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Geometric structure, electronic structure, and spin transition of several FeII spincrossover molecules Nguyen Anh Tuan Citation: Journal of Applied Physics 111, 07D101 (2012); doi: 10.1063/1.3670044 View online: http://dx.doi.org/10.1063/1.3670044 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/111/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Hysteresis and change of transition temperature in thin films of Fe{[Me2Pyrz]3BH}2, a new sublimable spincrossover molecule J Chem Phys 142, 194702 (2015); 10.1063/1.4921309 Spin-crossover molecule based thermoelectric junction Appl Phys Lett 106, 193105 (2015); 10.1063/1.4921165 Electronic structure of Fe- vs Ru-based dye molecules J Chem Phys 138, 044709 (2013); 10.1063/1.4788617 Assessment of density functional theory for iron(II) molecules across the spin-crossover transition J Chem Phys 137, 124303 (2012); 10.1063/1.4752411 Photoinduced phase transition in an iron(II) spin-crossover complex with a N O macrocyclic ligand Appl Phys Lett 86, 122511 (2005); 10.1063/1.1890478 [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 139.184.14.159 On: Tue, 11 Aug 2015 07:37:20 JOURNAL OF APPLIED PHYSICS 111, 07D101 (2012) Geometric structure, electronic structure, and spin transition of several FeII spin-crossover molecules Nguyen Anh Tuana) Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam (Presented November 2011; received 28 September 2011; accepted 30 September 2011; published online February 2012) We present a density functional study on the geometric structure, electronic structure, and spin transition of a series of FeII spin-crossover (SCO) molecules, i.e., [Fe(abpt)2(NCS)2] (1), [Fe(abpt)2(NCSe)2] (2), and [Fe(dpbo)(HIm)2] (3) with dpbo ¼ {diethyl(E,E)-2,20 -[1,2-phenylbis (iminomethylidyne)]bis[3-oxobutanoate](2–)-N,N’,O3,O3’}, and abpt ¼ {4-amino-3,5-bis(pyridin2-yl)-1,2,4-triazole} in order to explore more about the way to control SCO behavior of transition metal complexes Our calculated results show that the spin transition of these FeII molecules is accompanied with charge transfer between the Fe atom and ligands This causes change in the electrostatic energy (DU) as well as the total electronic energy of SCO molecules Moreover, our calculated results demonstrate an important contribution of the interionic interactions to DU, and there is the relation between DU and the thermal hysteresis behavior of SCO molecules These C 2012 American Institute of results should be helpful for developing new SCO molecules V Physics [doi:10.1063/1.3670044] I INTRODUCTION Transition metal complexes that exhibit a temperature dependent crossover from a low-spin (LS) state to a highspin (HS) state have been prepared as early as 1908.1 In the last few decades, research into the preparation and properties of complexes that exhibit this effect has been extensive after it was discovered that spin state can be switched reversibly by pressure or light irradiation in solid samples2 as well as in solutions.3 Spin crossover (SCO) complexes are now very potential candidates for applications such as molecular switches, display, and memory devices.4 Although the phenomenon of SCO is theoretically possible for octahedral d4–d7 ions, it is quite frequently observed in complexes containing FeII and FeIII,5,6 and to a lesser extent in CoII as well as MnIII complexes This situation highlights that to induce SCO in these complexes the ligands must impose a ligand field strength that results in a minimal difference between the octahedral splitting energy (D) and the electron spin pairing energy (P) in order for a minor perturbation results in switching between the LS and HS states The SCO phenomenon can be qualitatively explained by the ligand field model, however, designing transition metal complexes with expected SCO behavior is still a big challenge in the field of materials science In this paper, to explore more about the way to tailor the SCO behavior of transition metal complexes, the geometric structure, electronic structure and spin transition of three FeII spin-crossover molecules with different ligand configurations have been studied based on Density-functional theory, i.e., [Fe(abpt)2(NCS)2] (1), [Fe(abpt)2(NCSe)2] (2), and [Fe (dpbo)(HIm)2] (3) with dpbo ¼ {diethyl(E,E)-2,20 -[1,2-phenylbis(iminomethylidyne)]bis[3-oxobutanoate](2–)-N,N’,O3,O3}, and abpt ¼ {4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole} Our a) Electronic addresses: tuanna@hus.edu.vn and tuanna@vnu.edu.vn 0021-8979/2012/111(7)/07D101/3/$30.00 calculated results demonstrate an important contribution of the interionic interactions to SCO behavior of these molecules II COMPUTATIONAL METHOD All calculations have been performed by using the DMol3 code7 with the double numerical basis sets plus polarization functional For the exchange correlation terms, the generalized gradient approximation (GGA) PBE functional was used.8 The effective core potential Dolg-Wedig-StollPreuss was used to describe the interaction between the core and valance electrons.9 For better accuracy, the hexadecapolar expansion scheme was adopted for resolving the charge density and Coulombic potential The atomic charge and magnetic moment were obtained by using the Mulliken population analysis.10 The real-space global cutoff radius was ˚ for all atoms The spin-unrestricted DFT was set to be 4.6 A used to obtain all results presented in this study The charge density is converged to  10À6 a.u in the self-consistent calculation In the optimization process, the energy, energy gradient, and atomic displacement are converged to  10À5,  10À4, and  10À3 a.u., respectively In order to obtain both the geometric structures corresponding to the LS and HS states of FeII molecules, both the LS and HS configurations of the Fe2ỵ ion are probed, which are imposed as an initial condition of the structural optimization procedure In terms of the octahedral field, the Fe2ỵ ion could, in principle, has the LS state with configuration d6(t2 g6, eg0) and the HS state with configuration d6(t2 g4, eg2) III RESULTS AND DISCUSSION The schematic geometric structure of molecules [Fe(abpt)2(NCS)2] (1), [Fe(abpt)2(NCSe)2] (2), and [Fe(dpbo)(HIm)2] (3) is depicted in Fig In these molecules, the Fe2ỵ ion is located in nearly octahedron In molecules (1) 111, 07D101-1 C 2012 American Institute of Physics V [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 139.184.14.159 On: Tue, 11 Aug 2015 07:37:20 07D101-2 Nguyen Anh Tuan J Appl Phys 111, 07D101 (2012) FIG (Color online) Schematic geometric structure of molecules (1), (2), and (3) H atoms are removed for clarity and (2), two equivalent bidentate N2-coordinating ligands (abpt) stand in the equatorial plane and two equivalent terminal nitrile anions (X) complete the coordination sphere in trans position In the molecule (3), one tetradentate N2O22–coordinating Schiff base like ligand (dpbo) stands in the equatorial plane and two equivalent terminal neutral ligands (HIm) complete the coordination sphere The computed geometric structures of (1), (2), and (3) are slightly different from experimental data reported in Refs 11 and 12 as tabulated in Table I Here, it is noted that, these calculations have been carried out for isolated molecules in vacuum This approximation neglects interactions between neighboring molecules Calculations that not regard these interactions can therefore be different from the experiment Nevertheless, such calculations for isolated molecules in vacuum may reveal information about the molecular contribution to substituent-induced shifts of SCO characteristics This information can hardly be gained experimentally since any experiment with a solid sample will only reflect the combined influence of intra- and intermolecular interactions Also we succeeded in predicting the geometric structure of the LS state of molecules (1)–(3), which are not available from experiment so far The Fe-ligand bond lengths in the LS state are always shorter that those in the HS state for all these FeII molecules, as shown in Table I The Fe-ligand ˚ in the LS bond lengths are typically about 1.903 to 2.004 A state, increase by about 10% upon crossover to the HS state This can be explained in terms of ligand field theory Previous experimental studies reported that the SCO temperature (TSCO) of (1), (2), and (3) is 180, 224, and 314 K for (1), (2), and (3), respectively The TSCO can be estimated by a simple model13 that is restricted to isolated molecules and requires only the knowledge of the difference DE ¼ EHS – ELS between the total electronic energy of the ˚ ] of the LS and HS states of TABLE I Selected Fe-ligand bond lengths [A (1), (2), and (3) obtained from calculated results and experimental data [11,12] Experimental values are shown in italic (1) LS Fe-L1 Fe-L2 Fe-L3 Fe-L4 Fe-L5 Fe-L6 1.979 1.986 1.963 1.992 1.960 1.958 (2) HS 2.120 2.205 2.120 2.205 2.120 2.120 2.218 2.212 2.241 2.208 2.060 2.062 LS 1.968 1.991 1.977 1.990 1.955 1.955 (3) HS 2.189 2.105 2.189 2.105 2.131 2.131 2.219 2.205 2.218 2.205 2.073 2.073 LS 1.942 1.903 1.903 1.940 2.002 2.004 HS 2.011 2.088 2.078 2.048 2.239 2.198 2.005 2.095 2.105 2.011 2.276 2.260 HS and LS states In this model, the DE dependence of the TSCO can be written as TSCO $ DE From this relation, it is expected that the higher TSCO, the higher DE Indeed, our calculated results show that molecules (1), (2), and (3) have DE of 0.136, 0.177, and 0.338 eV, respectively It poses a question what makes the difference in DE between these molecules To shed light on this question, we carried out calculating energy components, including the kinetic energy (K), the electrostatic energy (U), and the exchangecorrelation energy (Exc) The difference in K, U, and Exc between the LS and HS states of (1), (2), and (3) is listed in Table II As shown in Table II, the kinetic energy difference (DK) and the electrostatic energy difference (DU) between the HS and LS states are significant in comparison to the total electronic difference (DE) for all these FeII molecules Interestingly, the DU is negative for (1) and (2), but it is positive for (3) The latter cannot be explained in terms of the electronic spin pairing energy Because, in terms of the electronic spin pairing energy, the DU must be negative due to advantage in the electron-electron repulsion energy of the HS state in comparison to the LS state As we known, the transition from the LS state to the HS state is accompanied with expansion of bond lengths, especially the Fe-ligand bonds This can cause redistribution of atomic charge, especially charge of the Fe and L1–L6 atoms To elucidate this, the atomic charge of (1), (2), and (3) has been calculated Our calculated results show that the charge of the Fe and L1–L6 atoms of (1), (2), and (3) in the HS state is significantly larger than that in the LS state, as tabulated in Table III For example, the charge of Fe atom in the HS state of (3) is over twice larger than that in the LS state, and the charge of L1–L6 atoms increases by about 1.16 to 1.21 times upon crossover from the LS to the HS state One may say that charge (electron) is transferred from the Fe ion to L1–L6 ions upon crossover from the LS to the HS state This causes the Fe ion becoming more positive and six anionic L1–L6 ions TABLE II The calculated energy differences [eV] between the LS and HS states of (1), (2), and (3), including the kinetic energy difference (DK), the electrostatic energy difference (DU), the exchange-correlation energy difference (DExc), and the total energy difference (DE) HS À LS DK DU DExc DE (1) (2) (3) 7.281 À7.123 À0.022 0.136 16.030 À15.786 À0.068 0.177 À6.406 6.640 0.103 0.338 [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 139.184.14.159 On: Tue, 11 Aug 2015 07:37:20 07D101-3 Nguyen Anh Tuan J Appl Phys 111, 07D101 (2012) TABLE III The charge of Fe and L1-L6 atoms in the LS state (nLS) and the HS state (nHS) of (1), (2), and (3) (1) Fe L1 L2 L3 L4 L5 L6 (2) (3) nLS (e) nHS (e) nHS/nLS nLS (e) nHS (e) nHS/nLS nLS (e) nHS (e) nHS/nLS 0.419 À0.230 À0.376 À0.227 À0.377 À0.334 À0.340 0.858 À0.299 À0.419 À0.301 À0.417 À0.419 À0.408 2.048 1.300 1.114 1.326 1.106 1.254 1.200 0.432 À0.228 À0.378 À0.230 À0.377 À0.202 À0.197 0.870 À0.305 À0.422 À0.305 À0.422 À0.289 À0.290 2.014 1.338 1.116 1.326 1.119 1.431 1.472 0.466 À0.434 À0.405 À0.407 À0.432 À0.359 À0.360 0.954 À0.523 À0.470 À0.477 À0.512 À0.422 À0.421 2.047 1.205 1.160 1.172 1.185 1.175 1.169 becoming more negative Hence, the coulomb attraction energy between the Fe and L1–L6 ions becomes more negative by transition from the LS state to the HS state, while the coulomb repulsion energy among the L1–L6 ions becomes more positive The significant difference in DU between (1), (2), and (3) can be understood by competition between these electrostatic interactions It is noted that these electrostatic interactions strongly depend on the ligand configuration of molecules For example, (1) and (2) with the configuration Fe-N6 have negative DU, while (3) with the configuration Fe-N4O2 has positive DU Moreover, it is known that the thermal hysteresis behavior of SCO materials can be controlled by electrostatic contributions.14 The molecule (3) with the positive value of DU ¼ 6.640 eV exhibits a 70 K wide thermal hysteresis loop,12 while the molecules (1) and (2) with negative values of DU has no thermal hysteresis behavior.11 To confirm the relation between DU and the thermal hysteresis behavior, we have calculated DU of several other FeII SCO molecules, i.e., [Fe(abpt)2(C(CN)3)2] (4) (Ref 15) and [Fe(pibp)(dmap)2] (5) (Ref 16) with pibp ¼ {([3,30 ]-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N0 ,O2,O20 } and dmap ¼ p-dimethylaminopyridine Our calculated results show that the molecule (4) with the negative DU ¼ –18.62 eV has no thermal hysteresis behavior,15 and the molecule (5) with the small positive DU ¼ 1.308 eV exhibits a narrow thermal hysteresis loop of K.16 IV CONCLUSION The geometric structure, electronic structure, and spin transition of a series of five FeII spin-crossover molecules have been studied based on density-functional theory in order to explore more about the way to regulate SCO behavior of transition metal complexes Our calculated results show that the transition from the LS state to the HS states is accompanied with charge (electron) transfer from the Fe atom to ligands This process makes change in the electrostatic energy (DU) as well as the total electronic energy of SCO molecules Moreover, our calculated results demonstrate an important contribution of the interionic interactions to DU, and there is the relation between DU and the thermal hysteresis behavior These results should be helpful for developing new SCO molecules ACKNOWLEDGMENTS We thank the Vietnam National University for funding this work within Projects QG-11-05 and TRIG The computations presented in this study were performed at the Information Science Center of Japan Advanced Institute of Science and Technology, and the Center for Computational Science of the Faculty of Physics, Hanoi University of Science, Vietnam This paper was written while the author was a visitor at the Department of Physics, Brown University within the sub-project TRIG A of Hanoi University of Science M Dele´pine, Bull Soc Chim Fr 3, 643 (1908) S Decurtins, P Guătlich, C P Koăhler, H Spiering, and A Hauser, Chem Phys Lett 139, (1984) J J McGarvey and I Lawthers, J Chem Soc., Chem Commun 16, 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E Kaps, J Weigand, C Carbonera, J.-F Letard, K Achterhold, and F.-G Parak, Inorg Chem 47, 487 (2008) [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 139.184.14.159 On: Tue, 11 Aug 2015 07:37:20 ... study on the geometric structure, electronic structure, and spin transition of a series of FeII spin- crossover (SCO) molecules, i.e., [Fe( abpt)2(NCS)2] (1), [Fe( abpt)2(NCSe)2] (2), and [Fe( dpbo)(HIm)2]... charge of the Fe and L1–L6 atoms To elucidate this, the atomic charge of (1), (2), and (3) has been calculated Our calculated results show that the charge of the Fe and L1–L6 atoms of (1), (2), and. ..JOURNAL OF APPLIED PHYSICS 111, 07D101 (2012) Geometric structure, electronic structure, and spin transition of several FeII spin- crossover molecules Nguyen Anh Tuana) Faculty of Physics,