Chapter 1

Chapter 1 Introduction

1.1 Background of Photochemistry

Photochemical processes are of the greatest importance to life on Earth. The well-

known 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

1

Chapter 1

electronically excited species are the central feature of photochemistry. For every atom

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 ground-

state 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.

2

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

3

Chapter 1

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.

4

S1

So

T1

kSR

kTR

kf kic kisckp

kisc

1 2735

4

6 4

'

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.

5

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)

6

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:

II

1 k [Q]o oq o

7

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).

8

Chapter 1

Figure 1.2 Schematic energy level diagram of the molecular orbitals and electronic

transitions in an octahedral coordination compound.

9

MetalOrbitals

SolventOrbital

*

Molecular Orbitals

LF LMCT

MLCT

CTTS

IL

10 Dq

np

ns

(n-1)d

LigandOrbitals

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.

10

Chapter 1

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 -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

12

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-

13

Chapter 1

earth metal ions which are efficiently chelated by agents of the ethylenediamine-

tetraacetate (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

14

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 involved in the structure.

15

Chapter 1

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

16

Chapter 1

a)

b)

c)

Figure 1.3 Examples of fluoroionophores based on cation control of photoinduced

electron transfer: (a) chelators and podands; (b) coronands; (c) cryptands.

17

N

N

N

N

N

O

O O

O

OH

N

N

NH

N N

CH3

OO

O O

O O

O

N

N

O

O O

O

O

OO

CH3

O O

CH3

N N

CO2

CO2 O2C

O2C_

_

_

_

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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