Download 1.4 Desirable Features of Inorganic Sensor Materials

Chapter 1
Chapter 1 Introduction
1.1 Background of Photochemistry
Photochemical processes are of the greatest importance to life on Earth. The wellknown photosynthesis uses the Sun's energy and creates carbohydrate from atmospheric
carbon dioxide, as well as liberating oxygen to the atmosphere. Light-induced chemical
changes in the gases of the atmosphere and the particles suspended there also modify the
chemical composition of the atmosphere and allow it to support life. Indeed, the
formation from the simplest elements of the complex organic molecular precursors of life,
and then the emergence of life itself, are intimately bound up with photochemical
processes. One of the most important senses for Man, and many other species besides, is
vision, which is also photochemical in origin. Nature thus relies on light to effect some of
her most essential chemistry. Man has also tried to harness light in his service, the
applications ranging from the synthesis of new and complex organic species, through
numerous kinds of imaging and photographic process, to the gathering and storage of
solar energy.
The significance of photochemistry is by no means limited to the use that Nature
and Man make of it. Rather, the chemistry itself is of profound interest at the most
fundamental level. The reactions, dissociations, isomerizations, and optical emissions of
electronically excited species are the central feature of photochemistry. For every atom
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Chapter 1
and molecule known to us in the ground state, there are likely to be one or more excited
states. Since these states possess different electronic structures from, and higher energies
than, their parents, their chemistry is almost inevitably distinct from that of the groundstate species. Whole new fields of chemistry are thus opened up by allowing light to
interact with the elements and their compounds.
Research in inorganic photochemistry has expanded enormously in scope and
importance. The monographs of Balzani and Carassitti on the photochemistry of
coordination complexes,[1] and the multiauthored volume edited by Adamson and
Fleischauer[2] and the most recent volume by Balzani and Scandola[3] on supramolecular
photochemistry can be cited as important milestones in the evolution of inorganic
photochemistry. Inorganic photochemistry can be discussed in two areas:
1) Photophysical studies devoted to characterization of the structure, bonding and
dynamics of electronically excited state both at the intermolecular and intramolecular
level.
2) Photochemical studies of uni- and bimolecular reactions of the excited states.
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Chapter 1
1.2 Photochemistry of Excited Molecule
Fundamentals
Figure 1.1 is a partial energy level diagram for a typical photoluminescent
molecule. The lowest heavy horizontal line represents the ground-state energy of the
molecule, which is normally a singlet state and is labeled So. The upper heavy lines are
energy levels for the ground vibrational states of excited electronic states. The lines on
the left represent the (S1) electronic singlet states. Those on the right (T1) represents the
energy of the first electronic triplet state. As is normally the case, the energy of the first
excited triplet state is lower than the energy of the corresponding singlet state. Numerous
vibrational energy levels are associated with each of the four electronic states, as
indicated by the lighter horizontal lines.
As shown in Figure 1.1, excitation of this molecule can be brought about by
absorption of radiation (So  S1). The molar extinction coefficients  were obtained from
Beer-Lambert’s Law. The absorbance, Abs, of a material with a light pathlength l is related
to the concentration, c, of the absorbant
Abs = log10(Io/It) = lc
Io and It are the incident and transmitted light intensities, respectively. The molar extinction
coefficient, , also referred to as the molar absorptivity, is a function of the oscillator
strength and is also dependent on the wavelength of the incident light. The Beer-Lambert’s
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Chapter 1
kSR
4
kisc
S1
6
4
T1
kf
1
2
kic
k'isc
kp
5
3
kTR
7
So
Figure 1.1 Schematic Jablonski diagram showing the various deactivation processes. kf,
kic, kisc, kp, and k'isc are the unimolecular rate constants for fluorescence, internal
conversion, S1  T1 intersystem crossing, phosphorescence, and T1  S0 intersystem
crossing, respectively.
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Chapter 1
Law cannot be obeyed if the incident light is polychromatic over a range in which  is not
constant.
The excitation process results in conversion of the molecule to any of the several
excited electronic states. Photophysical processes may be defined as transitions
interconverting the excited states with each other or with the ground state. The important
processes are elucidated as follows:
1) The first act of any photochemical and photophysical process is the absorption of a
photon by a molecule (A + h  A*). The excited configurations can be obtained
from the ground configuration by promoting one electron from occupied to vacant
molecular orbitals. The excited state that is formed in this way is a high energy,
unstable species which must undergo some types of deactivation.
2) Allowed emission which is termed as fluorescence (S1  So + h). The process
involves the dissipation of the energy of the excited state by radiative decay. The
lifetime of excited state is usually 10-7 to 10-9 second.
3) Forbidden, or triplet-singlet emission which involves a change in spin multiplicity,
called phosphorescence (T1  So + h). The lifetime of excited state is usually 10 -6
to 10-3 second.
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Chapter 1
4) A molecule may be promoted to any of the several vibrational levels during the
electronic excitation process. The excited molecule in its excited vibrational levels
will lose its energy as a consequence of collisions between the molecules of the
excited species and those of the solvent and drop back to the lowest vibrational level
via vibrational relaxation.
5) Allowed transition to a lower energy electronic states of the same spin without
emission of radiation, called internal conversion (e.g. S1  So + heat).
6) Intersystem crossing, a forbidden transition in which the spin of an excited electron is
reversed and a change in multiplicity of the molecule results. Intersystem crossing may
occur between excited states (e.g. S1  T1 + heat).
7) Intersystem crossing between triplet states and the ground states (e.g. T1  So + heat).
The excited state that is formed in this way is a high energy, unstable species
which must undergo some types of deactivation. Excited state deactivation can occur via:
i) Disappearance of the original molecule (photochemical reaction)
ii) Emission of light (luminescence)
iii) Degradation of the excess energy into heat (radiationless deactiviation)
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Chapter 1
iv) Some types of interaction with other species present in the solution (quenching
process)
Quenching process may occur if the lifetime of the excited state is long-lived
enough for the excited molecules to encounter a suitable species (quencher, Q). The
bimolecular quenching of an excited state molecule may take place by several
mechanisms. The most important bimolecular events are energy-transfer processes and
outer-sphere electron-transfer chemical reactions in which a redox process occurs. All of
the processes involve no bond cleavage and only energy or electron is transferred from
one reactant to the other.
M* + Q  M + Q*
energy transfer
M* + Q  M+ + Q¯ oxidative electron transfer
M* + Q  M¯ + Q+ reductive electron transfer
These processes quench the electronic excited state and compete with the
luminescence and other decay processes. The bimolecular quenching rate constants kq for
the photochemical processes are deduced according to the Stern-Volmer equation, given
by the following equation:
Io o

 1  k q  o [Q]
I

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Chapter 1
where Io and I are the luminescence intensity in the absence and presence of quencher
with concentration [Q], while  o and  are the luminescence lifetime in the absence and
presence of quencher.
Electronic States of Metal Complexes
Electronically excited states can undergo a wide variety of photochemical
reactions which are determined by the nature of the transition and the excited states
involved. The electronic transitions excited by the absorption of light are classified in
terms of the molecular orbitals between which the transition occurs, and often in terms of
whether there is some transfer of electronic charge.[4] Figure 1.2 depicts a simplified
orbital diagram of a typical octahedral metal complex. Typical electronic transitions are
as follows:
1) Metal-centred transitions
These are transitions between orbitals localized primarily on the metal centre.
Such interaction can be divided into ligand field (LF) transition (d-d and f-f transition
for d and f block elements, respectively), Rydberg transition (electronic transition
between orbitals with different principal quantum numbers) and intervalence charge
transfer transition (IVCT, only found in polynuclear bridged complexes and involve
transfer of electron from one metal atom to the other).
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Chapter 1
CTTS
np
MLCT
IL
p*
ns
LF
LMCT
10 Dq
(n-1)d
p
s
Metal
Orbitals
Molecular Orbitals
Ligand
Orbitals
Solvent
Orbital
Figure 1.2 Schematic energy level diagram of the molecular orbitals and electronic
transitions in an octahedral coordination compound.
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Chapter 1
2) Intraligand (IL) transitions
These are transitions between orbitals localized primarily on a coordinated ligand.
The coordination of the ligand to the metal usually causes a small perturbation of the
transition energy, though the lifetime and / or the quantum yield can be quite different
from that of the free ligand.[5] Besides, a mixed-ligand complex containing a reducing
ligand and an oxidizing ligand in the form Lred-M-Lox’ may display ligand-to-ligand
charge transfer (LLCT) transitions. If the ligand consists of both a reducing and an
oxidizing part, intraligand charge transfer (ILCT) transitions may result.
3) Metal-to-ligand charge transfer (MLCT) transitions
These are transitions in which electron density is transferred from a metal-centred
orbital to a predominantly ligand-based orbital. Such transitions are favoured by a
system composed of a reducing metal and a ligand containing a low energy acceptor
orbital. Octahedral low-spin d6 complexes with a reducing centre such as Re(I) and
Ru(II) and polypyridyl ligands such as 2,2’-bipyridine and 1,10-phenanthroline
represent the largest family of compounds with low-energy MLCT transitions.
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4) Ligand-to-metal charge transfer (LMCT) transitions
These are transitions in which electron density is essentially transferred from a
ligand-based orbital towards the vacant orbital of the metal centre (in the case of d10
metal complexes, s or p orbitals). Such transitions are favoured by oxidizing ligands
and easily reduced metal, as opposed to MLCT transition. LMCT for d6 complexes
usually occur at high energy region since the transition terminates at a s-antibonding
eg orbital. Examples of LMCT transition in d6 complexes include Fe(II) and Co(III),
which appear in UV region.[6] LMCT transitions are more commonly observed in d5
metal complexes.[7]
5) Charge-transfer-to-solvent (CTTS) transitions
These are transitions in which electron density is transferred from a metal-centred
orbital into the surrounding solvent. This transition is favoured by an easily oxidized
and electron rich metal centre, in the absence of a low energy ligand acceptor orbital,
and there are some low-lying unoccupied orbitals on the solvent molecule in the first
or second coordination sphere of the complex. Besides, CTTS are the most common
transitions in anionic complexes where coulombic repulsion favours charge separation.
Examples of CTTS transition include [CuC13]2–[8] and [Cu(CN)3]2–.[9]
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Chapter 1
1.3 Background of Luminescence Probe
Development of chemosensors has been the object of numerous investigations in
recent
years. Owing to
the sensitivity of luminescent
molecules to
their
microenvironment, information can be obtained on local physical and structural
parameters[10] (polarity, fluidity, order parameters, molecular mobility, distances at a
supramolecular level) as well as local chemical parameters[11] (pH, ion concentration).
Such a local information is seldom accessible by other techniques.
Ion recognition is a subject of considerable interest because of its implications in
many fields: chemistry, biology, medicine (clinical biochemistry), environment, etc. In
particular, selective detection of metal cations involved in biological processes (e.g.,
sodium, potassium, calcium, magnesium), in clinical diagnosis (e.g., lithium, potassium,
aluminum) or in pollution (e.g., lead, mercury, cadmium) has received much attention.
Among the various methods available for detection of ions, and more generally organic
and inorganic species, those based on luminescent sensors[12] offer distinct advantages in
terms of sensitivitiy, selectivity, response time and local observation (e.g., by
fluorescence imaging spectroscopy). Moreover, remote sensing is possible by using
optical fibers.[13-14] Recognition of ions requires special care in the design of luminescent
probes because attention should be paid to both recognition and signaling moieties. The
former is responsible for selectivity and efficiency of binding, which is relevant to the
field of supramolecular chemistry[15-18] and the latter converts the information into an
optical signal which should be as selective as possible of the species to be probed.
Therefore, selectivity must be viewed in terms of both selectivity of binding and
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Chapter 1
selectivity of photophysical effects. Furthermore, it should be emphasized that the
medium in which recognition takes place is of major importance: parameters such as the
nature of the solvent (polarity, hydrogen-bonding ability, protic or aprotic character), pH,
ionic strength, etc. play indeed a great role because they can affect not only the efficiency
and selectivity of binding, but also the photophysical characteristics of the fluorophore
(for instance, protonation may compete with cation binding). In many practical cases, and
of course for biological samples, aqueous solutions are mostly considered and water
soluble probes are desirable, but in some analytical applications (e.g., based on extraction)
the probe can be in an organic phase.
The photophysical changes of a luminescent probe on ion binding can involve
various photoinduced processes such as electron transfer, charge transfer with or without
concomitant internal rotation, energy transfer, excimer or exciplex formation or
disappearance. These changes on recognition should be of course as marked as possible.
Probes undergoing shifts of emission and/or excitation spectra (or appearance or
disappearance of bands) are preferable to those that undergo only changes in fluorescence
intensity. Indeed, after calibration, the ratio of the fluorescence intensities at two
appropriate emission or excitation wavelengths provides a measure of the ion or molecule
concentration which is independent of the probe concentration provided that the ion or
molecule is in excess.
Colorimetric determination of cations based on changes in color on complexation
by dye reagents started to be popular a long time ago, especially in the case of alkaline-
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Chapter 1
earth metal ions which are efficiently chelated by agents of the ethylenediaminetetraacetate (EDTA) type. Fluorimetric techniques being more sensitive than photometric
ones have been more widely used, and numerous fluorogenic chelating reagents were
studied and applied to practical cases.[19]. Among them, oxine (8-hydroxyquinoline) and
many of its derivatives occupy an important place in analytical chemistry and are still the
object of new applications but they are not very specific.[20] In contrast, luminescent
sensors of the EDTA type have been shown to exhibit high selectivity for calcium with
respect to the other ions present in living cells.[21-22]
The discovery of crown ethers and cryptands in the late sixties opened new
possibilities of cation recognition with improvement of selectivity, especially for alkali
metal ions for which there is a lack of selective chelators. Then, the idea of coupling
these ionophores to fluorophores, leading to the so-called fluoroionophores emerged
some years later.[23] In the design of a fluoroionophore, much attention is to be paid to the
characteristics of the ionophore moiety and to the expected changes in fluorescence
characteristics of the fluorophore moiety on binding. The complexing ability of the
ionophore will be considered first.
The stabilization constant of a complex between a given ionophore and an ion
depends on many factors, such as nature of the ion, nature of the solvent, temperature,
ionic strength, and pH in some cases. In ion recognition, complex selectivity (i.e. the
preferred complexation of a certain ion when other ions are present) is of major
importance. In this regard, the characteristics of the ionophore, i.e., the ligand topology
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Chapter 1
and the number and nature of the complexing heteroatoms or groups, should match the
characteristics of the cation, i.e., radius, charge, coordination number, intrinsic nature
(e.g., hardness of metal cations, nature and structure of organic cations, etc.) according to
the general principles of supramolecular chemistry.[15-18] The ionophore can be a chelator,
an open-chain structure (podand), a macrocycle (coronand, e.g., crown ether), or a
macrobicycle (cryptand). The relevant complexes are called chelates, podates, coronates,
and cryptates, respectively. The stability of chelates can be extremely different depending
on the structure of the chelator. Regarding the other ones, the stability of complexes with
alkali and alkaline earth metal ions increases in the following order: podates << coronates
<< cryptates. The high stability of the latter results from the three-dimensional
encapsulation, and the complexation selectivity is also usually higher because of their
small ability to be deformed. In the case of coronates and cryptates, the most stable
complexes are formed with ions having an ionic diameter close to that of the ligand
cavity. Another principle generally applicable in chemistry predicts that hard oxygen
centres combine with hard alkali metal ions, and soft sulfur or nitrogen centres with soft
transition metal ions. For aqueous solutions, an excellent review is devoted to the ligand
design for selective complexation of metal ions.[24] The connection between the
ionophore and the fluorophore is a very important aspect of probe design with in mind
the search for the strongest perturbation of the photophysical properties of the
fluorophore by the cation. The ionophore may be linked to the fluorophore via a spacer,
but in many cases some atoms or groups participating in the complexation belong to the
fluorophore. More than one ionophore and/or more than one fluorophore may be
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Chapter 1
involved in the structure. These types of probe, often called luminescent photoinduced
electron transfer (PET) sensors, have been extensively studied.[25-26]
It has long been discovered that the fluorescence of aromatic hydrocarbons is
quenched by aliphatic or aromatic amines because of photoinduced electron transfer from
the latter to the former. Various systems have been designed in which anthracene is
linked, via a spacer, to an ionophore containing a nitrogen atom. Figure 1.3 presents some
examples of fluoroionophores.[27-30] The anthracene fluorescence, which is efficiently
quenched by the lone pair of the nitrogen atom, is recovered on binding of a cation which
suppresses electron transfer.
1.4 Desirable Features of Inorganic Sensor Materials
In the last few years, the research in photochemistry has progressively moved
from molecular to supramolecular system. There is an increasing importance of using
luminescent transition metal complexes as sensor materials. These metal-based materials
have desirable features over organic molecular system. They can have long lifetimes
(hundreds of nanoseconds to tens of microseconds), which are much simpler to measure
than those of most organic fluorophores, which have short lifetimes in the range of
several to tens of nanoseconds. Their luminescence quantum yields are independent of
the excitation wavelength and can exceed 0.5. Most organic impurities absorb and emit at
a high-energy region. Therefore, the chromophore of a practical cation-probe system
should absorb at a lower energy region in the visible region, so that the interference of
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Chapter 1
a)
_
CO2
_
CO2
N
_
O 2C
_
O2C
N
O
N
N
O
CH3
N
N
b)
O
O
O
H
N
O
NH
N
O
N
c)
N
O
O
O
O
O
N
N
O
O
O
CH3
O
O
O
O
O
O
CH3
N
Figure 1.3 Examples of fluoroionophores based on cation control of photoinduced
electron transfer: (a) chelators and podands; (b) coronands; (c) cryptands.
17
Chapter 1
organic impurities can be minimized. This increases sensitivity, simplifies sensor design,
and expands the variety of excitation sources available. They also tend to be thermally,
chemically, and photochemically robust, which aids sterilization and extends sensor
lifetime. In view of these, an exploration into the use of luminescent transition metal
complexes as sensor materials may give rise to a better reporter for an ion recognition
system.
1.5 Aim of the Project
In this project, a number of receptor ligands have been coordinated to rhenium(I)
and ruthenium(II) metal centres. Rhenium(I) and ruthenium(II) polypyridine systems are
well known to exhibit intense MLCT absorptions and emissions. They have been most
extensively studied and most widely used in research laboratories. A unique combination
of chemical stability, redox properties, excited state reactivity, absorption and
luminescence characteristics (in particular, the environment-sensitive nature of MLCT
transitions), and excited state lifetime has attracted the attention of many research
workers on these complexes. The great interest generated by the studying of this class of
complexes has stimulated the growth of several branches of pure and applied chemistry.
In particular, the Ru(II) polypyridine complexes have played and are still playing a key
role
in
the
development
of
photochemistry,
photophysics,
photocatalysis,
electrochemistry, photoelectrochemistry, chemi- and electrochemiluminescence, electron
and energy transfer, and recently, analytical chemistry.
18
Chapter 1
Therefore it is the aim of the present project to explore the utilization of the rhenium(I)
and ruthenium(II) polypyridine systems through the appropriate design of the receptor
ligands, to serve as ion and molecular probes for a variety of substrates of interest. The
work collected in the thesis will focus on the design and synthesis of rhenium(I) and
ruthenium(II) complexes with various pendants, such as boronic acid which is well
known to have covalent interactions with diols and saccharides (Chapter 2), London
receptor and iminodiacetate chelates which could have ionic interactions with inorganic
metal cations (Chapter 3), dinitro-2,2’-bipyridine and histamine which could have
intermolecular interactions with iodo-compounds and dihydrogenphosphate, respectively
(Chapter 4), and their detailed receptor-substrate binding studies will also be described.
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