1
 Chapter 1 
Introduction 
 
1.1 Crystal engineering 
Crystal engineering is a branch of crystallography that entails the rational design of 
functional materials at the molecular level (Desiraju, 2003). It is able to achieve this 
by controlling the various molecular interactions such as non-covalent interactions 
and coordination complexes, which occur during the nucleation process 
(Hollingsworth, 2002). This allows researchers to tailor-make solids with desirable 
chemical and physical properties in both the molecular and bulk forms, thus making 
this field of prime interest to researchers (Zheng et al., 2005).  
 
The utilisation of these intermolecular interactions may vary between organic and 
inorganic systems. Organic systems primarily utilise non-covalent interactions, 
whereas inorganic systems are mainly made up of metal coordination complexes. The 
bonding interaction within these systems may vary from metal to ligand bonding to 
non-covalent interactions in the form of hydrogen bonding, electrostatic interactions, 
and van der Waals forces which include ????? interactions (Oshovsky et al., 2007, 
Lehn, 1988).  The approach of using non-covalent interactions to create new 
materials, make up a part of chemistry known as supramolecular chemistry. 
 
Supramolecular chemistry deals with the formation of superstructures through the 
design and utilisation of non-covalent interactions, by making use of small molecular 
building blocks (Ghosh et al., 2005). These non-covalent interactions are far weaker 
than covalent bonding and have been demonstrated in important concepts such as 
host-guest chemistry, molecular recognition, molecular self-assembly and folding 
(foldamers) (Oshovsky et al., 2007). Molecular self-assembly and molecular 
recognition allow for the formation of host guest complexes through non-covalent 
interactions and are studied extensively in supramolecular chemistry (Oshovsky et 
al., 2007, Lehn, 1988).   
 2
 O
 O H O
 OH N
 H
 O H
 O
 Their use in chemistry has been greatly exploited in crystal engineering due to the 
fact that they form supramolecular networks that can self assemble by directional 
molecular recognition between the molecules or ions (Bis et al., 2005). Interactions 
between molecules are often conservatively maintained in different supramolecular 
structures, as a consequence these molecular combinations may be referred to as 
supramolecular synthons (Desiraju, 1995). Supramolecular synthons can be classed 
into two different groups namely, homosynthons and heterosynthons (Walsh et al., 
2003, Etter, 1982). Homosynthons refers to interaction between the self 
complimentary functional groups e.g. carboxylic acids (Walsh et al., 2003, Etter, 
1982), whereas heterosynthons refer to interactions between different but 
complimentary functional groups e.g. acid-pyridines interactions (Vishweshwar, 
2003). Detailed knowledge of these non-covalent interactions is useful as they 
provide information for constructing supramolecular structures. Thereby allowing 
researchers to expand and exploit solid-state chemical research and engineering      
(Bis et al., 2005, Zaworotko, 2000). 
 
 
        
  
     
 
     Homosynthon         Heterosynthon 
Figure 1.1: Schematic representation of homosynthon (Bis et al., 2005) 
and heterosynthon (Walsh et al., 2003). 
 
1.2 Porous materials 
Industry and academia has provided particular interest into porous materials (Kitagwa 
et al., 2004), which was fuelled by the formation of highly rigid structures that are 
permeable (Barbour, 2006). Permeability plays an important role in the formation of 
porous materials as the removal of guest molecules should lead to the formation of a 
cavities, which form a network on interconnected channels within a rigid structure 
(Barbour, 2006). These vacant channels and cavities allow porous materials to be 
 3
 utilised in areas such as gas storage, drug delivery, sensor technologies and catalysis 
(Hermes et al., 2006, F?rey, 2008, Kitagwa et al., 2004, Volkringer et al, 2009). 
There are currently three known types of porous materials, namely zeolites, activated 
carbons and metal organic frameworks (MOFs) (Kitagwa et al., 2004), and each class 
has problems associated with them. The problems faced by zeolites and activated 
carbon materials is that they either have low porosity (zeolites) or broad pores 
(carbons), thus hindering gaseous uptake and storage (Kitagwa et al., 2004).  In 
addition to these problems, exhisiting synthetic methods do not allow for the design 
ablity, and tailoring of these structures (Kitagwa et al., 2004). However, the last and 
most recent form of porous materials MOFs (coordination polymers), has attracted  a 
lot  of attention recently amongst material scientists. This is due to current synthetic 
methods allowing for the creation and modification of regular repeating nano-sized 
pores within the framework (Eddaoudi et al., 2001, Kitagwa et al., 2004). Figure 1.2 
outlines the possible modification of these materials in terms of size, shape, surface 
functionality and dimensionality by altering their fundamental components. 
 
 
 
 
 
 
 
 
 
 
 
Figure 1.2: Possible alterations of MOF materials via current synthethic methods 
(Uemura et al., 2009). 
 
 4
 1.3 Metal Organic Frameworks (MOFs) 
The design, synthesis and characterisation of MOFs have shown exponential growth 
over the recent years (Kitagwa et al., 2004, Long et al., 2009). As a result there are 
close to 4000 fully characterised MOFs structures found in the CSD, with roughly a 
1000 journal articles being published every year since 2002 (Kitagwa et al., 2004, 
Long et al., 2009, Mueller et al., 2006).  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1.3: Exponential growth in MOF research over the past three decades (Long et 
al., 2009). 
 
The reason for this immense growth is due to the possibility of altering these 
materials in a systematic manner (Roswell et al., 2004). Thus allows researchers, 
molecular control of structural details by utilising concepts from both classic organic 
and inorganic solid-state structural chemistry (Ohashi, 1993). As a consequence it is 
possible to design materials that are able to self-assemble into 1D, 2D and 3D 
extended network structures (figure 1.4) (Roswell et al., 2005, Janiak, 2003).  
 
 
 
 5
  
                 
 
1D     2D             3D   
Figure 1.4: Representation of 1D, 2D and 3D network structures in MOFs. 
 
1.4 MOF components 
MOFs are hybrid materials that are mainly based on organic ligands (linkers) and 
cationic transition metal ions (connectors) (Kitagwa et al., 2004, Mueller et al., 
2006). Metals are used as they readily self assemble and directly influence the 
framework topologies formed by a MOF material (Czaja et al., 2009, Volkringer et 
al., 2009). The two main linker components that are extensively used for MOF 
production are usually based on a pyridyl and/or carboxylate functionality, which 
occurs at the both terminal positions of the organic molecule (Mueller et al., 2006, 
Hao et al., 2004). These functional groups are essential as they coordinate onto a 
metal thereby forming a metal cluster (figure 1.5 (b)), these clusters are then usually 
linked together by the organic molecule (linker), thereby forming the MOF 
framework.  
 
 
 
 
 
 
 
 
 
  (a)                             (b) 
Figure 1.5: Diagram of MOF-5 (Mueller et al., 2006), indicating (a) organic linker 
and pore volume and (b) metal-carboxylate cluster. 
 
Organic linker 
Metal-carboxylate cluster 
Pore volume 
 6
 1.4.1 Linkers 
Currently pyridyl and carboxylate groups are the two main functional groups used in 
linker components for MOF synthesis. Each linker provides its own unique structural 
characteristics to the MOF. The use of monodentate dipyridyl linkers leads to the 
formations of cationic frameworks with the addition of counterions (Eddaoudi et al., 
2001). Pyridyl linker components often form nonporous structures as the counterions 
tend to inhibit pore formation (Kim et al., 2001, Montney et al., 2007). Topology 
prediction is made extremely difficult due to the flexible nature of these linkers 
coupled with the presence of counterions (Kim et al., 2001). 
 
Carboxylate linkers in the form of dianionic dicarboxylates have been shown to be 
essential in the formation of neutral porous MOF materials without the incorporation 
of counterions (Montney et al., 2007, Kitagawa et al., 2004, Eddaoudi et al., 2001).  
It has also been noted by Eddaoudi and co-workers (2001), that the use of carboxylate 
groups (known to be essential in chelating metal ions) actually locks the metal into a 
respective position, thereby forming a secondary building unit (SBU) (figure 1.5 (b)) 
(Kitagawa et al., 2004). It does this by forming an M-O-C cluster, which has the 
ability to aggregate (Eddaoudi et al., 2001). These SBU?s are very rigid and as a 
result tend to be structurally stable and robust. Figure 1.6 indicates various pyridyl 
and carboxylate linker components. 
 
 
 
 
 
 
 
 
 
 
 7
  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1.6: Various pyridyl and carboxylate linker components used in MOF 
formation. 
 
1.4.2 Connectors 
The use of transitional metal-ions plays a large role in the formation of MOF 
materials as they are able to self assemble and act as coordination sites for a wide 
range of polyatomic bridging ligands (Czaja et al., 2009).  The oxidation state of the 
metal also influences the overall topology of MOFs by altering the coordination site, 
which could range from linear to octahedral (Kitagwa et al., 2004). This is illustrated 
by figure 1.7, whereby a linear linker component is utilised with metals of various 
oxidation states. The use of metals also enhances the physical properties by forming 
robust materials that have increased thermal and mechanical stability (Czaja et al., 
2009).   
 
 
 
 
 8
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1.7: Framework alteration with varying metal geometries using a linear linker 
(Kitagwa et al., 2004). 
 
1.5 Bonding interactions 
There are a large number of different framework structures possible due to the 
relative abundance of different metals and organic linkers. These frameworks are able 
to extend infinitely in all dimensions due to bonding interactions between the clusters 
(Janiak, 2003). Bonding interactions may vary from coordination bonding (CB), 
hydrogen bonding (HB), ????? interactions and other van der Waals interactions 
(Nattinen et al., 2003).  The use of weak interactions in conjuction with coordination 
bonding (CB) results in the formation of flexible and robust MOF structures (Kitagwa 
et al., 2004). 
 
 
 
 9
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1.8: Interactions resulting in overall extension of MOF structures (Kitagwa et 
al., 2004). 
 
1.6 Alterations 
The alteration of these crystalline porous materials enables researchers to develop 
materials with specific chemical functionality (Long et al., 2009, Hao et al., 2004). 
As a result, careful selection of linker and connector components makes it possible to 
tune the physical characteristics of the MOFs to suite specific applications (Janiak, 
2003). The large variety of ligand and metal ions results in large diversity in 
topologies in the metal building unit within the MOF (Janiak, 2003), thereby allowing 
for the design of very novel materials. 
 
Void formation and functionality (Roswell et al., 2004) within the framework is 
dependant on the size, length, and the functionality of the organic unit, as shown in 
figure 1.8 and table 1.1. Hence understanding the bonding interactions between the 
linker and connector components is essential, as it aids in understanding how these 
 10
 materials (MOF) come into existence, and so providing material designers with 
predictive capabilities (Yaghi et al., 2003).  
 
 
Figure 1.9: Effect of ligand size on the pore volume of MOFs with the same metal 
cluster (Kesanli et al., 2003). 
 
 
 
 
 
 11
 Table 1.1: Effect of linkers and joiners on the unit cell dimensions of MOF materials. 
(Rosi et al., 2005). 
 
MOF-70 MOF-72 MOF-73 
Linker 
CO2O2C
  
O2C CO2
  
CO2O2C
  
Metal Pb Cd Mn 
Unit cell:     
a(?) 8.36 13.69 24.78 
b(?) 17.99 18.25 10.58 
c(?) 9.96 14.91 17.42 
?(o) 90.00 90.00 90.00 
?(o) 102.687 101.01 129.93 
?(o) 90.00 90.00 90.00 
Volume (?3) 1462.40 3654.52 3504.1 
Z 4 8 8 
Space group P21/n C2/c C2/c 
Structure 
 
 
 
  
From table 1.1 and figure 1.8, it can clearly be seen that MOFs can be altered by 
simply changing the metal ion and/or linkers, thereby altering the MOFs chemical 
and physical (pore size) properties (Clausen et al., 2005). 
 12
 MOF networks provide attractive properties such as regioselectivity, stereoselectivity, 
shape selectivity and precise binding (Nattinen et al., 2003), and can be used in a 
number of applications, which are listed below: 
Current applications where MOFs have been used or considered: 
? Gas purification (Mueller et al., 2006). 
? Gas separation (Mueller et al., 2006). 
? Magnetic properties (Clausen et al., 2005). 
? Production of optically pure compounds (Lin, 2005). 
? Gas storage (Hydrogen and Hydrocarbons) (Hermes et al., 2006). 
? Zeolite type catalysts (Hermes et al., 2006). 
? Conductivity applications (Kepert, 2006). 
? Gas sensor applications (Achmann et al, 2009). 
 
1.7 Formation of MOF materials 
 
 
 
 
 
 
Figure 1.10: Vital components in MOF formation (Kitagwa et al., 2004). 
 
MOF synthesis usually requires the use of a metal salt and an anionic/neutral linker, 
with the relevant stoichiometry. In addition to this, various other auxiliary 
components can be used, in the form of blocking agents, counteranions and guest 
molecules (Kitagwa et al., 2004). Each of these components play a role in the 
eventual formation of the MOF material (Kitagwa et al., 2004). Blocking agents 
actually help in the formation of the SBU?s (Roswell et al., 2004), whereas 
counterions are used in the formation of cationic frameworks when pyridyl linkers are 
used (Eddaoudi et al., 2001). The use of non-bonding guests (such as solvents) aid in 
 13
 the formation of porous structures, by facilitating the formation of extended structures 
(Kitagwa et al., 2004). These guests can be displaced by various methods such as 
irradiation wherein more reactive species can be incorporated within the pore, 
provided that it is volatile enough and can fit into the cavity (Hermes  et al., 2006). 
 
1.8 Problems affecting MOFs  
The predictive capability associated with MOFs allows control of certain crystal 
properties to be achieved, by controlling the orientation of building blocks in the solid 
state (Kim et al., 2001). Unfortunately crystal engineering still faces immense 
problems over the control of overall topology, stability and reproducibility of a 
framework (Huang et al., 2006, Nattinen et al., 2003).  This has been noted by Yaghi 
and Wright, who synthesised MOF-2, in different labs that yielded different gas 
absorption properties (Nattinen et al., 2003).   
 
Synthesis still remains a key obstacle in the formation of these materials (Eddaoudi et 
al., 2001), as they are synthesised under relatively mild conditions (Zheng et al., 
2002). These mild conditions do not allow crystallisation to be greatly effected, hence 
making it difficult for crystalline materials to be obtained (Eddaoudi et al., 2001). 
 
The prediction of the resulting structure remains a great challenge, and when obtained 
are often difficult to reproduce (Kim et al., 2001). However, MOFs are more 
attractive than more conventional porous materials as their basic molecular building 
blocks are retained in the final assembled network (Clausen et al., 2005).  
 
1.9 Solid State Gas Sensors 
Solid state gas sensor technologies have become of increasing importance over the 
past decade (Capone et al., 2003). This is due to the need for improved monitoring 
devices for safety and environmental protection purposes. It is believed that solids 
state gas sensors are currently the best sensor technology available. The main reason 
for this is due to these sensors being able to provide real-time chemical analysis for a 
 14
 wide range of gaseous chemical compounds (Capone et al., 2003). These sensors are 
also typically of a compact design, portable, highly sensitive and most important 
inexpensive, all of which are highly desirable properties (Capone et al., 2003, 
Allendorf et al., 2009). This current technology provides numerous advantages over 
the more conventional analytical techniques, as real time data analysis is possible, 
sample preparation is not required and there is no investment in costly and bulky 
instruments (Capone et al., 2003). 
 
As with all new technology, current solid state gas sensors do suffer from a few 
problems, especially with regard to long term thermal and chemical stability (Capone 
et al., 2003, Achmann et al., 2009). However, these problems are being overcome 
through advances in nanotechnology. Nanotechnology looks at controlling and 
manipulating the material on a nanometer scale, and these alterations greatly affect 
the materials properties and characteristics (Capone et al., 2003). A material which is 
prone to manipulation on the nanometer scale is MOFs.  
 
MOFs are highly ordered nanoporous materials that contain extremely high surface 
areas (Achmann et al., 2009, Allendorf et al., 2009). These desirable properties have 
attracted a great deal of attention particularly with regard to gas separation and 
storage, thus making MOFs ideal candidates for gas sensor applications (Park et al., 
2008, Allendorf et al., 2009). They have also shown superior adsorption-desorption 
properties over the more conventional inorganic materials, as gases are not covalently 
bonded to the framework (Achmann et al., 2009). This allows the gas to be easily 
desorbed at relatively low partial pressures (Achmann et al., 2009).  
 
Current research by Allendorf and Hesketh (2009) has shown that this reversible 
process can be both selective and sensitive to a specific gas (Achmann et al., 2009, 
Allendorf et al., 2009). These include, testing for chemical weapons and being 
utilised as breathalysers that could determine pre-symptomatic infections due to 
 15
 exposure to environmental toxins (Capone et al., 2003, Allendorf et al., 2009). The 
possibilities are endless. 
 
1.10 Charge Transfer Complexes / Host-guest complexes 
Charge transfer (CT) complexes are electron-donor-acceptor based structures. They 
can either be formed intra-molecularly or inter-molecularly. The host-guest 
framework is made up of weak interactions (Lehn, 1990), which allows for flexibility. 
This enables it to accommodate for a large variety of electron rich systems (donor) 
(Kishikawa et al., 1999). A key feature of CT complexes is that they can be formed in 
gas, liquid and solid states of matter, and can emit a range of colors depending on the 
size and shape of the donor molecules. These colors occur in the visible range and so 
can be clearly seen with the naked eye.  
 
The formation of CT complexes is due to an electrostatic interaction between the 
donor (electron rich) and the acceptor (electron poor) aromatic molecules. This 
interaction (stabilising force) allows the aromatic rings to overlap such that there is a 
????? interaction between them (Hunter et al., 1990).  The strength of these non-
 covalent interactions between the donor-acceptor molecules are usually strong and of 
a short range order, and depends on the stacking arrangements (orbital overlap for 
donor to acceptor charge transfer) between the donor-acceptor molecules, (Hunter et 
al., 1990., Gabriel et al., 2002., Zheng et al., 2005)  
 
1.10.1 Host molecules 
The availability of host molecules for the purpose of studying CT interactions is 
currently very scarce due to lack of new acceptor molecules (Park et al., 2003). 
Acceptor molecules need to have an aromatic ring which is made electron deficient 
due to electron withdrawing groups close by. An example of such a molecule is        
2-(2-hydroxyethyl)isoindoline-1,3-dione (figure 1.11), a derivative of phthalimide. It 
is an interesting acceptor molecule, as it rapidly self assembles to form host-guest 
complexes. A reason for this rapid formation of host guest complexes is mainly due 
 16
 N
 O
 O
 OH
 N
 O
 O
 N
 O
 O
 O
 O
 OHOH
 to the molecule being relatively flat as well as containing polar carbonyl groups and ? 
acceptor sites (Barooah et al., 2003). These carbonyl and ? acceptor sites are able to 
stabilise host-guest framework by CH???? and CH???O, hydrogen bonding interactions 
(Barooah et al., 2003). 
 
 
 
 
 
Figure 1.11: Diagrammatic representation of 2-(2-hydroxyethyl)isoindoline-1,3-dione  
(Barooah et al., 2003). 
 
The acceptor molecule of choice to form host guest complexes in this study is N,N?- 
bis(glycinyl) pyromellitic diimide (figure 1.12). This molecule was chosen as it is a 
derivative of phthalimide, with the addition of an imide and terminal polar carbonyl 
groups (Barooah et al., 2006). The addition of these groups makes the ring highly 
electron deficient as it pulls electron density away from the carbon backbone. 
According to Barros and co-workers (1997), the addition of the second imide group 
lowers the first transition energy, S0?S1 which is due to the ?,?*. This indicates that 
the addition of the second imide helps to stabilise the ?,?* transition state and 
illustrates a strong ? conjugation between the carbonyl groups and the electron rich 
aromatic rings (Barros et al., 1997).  The pyromellitic diimide molecule, just like 
phthalimide, is also relatively planar in shape and as a result should be able to readily 
accommodate donor molecules to form host guest complexes.  
 
 
 
 
 
Figure 1.12: N,N?-bis(glycinyl)-pyromellitic diimide. 
 17
 D 
D 
D 
D 
D 
D 
D 
D 
D 
D 
D 
D 
A 
A 
A 
A 
A 
A 
A 
A 
A 
D 
(a) (b) 
 
These host molecules also allow for the formation of complexes with different donor 
acceptor ratios (D:A). The ratio between the D:A molecules may vary from 1:1, 
which leads to a D-A-D-A-D-A structure being stacked up in an alternating fashion as 
well as a 2:1 ratio which results in a sandwich type structure D-A-D D-A-D 
(Kishikawa et al., 1999) as illustrated in the figure below: 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
Figure 1.13: Diagram illustrating different stacking arrangements between donor and 
acceptor molecules, (a) alternating stacking 1:1 ratio of donor and acceptors and (b) 
sandwich stacking 2:1 ratio of donor and acceptors. 
 
Thus in this work, a number of guests were used in the formation of host guest 
networks with the pyromellitic diimide ligand. A variation of this ligand was used 
extensively by Kishikawa and his co-workers (1999), where they showed that in 
order for host-guest complexes to form in a stacking formation the following criterion 
has to be met: 
1.) The host molecules must contain an electron poor ?-plane at the centre of the 
molecule, and the guest molecules must be an electron rich aromatic 
compound. 
 18
 N
 O
 O
 N
 O
 O
 OHOH
 O
 O
 **
 * *
 *
 * *
 *
 2.) The ? plane on the host molecule must have the identical environment on 
each side to successfully undergo 1:2 complexations. 
3.) The formation of the stacking conformation requires the host molecule to 
contain four hydrogen bond acceptors, and the guest molecule to contain two 
hydrogen bond donors, so as to stabilise to host-guest framework. 
4.) The host-host interaction should be weaker than the host-guest interactions. 
 
                                                                                a) Electron poor ? plane 
 
 
 
 
 
 
c) * - Hydrogen bond acceptors   b) Lines of symmetry 
Figure 1.14: Diagrammatic representation of N,N?-bis(glycinyl)-pyromellitic diimide 
indicating a) electron poor ? plane, b) lines of symmetry and c) hydrogen bond 
acceptors. 
 
From the above figure, it can be seen that the N,N?-bis(pyridyl)-pyromellitic diimide 
ligand of choice meets the above criteria set out by Kishikawa and co-workers 
(1999). Hence it is likely that the pyromellitic diimide molecule will readily self-
 assemble with various electron rich aromatic systems to form host-guest complexes. 
 
Recent work by Barooah and co-workers (2006) has successfully shown the 
formation of colour when using various aromatic systems with their respective 
diimide host molecule. They also illustrated that the production of color is possible 
when non aromatic molecules are incorporated into the host-guest complexes. An 
example of this was the formation of red crystals when nitromethane (a clear liquid) 
incorporated itself into the CT complex formed between N,N?-bis(glycinyl)-
 19
 pyromellitic diimide (colourless) and 4,4?-dihydroxybiphenyl (beige). This 
expression of colour change in the form of CT complexes could have tremendous 
applications in the gas sensor applications, especially once incorporated into a MOF.  
 
1.11 Non-covalent network interactions. 
Interactions between host-guest complexes are usually made up of non-covalent 
bonds. These non-covalent bonding between the host and guest molecules allow for 
the formation of rather flexible networks to be produced (Kishikawa et al., 1999), due 
to the flexible nature of non-covalent bonds. Flexible networks have an advantage 
over more rigid networks as they can accommodate a variety of guest molecules and 
adjust the crystal lattice to form single crystals (Kishikawa et al., 1999).  
 
The weak non-covalent bonding interaction are primarily made up of hydrogen 
bonding and intermolecular forces, dipole-dipole and ion-dipole forces, hydrogen 
bonding and ????? interactions 
 
1.11.1 Hydrogen bonding 
Hydrogen bonding plays a vital role in chemistry, nature and in all biological systems 
(Jeffrey, 1997). They are essential as they help in the stabilisation of protein 
structures within the skin, muscles and other animal tissues (Brown et al., 2003). 
Hydrogen bond properties differ relative to the groups of atoms surrounding them 
(Jeffrey, 1997), and these can be characterised into three types; namely strong, 
moderate and weak interactions (table 1.2): 
 
 
 
 
 
 
 20
 Table 1.2: Characteristics of strong, moderate and weak hydrogen bonds (Jeffrey, 
1997). 
 
 
Due to the large number of different types of hydrogen bonding interactions 
occurring amongst molecules, a methodology in the form of graph set notation was 
proposed by Etter (1990). This methodology takes into account the formation of 
certain hydrogen bonding patterns without considering the chemical nature of the 
compounds involved (Bernstein et al., 1995, Jeffrey, 1997). This methodology allows 
all hydrogen bonding patterns to be broken down and indentified in terms of chains 
(C), rings (R), finite complexes (D) and intramolecular interactions (S), as indicated 
by fig. 1.15 (Bernstein et al., 1995, Jeffrey, 1997). In addition to this the number of 
hydrogen bond acceptor (a) and donor (d) sites can be described along with the 
number of symmetry independent hydrogen bonds (n) (Jeffrey, 1997), hence leading 
to the formation of a graph-set designation Gad(n) (Jeffrey, 1997). 
 
 
 
 
 
 
 21
  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1.15: Graph set notation of various hydrogen bonding molecules (Etter, 1991). 
 
1.11.2 ????? interactions 
????? interactions allows for the partial overlapping of aromatic rings (Hunter et al., 
1990). This formation is possible due to these organic molecules being planar 
(Barooah et al., 2003), hence allowing for overlapping of p-orbitals in ?-conjugated 
systems to occur. These systems do not directly overlap each other due to repulsion 
interactions between the rings and are aligned in an offset orientation relative to each 
other. The strength of these interactions are usually strong and of a short range order, 
and depends on the stacking arrangements of the molecules (Hunter et al., 1990., 
Gabriel et al., 2002., Zheng et al., 2005). 
1.12 Project Aims 
Development of MOFs materials has grown exponentially over the past decade due to 
the ease with which these materials can undergo physical and chemical change. The 
ability customise components with these materials has allowed them to be utilised for 
a number of applications. One application of current importance is with regard to 
 22
 solid state sensors technologies. A key feature of this technology is that it provides 
real time analysis for chemical compounds that occur in all different states of matter, 
i.e. solid, liquid and gas. With this in mind, the diimide linker component was chosen 
as it contained the correct terminal functionality to form MOF materials as well as 
having the ability to formed coloured materials. These features allow this type of 
linker to be incorporated within an MOF framework and undergo colour formation 
through CT complex formation. Therefore, the aims of this project can be summed up 
in the following points: 
? To design diimide linker components with the appropriate chemical 
functionality to be incorporated into a MOF framework as well as undergo 
colour formation for sensor technologies.  
? To develop methods for the synthesis and modification diimide linker 
components. 
? To determine the effect of diimide modification on color formation.  
? To form porous MOF materials using synthesised and modified linker 
components. 
? To incorporate various aromatic rings into porous frameworks. 
? To study the effect of CT complex formation within the MOF framework, and 
to determine the possibility of colour formation needed for sensor 
applications. 
? To fully characterise and compare all obtained structures through various 
experimental techniques. 
 
1.13 References 
Achmann , S.,  Hagen, G.,  Kita, J.,  Malkowsky, I. M., Kiener, C.,  Moos, R. (2009).  
Sensors, 9, 1574.  
 
Allendorf, M., Hesketh, P. (2009). SPIE Newsroom. DOI: 10.1117/2.1200905.1512.  
 
Barbour, L. J. (2006).  Chem. Commun., 1163. 
 
 23
 Barooah, N., Sarma, R. J., Batsanov,  A. S., Baruah,  J. B. (2006). J. Molecular 
Structure, 8, 608.  
 
Barooah, N., Sarma, R. J., Baruah, J. B. (2003). Crystal Growth & Design, 3, 639. 
 
Barros, T. C.,  Brochsztain, S.,  Toscano, V. G.,  Filho P.B., Politi, M. J. (1997). 
Journal of Photochemist' and Pholohiology A: Chemistry, 111, 97.  
 
Bernstein, J., Davis, R. E., Shimoni, L., Chang, N. L. (1995). Angew. Chem., Int. Ed. 
Engl., 34, 1555. 
 
Bis, J. A., Zaworotko, M.  J. (2005). Crystal Growth & Design, 5, 1169.  
 
Brown, T.L., LeMay, H. E., Bursten, B. E., Burdge, J. R. (2003). Chemistry, The 
Central Science, 9th edition, Prentice Hall, chapter 11. 
 
Capone, S., Forleo, A., Francioso, L., Rella, R.,  Siciliano, P.,  Spadavecchia, J.,  
Presicce, D. S., Taurino, A. M. (2003). Journal of Optoelectronics and Advanced 
Materials, 5, No. 5, 1335. 
 
Clausen, H. F., Poulsen, R. D., Bond, A. D., Chevallier, M. A. S., Iversen, B. B. 
(2005). J. Solid State Chem., 178, 3342.  
 
Czaja, A. U., Trukhan, N., M?ller, U. (2009). Chem. Soc. Rev., 38, 1284. 
 
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