Antenna Effect by Organometallic Chromophores in Bimetallic d–f

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Antenna Effect by Organometallic Chromophores in Bimetallic d-f Complexes Franklin Ferraro, Dayán Páez Hernández, Juliana Andrea Murillo-Lopez, Alvaro Muñoz-Castro, and Ramiro Arratia-Perez J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp406208e • Publication Date (Web): 23 Jul 2013 Downloaded from http://pubs.acs.org on August 5, 2013

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The Journal of Physical Chemistry

Antenna Effect by Organometallic Chromophores in Bimetallic d-f Complexes Franklin Ferraro, Dayan Páez-Hernández, Juliana A. Murillo-López, Alvaro Muñoz-Castro and Ramiro Arratia-Pérez* Departamento de Ciencias Químicas, ReMoPh group, Universidad Andrés Bello, Republica 275, Santiago 8370146, Chile. KEYWORDS: Antenna Effect, d-f complexes, Relativistic-DFT calculation, NIR technology. ABSTRACT: The nature of the intermetallic bond in a series of complexes of the type [Cp2TM−M-Cp2] (where TM = Re and M= Y, La, Lu, Yb, Ac; also TM= Os and M = Th; Cp= Cyclopentadienyl ligand) have been studied by relativistic two-component DFT calculations. The results obtained in this work show that the interaction between the transition metal and lanthanide atoms is mainly ionic in all cases, whilst for the case of actinide atoms this interaction becomes significantly more covalent. The effective direction of the electron transfer between the Re →Ac or Os → Th centers allow us to propose that the [Cp2ReAcCp2] and [Cp2OsThCp2] complexes are ideal candidates for NIR technologies since their absorption spectra shows some transitions over 600 nm. We also observed a shifting of the absorption spectrum of around 100 nm of the [Cp2Re] fragment when is compared against the absorption spectrum of the entire complex. This behavior allows us to argue that the [Cp2Re] fragment is a good antenna

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chromophore due to the possibility of charge transfer transitions from this fragment to f shell in lanthanide or actinide elements studied here. INTRODUCTION Since the past few years an increasing research activity has been devoted towards the design of lanthanides (Ln) based near-infrared (NIR) luminescent complexes sensitized by transition metal (TM) centers driven by the energy or charge transfer from organometallic chromophores.1,2 Since lanthanide ions exhibits low absorption coefficients (ε), because of Laporte-forbidden f-f transitions, the inclusion of adjacent strongly absorbing TM- chromophores is a usual strategy to stimulate luminescence from lanthanides centers.3,4 Long-lived near-infrared (NIR) narrow-bandwidth emissions are of technological interest in a wide range of applications with multifunctional and modulated properties, for example, in telecommunications optical networks based on silica fibers, because in this region exhibits high transparency, optical amplifiers to expand telecommunication networks bandwidth and also for making water-stable d–f systems which allow long wavelength excitation (>700 nm) to be used to generate long-lived NIR luminescence for biological imaging applications.5-9 The ability of strongly absorbing d-block chromophores to sensitize low-energy f–f states of NIR emitting lanthanides is an important criterion for optimizing energy-transfer. It has been observed that the charge transfer towards the Ln centers vary according to the TM center included as chromophore, for example, Re→Ln exhibits an energy-transfer two orders of magnitude faster than Ru→Ln energy-transfer.10,11 In this sense, the synthesis and characterization of the heterobimetallic [Cp2ReLnCp2] complex12 (Cp= Cyclopentadienyl ligand, C5H5−) offers an


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interesting system for the theoretical evaluation of d–f systems displaying high absorption coefficients followed by NIR emission centered at the Ln fragment. As a novelty, here we explore the interaction of organometallic chromophores with actinide (An) complexes to evaluate the nature of the intermetallic d-f bond and its effects in the charge transfer mechanism and their optical properties. With the aim to gain insights into the bonding nature and optical properties between the [Cp2TM] and [Ln/AnCp2] fragments, we employ relativistic density functional calculations for a series of hypothetical systems involving [ReCp2], [OsCp2] organometallic chromophores and [(Ln/An)Cp2] complexes. The ground states of the [Cp2Re-Ln/AnCp2] and the [Cp2OsThCp2] complexes, where Ln = Y, La, Yb, Lu, and An= Ac, Th, show a net dipole moment from the TM center towards the (Ln/An) center, thus their optical properties are expected to exhibit a high absorption that can be modulated almost anywhere in the IR or NIR region due to the TM chromophores bonded to it. This work deals with the calculation of the ground and excited states and the photophysical properties of d–f heteronuclear assemblies in which strongly light absorbing d-block chromophores are used as an antenna group to generate sensitized NIR luminescence. THEORETICAL MODELS AND COMPUTATIONAL DETAILS The calculations were done according to the molecular structures shown in Figure 1 that are based on experimental reports by Butovskii and coworkers.12 All of these organometallic complexes have a general structure [Cp2TM-Ln/AnCp2], where TM are the transition metals, Ln refers to the lanthanide atoms, An denotes the actinide atoms and Cp are the Cyclopentadienyl ligands. All the calculations were constrained to C2v symmetry and due to the inclusion of Spin-


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Orbit interaction we used the C2v* double point group with the aim of rationalizing the interpretation of the calculations.

Ln/An

Figure 1. Selected structure for each complex.

All structural and electronic properties were obtained using the Amsterdam Density Functional (ADF) code,13 where the relativistic scalar and spin-orbit effects were incorporated by the zerothorder regular approximation (ZORA Hamiltonian). All molecular structures were fully optimized by an analytical energy gradient method as implemented by Verluis and Ziegler, using the local density approximation (LDA) within the Vosko-Wilk-Nusair parametrization for local exchange correlations.14-16 The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation exchange-correlation functional was also used.17 The geometry optimizations of the ground states were calculated by a standard Slater-type-orbital (STO) basis set with triple-ζ quality double plus polarization functions (TZ2P) for all atoms.18 For [Cp2OsThCp2], [Cp2ReLaCp2] and [Cp2ReAcCp2] complexes a geometry optimizations of their first excited state in C2v* were done in order to study the possible conformational change of their emissive state which are related to the Stokes shift.19 Calculations on open-shell systems were performed using spin-unrestricted


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methods. In all cases, frequency analyses were performed after the geometry optimization, in which we obtained only positive frequencies, thus verifying local minima. We use the natural bond orbital (NBO) analysis of Reed and Weinhold20,21 for the calculation of the charge distribution. This method provides values for the atomic natural total charges using the oneelectron density matrix for deﬁning the shape of the atomic orbitals in the molecular environment, and derive molecular bonds from electron density between atoms.22 Timedependent density functional theory (TD-DFT) was employed to calculate the excitation energies.23-24 We also used the GGA SAOP (statistical average of orbitals exchange correlation potential) functional that was specially designed for the calculation of optical properties.25-26 In this case, the excitation energies were estimated by spin-orbit time-dependent perturbation density functional theory. Furthermore, the interaction between both fragments were analyzed in more detail by decomposing the bonding energy into three physically meaningful components, namely, electrostatic energy (∆EElstat) that corresponds to the classical electrostatic interaction between the unperturbed charge distributions of the isolated fragments as they are brought together at their final positions, Pauli repulsion (∆EPauli) that comprises the destabilizing interactions between occupied orbitals and is responsible for any steric repulsion, and bonding orbital interactions (EOrb) that accounts for electron pair bonding, charge transfer and polarization.27 Calculations of the electron localization function (ELF)28,29 were performed with the DGrid 4.6 program,30 and the results were visualized with MOLEKEL 5.4 software.31 The ELF function is defined as ELF={1+[T(r)/Th(r)]2}-1 where T(r) has the physical meaning of the excess local kinetic energy density due to Pauli Repulsion32 and Th(r) is the Thomas-Fermi kinetic energy density which can be regarded as a “renormalization factor”; this form of the ELF conﬁnes its


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values within the [0,1] interval, thereby facilitating its analysis and interpretation. A region of space with a high value of the ELF corresponds to a region where it is more probable to localize an electron or a pair of electrons. In this case, the Pauli repulsion has little influence on their behavior, and the excess local kinetic energy has a low value. In contrast, at the boundaries between such regions, the probability of repulsion of the electrons is rather high, the excess local kinetic energy has a high value, and the ELF has its minimum. RESULTS AND DISCUSSION Geometry and Electronic Structure The calculated geometries are in near agreement with experimental results, when available, as can be seen in Table 1 for [Cp2ReYCp2] and [Cp2ReYbCp2], which are reported for the ground state of each complex. In order to analyze the possible conformational changes at the emissive state we also report for some complexes their geometries at their first excited state (see Table 1). As shown in Table 1, the bond lenghts and bond angles of the first excited state of [Cp2OsThCp2] complex do not change as compared to its geometrical parameters of its ground state. For [Cp2ReLaCp2] and [Cp2ReAcCp2] we can see a variation in their geometrical parameters, an increment of about 0.2 Å in the intermetallic [Re- La/Ac] bond distance, therefore a non-negigible Stokes shift it would be predicted for the possible emission spectra of these systems. Comparing the ground states in the first column, the more affected parameter is the intermetallic distance, but the most interesting change is the diminution of the angle

Antenna Effect by Organometallic Chromophores in Bimetallic d–f

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