Charge Transfer in Model Bioinspired Carotene–Porphyrin Dyads

Article pubs.acs.org/JPCA

Charge Transfer in Model Bioinspired Carotene−Porphyrin Dyads Laura Orian,* Silvia Carlotto, Marilena Di Valentin, and Antonino Polimeno Dipartimento di Scienze Chimiche, Università degli Studi di Padova Via Marzolo 1, 35131 Padova, Italy S Supporting Information *

ABSTRACT: We present a computational study based on accurate DFT and TD-DFT methods on model bioinspired donor−acceptor dyads, formed by a carotenoid covalently linked to a tetraphenylporphyrin (TPP) at the ortho position of one of the TPP phenyl rings. Dyadic systems can be used in the construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion, due to the unique advantages they offer in terms of synthetic feasibility. This study aims to describe the influence of chemical modifications on the absorption spectra, in particular on the lowest energy charge transfer bands. Effects of different metals of biological interest, i.e., Mg, Fe, Ni, and Zn, and of H2O and histidine molecules coordinated to the metals in different axial positions are rationalized.

1. INTRODUCTION The past decade has seen an increasingly growing interest in the guided design and synthesis of molecular components of organic solar cells and efficient photocatalytic systems for solar energy conversion. One of the main goals is to build artificial devices mimicking the efficiency and negligible environmental impact of photosynthetic machinery.1 Artificial antennas, rection centers, and artificial photosynthetic membranes have been developed. In particular, dyadic/triadic prototypes are suitable molecular candidates for the advantages they offer in terms of synthetic feasibility and are therefore intensively studied.2 For artificial reaction centers, which mimic charge separation in photosynthesis, the challenge is to choose adequate donor and acceptor units with finely tuned electronic features and controlled mutual orientation, so that fast charge separation and very slow charge recombination occur. Among artificial photosynthetic systems, carotenoid-based molecular dyads have been extensively studied to elucidate the structural and electronic requisites of electron transfer as well as singlet− singlet and triplet−triplet energy transfer processes, which, in nature, involve the carotenoid pigments, both in terms of lightharvesting and photoprotection from singlet oxygen.3−5 Carotenoid chromophores play an important role in energy dissipation by quenching singlet excited states of chlorophylls in photosynthetic antennas and recent spectroscopic investigations have been focused on this specific role of the carotenoid moiety in carotenoid-based dyads.4−6 Three mechanisms have been identified as responsible of the tetrapyrrole singlet excited state quenching: intramolecular electron transfer, singlet energy transfer from the porphyrin to the carotenoid S1 state, and/or exciton coupling.4 The dominant mechanism depends on several factors as the molecular structure and the solvent, although a systematic experimental and theoretical understanding of the interplay of these processes is not at the moment available. In this article, we focus on charge transfer (CT) states in model bioinspired dyads. The parent system is formed by a © 2012 American Chemical Society

carotenoid polyene (C) covalently linked via an ester group to a tetraphenylporphyrin (P) at the ortho position of one meso aromatic ring. This dyad is here denoted CP (Scheme 1) and was first prepared by Moore et al.7 who proposed a folded conformation on the basis of 1H NMR measurements and semiempirical calculations.8,9 This folded arrangement mimics the mutual orientation of the pigments carotenoid/chlorophyll found in natural photosystems. Spectroscopic investigation on this system demonstrated both singlet−singlet and triplet− triplet energy transfer.8,10 Furthermore, effective quenching (75%) of the porphyrin fluorescence by the carotenoid moiety was found and tentatively interpreted as the possible formation of a charge separated state.7 It is worth to notice that the CT band of CP is not detected by UV−vis absorption spectroscopy since it is probably covered by the strong S2 ← S0 band of C, but the UV−vis spectrum of the dyad is not exactly the sum of the spectra of the separated P and C suggesting an interaction between them.7 Several computational studies on porphyrins11 as well as on carotenoids12 have been carried out at different levels of theory, highlighting different aspects of their structural and electronic properties, pointing out in particular the intrinsic difficulties of describing these apparently simple molecules, which have in common an extended conjugation and a rather rigid structure. To the best of our knowledge, no accurate computational study of donor−acceptor dyads combining these two moieties has been carried out: instead, triadic systems have been prepared and studied in which porphyrins are combined mainly with carotenes, quinones, and fullerenes.13 Extensive calculations have been performed on the interactions of carotenoid−(bacterio)chlorophyll couples, evaluating the energy of charge-transfer states, in the lightharvesting complexes of purple bacteria and higher plants.14 Only in some cases the charge transfer states were located at an Received: December 23, 2011 Revised: March 18, 2012 Published: March 19, 2012 3926

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Scheme 1. Dyad: CP, the ML group is replaced by two H atoms; CPM, M = Mg, Fe, Ni, Zn; CPM_Lexo/endo, M = Mg, Fe, Ni, Zn and L = H2O, His

Another common way of estimating HCP is from the energetic splitting of the HOMOs in systems formed by two molecules.23 The correctness of this procedure relies on the assumption that the spatial overlap between the HOMOs is equal to zero and that the site energies (diagonal matrix elements of the Hamiltonian) of the two molecules in the dimer are identical. In this work, we have used the MOs on the individual C and P fragments as a basis set in calculations on a system composed by the two fragments with the exact geometry and orientation they have in the corresponding dyadic compound (see computational methodology).24 The calculated overlap and coupling values give useful insight of the different efficiency of ET upon metal replacement in the porphyrin cavity and in the presence of the ligands H2O or His in the exo/endo position.

energy level suitable to take part to disexcitation processes of the chlorophyll pigment. This work is a systematic analysis performed at density functional (DFT)15 and time-dependent density functional (TD-DFT)16 level on the effects on the geometry, electronic structure, and lowest charge transfer absorption, produced by inserting a metal center in the porphyrin cavity (CPM; M = Mg, Fe, Ni, Zn) and subsequently adding a water molecule or a histidine residue (His) below the porphyrin plane (exo position) or sandwiched between the porphyrin and the carotenoid (endo position) (CPM_Lexo/endo; M = Mg, Fe, Ni, Zn and L = H2O, His) (Scheme 1). Magnesium is chosen because of its presence in chlorophyll; zinc is commonly used instead of Mg in synthetic systems, due to their strict analogy. Iron is the metal of heme, with a strong capacity of bonding H2O as well as His. Nickel, which is also a metal with a relevant biological importance, is chosen because the formation of pentacoordinated complexes of Ni(II) porphyrins with H2O and His is energetically disfavored.17 H2O and His are found as axial ligands of the chlorophyll pigments in the photosynthetic systems, and their role in the photosynthesis is still a matter of debate.18,19 Our main purpose is to analyze the geometric and the electronic factors determining the occurrence and the shift of charge transfer states involving the P and C moieties. The calculation of the excitation energies, once a reliable methodological protocol is established, represents a tool to recover information not accessible from the experiment and to explore novel CP systems selected to reproduce specific aspects of the natural photochemistry. Once the CT process has been identified, a critical parameter in determining the ET rate is the charge transfer integral, i.e., the off-diagonal matrix element of the Kohn−Sham Hamiltonian, ⟨ΨC|Ĥ |ΨP = HCP⟩ ; C and P label the initial and final diabatic states localized on the carotenoid-donor and on the porphyrin-acceptor, respectively. In the weak coupling nonadiabatic regime, the ET rate, is proportional to H2CP, and this descends from the Fermi Golden Rule from which the wellknown Marcus and Fö rster expressions are derived. 20 Computation of HCP requires some care, and it has been carried out in the literature using different approaches,21 among which the well-known generalized Mulliken−Hush scheme, which employs a linear combination of eigenstates to build charge localized states and treat them as diabatic states.22 The effect of nonzero spatial overlap between the orbitals of the molecular sites must be taken into account.

2. COMPUTATIONAL METHODOLOGY The geometries of P and C as well as those of the dyads were fully optimized in vacuo at the PBE1PBE/6-31G(d) level of theory25 as implemented in the software package Gaussian.26 The excitation energies were computed using a TD-DFT approach at the CAM-B3LYP/6-31G(d) level of theory.27 A larger basis set (6-31G+(d,p)) was tested for CP without significant change in the calculated absorptions. PBE1PBE,25 PC-wPBE,28 and wB97XD29 were also tested to calculate the absorption spectra of P, C, and CP, and the results in vacuo were compared to the experimental data available in the literature. The criteria for choosing the functional for TD-DFT calculations were the closest matching between the experimental and computed strong S2 ← S0 absorption of the carotenoid and the strongest Soret band of the porphyrin. With CAM-B3LYP, all the calculated excitations fall in the range of the measured absorptions. This is in agreement with the recent diagnostics by Tozer et al.30 that CAM-B3LYP density functional performs rather well for both local valence and CT excitations. Solvent effects were accounted for only in TD-DFT calculations using the polarizable continuum model (PCM).31 A standard cavity was used, and the dielectric constants of dichloromethane (DCM) and water employed were 8.93 and 78.35, respectively. The charge transfer and the overlap integrals were computed in vacuo using Amsterdam density functional (ADF2010.02) program.32 We have performed a direct calculation of the integral HCP and of the overlap integral S = ⟨ΨC|ΨP⟩, by using the MOs of the two isolated P and C fragments as a basis set in the calculations on dyads maintaining the exact geometry and 3927

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orientation they have in the corresponding covalently linked dyads, without invoking any additional approximation.24 The esther bond has been removed and the two fragments saturated with methyl groups. Both LDA and GGA approximations were tested for CP, CPMg, and CPZn sets of dyads with results differing by a few percent; for the set of Ni-based dyads, only GGA was employed. For the set of Fe-based dyads, the overlap and coupling was not calculated at the same level of theory since, while the ground state of the Fe-based dyads studied here is a singlet, the ground state of the Fe-porphyrin fragment is a triplet, also in the presence of the ligands; this resulted in SCF convergence problems.33 Thus, a different functional, i.e., OPBE0,34a was employed, which proved to be particularly good at describing the different electronic states of Fe(II) and Fe(III) porphyrins.34b An atomic basis set of Slater-type orbitals (STOs) of triple-ζ quality including one set of polarization functions (TZP) was used, which was found to give as much accurate results as larger basis sets in analogous problems.24 This type of orbitals gives a better description of the tails of the electron wave function as compared to Gaussian-type orbitals, thus resulting more suitable for calculations of charge transfer integrals. For LDA, the parametrization of electron gas data given by Vosko, Wilk, and Nusair (VWN)35 was used; instead for GGA, the nonlocal correction by Perdew−Wang to the exchange part of the functional (PW91x) and the correlation correction of Perdew−Wang (PW91c) were used.36

Table 1. Relevant Geometric Parameters of the Optimized Dyadsa CP CPMg CPFe CPNi CPZn CPMg_H2Oexo CPMg_H2Oendo CPMg_Hisexo CPMg_Hisendo CPFe_H2Oexo CPFe_H2Oendo CPFe_Hisexo CPFe_Hisendo CPNi_H2Oexo CPNi_H2Oendo CPNi_Hisexo CPNi_Hisendo CPZn_H2Oexo CPZn_H2Oendo CPZn_Hisexo CPZn_Hisendo

Rb

θ1c

θ2c

φd

4.3 3.8 3.8 4.6 3.8 4.4 4.9 4.6 6.4 3.8 4.9 4.2 6.3 4.5 5.4 4.6 6.7 4.5 4.9 4.7 6.3

103 103 103 105 103 103 100 104 92 103 100 107 93 105 107 110 96 103 101 105 91

91 88 88 105 88 97 82 102 72 91 85 97 74 105 77 73 73 98 83 103 73

31 29 28 20 29 38 46 42 100 33 25 47 81 20 22 35 97 38 47 49 101

a

Level of theory: PBE1PBE/6-31G(d). Distances are in Å and angles in deg. bR is the distance between the centroid of P and the centroid of the benzene ring of C (Figure 1). cThe angles θ1 and θ2 and the vector R are shown in Figure 1. dφ is the dihedral angle between average molecular planes of C and P.

3. RESULTS AND DISCUSSION In the parent dyad CP, the porphyrin ring and the carotenoid polyene are almost perfectly cofacial; the distance R measured from the centroid of P and the centroid of the benzene moiety of C (Figure 1) is 4.3 Å and is in excellent agreement with the

the dihedral φ between the planes containing P and C moieties and the values of the angles θ1 and θ2 (Figure 1, Table 1). It should be noted for sake of completeness that, in all the studied dyads, P and C are necessarily in a folded conformation due to the covalent link between P and C in the ortho position. It has been experimentally found, that the analogous meta and para conformers exist in solution in a partially open and completely unfolded arrangement, respectively.7,8 In Figure 2, the Kohn−Sham MOs involved in the three monoelectronic excitations mainly contributing to the S2 ← S0, Soret, and CT absorptions are shown for CPMg. In the lowest energy strong absorption, the dominant contribution involves a couple of MOs of opposite symmetry localized on the

Figure 1. Relevant angles and intermoieties distance for the description of the dyad geometry. R is the distance between the centroid of P and the centroid of the benzene ring of C; θ1 and θ2 are the angles between the two vectors lying in the average molecular plane of C (orange) and P (cyan), respectively.

crystallographic distance reported by Moore et al.7 It also compares favorably with the average distance encountered in the photosynthetic systems mentioned in the introduction. Notably, there was no need to include dispersion corrections in the functional, the stacked conformation of P and C being fully imposed by the covalent bond in ortho position. The insertion of a metal center in the porphyrin cavity changes R and the mutual orientation φ of P and C moieties, as reported in Table 1. When going from Mg to Zn, R remains constant except when Ni is present. This strong enhancement of the distance is due to the typical saddle-shaped form of the Ni(II) porphyrin, whose curvature pushes upward the cofacial carotenoid.37 A similar conclusion can be drawn considering

Figure 2. Kohn−Sham MOs of CPMg involved in the three monoelectronic excitations mainly contributing to the S2 ← S0, Soret, and CT absorptions. Level of theory: CAM-B3LYP/6-31G(d). For sake of clarity, occupied MOs (left) have been colored differently from empty MOs (right). 3928

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Table 2. Effect of the Metal on the Main Calculated Absorptions in Vacuo, in DCM, and in Watera S2 ← S0 bandb vacuum CP CPMg CPFe CPNi CPZn

485 486 488 485 486

(4.73) (4.71) (4.32) (4.71) (4.71)

DCM 511 512 513 511 512

(4.67) (4.69) (4.66) (4.63) (4.68)

CT bandb,c water 507 509 508 507 508

(4.69) (4.70) (4.69) (4.66) (4.69)

vacuum 395 380 372 357 379

(0.012) 81% (0.0508) 74% (0.1458) 60% (0.0646) 21% (0.0378) 76%

Soret bandb

DCM 408 394 385 380 391

(0.014) 82% (0.086) 76% (0.3335) 45% (0.3844) 51% (0.0606) 78%

water 413 397 412 384 394

(0.0086) (0.0311) (0.0058) (0.0661) (0.0266)

vacuum 83% 79% 79% 58% 80%

372 367 365 362 363

(0.97) (1.13) (0.69) (0.66) (1.18)

DCM 384 384 379 377 378

(1.36) (1.02) (1.14) (0.90) (1.40)

water 381 382 377 373 377

(1.31) (1.29) (1.22) (1.12) (1.32)

a

Level of theory: CAM-B3LYP/6-31G(d). All the wavelengths are in nm. bThe values in parentheses are the oscillator strengths. cThe percentage values in italics indicate the contribution of the monoelectronic excitation to the CT band analogous to that shown in Figure 2.

carotenoid moiety. The highest energy absorption of interest is one of the Soret bands of the porphyrin originating mainly from an electronic transition between two π MOs. The lowest CT band of interest involves mainly the filled MO, which is also involved in the S2 ← S0 transition, located on the carotenoiddonor, and the empty MO engaged also in the Soret transition, located on the porphyrin acceptor. Thus, the nature of the carotenoid-to-porphyrin charge transfer is clearly assessed. It is also worth of notice that the HOMOs and LUMOs computed for the isolated C and P are identical to the localized MOs centered on the carotenoid and porphyrin moieties of the dyad shown in Figure 2. The main excitation energies calculated in vacuo and in two solvents, i.e., DCM and water, are listed in Table 2. While the values for the S2 ← S0 and the Soret bands are almost unchanged in the series, the lowest CT band shifts to lower wavelengths with the increasing number of d electrons, going from Mg to Ni. The electronic valence configuration of zinc, analogous to that of magnesium, is the origin of the almost equal CT excitation energy computed for CPMg and CPZn. The surrounding dielectric medium induces a bathochromic shift of the CT band, which slightly increases with the polarity of the solvent (Table 2, Figure 3). In contrast, the two localized excitations are blue-shifted when going from DCM to water (Table 2).

above 50% with increasing polarity, with the exception of CPFe in DCM (Table 2). The net CT character of the band in Niand Fe-based dyads is less pronounced. The effects of the presence of a H2O molecule and His residue as ligands were investigated including in the dyadic system either the former or the latter in two different positions, i.e., below the porphyrin molecular plane (exo) or sandwiched between the porphyrin and the carotenoid moieties (endo). In principle, a H2O molecule can coordinate to the metal center through its oxygen, and His can coordinate through its acid imidazolic nitrogen.17 In the dyads, the exo coordination of both ligands has mainly an electronic effect and implies only slight geometric modifications to the dyad skeletal, as expected. The C−P distance increases upon coordination of an exo ligand, with the exception of Ni-based dyads because the ligand is not coordinated (Table 1). Instead, the endo coordination imposes a significant deformation to the dyad because of the steric hindrance of the ligand. In particular, upon endo coordination of His, a dramatic increase of the C−P distance occurs accompanied by a partial rotation of the carotenoid, which moves from the plane almost parallel to the porphyrin ring to a plane almost orthogonal to it. Notably, this apparently unusual (and intutitively less convenient for ET processes) mutual orientation of the pigments is present also in natural antenna systems like the Peridinin−Chlorophyll A Protein of dinoflagellates.38 It was possible to identify the electronic CT excitation involving mainly the MOs analogous to those of Figure 2 also in the presence of the ligands H2O and His. In our calculations, no electron density was found on H2O and His, excluding for these model systems a superexchange mechanism for the CT process involving these ligands. This observation does not exclude, of course, that ligands like H2O or His may have a role in other important photophysical processes of energy transfer SSET and TTET for transitions occurring via the Dexter mechanism.18,39 The computed CT absorptions in vacuo are shown in Table 3. Solvent effects on these CT bands are shown in Figure 4. For all the metal-based dyads, the effect of the ligands on the S2 ← S0 and Soret band is negligible. The solvents induce a bathochromic shift more pronounced in the Soret band. The effect of H2O or His ligands on the CT band is more complex, and a distinction must be done for Mg/Zn-based dyads and Nibased and Fe-based dyads. Analyzing first the data in vacuo, one can notice that, for Mg- and Zn-based dyads, the coordination of H2O in the exo position has almost no effect, while His in the same position induces an hypsochromic shift of about 10 nm. Upon endo coordination of both H2O and His, the energy of the CT band is significantly shifted to shorter wavelengths by approximately 50 and 60 nm, respectively. The same trend

Figure 3. Effect of the metal on the calculated CT absorptions in vacuo (empty diamonds), in DCM (cross diamonds), and in water (filled diamonds); level of theory, CAM-B3LYP/6-31G(d).

Notably, the metal does not participate directly; also in the case of CPZn, the d orbitals of Zn lie at lower energy. The scenario changes when Fe and Ni are coordinated to the porphyrin ring. In fact, in the lowest CT absoprtion of CPNi and CPFe, significant contributions from monoelectronic transitions involving metal-based MOs are found. As a result, the percentage contribution of the CT excitation represented in Figure 2 drops to 60% for CPFe and dramatically to 21% for CPNi (Table 2). In solvent, these percentages increase again 3929

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bands of CPMg_H 2 O endo and CPZn_H 2 O en do and CPMg_Hisendo and CPZn_Hisendo are approximately 30 and 40 nm, respectively. In Ni-based dyads, the coordination of both H2O and His to the metal center is not favored and does not occur; thus the energy of the CT band in vacuo does not change significantly when going from CP_Ni to CPNi_H2Oendo and CPNi_Hisexo. The shift calculated in vacuo for CPNi_H2Oexo is reduced in solvent. The remarkable exception is the energy of the CT band of CPNi_Hisendo, which is hypsochromically shifted by approximately 40 nm with respect to the CT band of CPNi in vacuo and by 25 nm in solvent. The data of Fe-based dyads have similar trends to those of Mg and Zn based systems both in vacuo and in solvent (Table 3, Figure 4). The hypsochromic shift of the CT band of CPFe_Hisendo with respect to that of the parent dyad CPFe in vacuo is almost 70 nm, the largest predicted if compared to the shift between the CT band of analogous Mg, Ni, and Zn dyads. Similarly to that found for Ni-based dyads, the contribution of the pure monoelectronic CT excitation predicted in Fe-based dyads is smaller if compared to the analogous values of Mg- and Zn-based dyads, due to the contribution of other monoelectronic excitations involving the metal d orbitals. Summarizing, Figure 4 shows clearly that (i) the effect of the ligand on the CT excitation energy exhibits the same trend in Mg-, Fe-, and Zn-based dyads in vacuo and the same trend is found almost entirely reproduced in both solvents; (ii) a ligand in the exo position has generally a less pronounced effect than a ligand in the endo position, a situation in which both electronic and steric effects are present; (iii) the effect of the ligand on the CT excitation energy of Ni-based dyads is rather small or negligible, except for CPNi_Hisendo; with increasing solvent polarity, the effect of the ligand becomes more and more negligible (all points become very close); (iv) the presence of His in the endo position in all dyads induces the largest hypsochromic shift of the CT band with respect to the CT band of the parent dyad without ligands. The peculiar effect of His in endo position has been investigated in detail by computing the overlap and electronic coupling integrals (Table 4). No significant overlap variation is predicted when going from CP to CPMg and CPZn; in addition, S2 values vary similarly upon coordination of the ligands in the Mg-based and Zn-based dyads, with the exception of CPZn_H2Oendo, the value of which is slightly lower than the value computed for CPMg_H2Oendo. The remarkably low value of S computed for CPNi is likely due to the deformed π system of the porphyrin ring. The high value of S computed for CPFe_H2Oexo is due to the short R. H2CP values follow the same trend of S when going from the parent dyad without ligands to the dyad with one ligand in the exo position. The presence of H2O in endo position imposes a deformation of the dyad (Table 1) but is accompanied by a negligible variation of the overlap and the coupling with respect to the parent dyad, except for CPFe_H2Oendo in which the presence of the sandwiched ligand has almost no effect on the dihedral angle φ. The presence of His in endo, which induces the largest deformation of the dyad, causes the largest increase of both overlap and coupling values, except for CPFe_Hisendo. An explanation can be found from the mutual orientation of the meso phenyl ring of C and one pyrrol ring of P in the deformed dyads, which in the presence of an endo His are in closer proximity due to a partial rotation of the C moiety. This makes

Table 3. Effect of the Ligand on the Calculated CT Absorptions in Vacuoa S2 ← S0 bandb CP CPMg CPMg_H2Oexo CPMg_H2Oendo CPMg_Hisexo CPMg_Hisendo CPFe CPFe_H2Oexo CPFe_H2Oendo CPFe_Hisexo CPFe_Hisendo CPNi CPNi_H2Oexo CPNi_H2Oendo CPNi_Hisexo CPNi_Hisendo CPZn CPZn_H2Oexo CPZn_H2Oendo CPZn_Hisexo CPZn_Hisendo

485 486 485 485 485 489 488 487 484 485 489 485 485 482 484 487 486 485 485 485 489

(4.73) (4.71) (4.74) (4.46) (4.82) (4.17) (4.32) (4.70) (4.47) (4.35) (4.19) (4.71) (4.70) (4.52) (4.87) (4.14) (4.71) (4.74) (4.47) (4.84) (4.17)

CT bandb,c 395 380 385 334 369 318 372 365 315 353 306 357 374 361 356 323 379 383 333 367 319

(0.0120) (0.0508) (0.0385) (0.0003) (0.1136) (0.0000) (0.1458) (0.1022) (0.0002) (0.0543) (0.0001) (0.0646) (0.0529) (0.0046) (0.0801) (0.0000) (0.0378) (0.0369) (0.0002) (0.0709) (0.0001)

81% 74% 78% 78% 61% 86% 60% 29% 76% 76% 76% 21% 49% 79% 72% 77% 76% 79% 80% 74% 84%

Soret bandb 372 367 369 370 373 373 365 360 356 362 362 362 362 364 363 365 363 366 367 374 376

(0.97) (1.13) (1.19) (1.24) (1.01) (1.28) (0.69) (0.42) (0.66) (0.46) (0.97) (0.66) (0.91) (0.89) (0.88) (0.82) (1.18) (1.18) (1.17) (0.99) (1.31)

a

Level of theory: CAM-B3LYP/6-31G(d). All the wavelengths are in nm. bThe values in parentheses are the oscillator strengths. cThe percentage values in italics indicate the contribution of the monoelectronic excitation to the CT band analogous to those shown in Figure 2.

(with slightly higher wavelengths of the CT band) is retained in both solvents where the hypsochromic shift upon coordination of His in exo is approximately 20 nm and those of the CT

Figure 4. Effect of the ligand on the calculated CT absorptions in vacuo (A), in DCM (B), and in water (C): parent dyads without ligands (triangles), dyads with H2O in exo/endo position (empty/ filled circles), and dyads with His in exo/endo position (empty/filled squares). Level of theory: CAM-B3LYP/6-31G(d). 3930

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Table 4. Square Values of Overlap Integral S and Electronic Coupling HCPa CP CPMg CPMg_H2Oexo CPMg_H2Oendo CPMg_Hisexo CPMg_Hisendo CPFe CPFe_H2Oexo CPFe_H2Oendo CPFe_Hisexo CPFe_Hisendo CPNi CPNi_H2Oexo CPNi_H2Oendo CPNi_Hisexo CPNi_Hisendo CPZn CPZn_H2Oexo CPZn_H2Oendo CPZn_Hisexo CPZn_Hisendo

S2 (10−6)

H2CP (10−4)b

H2CP/ (H2CP)CPc

1.8 1.9 1.5 1.4 0.040 3.8 3.5 4.0 0.044 0.35 1.4 0.012 1.1 0.49 1.6 2.9 1.9 1.6 0.98 0.0025 3.8

0.69 0.74 0.56 0.76 0.0022 1.8 1.4 1.7 0.052 0.15 0.47 0.15 0.78 0.42 0.78 1.2 0.73 0.62 0.56 0.0011 1.8

1 1.1 0.81 1.1 0.0032 2.6 2.0 2.5 0.075 0.22 0.68 0.22 1.1 0.61 1.1 1.7 1.1 0.90 0.81 0.0016 2.6

overlap between the interacting sites and the strongest coupling were obtained upon the endo presence of His, which produces the strongest deformation of the dyad and an increase of the donor−acceptor distance. This result can be explained by considering the two MOs involved in the CT. The presence of the bulky His ligand induces a partial rotation of the carotenoid moiety, so that the phenyl ring of the latter moves closer to one pyrrolic ring of the porphyrin moiety and the overlap of the π lobes of the MOs of C and P involved in the CT excitation becomes larger. In Fe-based dyads, the deformation due to the endo His is weaker, and this explains the lower values of S and HCP. Becaue of their structural simplicity and synthetical feasibility, novel dyadic systems are nowadays currently synthesized to investigate the details of the interactions between the carotenoids and the porphyrin derivatives (cf. the very recent papers by Moore and co-workers4,5). These studies indicate that carotenoids can quench tetrapyrrole singlet excited states in a variety of ways, and even modest differences in the molecular architeture can lead to different quenching mechanisms. In the systems considered in this study, as well as in other biological systems,14d the lowest CT band lies always at higher energy than the S2 ← S0 band. The band gap, defined as the energy difference between the CT and the S2 ← S0 bands, in our dyads is influenced by several factors: (1) Nature of the metal: in vacuo, the smallest band gap is found in the parent CP dyad; this gap increases going to CPMg and CPZn and to CPFe and CPNi where the largest hypsochromic shift is observed. (2) Solvent effect: the presence of a polar medium, in general, increases the band gap, an effect that is more evident in DCM than in water, due to a larger bathochromic shift of the CT combined to a smaller bathochromic shift of the S2 ← S0 band in water; hence, the increase of dielectric constant reduces the band gap.6 (3) Ligand effect: the smallest band gap is found in the parent CP dyad; in vacuo, the largest band gap is observed when His is present in the endo position, and the smallest band gap is observed when H2O is present in the exo position. (4) Distance effect: an analogous trend is recovered to that described in (3) since His in the endo position and H2O in the exo position induces the largest and the smallest C−P distance, respectively, in the dyads. Since the smallest band gap is observed in the parent CP dyad, we have run a TD-DFT calculation in vacuo by imposing a shorter C−P distance, i.e., 3.3 Ǻ , in the attempt to further reduce the band gap. The three computed absorptions are 497 nm (S2 ← S0), 477 nm (54%, CT), and 378 nm (Soret). The large bathochromic shift of the CT band drastically lowers the band gap (from 103 to 20 nm); this suggests that further reducing the distance might probably allow the ET disexcitation mechanism, but this requires a less hindered porphyrin acceptor than TPP with its out-of-plane phenyl rings. In addition, our results suggest that the optimal situation combining a low energy CT and high HCP is found in CP, CPMg, and CPZn and to some extent in CPFe, while the presence of nickel is undoubtedly unfavorable.

a

Level of theory: BPW91/TZP all electron and OPBE0/TZP all electron (for Fe-based dyads). bAll values are in eV2. cRatios between H2CP of a dyad and H2CP of CP taken as reference.

the overlap and, consequently, the coupling more efficient than in the other cases. This explains also the values of CPFe_Hisendo where the dihedral angle φ is smaller than in the other analogous dyads, corresponding to a weaker deformation. This is another example proving that the geometric dependence of ET processes must be considered accurately since it is accounted for not only by the geometrical distance between the donor and the acceptor but also by the mutual orientation of the interacting sites, which must provide an efficient overlap between the involved MOs.

4. CONCLUSIONS Charge transfer between a carotenoid donor cofacial to a porphyrin acceptor in model bioinspired dyads was studied employing state-of-the-art DFT and TD-DFT methods. The presence of different metals in the porphyrin cavity (Mg, Fe, Ni, and Zn) and the effect of coordinating an H2O or His molecule in the exo or endo position with respect to the donor/acceptor moieties were investigated systematically to highlight molecular and electronic features ruling the ET process. While Mg- and Zn-based dyads behave almost identically, Ni- and Fe-based dyads show specific peculiarities. In particular, Ni-based dyads are geometrically different, due to the saddle shape form of the porphyrin fragment, and no coordination of H2O and His occurs. Notably, in both Ni-based and Fe-based dyads, there is a non-negligible contribution of monolectronic transitions to the CT band in which the metal d orbitals are involved, and as a result, the net carotene to porphyrin charge transfer contribution is diminished. The presence of His in the endo position induces the largest hypsochromic shift of the CT band in all the dyads. To further explore this point, the overlap S and the electronic coupling matrix element HCP were computed using a fragment approach. In all cases, except in Fe-based dyads, the highest 3931

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ASSOCIATED CONTENT

S Supporting Information *

TD-DFT results at different levels of theory for the benchmark systems; data in DCM and H2O reported in Figure 4; overlap and coupling integrals at different levels of theory; figures of CP model dyad and of C and P fragments used for calculating the charge transfer integral; figures of CPNi, CPNi_H2Oexo, and CPMg_Hisendo; full references 26 and 32. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +390498275140. Fax: +390498275239. E-mail: laura. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support is acknowledged by the Ministero dell’Istruzione, Università e Ricerca (MIUR), grant 2008J9RNB3 PRIN 2008 TIME, and by the University of Padova, grant STPD08RCX5 “Progetto Strategico” HELIOS. Calculations were performed at LICC (Laboratorio Interdipartimentale di Chimica Computazionale) of the University of Padova.

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