27 329.21

(0.13) (0.09) (1.10) (0.88) (0.37) (0.13) (0.08) (0.25) (0.16) (0.09) (0.19) (0.11) (0.58) (1.35) (0.20) (0.24) (0.21) (0.07) (0.11)

Oscillator strength in parentheses.

dyads (C2 and C5), respectively. In Figures 4 and 5 the comparison between experimental and TD-DFT convoluted spectra are reported. It is evident that the behavior of the two hybrid and of the two long-range corrected functionals differs significantly. In looking at the porphyrin results, the most striking difference is 10739



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Table 2. Principal TD-DFT Vertical Transitions Wavelengths (nm) for C2 and C5a B3LYP

a

C2

725.18 615.73 604.94 525.20 489.96 462.99 444.86 441.13 428.21 414.45

(0.08) (0.09) (0.10) (0.75) (0.10) (0.06) (0.09) (0.06) (0.13) (0.40)

C5

639.88 529.03 484.63 478.63 455.27 446.71 411.85 397.75 393.44

(0.24) (0.49) (0.19) (0.12) (0.08) (0.17) (0.31) (0.06) (0.07)

PBE0 695.05 585.10 565.28 500.44 463.47 441.57 436.99 428.03 418.48 416.48 409.48 398.97 396.70 715.80 615.97 561.32 512.41 502.65 501.10 461.56 451.74 441.10 433.10 425.58 403.42 392.96 390.00 376.73

(0.08) (0.07) (0.13) (0.78) (0.12) (0.08) (0.13) (0.11) (0.09) (0.15) (0.08) (0.06) (0.29) (0.06) (0.24) (0.08) (0.10) (0.14) (0.38) (0.24) (0.13) (0.08) (0.15) (0.12) (0.10) (0.11) (0.23) (0.09)

CAM-B3LYP 618.83 550.08 479.96 467.51 438.44 429.19 426.13 400.26 398.03 389.85 374.59 322.76 320.02 626.87 569.12 562.00 454.78 428.77 424.78 406.00 395.14 385.75 378.02 346.65 335.32 334.54 330.28 309.11

(0.13) (0.15) (0.22) (0.17) (0.29) (0.28) (0.53) (0.25) (0.25) (0.57) (0.09) (0.17) (0.17) (0.12) (0.08) (0.20) (0.46) (0.49) (0.40) (0.27) (0.52) (0.13) (0.10) (0.12) (0.08) (0.10) (0.18) (0.09)

wB97XD 615.21 546.71 477.46 462.33 448.05 437.56 434.05 421.58 400.13 394.17 385.56 373.56 313.88 621.59 559.09 451.42 421.61 399.96 388.49 386.84 376.40 346.68 331.83 330.31 321.18 300.84

(0.12) (0.16) (0.22) (0.19) (0.08) (0.15) (0.07) (0.77) (0.26) (0.26) (0.74) (0.09) (0.19) (0.12) (0.27) (0.44) (0.78) (0.34) (0.73) (0.12) (0.09) (0.13) (0.06) (0.10) (0.20) (0.13)

Oscillator strength in parentheses.

Figure 4. TD-DFT convoluted spectra for P2 (left) and P5 (right) compared with experimental results.

the important blue-shift experienced by long-range functionals, with the Soret band being shifted by about 50 nm; the same trend is also evidenced in the Q-band. On the other hand the difference between functionals belonging to the same model (hybrid or long-range corrected, respectively) are quite marginal being of the order of some nm for the emplacement of the Soret band. This is the reason why in Figures 4 and 5 we report B3LYP and CAM-B3LYP results, only. This general trend is confirmed for both oxygen and sulfur systems, although the latter shows a much more complex spectral structure, and it

can be clearly seen on the dyad compounds, too. Indeed in the case of C1 and C5 compounds, both classes of functionals preview an important broadening of the Soret band, with an absorption covering a much larger spectral width, and an intensity at the maximum being strongly diminished compared to the parent porphyrin compounds. However, once again, the long-range functionals give blueshifted spectra, and the broadening of the band, compared to hybrid functional calculations and to experimental results, is much less important. It has to be noted that we performed the 10740



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Figure 5. TD-DFT convoluted spectra for C2 (left) and C5 (right) compared with experimental results.

Figure 6. B3LYP TD-DFT convoluted spectra for the porphyrins systems. Wavelengths in nm, intensities in arbitrary units.

spectral characteristics seem to indicate the presence of longrange charge transfer transitions due to a communication between the two subunits. This fact can be rationalized by an analysis of the overlap between the excited and the ground state wave functions, to get a more quantitative picture of the spatial spread of the charge transfer. One useful tool to perform such an analysis is the so-called Tozer’s index λ.61 We remind that λ indexes close to 1.0 indicates a well localized transition, while low values indicate an important charge-transfer nature of the transition, and therefore should require the use of long-range corrected functionals. All the principal (most intense) transitions for the porphyrin and the dyads have a λ index very close to unity and in any case not inferior to 0.7. Therefore all the transitions lay in the region where hybrid functionals, and in particular B3LYP and PBE0, correctly reproduce the excitation spectrum. Moreover, as nicely pointed out for instance by Mennucci46 or Adamo and Jacquemin,47 in the non pathological region the average performance of hybrid functionals has to be considered superior than the one of the long-range corrected ones. The problems of CAM-B3LYP in giving correct excitation energies has also been underlined, for instance by Scherbin and Ruud,62

calculation of the excited states as vertical transitions from ground state optimized geometry, following the Franck− Condon principle. Therefore the broadening of the band is only because different functionals give different relative energies, while in the experimental spectrum the broadening can also be due to vibronic or solvent effects. Indeed, if we compare the convoluted spectra with the experimental one, we can notice that as far as the positions of the bands are concerned hybrid functionals outperform the long-range corrected one. In particular in the case of porphyrin compounds the position of the Soret band is correctly reproduced with errors of less than 15 nm. The same trend can be seen on the case of dyads, where moreover only B3LYP and PBE0 functionals are able to reproduce the extensive absorption coverage observed experimentally. On the other hand B3LYP and PBE0 are, in some cases, not able to correctly reproduce the intensity ratio between the different bands. Indeed, in the case of P5 the Soret band at 450 nm is quite underestimated, while in the case of C2 the 550 nm band is severely overestimated. The overall better behavior of hybrid functionals versus longrange corrected ones can be quite surprising, especially in the case of the dyads, that is, supramolecular systems whose 10741



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Figure 7. B3LYP TD-DFT convoluted spectra for the dyad systems. Wavelengths in nm, intensities in arbitrary units.

between the subunits in the dyad. The same main characteristic can be evidenced for the Soret-band transition, that, as expected, shows a more important participation of the coordinating metal in the occupied orbital. Indeed the communication and the combined effect of the two units is clearly evidenced by the NTOs of the dyad represented in Figure 9. The lowest energy transition looks

in the context of circular dichroism calculations and by some of us with ruthenium organo-metallic compounds.20,50 In Figures 6 and 7 we report the convoluted TD-DFT absorption spectra, computed with the B3LYP functional, for all the porphyrins and the dyads. One can notice that, coherently to what was found for the geometry optimization, the porphyrin metal plays quite a marginal role: all the spectra showing coherent features, in particular the red-shift due to the presence of sulfur, is always confirmed, as well as the wide broadening of the absorption band leading to the very large spectral coverage due to the coordination with ruthenium. Absorption maxima are indeed only marginally affected by the change in the porphyrin unit. The only relatively more important difference can be ascribed to the apparently more intense absorption band in the case of the Zn containing systems, an evidence that seems to be confirmed experimentally. Excited State Analysis. The nature of the excited states is analyzed by means of NTOs, in particular to clearly assess the communication between the two subsystems and the extension of the antenna effect of the porphyrin unit on the Ru properties. The transitions involving the parent porphyrin compound are represented in Figure 8, where the principal excited states giving raise to the Q- and the Soret-band are represented. Not surprisingly the Q-band transition has a π−π* nature and is extended to all the conjugated system. The important participation of the region of the bridge units (S and N atoms) has also to be noted since this occurrence can be extremely important to ensure an efficient communication

Figure 9. NTOs for some of the principal transition of the C5 molecule.

similar to a Soret band but now with a strong participation of the Ru atom in the occupied orbital, and of the bridging unit in the virtual one. The same characteristics are also present on the transition occurring at about 530 nm that could almost being classified as a Ru to porphyrin transition. Finally in the 450 nm transition one can see a transition from the porphyrin subunit (similar to a characteristic Soret-band) inducing a strong electronic density accumulation on the bipyridine. Note that this charge redistribution can be extremely important in the framework of DSSC solar cells, since the dyad could be grafted to the semiconductor surface by covalently binding of the (functionalized) bipyridine unit. The excited state charge-accumulation close to the semiconductor surface could then most probably favor the photoinduced electron injection. Note also that B3LYP (as well as PBE0) is usually known to somehow overestimate electron delocalization; it is therefore possible that our NTOs are showing a too pronounced electron delocalization between the units.

Figure 8. NTOs for two of the principal transition of the P5 molecule. Q- and Soret band respectively. 10742



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However, these results are coherent with the experimentally observed spectroscopic evolution in passing from porphyrins to supra-molecular systems, and CAM-B3LYP NTOs also show an evident, though less intense delocalization. We, therefore, believe that the previous orbitals are actually reproducing well the inherent characteristic of the excited state nature.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Continuous financial support from the CNRS, the Universities of Strasbourg and Lorraine, is acknowledged. A.M. thanks the CNRS for the support of the “Chaire d’excellence”. A.M. and X.A. acknowledge financial supports from the ANR Blanc PhotBioMet grant.

CONCLUSIONS We reported the study of the excited and ground state properties of some dyads based on the bridging of a metal porphyrin unit and a ruthenium bipyridinyl moiety. The electronic communication between the two fragments is clearly evidenced by the strong enlargement of the typical porphyrin absorption bands in the UV/vis spectrum. Quantum chemical calculations clearly show the emergence of electronic transitions presenting some amount of charge transfer between the units, therefore confirming the cooperative effects of the supramolecular complex. We also underline how, for these systems, the correct and balanced choice of the exchangecorrelation functional is crucial to correctly reproduce the excitation spectra by DFT and TD-DFT methods. In particular we show how, even in presence of charge transfer transition, long-range corrected functionals are outperformed by hybrid ones. From a more physical point of view the antenna effect induced by the porphyrin moiety has been proved to be quite intense, especially when the bridge is assured by a sulfur atom, leading to a very large spectral width, covering practically all the visible spectrum, and extending into the near-infrared region. These results are extremely promising for a possible use of such complexes as sensitizers in DSSC, since in particular NTO analysis of the excited states has shown the presence of some transitions leading to a charge accumulation on the bipyridine ligand, that is, on the unit that in DSSC applications will ensure the grafting to the semiconductor interface. The latter transition will, therefore, facilitate the charge injection process. The communication between the two units could also be exploited to facilitate the dye-regeneration process by the redox mediator present in DSSC devices. In that case one should coordinate the porphyrin with a metal, for instance Copper, having two accessible oxidation states separated by a suitable redox potential, thus allowing the reduction of the porphyrin by the mediator and subsequently the intramolecular electron transfer to the ruthenium moiety to completely regenerate the dye. This system will have the advantage to keep the mediator far away from the semiconductor and therefore to minimize recombination processes. In the future we plan to better study this aspect, also tackling the problem of the intramolecular electron transfer for instance using constrained DFT techniques. From an experimental point of view, since the preliminary results appears encouraging, we plan to synthesize the whole series C1 to C6 of complexes also functionalizing the bipyridine ligand with carboxylic groups to allow the grafting on semiconductors surfaces. The efficiency parameters of DSSC performance could then be directly determined in protypical devices.

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Ground and Excited State Properties of New Porphyrin Based Dyads

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