1 Introduction

Photochemistry is concerned with the absorption, excitation and emission

of photons by atoms, molecules and ions etc. The interaction of an electronically

excited molecule with a second species in its ground state often leads to new

interesting, photophysical and photochemical processes [1-3]. Each excited state

has a definite energy, lifetime and structure. The excited states are chemical

entities having strange properties than the ground state and behave differently.

Photosynthesis in plants provides the most obvious example of chemistry driven

by light that at the present stage of evolution forms a vital link between the

utilization of solar energy and the survival of life. The production of

carbohydrates that can be used as energy sources by living organisms is one part

of story and the liberation of oxygen, a major components of our atmosphere, is

another. The interaction of light with matter thus gives access to an enormously

rich extension of dark chemistry. The selective nature of photochemical activation

differentiates it from thermal activation.

Photochemistry also provides some fascinating insights into the way in

which chemical reactions occur. But the subject has an important just far beyond

the study of chemistry. Our atmosphere, which supports life and shields us from

damaging ultraviolet and other cosmic radiations, has its specific composition

determined by photochemistry. Photochemistry also finds many applications

including solar energy conversion, photocatalysis, photography and

photopolymerization (the production of photodegradable polymers) and in organic

synthesis and an exciting and developing area is the use of photochemistry to

provide novel medicinal treatments.

Introduction … 2

1.1 Relaxation mechanism of excited state molecules

Once a molecule has absorbed energy in the form of electromagnetic

radiation, there are a number of routes by which it can return to ground state.

The following diagram, termed Jablonski diagram [4], shows a few of these

processes.

Figure 1.1: Jablonski diagram and illustration of the relative positions of absorption,

fluorescence and phosphorescence. (S0; Ground state, S1; Singlet excited state and

T1; Triplet excited state).

The processes are:

(i) Radiative decay processes namely

(a) Fluorescence (hf)

(b) Phosphorescence (hp)

(ii) Non-radiative decay processes namely

(a) Internal conversion (IC)

(b) Intersystem crossing (ISC)

Introduction … 3

S0 + S1 Excitationh

S1

kICS0 + heat Internal conversion

S1

kISCT1 + heat Intersystem crossing

S1

kf S0 + h Fluorescence

T1

kp S0 + h Phosphorescence

T1

kRISCS0 + heat Reverse Intersystem crossing

Jablonski diagram (Figure 1.1) illustrates the electronic states of a

molecule and the transitions between them. The states are arranged vertically by

the energy order and grouped horizontally by spin multiplicity. Radiative

transitions are indicated by straight arrows and nonradiative transitions by wavy

arrows. The vibrational states of each electronic state are indicated with thick

lines, the higher vibrational states with thinner lines.

Luminescence is an emission of ultraviolet, visible or infrared photons

from an electronically excited species. The word luminescence, which comes

from the Latin (lumen = light) was first introduced as luminescenz by the physicist

and science historian Eilhardt Wiedemann in 1888, to describe “All those

phenomena of light which are not solely conditioned by the rise in temperature”,

as opposed to incandescence. Luminescence is cold light whereas incandescence

is hot light.

Luminescent compounds can be of different kinds:

Organic compounds: aromatic hydrocarbons (naphthalene, anthracene, pyrene,

perylene, phenanthrene etc.), fluorescein, rhodamines, coumarins, oxazines,

polyenes, diphenylpolyenes and amino acids (tryptophan, tyrosine, phenylalanine)

etc.

Inorganic compounds: uranyl ion (UO2+), lanthanide ions (e.g. Eu3

+, Tb3

+), doped

glasses (e.g. with Nd, Mn, Ce, Sn, Cu, Ag) and crystals (ZnS, CdS, ZnSe, CdSe,

GaS, GaP, Al2O3/Cr3+ (ruby)) etc.

Introduction … 4

Organometallic compounds: ruthenium complexes (e.g. [Ru(bpy)3]2+

), complexes

with lanthanide ions and complexes with fluorogenic chelating agents

(e.g. 8-hydroxyquinoline, also called oxine), etc.

Luminescence is formally divided into two categories i.e., fluorescence and

phosphorescence depending on the nature of the excited states. Fluorescence

(S1S0) is the emission of light from singlet excited states to ground state in

which the electron in the excited orbital has the different spin orientation as the

ground state electron. The emission rates of fluorescence are typically at 108 s

1,

so that a typical fluorescence lifetime is 10 ns. Phosphorescence (T1S0) is the

emission of light from triplet excited states to ground state in which the electron in

the excited orbital has the same spin orientation as the ground state electron.

Transitions from triplet state to the ground states are forbidden and the emission

rates are very slow (103 to 10 s

1), so that phosphorescence lifetimes are typically

in the range of milliseconds to seconds.

Three nonradiative deactivation processes are also significant here such as

internal conversion (IC, radiationless transition between energy states of the same

spin state), intersystem crossing (ISC, radiationless transition between different

spin states) and vibrational relaxation (Figure 1.1). Vibrational relaxation, the

most common of the three, occurs very quickly (<1 x 1012

s) and is enhanced by

physical contact of an excited molecule with other particles with which energy, in

the form of vibrations and rotations, can be transferred through collisions. This

means that most excited state molecules never emit any energy because in liquid

samples the solvent or, in gas phase samples, other gas phase molecules that are

present “steal” the energy before other deactivation processes can occur. The

possible de-excitation pathways of excited molecules are shown in Scheme 1.

Introduction … 5

Intersystem

crossingDelayed

fluorescencePhosphorescence

Fluorescence

emissionInternal

conversion

Intramolecular

charge transfer

Conformational

change

Electron

transfer

Proton

transfer

Energy

transfer

Excimer

formation

Exciplex

formation

Photochemical

transformation

h

EXCITED

MOLECULE

Scheme 1: Possible de-excitation pathways of excited molecules

1.2. Fluorescence quenching

Fluorescence quenching is a process which decreases the intensity of the

fluorescence emission. The quenching of fluorescent molecules has for many

years provided useful information on the nature of bimolecular interactions in

solution. There are large numbers of parameters influencing the emission of

fluorescence. Some of them are shown in Scheme 2. Among these parameters we

are concentrating more about the quenchers.

Introduction … 6

MOLECULAR

FLUORESCENCE

Polarity

pH

Hydrogen

bonds

Pressure

ViscosityTemperature

Quenchers

Electrical

potential

Ions

Scheme 2: Various parameters influencing the emission of fluorescence

1.2.1. SternVolmer equation

F0/F = 1 + KSV [Q] (1)

= 1 + kq.0 [Q] (2)

where,

F0 and F are the fluorescence intensity in the absence and presence of quencher

kq is the quenching rate constant

KSV is the Stern-Volmer constant

0 is the lifetime of fluorophore in the absence of quencher

[Q] is the concentration of quencher.

1.2.2. Types of quenching

1. Collisional or Dynamic quenching

2. Static quenching

3. Combined dynamic and static quenching

Introduction … 7

1.2.2.1. Collisional or Dynamic Quenching

In the case of collisional quenching, the quencher must diffuse with the

fluorophore during the lifetime of the excited state. Upon contact, the fluorophore

returns to the ground state without emission of photon.

S S*

S* + Q S + Q+_

Dynamic quenching is described by the Stern-Volmer equation,

F0/F = 1 + KSV [Q]

kq = KSV/0 (3)

Quenching data are usually presented as plots of F0/F versus [Q]. This is because

F0/F is expected to be linear depending upon the concentration of quencher. From

the slope of the Stern-Volmer plot, the bimolecular quenching rate constant can be

calculated by using equation (3).

[Q]

F0/F

0/

or

Higher temperature

Slope = kq 0 = KSV

Figure 1.2: Stern Volmer plot for dynamic quenching

It is important to recognize that a linear Stern-Volmer plot does not prove

that collisional quenching has occurred. Sometimes static quenching also results

in linear Stern-Volmer plots. Static and dynamic quenching can be distinguished

by their dependence on temperature, viscosity and preferably by lifetime

measurements. Higher temperatures result in faster diffusion and hence large

Introduction … 8

amount of collisional or dynamic quenching occur [5] (Figure 1.2). For pure

dynamic quenching F0/F = 0/.

1.2.2.2. Static quenching

In static quenching, a non-fluorescent ground state complex is formed

between the fluorophore and quencher. When this complex absorbs light it

immediately returns to the ground state without emission of photons.

S S*

S* + Q [S*.....Q]

[S*.....Q] S + Q+_

For static quenching the dependence of the fluorescence intensity upon

concentration of the quencher is easily derived by consideration of the association

constant for complex formation [6]. This constant is given by

[F...Q]KS =

[F] [Q] (4)

where,

[F…Q] is the concentration of the complex

[F] is the concentration of uncomplexed fluorophore

[Q] is the concentration of quencher

If the complexed species is non-fluorescent, then the fraction of fluorescence that

remains (I/I0) is given by the fraction of total fluorophores that are not complexed.

The total concentration of the fluorophore [F0] is given by

[F0] = [F] + [F…Q] (5)

by substituting the equation (5) in (4)

Introduction … 9

[F0] [F]KS =

[F] [Q]=

[F0]

[F] [Q]

_ 1

[Q]

_

(6)

On substituting the fluorophore concentration for fluorescence intensities

and rearranging the equation (6)

F0/F = 1 + KS [Q] (7)

KS is the association constant. The plot of I0/I versus [Q] is linear, which is

identical to that observed for dynamic quenching, except that the quenching rate

constant is now the association constant. The magnitude of KS can sometimes be

used to demonstrate that dynamic quenching cannot account for the decrease of

intensity. The measurement of fluorescence lifetime is the most definitive method

to distinguish static and dynamic quenching (Figure 1.3). For static quenching

F0/F 0/.

[Q]

F0/F

0/

Higher temperature

Slope = KS

Figure 1.3: Stern-Volmer plot for static quenching

1.2.2.3. Combined dynamic and static quenching

In many instances the fluorophore can be quenched both by collisions as

well as complex formation with the same quencher. The characteristic feature of

the SternVolmer plots in such circumstances is an upward curvature,

concave towards the y-axis (Figure 1.4). The modified SternVolmer equation for

this type of quenching is given as

Introduction … 10

F0/F = (1 + KD [Q]) (1 + KS [Q]) (8)

This modified form of Stern-Volmer equation is second order in [Q], which

accounts for the upward curvature observed when both static and dynamic

quenching occur for the same fluorophore [6]. The dynamic portion of the

observed quenching can be determined by lifetime measurements. Otherwise the

above equation can be modified by multiplying the terms in the parenthesis which

yields:

F0/F = 1 + (KD + KS) [Q] + KDKS[Q]2

F0/F = 1 + Kapp[Q]

Kapp =F0

F 1

1

[Q]= (KD + KS) + KDKS [Q]_

(9)

The apparent quenching constant is calculated at each quencher

concentration. A plot of Kapp versus [Q] yields a straight line with an intercept of

KD + KS and a slope of KDKS. The individual values of KD and KS can be

obtained by substituting in the quadratic equation.

Figure 1.4: Stern-Volmer plot for combined dynamic and static quenching

1.3. Steady state and time resolved fluorescence spectroscopy

Fluorescence measurements can be broadly classified into two types:

steady state and time resolved [6]. Steady state measurements, the most common

type, are those performed with constant illumination and observation. The sample

Introduction … 11

is illuminated with a continuous beam of light and emission intensity is recorded.

The second type of measurement is called time resolved, which is used for

measuring intensity decays or anisotropy decays. For these measurements the

sample is exposed to pulse of light, where the pulse width is typically shorter than

the decay time of the sample.

Time resolved measurements provide more information than the steady

state measurements. Static and dynamic quenching can be distinguished by time

resolved measurements, which is not possible in steady state measurements.

Formation of static ground state complexes do not decrease the decay time of the

uncomplexed fluorophores because only the unquenched fluorophores are

observed. Resonance energy transfer is also best studied using time resolved

measurements. One important application is cellular imaging using fluorescence

miscroscope. Fluorescence lifetime imaging microscopy (FLIM) has now become

an accessible and increasingly used tool in cell biology.

1.4. Introduction to semiconductor nanoparticles

Numerous small organic fluorophores have been characterized and are

commercially available. The majority of these probes have extinction coefficients

ranging from 10,000 to 100,000 M–1

cm–1

and decay times ranging from

1 to 0 ns. Some of these probes are photostable, but all the organic fluorophores

display some photobleaching, especially in fluorescence microscopy with high

illumination intensities. We now describe different types of luminophores that are

mostly inorganic or display unusually long lifetimes. These classes of probes are

semiconductor nanoparticles.

Starting in 1998 [7] there has been rapid development of fluorescent

semiconductors nanoparticles. The main component of these particles is usually

cadmium selenide (CdSe), but other semiconductors are also used. Particles with

Introduction … 12

diameters ranging from 3 to 6 nm can display intense fluorescence. The optical

properties of nanoparticles are similar to a quantum mechanical particle in a box.

Absorption of light results in creation of an electron-hole pair. Charge transfer

process at a reactive semiconductor surface is shown in Scheme 3. The energy of

the excited state decreases as the particle size increases [8]. The energy of the

excited state also depends on the material.

VB

CB

(a) (b)

A

A_

D

D+

S

S*

S_

Products

h Eg

VB

CB

=

e

h

Scheme 3: (a) by direct bandgap excitation of the semiconductor and (b) by charge

injection from excited state of the adsorbed molecule into the conduction band of the

semiconductor.

1.4.1. Desirable properties of colloidal semiconductors

Semiconductor particles of colloidal dimensions are sufficiently small to

yield transparent solutions. Transparent nature allows the easy detection of short

lived intermediates by fast kinetic spectroscopy [9]. In semiconductors only there

is a suitable positioning of valence and conduction bands and hence it is easy to

achieve high efficiencies in light energy conversion processes. There is a

possibility to modify the surface of semiconductor particles by chemisorption.

Semiconductors having feasible band gap energy (Eg 2.6-1.7 eV) absorb in the

visible region and find applications in the field of solar cells and photocatalysis

[10].

Introduction … 13

1.4.2. Semiconductor Quantum Dots (QDs)

Quantum dots are the very first extensively researched nanoparticle

systems discovered by Louis E. Brus [11]. Their optical, photophysical,

photochemical, biological and catalytic properties have opened up numerous

application possibilities. Several of these have been realized such as the dye

sensitized solar cells which utilize the electronic properties of these materials.

Semiconductor quantum dots signify a class of materials in which quantum

confinement effects are investigated in greater detail. They are also referred to as

“semiconductor nanocrystals”. ‘Quantum dots’ is a term referred only to

semiconductor particles, while ‘nanocrystals’ can be any inorganic entity in which

there is a crystalline arrangement of constituent atoms/ions [12]. The particles are

called as quantum dots as their electrons are confined to a point in space. They

have no freedom in any dimension and electrons are said to be localized at a point,

implying that a change in all directions changes the properties (in reality, a dot is a

three-dimensional object comprising several hundreds or thousands of atoms, with

a finite shape). QDs find applications in a larger number of areas. A summary of

the various applications is presented in Scheme 4.

Quantum

dots

Non-linear optical effects

Biological labels

Photocatalysis

BioconjugatesPhoto and electrochromic

devices

Drugs

Photovoltaics

Scheme 4: Diverse applications of quantum dots.

Introduction … 14

QDs are small (<10 nm) inorganic nanocrystals that possess unique

luminescent properties which can be tuned by varying the particle size or

composition. They are generally composed of atoms from groups IIB-VI or III-V

of the periodic table and are defined as particles with physical dimensions smaller

than the exciton Bohr radius [13]. Examples for QDs are CdSe (cadmium

selenide) and CdTe (cadmium telluride). The salient features of semiconductor

nanocrystals are as follows:

1. Size-tunable emission (from the UV to the IR) of quantum dots

2. Narrow spectral line widths

3. High luminescence

4. Continuous absorption profiles

5. Stability against photobleaching

6. Ideal immuno-lables for in vitro and in vivo fluorescent imaging.

1.4.3. Spectral properties of QDs

QDs display several favorable spectral features. They do not display the

long-wavelength tail common to all fluorophores. These tails interfere with the

use of multiple fluorophores for imaging or multi-analyte measurements.

The emission spectra of QDs are roughly symmetrical on the wavelength scale

and do not display such tails. For this reason QDs are being used for optical bar

codes for multiplexed assays [14-15]. An important spectral property of QDs is

their absorption at all wavelengths shorter than the onset of the absorption [16].

Many of the commonly used organic fluorophores display strong long-wavelength

absorption, but much less absorption at shorter wavelengths. In contrast, the QDs

absorb at these shorter wavelengths. This spectral property allows excitation of a

range of nanoparticle sizes using a single light source, which is needed for

practical multiplex assays. The wide absorption spectra also allow excitation with

a spectrally wide light source. The QDs also have large extinction coefficients (ε)

that on a molar basis can be up to tenfold larger than Rhodamine 6G [17-18].

Introduction … 15

Smaller QDs have ε values similar to that of Rhodamine 6G, near

200,000 M–1

cm–1

. Larger QDs can have ε values as large as 2 x 106 M

–1 cm

–1.

Finally QDs can be highly photostable, making them useful probes for

fluorescence microscopy.

Emission of QDs arises from the recombination of electron-hole pair [19].

When a photon of visible light hits such a QDs some of their electrons are excited

into higher energy states. When they return to their ground state, a photon of a

frequency characteristic of that material is emitted (Scheme 5). Two types of

emission such as (i) Excitonic emission, which occurs from the core of QDs

(excitonic radiative emission is near the same wavelength as the absorption) and

(ii) Defect emission, which is due to presence of surface traps (the surface traps

formed within the band gap can cause luminescence at significantly longer

wavelengths and act as non-radiative recombination centers that lower the

efficiency of photoluminescence) [20].

h+

eCB

VB

h

_

Excitonic radiative emission

Radiative emission at surface trap

Non-radiative recombination

Scheme 5: Emission nature of QDs

Introduction … 16

1.4.4. Capping agents

Cadmium based QDs are toxic, so it was not known if they would be useful

for cell labeling. They can be cytotoxic under some conditions [21-22] but it

seems that the core–shell particles are nontoxic. Semiconductor nanocrystals can

be capped with inert oxides or sulphides and can be attached to a biomolecule

with a specific function. Some of the drawbacks of bare core nanocrystals are

their crystal structure which lends itself to imperfections resulting in emission

irregularities, blinking etc. The cores are highly reactive due to their large surface

area to volume ratio, resulting in a very unstable structure which prone to

degradation [23].

Capping agents are needed to prevent the non-radiative recombination of

electron and hole at surface sites of QDs, to control growth kinetics and to prevent

aggregation via steric hindrance. Capping molecules cover the surface which

passivate the “Dangling bonds” (When an atom is missing a neighbour to which it

would be able to bind is called dangling bond. Such bonds are defects that disrupt

the flow of electrons and that are able to collect the electrons, it is a broken

covalent bond) [24]. Toxicity of QDs is due to release of Cd2+

with subsequent

generation of radicals [25,26]. So in order to prevent cells and tissues from

exposure to cadmium, QDs are stabilized with a shell [27].

The difficulty encountered with QDs is that their surface is hydrophobic in

nature which makes its interaction with water-friendly molecules extremely

difficult like proteins. Research is being carried out to modify the surface of QDs

so as to make them more biocompatible [28]. Some of the examples for the

capping agents are polymers, amines, thiols, trioctyl phosphine oxide, silica and

amino acids [29-37]. Among these capping agents thiol group has been widely

used as capping agent because it binds efficiently to the QDs surface and it gives

high quantum yield [38].

Introduction … 17

1.4.5. Introduction to Cadmium

Cadmium is a soft, malleable, bluish-white, bivalent metal and is used

mainly for batteries (predominantly for rechargeable nickel-cadmium batteries),

electroplating, pigments and stabilizers for plastics [39,40]. Because of its

toxicity, cadmium is an environmental hazard associated with the above industrial

processes. There are two avenues of cadmium poisoning: in-breathing

cadmium-containing fumes and ingesting food and water contaminated by

cadmium. Long-term exposure to cadmium initially results in metal fume fever

but may progress to chemical pneumonitis, pulmonary edema, renal abnormalities

and death [41,42]. Understanding the physicochemical and biological behaviours

of heavy metals such as cadmium can be furthered by researching their

mechanisms of toxicity at the molecular level [43]. Indeed, it has been reported

that CdTe quantum dots have much lower toxicity when they are purified by

removing the free Cd2+

ions [44].

1.4.6. Electronic states of quantum dots

In a bulk semiconductor an energy bandgap between its valence and

conduction bands is a fixed factor determined by the nature of the semiconductor

material. Sizing down the semiconductor crystal to less than Bohr radius,

a quantum confinement effect occurs, when electronic excited states respond to

the particle boundaries by adjusting their energy states (Scheme 6). Therefore, the

particles that exhibit such effect are termed QDs [45]. Decreasing or increasing

size of QDs leads to respectively increase or decrease in energy gap, this changes

their absorption spectra (appearance of colour) and correspondingly shifts their

emission wavelength to the blue or red spectral region [46-49].

Introduction … 18

e_

e_

e_

e_

e_

e_

e_

e_

h+ h+

Conduction band

Valence band

Quantum dot Bulk semiconductor

Band gapEg (QD) Eg

2sh

1dh

1ph

1sh

.....

1se

1pe

1de

2se

.....

Scheme 6: Simplified diagram illustrating energy levels of a quantum dot compared to

its semiconductor material in a bulk crystal. Electrons and holes in the quantum dot and

the semiconductor crystal obey Pauli's exclusion principle when filling the energy states

and their positions are shown schematically only.

1.4.7. Nanoparticles and QDs in solar cells

Increasing demand for energy soon will force us to seek environmentally

clean alternative energy resources [50-52]. Renewable energy such as solar

radiation is ideal to meet the projected demand but requires new initiatives to

harvest incident photons with higher efficiency, for example, by employing

nanostructured semiconductors and molecular assemblies. Semiconductors such

as CdS, PbS, Bi2S3, CdSe and InP [53-60] which absorb light in the visible, can

serve as sensitizers as they are able to transfer electrons to large band gap

semiconductors such as TiO2 or SnO2.

Introduction … 19

Scheme 7: (a) Linking CdSe QDs to TiO2 Particle with bifunctional surface modifier;

(b) Light Harvesting Assembly Composed of TiO2 Film Functionalized with CdSe QDs

on optically transparent electrode (Not to Scale).

Semiconductor quantum dots (QDs) such as CdSe with its tunable band

edge offer new opportunities for harvesting light energy in the visible region of

the solar spectrum [61,62]. Few recent studies reported the applications of QDs in

organic photovoltaic cells [63-65]. Chemically and electrochemically deposited

CdS and CdSe nanocrystallites are capable of injecting electrons into wider gap

materials such as TiO2, SnO2 and ZnO [66-70] generating photocurrents under

visible light irradiation. To explore the salient features of QDs, Kamat and

coworkers have assembled TiO2 and CdSe nanoparticles using bifunctional

surface modifiers of the type HS-R-COOH (Scheme 7). Photochemical processes

that follow the excitation of CdSe QDs, as probed by photoelectrochemical and

transient absorption measurements are presented [71].

1.4.8. Nanoparticles and QDs in the life sciences

In life sciences semiconductor nanomaterials are widely used in separation

technologies, histological studies, clinical diagnostic assays and drug delivery

systems (DDS) [72-75]. In the DDS applications, nanoparticles have several

Introduction … 20

merits, such as ease of purification and sterilization, drug targeting possibilities

and a sustained release action. In recent years, an increasing number of

researchers have considered the possibility of using nanoparticles in

photodynamic therapy (PDT) [76,77]. PDT offers the advantage of an effective

and selective method of destroying diseased tissues without damaging the

surrounding healthy tissues. It is based on the concept that photosensitizer (PS)

molecules can be preferentially localized in tumor tissues upon systemic

administration. Reactive oxygen species, such as singlet oxygen (1O2), or free

radicals are the main cytotoxic substances, which can irreversibly damage the

treated tissues [78]. General mechanism of action of PDT is shown in Scheme 8.

Nanoparticles can be ideal carriers of PS molecules in PDT.

PDT requires three elements: light, a photosensitizer and oxygen. When the

photosensitizer is exposed to specific wavelengths of light, it becomes activated

from a ground state to an excited state. As it returns to the ground state, it releases

energy, which is transferred to oxygen to generate reactive oxygen species (ROS),

such as singlet oxygen and free radicals. These ROS mediate cellular toxicity

[79].

Photosensitizer

(ground state)

Photosensitizer

(excited state)

Tissue

oxygen

Free radicals,

singlet oxygen

Cellular

toxicity

Light

Scheme 8: Mechanism of action of photodynamic therapy (PDT).

Introduction … 21

During the past few decades, various types of PS molecules have been

synthesized [80]. Many organic dyes, porphyrins and their derivatives, flavins

and other biomolecules are used as efficient sensitizers. However, there are many

problems for the clinical application of existing photosensitzers: (1) Most PS

molecules are hydrophobic and can aggregate easily in aqueous media where the

PS aggregation will result in the decrease of its quantum yield () [81]. Moreover,

the aggregated PS cannot be simply injected intravenously. (2) Selective

accumulation of the PS molecules in diseased tissues is required to avoid

collateral damage to healthy cells. Although third generation PS have been

prepared for selective targeting, their selectivity is not high enough for clinical

application. Because of these problems, the construction of an effective PS carrier

becomes critical and remains a major challenge in PDT. It is therefore not

surprising that there is increasing interest in using nanoparticles as PS carriers.

Nanomaterials are promising in that (1) they could be made hydrophilic

(2) possess enormous surface areas, and their surface can be modified with

functional groups possessing a diverse array of chemical or biochemical

properties (3) owing to their sub-cellular and sub-micron size, nanoparticles can

penetrate deep into tissues through fine capillaries, cross the fenestration present

in the epithelial lining (e.g. as in the liver), and are generally taken up efficiently

by cells (4) since numerous strategies for the preparation of nanomaterials are

already in place [82], PS loaded nanoparticles can be desirously made by

numerous different methods, e.g. chemical covalent grafting or self-assembly.

QDs are generating strong research interest in biology due to their

fluorescence property seen when they are excited by a laser. Their fluorescence

intensity is also significantly higher and are more stable compared to conventional

fluorescent markers. QDs have fairly broad excitation spectra which can be tuned

by varying the physical size and composition. Additionally, QDs have narrow

emission spectrum which means that it is possible to resolve the emissions of

Introduction … 22

different nanoparticles simultaneously with minimal overlap. Finally QDs are

highly resistant to degradation [83].

Overall, an ideal PDT photosensitizer should possess the following

characteristics: it should 1) have a constant composition, 2) be simple to

synthesize and easily available, 3) be non-toxic in the absence of light, 4) exhibit

target specificity, 5) possess the energy of its excited state higher than 94 kJ/mol

(0.97 eV, energy of singlet oxygen), 6) be quickly cleared from the body,

7) possess minimal self-aggregation, and 8) be photostable (no photobleaching).

Quantum dots satisfy the first five criteria and are therefore promising new

generation photosensitizers [84].

QDs can also be used as the photosensitizer in photodynamic therapy

(PDT). A PDT agent works by collecting near diseased tissue and releasing highly

reactive singlet oxygen when stimulated by light. The singlet oxygen reacts with

the tissue and kills it. QDs may be attached to PDT molecules, for imaging

purposes, but may also act as the PDT agent, releasing singlet oxygen. Thus, the

quantum dots may be used for both imaging and PDT, with some flexibility in the

design of the spectral response [85].

1.5. Introduction to Porphyrins

The simplest parent ligand is called porphine. Various porphyrin systems

(Figure 1.5) are considered as being formally derived from this porphine

framework. Porphyrins with one hydrogenated pyrrole unit are called chlorins

PH2. Those with two opposite hydrogenated rings are called bacteriochlorins PH4.

Phlorins are isomeric derivatives of chlorin with hydrogen atom added to a

methane bridge of a porphyrin.

Introduction … 23

N

NH N

HN

Porphine

N

NH N

HN N

NH N

HN

H

H

H

H

H

H

N

NH N

HN

H

H

Chlorin Bacteriochlorin Phlorin

Figure 1.5: Structure of various porphyrin systems.

Porphyrins are highly coloured molecules with strong absorption in the

visible region due to the cyclic conjugated tetrapyrrole chromphore. The methane

bridges are also called meso positions and hence porphyrins with substituents at

methylene bridging positions are known as meso (or) -substituted porphyrins.

The presence of electron donating substituents in the -position increases the

basicity of porphyrin. The basicity of pyrrole nitrogens is further increased by

negatively charged subsitituents, such as carboxylate or sulfonato groups, either

on beta pyrrole or meso positions [86].

The near UV and Visible absorption bands of porphyrins are ascribed to

-* electronic transitions, although n-* and -* transitions due to the pyridine

and pyrrole type electron lone pairs on the central nitrogen atoms may also occur.

The electronic spectra of porphyrins consist of several bands.

B bands: An extremely intense band at about 380420 nm (sometimes called the

Soret band). It is the origin of the second excited singlet state and has a molar

extinction coefficient in the range of (24) x 105 M

1cm

1.

Introduction … 24

Q bands: The lowest energy band is the electronic origin of the lowest excited

singlet state. The Q band has a molar extinction coefficient in the narrow range

(12) x 104 M

1cm

1.

N,L,M bands: Several other less intense bands labeled N,L and M occur at shorter

wavelengths (N about 325 nm; M about 215 nm and L anywhere in between the

M and N bands). The shape and positions of the Soret band are affected by the

formation of porphyrin aggregates. The aggregation process occurs extensively

for porphyrins in aqueous solutions, as a consequence of the strong hydrophobic

and -* interactions between the aromatic moieties. Porphyrins and

metalloporphyrins represent vital components of light absorbing pigments in

cascaded energy and electron transfer pathways involved in photosynthesis and

photodynamic therapy [87,88].

1.6. Photoredox Reactions of porphyrin molecules

Light induced electron transfer reactions constitute the major

photoreactions of porphyrin molecules. Some of the key concepts as related to

porphyrin photochemistry will be discussed here. Scheme 9 represents different

stages involved in photoinduced electron transfer from excited porphyrin

molecules in solution, formation of an encounter complex, electron transfer within

the complex and competitive separation of the redox products along with

recombination of a geminate pair.

P + Q

P* + Q (P*.....Q) (P +.....Q )

P + + Q

h

k -d

kd

k -et

ket

kb

_

_

ksk-s

Scheme 9: General scheme for photoredox reactions

Introduction … 25

With reference to general scheme for photoredox quenching, the course of the

reaction can be pictured as a continuum of states:

P* + Q (P*Q) (PQ)* (P+Q

)cip P

+sQ

s

A B C D

The first two refer to excited state complexes where the excitation energy

is localized on the chromophore (A) or delocalized on the chromophore pair (B).

These two are also referred to as exciplexes. Forster, who originally detected

excited state complexes of type B, gave a general definition to the term “exciplex”

refer to “Any kind of well defined complex which exists in the electronically

excited states”. In exciplexes, there is no ground state interaction between P and Q

(the absorption spectrum is merely the sum of the spectra of the separated

components), and they often have a broad, structureless, red-shifted emission.

C and D refer to the product states that can exist after the electron transfer event.

The former (C), still in the geminate cage, is referred to as the contact ion pair.

D is the solvent separated ion pair and these are the final stages where products

freely diffuse away. D is also referred to as the radical ion pair state.

Photoinduced electron transfer rate constants inferred that the fluorescence

intensity and/or lifetime measurements reflect the first two steps (A,B), while the

kinetics of formation of the radical ion pair (CD) is measured by fast kinetic

(picosecond) transient absorption spectroscopy.

Depending on the nature of the donor-acceptor pair (which determines the

energetics of the reaction) and the environment (solvent, polarity, represented by

the dielectric constant or related parameters), the quenching process can proceed

all the way to the right, or the system can return to the ground state from any of

the intermediate states A, B or C. In general, non-polar solvents such as benzene

favour the existence of the exciplexes. Solvation plays an important role in

controlling the stability of the intermediates and products. Increasing solvent

Introduction … 26

polarity leads to increasing red shift of the exciplex emission but to decreasing

intensity. In strongly polar solvents, exciplexes are not observed, but separated

redox products can be detected, often with the high quantum yield.

Typical examples conforming to the above behaviour are

pyrene-N,N-dimethylaniline and pyrene-p-dicyanobenzene. Behavior along the

lines indicated above has also been observed during the excited state quenching of

weak electron donor-acceptor (EDA) complexes. In cases where the charge

transfer interaction between the donor and acceptor is strong, distinct absorption

and fluorescence (charge transfer fluorescence) can be observed. In the case of

intramolecularly linked pyrene-DMA systems, fluorescence in non-polar solvents

is emitted from the locally excited state (excited state localized in the pyrene part)

while charge transfer fluorescence is observed in polar solvents.

Porphyrins are photoactive compounds with high extinction coefficients in

the visible region and are of great importance in the fields of catalytic chemistry,

biochemistry and photochemistry, which are unique among these dyes. Several

groups have studied physical and chemical processes of the porphyrins on

colloidal surfaces [89].

1.7. Introduction to Xanthene dyes

The photophysics and photochemistry of dyes in general are of

considerable interest in the appreciations of various phenomena in pico to

micro-second range, viz, fluorescence, phosphorescence, long and short range

excitation energy transfer, electron transfer and various modes of quenching. Dyes

in general have planar hydrophobic centres with extended systems of single and

double bonds and hydrophilic groups in the peripheral regions [90-96].

Xanthene dyes are derived from xanthene nucleus (Figure 1.6). This class

of dyes is divided into three subgroups: Fluorenes, fluorones and rhodols.

Introduction … 27

Fluorenes and fluorones containing dyes of importance in histotechnology but the

rhodols do not have the same. The fluorenes are further subdivided into five

groups. Among these the pyronins and rhodamines include dyes we use. The

others do not [97].

O

Figure 1.6: Structure of Xanthene

The xanthene dyes are probably the most intensely studied class of

luminescent dyes, interest has been spurred both by the special spectral

characteristics of the dyes and by their wide range of applications as biological

stains, sensitizers, tracing agents, photochromic and thermochromic agents and

laser dyes. Several members of this class have been recommended as luminescent

standards, while others have found to use as fluorescent probe indicators of

microscopic environments, especially in enzyme and membrane studies. For many

of these applications and especially for their roles as fluorescent probes, it is

important that the factors determining their excited state behaviors be well

understood [98].

Currently, the investigation of efficient molecular photosensitizers for the

development of nanocrystalline semiconductors based dye sensitized photovoltaic

(DSPV) and dyes sensitized solar cells (DSSC, Scheme 10) is a very active area

of research due to their high performance and low-cost of production [99-109].

Introduction … 28

Scheme 10: (a) Schematic diagram of DSSC and (b) molecular structure of Rose bengal.

1.8. Introduction to Albumins

The biochemical applications of fluorescence often utilize intrinsic protein

fluorescence. Among biopolymers, proteins are unique in displaying useful

intrinsic fluorescence. Proteins contain three amino acid residues that contribute to

their fluorescence which are usually described by their three letter abbreviations.

These are tyrosine (tyr), tryptophan (trp) and phenylalanine (phe). The structure of

these amino acids is shown in Figure 1.7. Emission of proteins is dominated by

tryptophan which is generally present at about 1 mole % in proteins absorbing at

the longest wavelength and displays the largest extinction coefficient. Energy

absorbed by phenylalanine and tyrosine is often transferred to the tryptophan

residues in the same protein. Phenylalanine displays the shortest absorption and

emission wavelengths. Phenylalanine displays a structured emission with a

maximum near 282 nm. The emission of tyrosine in water occurs at 303 nm and is

relatively insensitive to solvent polarity. The emission maximum of tryptophan in

Introduction … 29

water occurs near 350 nm and is highly dependent upon polarity and local

environment [110].

NH2

NH

O

OH

Tryptophan

NH2

OHO

HO

Tyrosine

H2N

O

OH

Phenylalanine

Figure 1.7: Structure of amino acid residues.

1.9. Scope of the Research Work

Studies on photochemical reactions are important from the view point of

their applications in synthesis, biology and solar energy research work.

Semiconductor quantum dots and nanoparticles (eg: CdS, CdSe and CdTe)

mediated photoreactions find importance in the above aspects. Particularly for

their applications in solar energy conversion as well as biological imaging

quantum dots assume much importance.

The combination of nanoparticles and biological molecules has attracted

tremendous attention in biotechnological applications such as luminescence

tagging, immunoassay, drug delivery and cellular imaging. Based on the above,

investigations on photoinduced reactions of certain porphyrins and xanthene dyes

with colloidal CdS nanoparticles are of much significance in the mechanistic as

well as application point of view. Due to certain advantages of porphyrins, serum

albumins and lysozyme i.e., anchoring groups, commercial availability, unusual

Introduction … 30

binding properties and their water solubility, it is relatively worthwhile and

convenient to investigate such photoinduced reactions of these molecules with

CdSe and CdTe QDs. To our knowledge there are no reports available on the

photoinduced reactions of CdS with porphyrins, albumins, CdTe with porphyrins

and albumins in the literature. Hence the present investigations were undertaken

with the following objectives.

1.10. Objectives

(i) To prepare colloidal CdS, CdSe and CdTe nanoparticles and quantum dots and

characterize them using SEM, TEM and XRD analysis.

(ii) To investigate the fluorescence quenching of certain porphyrins by colloidal

CdS nanoparticles.

(iii) To check the possibility of the ground state complex formation using

absorption measurements.

(iv) To calculate the apparent association constant and analyze the mechanism of

quenching process by applying Rehm-Weller equation.

(v) To study the interaction between xanthene dyes and colloidal CdS

nanoparticles using spectroscopic techniques.

(vi) To investigate the interaction of water soluble CdTe QDs with certain

porphyrins.

(vii) To investigate the effect of subsitituent on the porphyrins for electron

transfer or energy transfer from QDs to porphyrins and prove by transient

absorption measurements.

(viii) To study the interaction of bovine and human serum albumins and lysozyme

with QDs by using absorption, steady state, time resolved and synchronous

fluorescence measurements.

(ix) To calculate the binding constant and binding sites for albumins-CdX

systems.

(x) To calculate the energy transfer parameters such as distance between donor

and acceptor, critical energy transfer distance.

Introduction … 31

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