Geometric structure, electronic structure, and spin transition of several FeII spin-crossover molecules Nguyen Anh Tuan Citation: Journal of Applied Physics 111, 07D101 2012; doi: 10.106
Trang 1Geometric structure, electronic structure, and spin transition of several FeII
spin-crossover 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
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Trang 2Geometric 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 3 November 2011; received 28 September 2011; accepted 30 September 2011;
published online 3 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 dpbo¼ {diethyl(E,E)-2,20-[1,2-phenylbis
(iminomethylidyne)]bis[3-oxobutanoate](2–)-N,N’,O3,O3’}, and abpt¼
{4-amino-3,5-bis(pyridin-2-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
results should be helpful for developing new SCO molecules V C 2012 American Institute of
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
high-spin (HS) state have been prepared as early as 1908.1In 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 samples2as 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
possi-ble for octahedrald4–d7ions, 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
per-turbation 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
com-plexes 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 dpbo¼ {diethyl(E,E)-2,20
-[1,2-phenyl-bis(iminomethylidyne)]bis[3-oxobutanoate](2–)-N,N’,O3,O3},
and abpt¼ {4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole} Our
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 DMol3code7with the double numerical basis sets plus polar-ization functional For the exchange correlation terms, the generalized gradient approximation (GGA) PBE functional was used.8 The effective core potential Dolg-Wedig-Stoll-Preuss was used to describe the interaction between the core and valance electrons.9For better accuracy, the hexadecapo-lar expansion scheme was adopted for resolving the charge density and Coulombic potential The atomic charge and magnetic moment were obtained by using the Mulliken pop-ulation analysis.10 The real-space global cutoff radius was set to be 4.6 A˚ for all atoms The spin-unrestricted DFT was used to obtain all results presented in this study The charge density is converged to 1 106 a.u in the self-consistent calculation In the optimization process, the energy, energy gradient, and atomic displacement are converged to
1 105, 1 104, and 1 103a.u., respectively In order
to obtain both the geometric structures corresponding to the
LS and HS states of FeIImolecules, both the LS and HS con-figurations of the Fe2þion are probed, which are imposed as
an initial condition of the structural optimization procedure
In terms of the octahedral field, the Fe2þion could, in princi-ple, has the LS state with configurationd6(t2 g6,eg) and the
HS state with configurationd6(t2 g4,eg)
III RESULTS AND DISCUSSION The schematic geometric structure of molecules [Fe(abpt)2(NCS)2] (1), [Fe(abpt)2(NCSe)2] (2), and [Fe(dp-bo)(HIm)2] (3) is depicted in Fig.1 In these molecules, the
Fe2þ ion is located in nearly octahedron In molecules (1)
a) Electronic addresses: tuanna@hus.edu.vn and tuanna@vnu.edu.vn.
JOURNAL OF APPLIED PHYSICS 111, 07D101 (2012)
Trang 3and (2), two equivalent bidentate N2-coordinating ligands
(abpt) stand in the equatorial plane and two equivalent
termi-nal 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
geo-metric structures of (1), (2), and (3) are slightly different
from experimental data reported in Refs 11 and 12 as
tabu-lated in TableI 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 do 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
infor-mation can hardly be gained experimentally since any
experiment with a solid sample will only reflect the
com-bined 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
bond lengths are typically about 1.903 to 2.004 A˚ in the LS
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
DE¼ EHS –ELS between the total electronic energy of the
HS and LS states In this model, the DE dependence of the
TSCOcan be written asTSCO 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 cal-culating energy components, including the kinetic energy (K), the electrostatic energy (U), and the exchange-correlation energy (Exc) The difference in K, U, and Exc
between the LS and HS states of (1), (2), and (3) is listed in TableII As shown in TableII, 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 pos-itive for (3) The latter cannot be explained in terms of the electronic spin pairing energy Because, in terms of the elec-tronic 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, espe-cially 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 signifi-cantly 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
FIG 1 (Color online) Schematic geometric structure of molecules (1), (2), and (3) H atoms are removed for clarity.
TABLE I Selected Fe-ligand bond lengths [A ˚ ] of the LS and HS states of
(1), (2), and (3) obtained from calculated results and experimental data
[ 11 , 12 ] Experimental values are shown in italic.
Fe-L1 1.979 2.120 2.218 1.968 2.189 2.219 1.942 2.011 2.005
Fe-L2 1.986 2.205 2.212 1.991 2.105 2.205 1.903 2.088 2.095
Fe-L3 1.963 2.120 2.241 1.977 2.189 2.218 1.903 2.078 2.105
Fe-L4 1.992 2.205 2.208 1.990 2.105 2.205 1.940 2.048 2.011
Fe-L5 1.960 2.120 2.060 1.955 2.131 2.073 2.002 2.239 2.276
Fe-L6 1.958 2.120 2.062 1.955 2.131 2.073 2.004 2.198 2.260
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 differ-ence (DE xc ), and the total energy difference (DE).
Trang 4becoming 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
cou-lomb 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
electro-static interactions It is noted that these electroelectro-static
interac-tions 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
DU¼ 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.11To 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 pibp¼ {([3,30
]-[1,2-phenylenebis(iminomethyli-dyne)]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 DU¼ –18.62 eV has no
thermal hysteresis behavior,15 and the molecule (5) with the
small positive DU¼ 1.308 eV exhibits a narrow thermal
hysteresis loop of 9 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
accompa-nied 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 computa-tions presented in this study were performed at the Informa-tion 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-pro-ject TRIG A of Hanoi University of Science
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TABLE III The charge of Fe and L1-L6 atoms in the LS state (n LS ) and the HS state (n HS ) of (1), (2), and (3).