Theoretical Method for an Accurate Elucidation of Energy Transfer

Article pubs.acs.org/IC

Theoretical Method for an Accurate Elucidation of Energy Transfer Pathways in Europium(III) Complexes with Dipyridophenazine (dppz) Ligand: One More Step in the Study of the Molecular Antenna Effect María J. Beltrán-Leiva,† Plinio Cantero-López,† César Zúñiga,‡ Ana Bulhões-Figueira,§ Dayán Páez-Hernández,*,†,‡ and Ramiro Arratia-Pérez*,†,‡ †

Relativistic Molecular Physics (ReMoPh) Group, Ph.D. Program in Molecular Physical Chemistry, Universidad Andrés Bello, Av. República 275, Santiago 8370146, Chile ‡ Centro de Nanociencias Aplicadas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Av. República 275, Santiago 8370146, Chile § Centro Universitário Estácio de Ribeirão Preto, Rua Abrahão Issa Halach, 980 Ribeirânia, Ribeirão Preto, Sao Paulo 14096-160, Brazil S Supporting Information *

ABSTRACT: A theoretical protocol to study the sensitization and emission mechanism in lanthanide compounds on the basis of multireference CASSCF/PT2 calculations is proposed and applied to [Eu(NO3)3(dppz-CN)] and [Eu(NO3)3(dppz-NO2)] compounds synthesized and characterized herein. The method consists of a fragmentation scheme where both the ligand and the lanthanide fragments were calculated separately but at the same level of theory, using ab initio wave-function-based methods which are adequate for the treatment of quasi-degenerate states. This is based on the fact that the absorption is ligand-localized and the emission is europium-centered. This characteristic allowed us to describe the most probable energy transfer pathways that take place in the complexes, which involved an ISC between the S1 to T1 ligand states, energy transfer to 5 D2 in the lanthanide fragment, and further 5D0 → 7FJ emission. For both compounds, the triplet and 5D2 states were determined at the CASPT2 level to be around ∼26000 and ∼22400 cm−1, respectively. This difference is in the optimal range for the energy transfer process. Finally, the emissive state 5D0 was found at ∼18000 cm−1 and the emission bands in the range 550−700 nm, in quite good agreement with the experimental results. cancer,14,15 that has emerged as a powerful alternative due to its noninvasive and selective nature, in which a photoactivatable drug becomes toxic only for cancerous cells, leaving the unexposed normal cells intact. This procedure involves a photosensitizing agent that, in this case, is constituted by both antenna and lanthanide fragment. Thus, upon UV−vis irradiation, the antenna is excited and the energy transferred to the LnIII moiety. At the same time, the generation of reactive oxygen species (ROS) occurs, which causes oxidative cell death only to the region under irradiation. Therefore, the choice of an appropriate photosensitizing antenna is fundamental in order to obtain promising complexes to be used in PDT applications. In this context, the dipyridophenazine (dppz) ligand has been widely used as a photosensitizer in the synthesis of lanthanide complexes. The choice of the dppz ligand as antenna is based on (i) its ability to generate photoinduced 3(n → π*) and/or 3(π → π*) states which, in turn, transfer their energy to molecular oxygen to form ROS, (ii) their efficient energy transfer to the LnIII ion for generating emissive excited states,

1. INTRODUCTION In recent decades, the synthesis of new lanthanide(III) complexes has become a hot area because of their potential use as materials for biological immunoassays, lasers, cathode ray tubes, lighting systems, electroluminescent devices, sensors, dosimeters, imaging agents, display applications, decoration purposes, and light-emitting diodes (LEDs).1−6 The growing interest in these kinds of compounds is due to their unique spectroscopic properties attributed to the characteristic f−f transitions in their inner 4f shell, which is shielded from the influence of the environment by the outer 5s and 5p shells. These transitions are Laporte forbidden with low absorption coefficients, which makes the direct excitation of the LnIII ion inefficient. Therefore, the luminescence efficiency in these ions is mainly governed by their strongly absorbing ligands: a ligand absorbs light in the ultraviolet region and transfers energy from its excited level to the resonant level of the LnIII ion, which can emit light or decay nonradiatively (antenna effect).7−10 One of the most recent and significant applications of these compounds is the design of multimodal DNA photocleavage agents in photodynamic therapy (PDT).11−13 PDT is a treatment modality for a variety of diseases, including © 2017 American Chemical Society

Received: May 17, 2017 Published: July 10, 2017 9200

DOI: 10.1021/acs.inorgchem.7b01221 Inorg. Chem. 2017, 56, 9200−9208

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Inorganic Chemistry

determination of their energy transfer pathways.33,34 In the present work, we propose the application of a systematic ab initio approach to predict transition energies for low-lying ligand-localized excited states of LnIII complexes using a fragmentation scheme, where the ligand and the lanthanide are calculated separately but at the same level of theory. The approach employs wave-function-based methods adequate for quasi-degenerate states. It is based on a multireference CASSCF/PT2 technique for the calculations of the ground and excited states in LnIII complexes. On the basis of the above considerations, we performed a theoretical study of two EuIII complexes, viz. [Eu(NO3)3(dppzR)], where R = H (1), CN− (2), NO2− (3) (see Scheme 1),

and (iii) their high binding affinity to DNA due to the presence of an extended planar aromatic moiety, which can intercalate strongly with the planar basis pairs in DNA.16,17 Recent developments in the chemistry of lanthanide-based PDT agents include the research work of Yuasa et al.,11,13 who synthesized lanthanide nanoparticles (LNP) employing different coatings in order to combat cancer. They observed that Er(NP) with 5-aminolevulinic acid (ALA) as a coating gave rise to a dramatic enhanced PDT effect, reaching almost perfect lethality in comparison with other complexes under study. Furthermore, they reported that Er(NP) conjugated with monosaccharides exhibits greater photodamaging effects toward the corresponding cells, which demonstrates that carbohydrates can be used as selective ligands for cancer cells in PDT. In 2010, Chakravarty et al.18,19 reported the photoinduced DNA cleavage activity of a series of LaIII and GdIII complexes with phenanthroline, dipyridophenazine (dppz), pyridylphenanthroline, and terpyridine bases as photosensitizers, which exhibited significant photocytotoxic effects in HeLa cancer cells. In addition, in 2016 Patra et al.20−22 also reported the synthesis and structural characterization of a series of promising EuIII, TbIII, SmIII, and ErIII compounds employing both dppz and dipyridoquinoxaline (dpq) as antennas. They observed that, upon photoexcitation, both ligands showed efficient energy transfer to lanthanide fragment along with generation of ROS involved in DNA damage activity. Nevertheless, dppz complexes exhibited higher binding affinity to DNA in comparison to their dpq analogues. There have been multiple experimental studies on lanthanide complexes with dppz ligands, which have demonstrated their effective application in PDT. However, despite the importance and effect of these studies and with the purpose of contributing in this field, it is essential to have a good description of the transfer mechanisms that take place between both lanthanide and antenna fragments and the electronic states involved on it. Quantum chemical calculations provide important information about the excited ligand levels, which can be used to estimate the efficiency of the energy transfer. On the basis of these calculations, several energy transfer mechanisms and their corresponding kinetic models have been proposed in the literature to calculate energy transfer rates and emission quantum yields.23−26 However, the accuracy of the excitation energies is of the utmost importance, and the methodology needs to be selected carefully. In many works, the positions of the ligand-localized triplet state of LnIII complexes were calculated by DFT and TDDFT; however, DFT-based methods can underestimate the relative stability of high spin states in molecules and produce excessive delocalized charge distribution even with the Hartree−Fock exchange partially included.27−30 This means that DFT cannot predict the correct localization of the triplet excited state. Because of that, higher level computational methods become necessary in order to understand in detail all of the processes involved in the emission of LnIII complexes.31,32 Multireference ab initio methods have been successfully used to estimate the energy transfer pathways from antenna ligands to LnIII ions. This approach is more accurate and qualitatively correct than DFT methods. Multireference methods make possible the treatment of singlet and triplet states with equal accuracy, which is crucial for further calculations of spin−orbit matrix elements and the corresponding rate constants. Unfortunately, due to the time demand and the complexity of modeling lanthanide compounds, just a few theoretical studies have been reported, particularly on the

Scheme 1. Structural Formulas for the dppz-R Ligand and [Eu(NO3)3(dppz-R)] Complex

which were synthesized and characterized following a robust synthesis protocol widely tested in these kinds of compounds.20,35−40 For this, a computational technique is presented using ab initio methods that allows excited states in lanthanide complexes to be studied and energy transfer pathways from the antenna to lanthanide ion to be elucidated. In order to examine the effect of a possible coordination of solvent molecules to the lanthanide ion, we also performed calculations on solvated [Eu(NO3)3(dppz-R)(H2O)2] complexes.

2. COMPUTATIONAL DETAILS All calculations employed molecular models that were designed on the basis of the experimental reports of similar compounds and on evidence provided by our own experimental results (Figure 1). In a first step, all structures were optimized using the Amsterdam Density Functional (ADF) package.41,42 The scalar relativistic effects were incorporated by means of a two-component Hamiltonian with the zeroth-order regular approximation (ZORA). The BP86 generalized gradient approximation exchange-correlation functional was used with the standard Slater-type orbital (STO) basis set with the triple-ξ quality double plus polarization function for all of the atoms (TZ2P).43−45 To analyze the possible conformational change in the emissive state that could lead to a variation in the excitation energies, all compounds were fully optimized in their ground and first excited state, which was selected from the most common emissive state in europium (5DJ). At the DFT level both states were built modifying the spin polarization of the molecule. In all cases, the frequency calculations were carried out to verify the quality of the minimum found in the optimization process. To understand the origin of the emission in these kinds of molecules, a fragmentation scheme, where the antenna ligand was separated from the lanthanide moiety, was employed. This is possible because in all complexes the absorption bands are localized on the dppz ligand, regardless of whether the complexes do or do not have water molecules coordinated to the lanthanide ion. The fragments used were [Eu(NO3)3]/[Eu(NO3)3(H2O)2] and [dppz-R]. All of the optical properties of the antenna ligand were calculated using scalar relativistic time-dependent density functional theory (SR-TDDFT) 9201

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Figure 1. Molecular models for the complexes: (a) [Eu(NO3)3(dppz-R)]; (b) [Eu(NO3)3(dppz-R)(H2O)2], R = H (1), CN− (2), NO2− (3). The fragments used in the calculations are presented with different colors. with the XC functional of van Leeuwen and Baerends (LB94),46 which was specially designed for the response property calculations, particularly for spectroscopic properties, since the model has asymptotic corrections.47−49 The solvent effects were included via the COSMO model using the dichloromethane parameters.50,51 These calculations, performed on the ligand, are useful in the validation of the proposed theoretical methodology. However, in lanthanide(III) compounds many low-lying states are derived from a 4fn configuration and the wave functions for the ground and excited states have a large multireference character; therefore, a more sophisticated level of theory is necessary. The complete active space self-consistent field (CASSCF) approximation is a robust alternative methodology which describes in a good approach the electronic correlation effects and allows simultaneous determination of the ground and excited states.52,53 All of the correlated calculations were performed employing the MOLCAS 8.0 program package.54 The second-order Douglas−Kroll− Hess scalar relativistic Hamiltonian was employed in the calculations without spin−orbit (SO) coupling.55 All-electron ANO-RCC Gaussian-type basis sets contracted to TZP quality were used.56,57 SO coupling was treated by state interactions between the CASSCF wave functions, using the restricted active space state interaction (RASSI) program.58 The SO operator matrix was calculated from an atomic mean-field (AMFI) approximation.59 The dynamic correlation was included at the second order of perturbation using the multistate CASPT2 method.60,61 For the lanthanide ion the active space selected was (on the basis of the previous work and literature)62−68 CAS(6,7), which corresponds to the six unpaired electrons in the seven 4f orbitals. All of the possible multiplicities for EuIII ion were considered in the calculations as follows: 7 septuplets, 60 quintuplets, 21 triplets, and 21 singlets. The role of the highest energy multiplicities is discussed. The antenna ligand was also treated at the same level of theory using an active space selected in order to properly describe its electronic transitions. It consists of 10 electrons in 10 orbitals; 1 of nonbonding nature and 9 with π character (CAS(10,10)). The calculations were carried out for the optimized geometry of the antenna in the ground (S0) and first excited (S1) singlets and for a first excited triplet (T1). Additionally, to elucidate the possible energy transfer pathways from the antenna to the lanthanide ion, the same calculations were performed for the antenna ligand as for a fragment obtained from the optimized geometry of the entire complex. To facilitate the interpretation of the results, we designed the location of the multiplet states in cm−1 and the absorption/emission bands in nm.

All of the complexes were isolated in good yields and characterized by elemental analysis (section 4 in the Supporting Information), FT-IR and UV−visible spectra, and ESI-MS. Because of the poor solubility of the samples, crystal structures could not be obtained, and only X-ray powder diffraction measurements were made.

4. RESULTS AND DISCUSSION Commonly, accepted antenna models of energy transfer in LnIII complexes are based on the assumption that the excitation in the complexes is ligand localized. This assumption is supported by a number of theoretical considerations and experimental observations.69−71 Thus, in different lanthanide complexes containing the same organic ligand, the positions of the excitation and emission bands associated with this ligand differ only slightly from each other. This indicates that these excited states are only slightly affected by the nature of the central ion and other ligands and that the excitation is localized on the corresponding ligand,29,30 which constitutes the basis of the developed methodology in the present work. The excitation energy is transferred to the lanthanide ion from the first excited triplet state of the ligand, which means that the quality of our model is directly related with the accuracy in the determination of this state. The following results are presented separately: first the ligands are characterized and discussed, and second the possible energy transfer mechanism is presented. 4.1. Spectroscopic Properties of the Ligands. The UV−visible absorption spectra of the europium complexes [Eu(NO3)3(dppz-CN)] (2) and [Eu(NO3)3(dppz-NO2)] (3) in DMF are shown in Figure 2. The europium complexes show two bands at 369−390 nm for 1 and 374−393 nm for 2, respectively. According to the experimental data and calculated transitions, these bands can be ascribed mainly to n−π*/π−π* transitions of the dppz-R ligands. It is important to note that the absorption spectrum of the given complex is very similar to that of the free ligand,17,72 which indicates that the absorption depends mainly on the dppz-R ligand. This suggests a possible sensitization process between the ligand and the lanthanide ion. TD-DFT calculations reproduce the most important transitions in correct agreement with the experimental reports. All of the ligands show absorption bands between 350 and 400 nm assigned as n−π*/π−π*. The inclusion of the solvent in the calculations keeps almost unaltered all transitions in the computed spectra, with a bathochromic shift in all cases (section 2 of the Supporting Information). In order to validate the application of the CASSCF/PT2 methodology in the description of the optical properties (especially emission), the dppz ligand was used as a model compound because its optical properties have been widely reported. The first excited triplet state was optimized for the three ligands, and the resulting

3. EXPERIMENTAL DETAILS Lanthanide(III) complexes were prepared using a general synthetic procedure by reacting an ethanolic solution of [Eu(NO3)3]·5H2O (1 mmol) with the corresponding dipyridophenazine derivatives (2 mmol) dissolved in a 2/1 CHCl3/CHCl2 mixture (18 mL). The ethanolic solution (2 mL) was added dropwise to the ligand solution and stirred for 60 min. The pH of the resulting solution was adjusted between 6 and 7 with dilute NaOH (1 mol dm−3) solution. The precipitate was removed by filtration, dried, and washed with ethanol. 9202

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Figure 3. Phosphorescence spectra of [Gd(NO3)3(dppz-CN)] (λex 390 nm) and [Gd(NO3)3(dppz-NO2)] (λex 393 nm) at 77 K.

experimental results, which is evidence of the good performance of the selected active space. The values for the optimized substituted ligands were 21355 and 20630 cm−1 (for CN and NO2, respectively). 4.2. Electronic States of the EuIII Fragment. Because of the strongly localized character of the 4f orbitals in lanthanides, the 4f−4f excitations cannot be considered directly in the calculations. However, an accurate estimation of the excited states in the europium fragment is crucial in order to determine the possible energy transfer pathways in the entire complex. Several authors73−78 have observed that the largest quantum yields occur when the triplet state energy is close to resonant levels of the lanthanide ion (Ln*), and a “safe” energy difference has been established between 2500 and 4000 cm−1 in order to efficiently sensitize the luminescence in lanthanide compounds. When this difference is lower, the back energy transfer processes are activated due to thermal repopulation of the ligand triplet state. The emissive state in EuIII, 5D0, appears around 17300 cm−1, which ensures the correct sensitization of the metal-centered (MC) luminescence. Luminescence excitation spectra (Figure 4) of europium complexes are characteristic of the metal in the 550−700 nm region and exhibit five well-resolved emission bands with different intensities at around 579, 593, 615, 648, and 685 nm, respectively. These bands correspond to EuIII-centered transitions (5D0 → 7 F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4). The most intense transition in the emission spectrum is 5D0 → 7F2 located at 615 nm; this is the so-called hypersensitive transition79 which is an electric dipole induced transition, sensitive to the coordination environment of the EuIII ion and responsible for the brilliant red emission color of the complexes obtained in this work (Figure S9 in the Supporting Information). This transition is more intense than 5D0 → 7F1, which indicates that the coordination environment of the EuIII ion does not have an inversion center. Emission bands located at 579 and 648 nm are weak, because these are usually known as forbidden in magnetic and electric dipole fields. The band at 593 nm is relatively more intense, as it is a magnetic dipole transition which is independent of the coordination environment of the EuIII ion.

Figure 2. UV−visible absorption spectra of [Eu(NO3)3(dppz-R)] in DMF solution (2.10 × 10−3 M).

geometries were employed as input for CASSCF/PT2 calculations, including 20 singlets + 20 triplets. These calculations allowed us to find the emission energies at 536, 503, and 505 nm for dppz, dppz-CN, and dppz-NO2, respectively, which are in accordance with the experimental value of 538 nm for dppz. TD-DFT calculations were used to describe the S0 → Sn transitions; however, the state responsible for the energy transfer is an excited triplet state. Therefore, the next step was the localization of this state. For this purpose, the gadolinium complex [Gd(NO3)3(dppz-R)] was selected, in order to enhance the phosphorescence/luminescence ratio (ϕph/ϕlum > 1). Gadolinium(III) was selected because it has the highest energy gap between ground and excited states in the lanthanide series, and therefore the energy transfer from the antenna is locked. The phosphorescence spectra of the gadolinium complex with dppz-CN and dppz-NO2 were measured under N2 (liquid nitrogen) to get the complete emission band from the triplet states (see Figure 3). For both ligands, the energy of the triplet state was determined at ∼26667 cm−1 according to the maximum of phosphorescence bands, which is not surprising because of their similar structural characteristics. In order to transfer energy from the ligand to the lanthanide ion, the triplet state energy needs to be higher than the resonance level of the metal ion (EuIII: 5D0, 17300 cm−1). Thus, the selected ligands obey this rule. From a theoretical point of view, CAS(10,10)SCF/PT2 calculations were performed using two starting geometries: first the optimized ground singlet and second the ligand as a fragment obtained from the optimized entire complex. The calculations show that the first triplet appears at 28044 cm−1 for the dppz fragment, which is 230 cm−1 less than the value for the optimized ligand. In the case of both dppz-CN and dppz-NO2 as fragments, the triplet states were obtained at 26024 and 26021 cm−1, respectively, in almost perfect agreement with 9203

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These are spin-adapted into configurational state functions (CSFs) of definite spin quantum numbers. For each spin manifold, a number of CASSCF wave functions are then optimized. At this level, the dynamic correlation effects are included via the CASPT2 method. SO coupling is introduced in the second step by diagonalizing the SO operator on the basis of the optimized CASSCF wave functions. The dynamic correlation is included on replacing the CASPT2 energies into the diagonal of the SO matrix. According to our calculations, the SO-CASSCF predicts (for both compounds) emission bands between 480 and 580 nm with a general error in a range of 100−120 nm. The calculations show that the position of 5D0 is sensitive to the inclusion of the highest energy multiplicities. When only septuplets and quintuplets are taken into account, the state appears at 22890 cm−1; however, if triplets and singlets are included, it appears at 20473 cm−1. Despite the fact that an important stabilization is produced, it is not enough to reproduce the experimental emission spectra. The inclusion of the dynamic correlation allowed us to correct this value to 18017 cm−1, which reproduces the emission bands between 550 and 700 nm, with a general error of less than 20 nm; the most intense band (5D0 → 7F2) is calculated at 596 nm. When the solvent molecules were included, no changes were produced in the position of the 5D0 state; however, 2% a contribution of quintuplets was observed in the wave function of the 7F2 state. The Judd−Ofelt80,81 theory offers an explanation to understand the parity-forbidden 4f−4f transitions, primarily on the basis of the mixture of states of different multiplicities generated by SO coupling, which leads to a partially allowed transition. Under this assumption, the inclusion of the water molecules possibly contributes to the allowed character of the 5D0 → 7F2 transition and, consequently, to a faster deactivation of the 5D0 state. Energies of all calculated SO states are presented in Table S3 in the Supporting Information. The 5D0 lifetime of EuIII was determined by monitoring the emission decay curves within the 5D0 → 7F2 transition (615 nm) at 298.15 K. In both complexes, the luminescent decay rates are monoexponential. This indicates that only one

Figure 4. Emission spectra of [Eu(NO3)3(dppz-CN)] (λex 370 nm) and [Eu(NO3)3(dppz-NO2)] (λex 360 nm), both in the solid state.

From a theoretical point of view, calculations at the highaccuracy SO-CASSCF/PT2 level were done. This is a two-step method to determine the ground and excited states of the lanthanide compounds. The purpose of the first step is to apply the CASSCF method to obtain wave functions that can be thought of as corresponding to the atomic Russell−Saunders terms, whose degeneracies are weakly split by the presence of the ligand environment. For a EuIII complex whose formal configuration is 4f6, this is achieved by choosing the active space to consist of six electrons in the seven 4f-like orbitals.

Figure 5. Energy diagrams for possible sensitization and emission pathways in complexes 2 and 3. All of the states were determined at the CASPT2 level of theory, employing an active space of CAS(10,10) for the ligand and CAS(6,7) for the lanthanide fragment. Definitions: IC, internal conversion; ISC, intersystem crossing; ET, energy transfer; VR, vibrational relaxation. All the Δ values in the diagram are in cm−1. 9204

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Table 1. Comparison among Calculated Emission Bands for EuIII Fragments and the Model Compound [EuCl6]3− and Experimental Energiesa transition

[Eu(NO3)3]

[Eu(NO3)3(H2O)2]

[EuCl6]3−

exptlb

exptlc

→ → → → → → →

18239 17521 16742 15805 14685 13493 12303

18017 17523 16743 15878 14667 13607 12549

18270 17568 16791 15854 14744 13542 12397

17271 16863 16260 15432 14598

17241 16949 16313 15384 14492 14084

5

D0 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5

a

7

F0 F1 7 F2 7 F3 7 F4 7 F5 7 F6 7

All values are given in cm−1. bExperimental values obtained in this work. cThese values are reported in ref 9.

reported by other authors.17,72 At this point, an internal conversion (IC) mechanism is proposed, in order to reach the S1 state, which in the case of complex 1 appears at 29253 cm−1, while for complexes 2 and 3, it appears at 27072 and 26980 cm−1, respectively. A stabilization of the S1 state can be observed with the inclusion of the electron-withdrawing groups in complexes 2 and 3, which is in accordance with the experimental reports of Saldivia et al.17 Intersystem crossing (ISC) from S1 can lead to the T1 state which, as was previously said, has been determined (for all ligands) to be in almost perfect agreement with experimental results. Excitation energy is transferred to the lanthanide ion from this state; therefore, it is important to emphasize at this point that the way in which these triplet states were determined has fundamental importance in order to predict properly the sensitization mechanism. Under this assumption, and considering that the “safe” energy gap T1−Ln* is around 2500−4000 cm−1, the most probable pathway is in all cases T1 → 5D2, which has a theoretical gap (ΔE1 in Figure 5) of 1881 cm−1 for compound 1 and ∼3460 cm−1 for 2 and 3. After this process, a vibrational relaxation (VR) can lead to the emissive 5D0 state, from which the emission to 7FJ states is produced. As can be observed, in the substituted complexes, an optimal energy gap is produced, which favors the sensitization of the lanthanide moiety, with a low probability of back energy transfer. Nevertheless, the fact that the energy is transferred to levels higher than 5D0 produces a significant nonradiative deactivation of EuIII. An interesting fact is the similar behavior showed by compounds 2 and 3, which indicates the relatively weak influence of the ligand environment on the properties of the 4f shell in the lanthanide ion. Same behavior is observed in the case of solvated and nonsolvated molecules. In Table 1 calculated and experimental energy levels of lanthanide complexes under study are shown. In addition, [EuCl6]3− is included as a model compound in order to compare the influence of the EuIII environment on its energy levels. Theoretical calculations show only small differences between the electronic states in the three molecules, which are quite comparable with the experimental reports obtained in this work and with other reported studies of EuIII in solution. These results corroborate the assumption that the ligand environment does not generate meaningful differences between the energy levels in the compounds; however, as was discussed above the inclusion of the polar solvent molecules in the coordination sphere introduces some quintuplet contribution into the septuplet ground state, with consequent relaxation of the Laporte rule. In the case of the model [EuCl6]3− complex, it was possible to come to similar conclusions.

deactivation phenomenon occurs. On the basis of the respective luminescent decay profiles by fits with monoexponential decay curves, the luminescence lifetime was found to be 0.1632 ms for [Eu(NO3)3(dppz-CN)] and 0.140 ms for [Eu(NO3)3(dppzNO2)] (see Figures S7 and S8 in the Supporting Information). It is well-known that for EuIII ion containing compounds, a simplified equation leads to the radiative lifetime: ⎛I ⎞ 1 = AMD,0n3⎜ tot ⎟ τrad ⎝ IMD ⎠

(1) −1

where AMD,0 is a constant equal to 14.65 s and represents the spontaneous emission probability of the magnetic dipole 5D0 → 7 F1 and n is the refractive index. An average index of refraction equal to 1.5 was employed in the calculation (Lorentz local field correction term).82,83 Itot is the integrated emission of the 5 D0 → 7FJ (J = 0−4) transition, and IMD is the integrated emission of the 5D0 → 7F1 transition. If the radiative lifetime τrad is known, the quantum yield of the luminescence (Qln) can be calculated using the observed luminescence lifetime τobs: τ Q ln = obs τrad (2) The intrinsic quantum yields (Qln) were found to be 8.29% and 7.16% for 2 and 3, respectively; the intrinsic quantum yields of these compounds are not high. It is well-known that the quantum yield essentially depends on the energy gap between the emissive state of the metal and the highest sublevel of its ground, or receiving, multiplet; if this gap is small, it is easier to deactivate it by nonradiative processes generated by highenergy vibrations. In this case, the presence of O−H (ν̃ 3600 cm−1) oscillators included in the water molecules coordinated to EuIII could produce a quenching of the luminescence.73 According to Bünzli84 the energy gap between 5D0 and 7F6 states is ∼12300 cm−1 (calculated theoretical value 12549 cm−1); therefore, four O−H oscillators are enough to induce a nonradiative deactivation.73 In this context, better results could be obtained if another lanthanide ion with a larger energy gap such as terbium(III) were used or if the first coordination sphere of EuIII ion were saturated with bulky and rigid ligands, in order to protect the LnIII ion from solvent interactions. 4.3. Energy Transfer Pathways. Figure 5 shows the energy diagrams for the possible sensitization and emission pathways for substituted complexes. According to CASSCF/ PT2 calculations and on the basis of the theoretical oscillator strengths, ligand-centered excitation (vertical black arrow) leads to the S3 state in all compounds. In the case of complex 1 this state is located at 35994 cm−1, while in the others it is located at ∼30000 cm−1. All of these values are in agreement with experimental excitation energies obtained in this work and 9205

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Recently, Romanova et al.33,34 have reported a theoretical work in which they affirm that the ligand-field effects on the 4f states are negligible and that there is no need to perform ab initio calculations for 4f levels of LnIII. In our opinion, this is not completely correct, because although the calculated values are similar to the experimental data, the compositions of the wave functions for the states are influenced by the nature of the ligand surrounding the lanthanide ion. Then, a correct interpretation of the sensitization mechanism and emission in molecules such as those studied here require a correct determination (even including dynamic correlation) of the electronic states of the lanthanide fragment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01221. All calculated structural parameters, TD-DFT calculations, and experimental characterization (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*D.P.-H.: e-mail, [email protected]; tel/fax, +56-2-27703352. *R.A/-P.: e-mail, [email protected].

5. CONCLUSIONS The fragmentation scheme proposed in this work has successfully reproduced excitation and emission bands for the complexes under study. In order to properly apply this methodology, it is crucial to take into account two important factors: first, to have a ligand-localized excitation and a lanthanide-centered emission, to ensure that the bands involved in the sensitization mechanism are not affected by the fragmentation process, and second, to treat properly the fragments. We stated above that an optimized antenna does not reproduce the experimental values for the electronic states; thus, it is necessary to employ the fragments obtained from the optimized geometry of the entire complex. Both the antenna ligand and lanthanide fragment states were determined in good agreement with experimental results. At this point, the role of the dynamic correlation must be highlighted, because its inclusion allowed us to reproduce with a minimal error the energies for all involved states. In general, both complexes showed similar sensitization and emission mechanisms; nevertheless, some differences were noted with respect to the unsubstituted compound: the inclusion of the electronwithdrawing groups extended the energy gap T1−Ln* to an optimal range for the energy transfer process. In relation to the solvation of complexes, after the incorporation of water molecules in the coordination sphere of EuIII, a small contribution of quintuplets was observed in the septuplet ground state, which possibly contributes to the allowed character of the 5D0 → 7FJ transition and consequently to a faster deactivation of the 5D0 state. Finally, the influence of the ligand environment on the EuIII states was analyzed, and despite the fact that no significant changes were observed between the different complexes, we can conclude that the ligand-field effects are non-negligible, because the compositions of the wave functions for the involved states are influenced by the nature of the ligand surrounding the lanthanide ion. Thus, the calculations performed on the lanthanide fragment made it possible to understand the solvent role and to explain properly the deactivation process. Therefore, for a correct elucidation of the energy transfer pathways, a precise determination of the electronic states of the lanthanide fragment is essential. Furthermore, despite the fact that the ligands do not seem to be good candidates for photosensitizing agents, these studies allowed us to validate the proposed methodology, which can be considered as a helpful tool in the correct prediction of the energy transfer pathways and emissions in similar compounds and can even be extended to other systems such as intermetallic compounds.

ORCID

Dayán Páez-Hernández: 0000-0003-2747-9982 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Grants FONDECYT Nos. 1150629 and 11140294. P.C.-L. and M.J.B.-L. acknowledge CONICYT/Doctorado Nacional 2013/63130037 and 2015/21151553 for their Ph.D. fellowships, respectively. Furthermore, P.C-L. acknowledges Andrés Bello University (internal project DI-712-15/I). The authors thank Ph.D.s Osvaldo Serra and Paulo Cesar de Sousa Filho at the Rare Earths Laboratory, FFCLRP, University of São Paulo, for the luminescence measurements.

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