Journal of Optics Applications October 2013, Volume 2, Issue 4, PP.56-62
A Theoretical Investigation on Photo-Electronic Property of Rac-Catena-Poly [nickel (II)-di-μ-tryptophanato] Ran Zhang1,3, Ruojing Song2, Yuxi Sun2,3,4,#, Qingli Hao4 1 Institute of material Chemistry, Binzhou University, Binzhou, 256600, P. R. China 2 Key Laboratory of Photoinduced Functional Materials, Mianyang Normal University, Mianyang 621000, P.R. China 3 Key Laboratory of Life-Organic Analysis, Qufu Normal University, Qufu, 273165, P. R. China 4 Key Laboratory of Education Ministry for Soft Chemistry and Functional Materials, Nanjing University of Science and Technology, Nanjing 210094, P. R. China # Email: yuxisun@163.com
Abstract In this paper, density functional theory calculation was used to reveal the electron characteristics of rac-catena-poly [nickel (II)-di-μ-tryptophanato]. In the studied molecule, the Mülliken atomic charges are distributed heterogeneously, and the electrons in natural bond orbitals are transferred easily among the separated indolyl and Ni-coordinated segments. The electrons are transferred directionally from the outsider indolyl groups to the medial nickel-coordinated moiety with obvious threshold energies with absorbed lights of 638.11, 723.77 and 831.48 nm due to the linking bridge composed of the C2-C3-C5 bond, while the electrons of the medial Ni-coordinated moiety can be redistributed easily in the medial plane with the lowest energy gap 0.3582 eV and the strongest oscillator strength 0.0444. The results indicate the studied compound is a good candidate of potential photoelectric or bioelectric materials. Keywords: Nickel-Trytophanato Complex; Photo-Electronic Property; NBO; TD-DFT
1 INTRODUCTION The molecule with directionally electron-transferred characteristics is the micro-structural foundation of the bioelectric or photovoltaic material. For example, the photosynthetic organelles of membrane proteins can transport charges directionally with absorbed ambient photons, and the photovoltaic devices can transfer electrons directionally to separate charges into different moieties, as result, these materials will present the photosynthetic and photovoltaic functions. It has been reported that the heterojunction substructure is the key component to perform the charge separations by the mode of long-range electron transitions in the bioelectric and photovoltaic materials [1-12]. Then the molecular architecture should be the functional foundation toward striking the charge balance and impact on the charge-transfer interactions to achieve electron separation. So, it is a significant work to find out the key functional unit in the photo responsive material field. Biological substances, such as amino acids, peptides, purines, DNA etc with their metal complexes, are generally accepted as model compounds in biological, medical and chemical fields due to their comprehensive properties [7-19]. Numerous aromatic compounds have been used to construct the photovoltaic devices [20-22]. In particular, the metal-organic complexes have shown the promising photoelectric functions demonstrated in previous studies [22-26]. Then, it should be a remarkable work to theoretically investigate the electron-transferred structural characteristics of the metal-organic complexes made by aromatic biological substances and metal ions, which will maybe provide us a new idea in finding or exploring the photo functional compounds. These reasons mentioned-above trigger the theoretical exploration of the photo responsive characteristics of the metal-bioorganic complexes. The tryptophan is the most photosensitive compound among the bioorganic units - 56 www.joa-journal.org
including amine acids, DNA/RNA bases etc. Since it is very difficult to form metal-organic complexes in the natural experimental condition for tryptophan, the experimental structures of the complexes formed by tryptophan have been reported rarely. Examining the corresponding references, the crystal parameters of the nickel-tryptophan complex: rac-catena-Poly [nickel (II)-di-μ-tryptophanato] ([Ni (L-trp)(D-trp)]n) was reported by Wang in 2009 [27], which provides the structural foundation in the theoretical calculation. Therefore, the complex was selected in this work, and its intramolecular electronic properties were revealed by a DFT method. As a result, the promising photovoltaic characteristics of the studied complex were revealed in this work.
2 COMPUTATIONAL METHOD Density functional theory (DFT) calculations have successfully carried out to investigate the molecular structures, vibrational frequencies, NMR, photoelectric properties in chemical, medical and biological fields. The LANL2DZ basis set has an advantage in the calculations for metals [28]. For meeting the requirements of computing accuracy and economy, the Becke’s three parameter hybrid functional method combined with the Lee-Yang-Parr correction functional of DFT (DFT/B3LYP) as well as LANL2DZ basis set was adopted in this work. As it is well known that the three-dimension geometry taken from X-ray crystallographic data can provide us an important structural foundation in the theoretical research on complicated molecules. In order to research on the Ni-trp complex better in this work, the crystallographic data [27] was directly used as the spatial coordinate positions in the theoretical calculations, and the natural bond orbital (NBO) and time dependent-density functional theory (TD-DFT) were applied to reveal the photo responsive characteristics relating to the electron transition potentials and photoelectric properties. All the calculations were done with Gaussian 03W program software package criteria on a Dell Core computer.
[29]
using the default convergence
3 RESULTS AND DISCUSSION
Graphical Abstract FIGURE 1 THE MOLECULAR DIAGRAM OF THE STUDIED COMPLEX WITH ATOMIC NUMBERING a a The unlabeled atoms in the smallest symmetric unit are obtained by symmetrical operations with symmetry codes: (i) -x, 1-y, 1-z; (ii) –x, -0.5+y, 0.5-z; (iii) x, 1.5-y, 0.5+z. - 57 www.joa-journal.org
In order to state clearly in the context, the smallest molecular unit with the atom numbering scheme for the studied complex is shown in Figure 1. A central nickel (Ni) atom adopts a six coordinated mode with adjacent four tryptophans to form a slightly distorted octahedral configuration (Ni1-O1, 2.037(3) Å; Ni1-N1, 2.096(4) Å; Ni1-O2ii, iii , 2.098(3) Å (ii: –x, -0.5+y, 0.5-z; iii: x, 1.5-y, 0.5+z)) [27]. The coordinated moieties connected by carboxyl groups form a two-dimensional planar structure, and the neighboring indolyl groups are distributed by a nonparallel mode on both sides of the planar substructure. The geometry provides us with a good representative model structure for the investigation on photoelectric properties of metal-organic compounds.
3.1 MÜLliken Atomic Charges The Mülliken atomic charges of the non-hydrogen atoms for the studied complex are listed in Table 1. Comparison of these atomic charges shows that the C1 and C5 atoms have relatively positive charges with 0.356437 e and 0.262504 e, respectively, while the N1, C3 and C4 atoms give relatively negative charges with -0.574554 e, -0.576340 e and -0.509070 e respectively. TABLE 1 MULLIKEN ATOMIC CHARGES OF THE STUDIED COMPLEX
Atoms Ni1 N1 O1 O2
Charges 0.174010 -0.574554 -0.313027 -0.334249
Atoms C1 C2 C3 N2 C4 C5
Charges 0.356437 -0.111396 -0.576340 -0.321723 -0.509070 0.262504
Atoms C6 C7 C8 C9 C10 C11
Charges 0.090554 0.082132 -0.395894 -0.324639 -0.292561 -0.468301
FIGURE 2 THE TOTAL CHARGE DENSITY SURFACE IN ELECTROSTATIC POTENTIAL FOR HEXAPLOID ASYMMETRIC UNIT OF THE STUDIED COMPLEX
On the whole, the indolyl groups and Ni-central coordinated atoms are charged negatively, and the total charge density surface of the complex is shown in Figure 2 by using ChemOffice 2004 software package of version 8.0 [30]. The medial planar moiety is bridged by the electron-richer carboxyl oxygen atoms, and the photosensitive indoles with conjugated atoms are separated with each other. Consequently, the electrons may be transferred from the indolyl moieties to the Ni-carboxyl-bridged plane with the lights absorbing. Yet, the indolyl and Ni-coordinated moieties are linked by single bonds of C2-C3 and C3-C5, resulting in threshold energy when electrons are - 58 www.joa-journal.org
transported between them. The structure is satisfied with the structural characteristics of the photovoltaic transition molecular device [31], and it is beneficial to transfer electrons in a long range along with a photon absorption/emission. In a word, the charge architecture of the studied complex is possible to transfer electrons in a long range because of the characteristics of atomic charges.
3.2 Natural Orbital Analysis According to the natural bond orbital (NBO) basis [32, 33], the orbital interactions result in a loss of occupancy from the localized NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the delocalization i→j is estimated as follows: F2 F2 E (2) qi i , j qi i , j j i i , j Here, qi is the donor orbital occupancy, εi and εj are diagonal elements (orbital energies), Δεi,j is energy gap between donor (i) and acceptor (j) NBO orbitals, and Fi, j is the off diagonal NBO FOCK matrix element. The NBO data provides the quantitative energies of adjacent orbital interactions with the most accurate ‘natural Lewis structure’, thus, which can be used to estimate the instantaneous direction for the electron migration of substructure, furthermore, to reveal the characteristics of intramolecular charge transition with external perturbations for a studied molecule. In order to investigate the electron interactions of the studied molecule, the hexaploid structure as shown in figure 1 was computed at DFT/B3LYP-LANL2DZ level and the representative couples of NBO interactions of [Ni(L-trp) (D-trp)]3 were listed in Table 2. TABLE 2 SELECTED NATURAL BOND ORBITAL INTERACTIONS OF THE STUDIED COMPLEX
Donor NBO(i) LP (1) N1 LP (1) N1 LP (1) O1 LP (2) O1 LP (2) O1 LP (3) O1 LP (1) O2ii LP (2) O2ii LP*(6)Ni1 BD (1) C1ii-O2ii BD (1) C1ii-O1ii BD (1) C1ii-O2ii BD (2) C1ii-O1ii BD (2) C1ii-O1ii BD (2) C1ii-O1ii BD (2) C1ii-O2ii BD (2) C1ii-O2ii LP (2) O2ii LP (2) O2ii BD (2) C10-C11 BD (2) C10-C11 BD (2) C9-C8 BD (2) C9-C8 LP (1) C6 LP (1) C6 LP (1) C6 BD (2) C7-N2 BD (2) C7-N2 BD (2) C4-C5
EDi/e 1.78512 1.78512 1.95464 1.74840 1.74840 1.60535 1.95522 1.86166 0.25860 1.82957 1.92695 1.82957 1.82185 1.82185 1.82185 1.87749 1.87749 1.86166 1.86166 1.97606 1.97606 1.74311 1.74311 1.06081 1.06081 1.06081 1.89298 1.89298 1.85492
Acceptor NBO(j) LP*(5)Ni1 LP*(6)Ni1 LP*(6)Ni1 LP*(5)Ni1 LP*(6)Ni1 LP*(5)Ni1 LP*(6)Ni1 LP*(6)Ni1 BD*(1) C1ii-O2ii BD*(2) C1ii-O1ii BD*(2) C1ii-O1ii BD*(2) C1ii-O2ii BD*(1) C1ii-O2ii BD*(2) C1ii-O2ii BD*(1) C1ii-O1ii BD*(1) C1ii-O2ii BD*(2) C1ii-O1ii BD*(1) C1ii-O1ii BD*(1) C2ii-C1ii LP (1) C6 BD*(2) C9-C8 BD*(2) C10-C11 BD*(2) C7-N2 BD*(2) C7-N2 BD*(2) C10-C11 BD*(2) C4-C5 BD*(2) C4-C5 LP (1) C6 LP (1) C6
EDi/e 0.68055 0.25860 0.25860 0.68055 0.25860 0.68055 0.25860 0.25860 1.82957 0.23796 0.23796 0.11779 0.18610 0.11779 0.11303 0.18610 0.23796 0.11303 0.08905 1.06081 0.31995 0.30641 0.74936 0.74936 0.30641 0.31146 0.31146 1.06081 1.06081
E(2)/kJ/mol 101.75 124.52 40.08 109.20 91.17 20.00 55.35 31.38 67.20 162.34 105.44 99.41 127.32 59.12 58.62 134.39 116.19 69.87 68.12 163.93 99.29 88.41 131.46 1183.70 286.90 321.21 74.27 40.58 147.90
Δεi,j/a.u. 0.18 0.59 0.81 0.17 0.58 0.09 0.93 0.54 0.04 0.84 1.22 0.92 0.70 0.81 0.79 1.01 1.04 0.66 0.66 0.15 0.30 0.30 0.23 0.07 0.14 0.12 0.38 0.26 0.18
Fi,j/a.u. 0.068 0.119 0.083 0.067 0.100 0.020 0.105 0.058 0.047 0.164 0.164 0.134 0.131 0.097 0.095 0.164 0.156 0.094 0.094 0.089 0.076 0.072 0.088 0.129 0.105 0.105 0.077 0.061 0.096
BD represents bond density, BD*represents anti-BD; LP represents lone pair, LP* represents anti-LP; ED presents electron density; E(2) is interaction energy of adjacent orbitals; Δεi,j is energy gap between donor (i) and acceptor (j) NBO orbitals; Fi,j is the Fock matrix element between i and j NBO orbitals; Symmetry code: (ii) –x, -0.5+y, 0.5-z. - 59 www.joa-journal.org
As seen from Table 2, the lone pair electrons of oxygen and nitrogen atoms coordinated with nickel ions present strong charge transfer potentials (226.27 kJ/mol for N1, 260.45 kJ/mol for O1 and 86.73 kJ/mol for O2). The bonds C1-O1 and C1-O2 exhibit the characteristics of easily transferring electrons to their antibonds, furthermore, the lone pair electrons of O2 atom bring strong transfer potentials to adjacent antibonds C1-C2 and C1-O1. These results indicate that the Ni-carboxyl-bridged two-dimensional structure possesses a good charge transfer characteristics. In the indolyl moiety, the benzene and pyrrole rings exhibit p-electrons conjugated structural characteristics. It is remarkable that the electrons of the indole present an unequal charge transfer potential, as a result, the charges of the benzene take away to the pyrrole with a good potential. Overall, the NBO results show that the electrons-active regions of indolyl and Ni-coordinated moieties of the complex are also separated from each other due to the connection of single bonds C2-C3 and C3-C5. The results indicate that the investigated molecule maybe benefit to the formation of a threshold energy when it transfers electrons, in other words, the electrons may be transported from one conjugated moiety to another when the molecule absorbs certain photons to overcome the threshold energy.
3.3 Photoelectric Effect According to frontier molecular orbital (FMO) theory, the electron transitions of compounds are most likely to occur between two interactive FMOs. In the process of the interaction, electrons move from an occupied molecular orbital to an unoccupied one along with absorption or emission of photon. The FMOs in the electrons-jumping conditions are following the rules: (i) they should have the same symmetry as far as possible; (ii) their energies should be close to each other; (iii) the moving direction of electrons should benefit to weak old bonds and the formation of new bonds. According to the previous literatures reporting that the aromatic structures possess photon- and electron-absorption properties [34-38], these indolyl groups of the investigated complex may be good photosensitive components while the complex acts as a functional photoelectric material. In order to reveal the photoelectric properties, the TD-DFT method was adopted to compute [Ni (L-trp)(D-trp)]3 as the representative structure of the studied complex and the effective electron-transfer details calculated at DFT/B3LYP-LANL2DZ level are listed in Table 3, including the calculated excitation wavelengths (λ), the vertical absorption values (Ω), oscillator strength (f) and the mainly contributing orbital transitions. TABLE 3 SELECTED ELECTRONIC ABSORPTION VALUES FOR THE STUDIED COMPLEX
λ /nm 3461.01 831.48 723.77 638.11
Ω /eV 0.3582 1.4911 1.7130 1.9430
Oscillator strengths (f) 0.0444 0.0019 0.0023 0.0121
Electronic transitions HOMO→LUMO (100%) HOMO-1→LUMO+1 (75.9%) HOMO-6→LUMO (74.0%) HOMO-9→LUMO (57.4%)
A long-range electron transition will produce a p-n junction so that it is the most concern to the researcher of photovoltaic systems. In order to study the electronic distributions of active orbitals, these molecular orbitals listed in Table 3 are shown in Figure 3 by GaussView 3.0 software [39]. As seen from the Figure 3 and Table 3, all the transitions comply with the electron-transfer principle, and the strongest electron transition is shown by the highly symmetric HOMO and LUMO orbitals with oscillator strength 0.0444, while other transitions present lower oscillator strengths. By comparison of these excitable FMOs, it can be found that the electron distributions of the orbitals are different from each other. As expected, the LUMO+1 and LUMO as acceptor orbitals are essentially localized at the Ni-coordinated medial moiety, and HOMO-1, HOMO-6 and HOMO-9 as donor orbitals are principally localized in the indolyl groups. Thus, the complex can theoretically absorb the lights of 638.11 nm, 723.77 nm and 831.48 nm, accompanying with the transporting electrons from outside indoles to medial Ni-coordinated moiety with respective weaker oscillator strengths 0.0019, 0.0023 and 0.0121 indicating obvious threshold energies, while the electrons of the medial Ni-coordinated moiety can be distributed easily in the medial plane as shown by the HOMO and LUMO with the lowest energy gap 0.3582 eV and the strongest oscillator strength 0.0444. The results indicate that the complex has the promising photovoltaic characteristics with the - 60 www.joa-journal.org
electron-migration pattern of the electron transition from outside indoles to the medial Ni-coordinated moiety and the electron redistribution in the medial plane.
FIGURE 3 SURFACES OF FMOs FOR THE STUDIED COMPLEX
4 CONCLUSIONS The rac-catena-Poly [nickel (II)-di-μ-tryptophanato] was objectively selected as a photovoltaic model of metal-organic compounds, and its photoelectric properties were revealed by the theoretical data calculated at DFT/B3LYP-LANL2DZ level. The mülliken atomic charge distributions are heterogeneous, and the electrons in natural bond orbitals are transferred easily among the separated indolyl and Ni-coordinated segments due to the connection of single σ-type bonds C2-C3 and C3-C5. As expected, the investigated complex shows a promising photovoltaic property with electron transitions from outside indoles to the medial moiety with absorbing lights of 638.11, 723.77 and 831.48 nm. The reported results are of assistance for theoretical evidences in the quest of new functionally photovoltaic and bioelectric materials, and promote the application of metal-organic compounds in the new energy and bioelectronic fields.
ACKNOWLEDGEMENTS This work was supported by Natural Science Foundations of China (Nos. 21073092 & 21103092), Sichuan Education Department Fund (No. 12ZA080), Scientific Research Foundation for Excellent Plan of Binzhou University (BZXYQNLG200704) and Mianyang Normal University for Excellent Plan Fund (No. QD2012A06).
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