Advanced Inorganic Chemistry I: Report
Engineering the Optical Response of the Titanium-MIL-125 Metal-Organic Framework through Ligand Functionlization

, 2013, 135 (30), pp 10942–10945
Advanced Inorganic Chemistry I: Report
Engineering the Optical Response of the Titanium-MIL-125 Metal-Organic Framework through Ligand Functionlization

, 2013, 135 (30), pp 10942–10945
Valeriya Chernikova
PhD student
KAUST
November 2013

Valeriya Chernikova
PhD student
KAUST
November 2013

Conductive metal-organic frameworks

Conductive metal-organic frameworks

Conductivity in metal-organic frameworks
Metal–organic frameworks (MOFs), are a class of crystalline materials whose crystal structure is made up from metal-containing clusters connected by multidentate organic linkers1,2. MOFs are attracting considerable attention due to the possible rational design of crystal structures of coordination frameworks with versatile metal ions and organic ligands. In principle, MOFs topologies along with intermolecular distances between various building blocks can be controlled using the fundamentals of reticular chemistry. This offers a great potential for tailoring MOFs properties for a wide scope of high-tech applications (high-capacity adsorbents, membranes, thin film devices, catalysis, biomedical imaging, etc.).
MOFs have traditionally been used for gas storage and separation, and much less attention has been devoted to their electronic properties3. This has changed in the past decade and MOFS with high electrical conductivity, charge carrier mobility and charge storage capacity became an emerging area of research. The physico-chemistry of inorganic (molecular) and organic (coordination polymers) conductivity are well understood by condensed matter researchers; however, this area is diverse and full of complexity. Thus, combination of both will represent a new paradigm.
Considering MOFS, conductivity can come from the metal, i.e. inorganic part ( metal chain, quantum dot), ligand, or incorporating in the pore. In addition, when we are speaking about electronic conductivity, it should be mentioned that electrons do not have an absolute monopoly on electrical conduction in solids. In literature still possible to meet a great uncertainty as to whether typical measurements allow researcher to conclude that conduction in a given m is due predominately to ions or to electrons. Many researchers have assumed that some of the MOFS can be treated as wide band gap semiconductors but other scientists have believed that most of known conductive structures conduct by the movement of ions.
It is of great interest to find the methodology to design new MOF with good charge transport or improve known system, in terms of conductive properties. Two main approaches reported so far. One is the “through-space” approach, which relies on π-stacking interactions between electroactive moieties. tetrathiafulvalene (TTF)-based MOF with high charge mobility.
3c The second approach relies on a “through-bond” formalism, where both symmetry and energy overlap between the covalently bonded components must exist to promote good charge transport.
Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation that two main approaches can be employed to construct new MOFs with good charge transport properties. 1. C. Janiak, Dalton Trans., 2003, 2781–2804. 2. A. K. Cheetham, C. N. R. Rao and R. K. Feller, Chem. Commun., 2006, 4780. 3. A. K. Cheetham and C. N. R. Rao, Science, 2007, 318, 58–59.

4

Band gap modification of MIL-125

Christopher H. Hendon, Davide Tiana, Marc Fontecave, Clément Sanchez, Loïc D’arras, Capucine Sassoye §, Laurence Rozes, Caroline Mellot-Draznieks, and Aron Walsh. Engineering the Optical Response of the Titanium-MIL-125 Metal−Organic Framework through Ligand Functionalization
J. Am. Chem. Soc., 2013, 135 (30), pp 10942–10945

According to the Silva and oth.1 the general structural requirements for MOFs to act as semiconductors are the irradiation with photons of energy larger than their bandgap and ability to generate either the electron or the whole (charge careers) which can easily migrate through the whole particle, i.e. photoinduced charge separation. Direct detection of electrons and holes or their trapping constitute firm, for example, by using absorption and emission spectroscopies, proof of MOF’s semiconductor nature. Therefore, material can potentially be applicable as photocatalysis, element of photovoltaic cells or electroluminescence diodes.
This reporting state-of-the-art article deals with the exploring methods to control photoconductive properties (the size of band gap) of metal-organic frameworks.
To avoid confusing terminology, it should be mentioned, that in some systems, including organic semiconductors, it is possible for a photon to have just barely enough energy to create an exciton (bound electron–hole pair, which can transport energy without transporting net electric charge), but not enough energy to separate the electron and hole (which are electrically attracted to each other). In this situation, there is a distinction between "optical bandgap" and "electrical band gap" (or "transport gap"). The optical bandgap is the threshold for photons to be absorbed, while the transport gap is the threshold for creating an electron–hole pair that is not bound together. The optical bandgap is at a lower energy than the transport gap. However, in almost all inorganic semiconductors distinction between them is ignored (as well as in reporting paper).
The authors of the reporting article choose MIL-1252, which porous structure is composed of octameric rings of Ti-O polyhedra connected by aromatic dicarboxylate linkers (fig. 1), as a parent material because of two reason. First, as synthesized, MIL-125 possess a photochromic transition (reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation) under UV, i.e. material exhibit photonic sensitivity, and is an active photocatalyst for the oxidation of alcohols to aldehydes. Hence, MIL-125 is semiconductor. Second, The higher band gap along with photoinduced charge separation for the MIL-125-NH2 with bdc-NH2 linker was reported4,5,6. Authors assumed that band gap modification can be done by changing the ligand.

solvothermal synthesis: DMF
MeOH
150˚C 15h

solvothermal synthesis: DMF
MeOH
150˚C 15h

+

+

Pic.1. Synthesis of MIL-125.

They followed logical sequence of the experiment: 1. In order to understand the change in the band gap in response to increasing the concentration of monoaminated bdc-NH2. Structure with following composition were synthesized and computed. The results and conclusion can be summarized as: | | syntesized | | 1. Values are deferent because modified MIL-125 breaks original symmetry, but in order to obtain qualitative characteristic for materials authors used lattice constants fixed at the equilibrium values of the parent MIL-125 for all structures.
1. Values are deferent because modified MIL-125 breaks original symmetry, but in order to obtain qualitative characteristic for materials authors used lattice constants fixed at the equilibrium values of the parent MIL-125 for all structures. computed | bdc/bdc-NH2 | bdc-NH2 per unit cell | color | band gap (eV) | band gap (eV) | 100% | 0 | | 3.68 | 3.7 | 10% | 1 | | 2.75 | 2.5 | 50% | 6 | | 2.62 | 2. Band gap doesn’t change in a presence of bdc-NH2 from 2 to 12 per unit cell: a single −NH2 group electronically saturates MIL-125, band gap and others(2-12) – independent of amount NH2 group. It reflects on color (3).
2. Band gap doesn’t change in a presence of bdc-NH2 from 2 to 12 per unit cell: a single −NH2 group electronically saturates MIL-125, band gap and others(2-12) – independent of amount NH2 group. It reflects on color (3).
2.3 | 100% | 12 | | 2.72 | 2.3 | bdc-(NH2)2/ bdc-NH2 | | | | | 10% | 1 | | 1.23 | 1.5 |
4. Increasing electron density reflects decreasing band gap.
4. Increasing electron density reflects decreasing band gap.
3. Color increased in intensity. Increasing the molar ratio reflected in the physical crystal appearance.
3. Color increased in intensity. Increasing the molar ratio reflected in the physical crystal appearance.

2. From the 1. authors learned, that by increasing the electron density on the ring the value of band gap decreasing. NH2 is strong electron-donating group changing the bend gap from 3.68 to 2.75 eV ( 1.23 in case of bdc-(NH2)2), thus, to fit to the stated purpose (engineering of the optical response) it is quite logical to investigate the influents of the other substitution species on the ligand, and then on the band gap in MOF. Result: weak

Electron donation groups
Electron density

strong weak Electron donation groups
Electron density

strong

The authors, relying on the computational data defined the promising structures.
The main conclusion is that the optical properties can be controlled by the substitution of the aromatic motifs, which led to a change of the valence band, and the main contribution is that this methodology can be further extended to the large range of aromatic linkers used in MOF.
1. Dan-Hardi, M.; Serre, C.; Frot, T.; Rozes, L.; Maurin, G.; Sanchez, C.; Férey, G. J. Am. Chem. Soc. 2009, 131, 10857. 2. C. G. Silva, A. Corma and H. Garcia, J. Mater. Chem., 2010, 20,3141–3156. 3. Zlotea, C.; Phanon, D.; Mazaj, M.; Heurtaux, D.; Guillerm, V.; Serre, C.; Horcajada, P.; Devic, T.; Magnier, E.; Cuevas, F.; Férey, G.; Llewellyn, P. L.; Latroche, M. Dalton Trans. 2011, 40, 4879. 4. Fu, Y.; Sun, D.; Chen, Y.; Huang, R.; Ding, Z.; Fu, X.; Li, Z. Angew. Chem. Int. Ed. 2012, 51, 3364−3367. 5. Horiuchi, Y.; Toyao, T.; Saito, M.; Mochizuki, K.; Iwata, M.; Higashimura, H.; Anpo, M.; Matsuoka, M. J. Phys. Chem. C 2012, 116,20848.

Appendix
Appendix
Electrical Conduction
In general, conductivity is the rate at which matter or energy can pass through a given material. In metals, the current is carried by electrons, and hence the name electronic conduction. In ionic crystals, the charge carriers are ions, thus the name ionic conduction.

When an electric potential V is applied across a material, a current of magnitude I flows. Ohm's law:I = V/RR is the electrical resistance. | R depends on the intrinsic resistivity of the material and on the geometry (length l and area A through which the current passes).R = l/A | The electrical conductivity is the inverse of the resistivity: =1/ |

Energy Bands in Solids
When atoms come together to form a solid, their valence electrons interact due to Coulomb forces, and they also feel the electric field produced by their own nucleus and that of the other atoms. Due to these interaction energy levels of the atoms must split into bands of discrete levels so closely spaced in energy, that they can be considered a continuum of allowed energy. The process of splitting of energy levels can be understood from silicon lattice, which is shown on the picture. .The bands are separated by gaps, where electrons cannot exist. Strongly bonded materials tend to have small interatomic distances between atoms. Thus, the strongly bonded materials can have larger energy band gaps than do weakly bonded materials.
Spacing is progressively decreasing:

1) No interatomic separation. Each atom in the crystal behaves as free atom.
2N ē in 2N 3s levels 2N ē in 6N 3p levels
2) The energy of electrons of each atom starts changing, whereas the energies of electrons in the inner shell do not change. Instead of a single 3s or 3p levels, we get a large number of closely packed levels.
e) The gap between 3s and 3p completely disappear and the energy levels are continuously distributed.
4N levels - filled 4N levels - empty
f) Actual spacing in the crystal: the 4N filled energy levels (valence band) are separated from 4N unfilled energy levels (conduction band).

Spacing is progressively decreasing:

1) No interatomic separation. Each atom in the crystal behaves as free atom.
2N ē in 2N 3s levels 2N ē in 6N 3p levels
2) The energy of electrons of each atom starts changing, whereas the energies of electrons in the inner shell do not change. Instead of a single 3s or 3p levels, we get a large number of closely packed levels.
e) The gap between 3s and 3p completely disappear and the energy levels are continuously distributed.
4N levels - filled 4N levels - empty
f) Actual spacing in the crystal: the 4N filled energy levels (valence band) are separated from 4N unfilled energy levels (conduction band).

Picture. Process of splitting of energy levels. N Silicon atoms with 1s2 2s2 2p6 3s2 3p2
Appendix
Appendix
Bend theory
Depending on the band gap energy, solid-state materials can be classified into three groups:

For metals, the electrons can jump from the valence orbits to any position within the crystal (free to move throughout the crystal) with no “extra energy needed to be supplied”.
For insulators, it is very difficult for the electrons to jump from the valence orbits and requires a huge amount of energy to “free the electron” from the atomic core.
For semiconductors, the electrons can jump from the valence orbits but does require a small amount of energy to free the electron” from the atomic core.
Semiconductors can be intrinsic or extrinsic. Intrinsic means that electrical conductivity does not depend on impurities (intrinsic – pure). Pure semiconductor, however, is not very useful, as it is neither a very good insulator nor a very good conductor. However, one important feature of semiconductors is that their conductivity can be increased and controlled by doping with impurities and gating with electric fields. Doping and gating move either the conduction or valence band much closer to the Fermi level, and greatly increase the number of partially filled states. An extrinsic semiconductor may have different concentrations of holes(p) and electrons(n). It is called p-type if p>n and n-type if n>p .

Appendix
Appendix
Organic semiconductors

The electrical and optical properties of organic materials originate to a large extent from the arrangement and occupation of the pi orbitals. The pz atomic orbitals of all carbon atoms are oriented perpendicular to the chain and overlap, therefore the electrons in these orbitals are delocalized along the chain. The number of molecular pi and pi* orbitals is equal to the number of carbon atoms, thus energy levels split up with increasing chain length. For long chains the difference between energy levels becomes so small that the levels may be treated as continuous bands. In polyacetylene the highest occupied molecular orbital (HOMO) is the top of the pi band while the lowest unoccupied molecular orbital (LUMO) is the bottom of the pi* band. If all bond lengths were equal in polyacetylene, the pi and pi* bands would have form a continuous band and the material would appear as a quasi-metal. However, this is not the case sine Peierl’s theorem state that this configuration is not energetically stable. Instead, the bond length alternates between double and single bonds, which creates a band gap Eg between the HOMO and LUMO levels. A material with a relatively small band gap is referred to as a semiconductor.