design, synthesis and photophysics of fluorescence turn-on conjugated polymer chemosensors

The conjugated polymers were designed as fluorescence “turn-on” chemosensors based on a photoinduced electron transfer PET mechanism in which the polymer fluorescence is quenched in the

DESIGN, SYNTHESIS AND PHOTOPHYSICS OF FLUORESCENCE

“TURN-ON” CONJUGATED POLYMER CHEMOSENSORS

BY

LI-JUAN FAN

B.S Nanjing University, 1994 M.S Fudan University, 1997

2006

UMI Number: 3214759

3214759 2006

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© Copyright by Li-Juan Fan 2006 All Rights Reserved

Accepted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry

in the Graduate School of Binghamton University State University of New York

2006

Wayne E Jones, Jr. _ April 27, 2006 Advisor, Chemistry Department

Susan L Bane _ April 27, 2006

Chair, Chemistry Department

Alistair J Lees _ April 27, 2006 Chemistry Department

Mark D Poliks _ April 27, 2006 Chemistry Department

Susannah Gal April 27, 2006 Outside Examiner, Department of Biological Sciences

ABSTRACT

This dissertation explores the synthesis, characterization, and application of conjugated polymers as fluorescence “turn-on” chemosensors A series of conjugated polymers using the poly[p-(phenyleneethynelene)-alt-(thienylene-ethylene)](PPETE) polymer backbone were prepared using N,N-diethylamino (dea) and N,N,N’-trimethylethylenediamino (tmeda) groups as receptors The conjugated polymers were designed as fluorescence “turn-on” chemosensors based on a photoinduced electron transfer (PET) mechanism in which the polymer fluorescence is quenched in the absence

of coordinating analytes A chelation-enhanced fluorescence (CHEF) phenomenon results upon coordination of a cation to the redox active receptor as a result of termination of the fluorophore quenching process The polymers were fully characterizated by NMR, FTIR, Gel Permeation Chromatography (GPC) and elemental analysis

Detailed photophysical studies of dea-PPETE and tmeda-PPETE demonstrated relatively weak emission at λmax= 488 nm with quantum yields of 0.11 and 0.09 Room

temperature emission studies show that tmeda-PPETE exhibited a fluorescence “turn-on” response in the presence of many cations at less than 500 nM concentrations. For example, Hg2+ in aqueous solution causes the fluorescence of tmeda-PPETE to increase

by a factor of 2.7 at less than millimolar concentrations This represents the first example

of a conjugated polymer applied as a fluorescence “turn-on” chemosensor based on the PET mechanism

The competitive role of PET and energy migration is critical to sensor function This was investigated by synthesizing a series of PPETE’s with different amino receptor

loadings Theoretical and experimental studies revealed that the limited sensitivity achieved in this system may be attributed to relatively slow energy migration (<109s-1) along the polymer backbone relative to the emissive lifetime (∼10-10

To my family

ACKNOWLEGMENTS

My gratitude begins with my advisor, Dr Wayne E Jones, Jr., who gave me the opportunity to explore in this interesting field freely I appreciate his guidance, encouragement and patience with me through the past several years His unique way of mentoring student has helped me to come to know how science truly works and like science more than ever before I have gained more confidence and achieved more independence in doing research under his supervision I also learned a great deal from him in the way of thinking, solving problems and communicating with others in research and teaching His insight into science and friendly attitude towards students also made the entire journey very fruitable and enjoyable I also thank him for his support and understanding in other aspects of life besides research

I would also like to extend my thanks to my committee: Dr Susan Bane, Dr Alistair Lees, Dr Mark Poliks and Dr Susannah Gal for taking the time to read my dissertation, useful discussion and all other help throughout my graduate study in Binghamton University Dr Scott Handy should also be thanked for serving as the committee member for my preliminary oral exam

I appreciate Dr Brendan Flynn and Professor Emeritus, Dr Stanley K Madan for devoting their precious time in reading and correcting this dissertation and all my manuscripts before submitting to different journals I benefited a lot for my writing from

Dr Flynn’s sharp and picky eyes as a teacher in reading my writing Dr Mandan’s discussion about coordination chemistry and other aspects of life seems also became a part of my everyday life in the past several years

I also thank Dr Mark Poliks and Dr Barbara Poliks to be my mentors and also friends during the past several years I got to know Dr Mark Poliks in his polymer and NMR class I was impressed by his excellent teaching skill, broad knowledge and great responsibility towards students I also thank him for the instruction in the NMR project related to my research Dr Barbara Poliks is such a warm-hearted lady and I really enjoyed the discussion with her about many things I feel very lucky they are always there for me whenever I needed help and the help was always given very timely

I would like to give my special thanks to my dear friends, Dr Yan Zhang and Dr Zhengtao Zhu They brought me here and through them I got to know where Binghamton University was and also the Jones group I am grateful that they assisted me to settle down, allowed me to be their “dependent” for the first year I was here and for continuous support even after they left

I have so many friends and colleagues in the Chemistry Department that have made this journey enjoyable First, I wish to thank the whole Jones group, including previous and current members Dr Biwang Jiang, who started this conjugated polymer project, came to my poster at ACS meeting in New York City and then became my friend

I was surprised that he still had many good suggestions about my research after so many years away from our group Dr Szu-wei (Steve) Yang, who graduated in the first year I joined the group, still was around and willing to provide help whenever I asked for Dr Yan Zhang helped me to start the synthesis and Dr Cliff Murphy helped me to start the photophysics studies Dr Dave Sarno helped me to get familiar with the lab during the first summer research Dr Cliff Timpson helped me with my E-Chem experiments during his stay here for the sabbatical semester Special thanks owe to Justin Martin, a really

smart guy, who was my computer specialist and consultant for the past several years and also brought a lot of laughter to our lab Thanks also should be given to Dr Hong Dong

to be my Chinese-Speaking partner in the lab I’d like to thank all other members in our group, namely, Dr Ed Fey, Fredrick Ochanda, Jasper Chiguma, Wenlong Gui, Sarah Angell, Matt Parker, Kat Minerly and many undergraduate students in our lab Thank you all for helping me with the research and all other fun we had in the lab besides the research

Some other people outside our group must be thanked for helping with my research I would like thank Dr Yanan Zhang for acting as my organic synthesis consultant; Dr Jürgen Schulte for helping me with the NMR; Dr Tatini Radhakrishnafor helping me with the GPC; Dr Nikolay Dimitrov for discussion about E-Chem; Dr Robert Ben and Dr Scott Handy for discussion about some synthesis when they were still in the department; Dr David Doetschman and Dr Steve Yang for the EPR experiment and related discussion Dr Tom Troxler at the University of Pennsylvania must also be thanked for running all the lifetime experiments for us

Many people in the chemistry department must be thanked for their support besides research, such as Richard Quest, Dr Bob Kematic, Dr Alexa Silva, Mary Bridge , Renee Sersen, Pat Gorman, Linda Schaffer, Bob Gonzales, Dat Tran, Daniel Brennan Elizabeth Brown in the Science Library should also be thanked for her assist in literature searching

I also want to thank all the Chinese people in the Chemistry Department besides I already mentioned above, Namely, Chunmei Ban, Chen Chen, Quan Fan, Dr Xiaojuan Fan, Li Han, Yan Lin, Shuhuai Liu, Dr Jin Luo, Dr Yanning Song, Jie Xiao, Linyan

Wang, Dr C J Zhong, et al I also like to thank many other Chinese people outside our

department or even outside the university, such as Lin Tan, Yuanyuan Song, Jie Zhou and Yuening Li couple, Carol Tang, Xueping and George Lee couple, and so on We enjoyed Chinese food together, celebrated our traditional Chinese holidays or non-Chinese holidays and shared many other things together Their companionship has made

me less homesick and my staying here more enjoyable

I dedicate this dissertation to my father He was so happy when I started this journey However, he was unable to wait until I finished it I believe if there really is a heaven, he must be looking down there, smiling at me as he used to I am very proud of

my mother She is still being so strong though experienced so many problems during the past several years I also thank her for taking care of my daughter All the people in my family and family-in-law must be appreciated for taking care of my daughter when I was not around her

The last but the most important people I must thank are my husband, Zhigang (Louis)

Lu and my daughter Minyuan (Betty) Lu I feel so grateful to my husband for his love, support and understanding for so many years I also thank my daughter She is so cute and brought me so much fun, whether I was with her or not with her Though I was physically away from them most of time for the past four and half years, we kept in touch with each other every day and shared details of our everyday life We feel like we were together all the time.I really thank them for giving me a home, both in reality and in spirit

Finally, this work was funded by the National Institute of Health (Grant No 1R15-ES10601-01) and Research Foundation of the State University of New York at Binghamton

TABLE OF CONTENTS LIST OF TABLES

LIST OF FIGURES

LIST OF SCHEMES

LIST OF SYMBOLS AND ABBREVIATIONS

Chapter 1 Introduction to Fluorescent Chemosensors 1

1.2.1 Photoinduced Electron Transfer 4

1.3 Conjugated Polymer as Fluorescence Chemosensor 8 1.4 Fluorescence “Turn-off” Sensor for Transitional Metals 13

1.5.1 Small Molecular “Turn-on” Sensors 15 1.5.2 Examples Polymer “Turn-on” Sensors 20

Chapter 2 Synthesis and Characterization of

Poly(p-phenyleneethylene)-alt-(thienyleneethylene) with Amino Pendant Groups

32

2.3.1 Synthesis and Characterization 40

Chapter 3 Photoinduced Electron Transfer and Energy Migration in

PPETEs with Varying Receptor Loading

64

3.3.1 Synthesis and Characterization 70

Chapter 4 A Highly Selective and Sensitive Inorganic/Organic Hybrid

polymer Fluorescence as “Turn-on” Chemosensor for Iron

4.3.1 Exceptional Copper (II) Quenching of

tmeda-PPETE

102 4.3.2 Fluorescence Enhancement with Iron (II) 1084.3.3 High Selectivity toward Iron Cations 1114.3.4 Mechanism for the Selectivity 1184.3.5 Direct Observation of Binding by EPR 121

5.3.2 Sensor for Carbonyl Compounds 140

Table 2-3 Stepwise associate constants between some cations and ligands 58

Table 3-1 Theoretical and Measured Percentages of Nitrogen in

(x%-tmeda)-PPETE

74

Table 3-2 Molecular Weight and Polydispersity of (x%- tmeda)-PPETE 74

Table 3-3 Photophysical properties of (x%-tmeda)-PPETEs in THF solution

at room temperature

76

Table 3-4 Fluorescence quenching in (x%-tmeda)-PPETEs 84

Table 3-5 Maximum fluorescence enhancement factor for

(x%-tmeda)-PPETEs upon titration of different cations

92

Table 4-1 Quantum yields PPETE, model PPETE, and

tmeda-PPETE with Cu2+

104

Table 4-2 Electronic configurations of related cations 111

Table 4-3 Calculated pH values of cations at two difference concentrations 119

Table 4-4 Hydrolysis equilibrium constants and solubility parameter

constants for cations

120

LIST OF FIGURES

Figure 1-2 Schematic illustrate for fluorescent chemosensor 4

Figure 1-3 Orbital energy diagrams for fluorescent “turn-off” PET sensors

before and after binding analyte

5

Figure1- 4 Orbital energy diagrams for fluorescence “turn-on” PET sensors

before and after binding analyte

6

Figure 1-5 Orbital energy diagrams for double exchange transfer between

the excited fluorophore to the analyte bound by receptor

7

Figure 1-6 Plot of log(kET) versus distance, r, for both Dexter and Föster

energy transfer mechanism, absent any criteria other than distance

8

Figure 1-8 Basic structures of several fluorescent conjugated polymers 10

Figure 1-9 Structures of several conjugated polymers as fluorescent sensors 11

Figure 1-10 Schematic representation of molecular wire approach to sensory

signal amplification

12

Figure 1-11 Structure of PPETE polymers with oligo-pyridine pedants 14

Figure 1-12 Examples of small molecules as fluorescence “turn-on” sensor

with amino receptors

17

Figure 1-13 An example of small molecule sensor shows fluorescence

“turn-on” upon anions

18

Figure 1-14 Reaction of aldehyde and ketone with

N-amino-N-(1-hexylheptyl)perylene-3,4:9,10-tetracarboxylbisimide

18

Figure 1-15 Examples of sensors with thiourea group and modified thiourea

group

19

Figure 1-16 Glucose PET sensors with boronic acid as the receptor 20

Figure 1-17 Structures of polymer as fluorescence “turn-on” sensors 21

Figure 1-18 Structure of a conjugated polymer showed fluorescence

“turn-on” upon trysin

22

Figure 1-19 Structure of the two polymer components for fabricating a

sensor sandwich

22

Figure 1-20 Structure of a conjugated polymer as fluorescence “turn-on”

chemosensor for anions

23

Figure 2-1 Orbital energy diagram for fluorescence “turn-on” PET sensors

with amino receptor before and after binding cation

33

Figure 2-2 Structures of PPETE polymers with pendant amino receptor 34

Figure 2-3 NMR spectra of 1,4-diethyl-2,5-didodecyloxybenzene

,2,5-dibromo-thiophen-3-ylmethyldiethylamine and thiophen-3-ylmethyl)-N,N,N'-trimethylethane-1,2-diamine

N-(2,5-dibromo-47

Figure 2-5 FTIR of tmeda-PPETE and its monomers 50

Figure 2-6 NMR spectra of tmeda-PPETE and dea-PPETE 51

Figure 2-7 Comparison of routine 13C NMR and 13C DEPT of dea-PPETE 52

Figure 2-8 2D Heteronuclear (C,H)-Correlated NMR of dea-PPETE 52

Figure 2-9 UV-Vis and emission spectra of model PPETE, dea-PPETE and

tmeda-PPETE in THF solution at room temperature

55

Figure 2-10 Fluorescence enhancement of tmeda-PPETE in THF upon

addition of metal cations

56

Figure 2-11 Fluorescence enhancement of dea-PPETE in THF upon addition

Figure 3-4 Absorption and emission spectra of (x%-tmeda)-PPETEs in

Figure 3-6 Representation of process leading to energy-migration enhanced

quenching with a PET mechanism along the conjugated polymer backbone

81

Figure 3-7 Quantum yield ratios between the model PPETE and

(x%-tmeda)-PPETE vs percent loading of amino group

83

Figure 3-8 Schematic illustration of response with binding cations for

monoreceptor loaded and polyreceptor loaded conjugated polymers

86

Figure 3-9 Fluorescence enhancement of (x%-PPETE) in THF upon

addition of metal cations

90

Figure 3-10 Enlarged parts of I/I0 vs cation concentration curves (Figure 3-8)

at low cation concentrations

91

Figure 4-2 Fluorescence quenching of tmeda-PPETE upon titration of Cu2+

aqueous solution

103

Figure 4-3 UV-Vis spectra of tmeda-PPETE upon titration of Cu2+ aqueous 103

solution

Figure 4-4 Fluorescence response following excitation at 408 nm from

tmeda-PPETE/Cu2+ solutions upon addition of Fe2+ aqueous solution

104

Figure 4-5 The UV-Vis spectra of titration of FeCl2 aqueous solution into

tmeda-PPETE/ Cu2+ hybrid system

105

Figure 4-6 Fluorescence response following excitation at 408 nm from

tmeda-PPETE solutions upon addition of Fe2+ aqueous solution

107

Figure 4-7 Suggested mechanism (schematic diagram) of fluorescence

enhancement of tmeda-PPETE/Cu2+ upon titration of Fe2+

109

Figure 4-8 Emission spectra of tmeda-PPETE; tmeda-PPETE/Cu2+and

tmeda-PPETE/Cu2+ system titrated by Ca2+, Zn2+, Ni2+ and Mn2+

109

Figure 4-9 Emission spectra of tmeda-PPETE; tmeda-PPETE/Cu2+(1:1) and

tmeda-PPETE/Cu2+ system titrated by Fe3+, H+, Hg2+ and Co2+

110

Figure 4-10 Fluorescence intensity enhancements upon various cations in the

tmeda-PPETE/ Cu2+ sensory system

Figure 4-14 Fluorescence response of tmeda-PPETE/Cu2+ (green) or

tmeda-PPETE (red) to various 10 µM cations in room temperature solution

Figure 4-17 Dexter (double-electron exchange) energy transfer between an

excited fluorophore and Cu2+

119

Figure 4-18 Electron transfer between an excited fluorophore and Cu2+ 123

Figure 4-19 Room temperature and low temperature emission spectra for

model-PPETE, tmeda-PPETE and tmeda-PPETE/Cu2+

124

Figure 5-1 Schematic diagram of the –(A-(B-X))n- component

chemosensory polymer system

130

Figure 5-2 Structure of compounds discussed in Chapter 5 131

Figure 5-3 NMR spectra of amino receptor loaded phenyl building blocks 138

Figure 5-4 Targeted polymer (a-PPETE) for carbonyl group sensor and the

Figure 5-8 Orbital energy diagram for PET sensors for anions 149

Figure 5-9 Schematic diagram of anion sensors with thiourea group 150

Figure 5-10 1H NMR of 2, 5-dibromo-3-(phenyl-thioureidomethyl)

thiophene in CDCl3 and DMSO

151

LIST OF SCHEMES

Scheme 2-2 Polymerization for PAE polymers under Sonoghshira coupling

proctol

43

Scheme 2-3 Synthesis of 1,4-diethylnyl-2,5-didodecyloxybenzene 44

Scheme 2-5 Synthesis of conjugated polymers containing amino receptors 48

Scheme 3-1 The Synthetic Route of (x%-tmeda)-PPETEs with Different

Receptor Loading

70

Scheme 5-1 Synthesis of amino receptor loaded phenyl building blocks 137

Scheme 5-2 Unsuccessful synthesis of dea-PPE and tmeda-PPE 139

Scheme 5-3 Synthesis of 2, 5-dibromo-thiophene-3-ylmethylamine 142

Scheme 5-7 Synthesis of thiourea group loaded thienyl building block 150

LIST OF SYMBOLS AND ABBREVIATIONS

EET Electronic Energy Transfer

EPA Environmental Protection Agency

EPR Electron paramagnetic resonance

F fluorophore

FTIR Fourier Transform Infra-Red

GPC Gel Permeation Chromatography

HETCOR Heteronuclear Correlation NMR spectroscopy HOMO Highest occupied molecular orbital

I Intensity

I0 Initial intensity

kf rate constant of fluorescence

knr rate constant of non-radiative decay

kPET rate constant of photoinduced electron transfer

KSV Stern-Volmer constant(static)

LUMO Lowest unoccupied molecular orbital

mm millimeters

Mn Number-averaged molecular weight

Mw Weight-averaged molecular weight

PMMA polymethylmethacrylate

PPETE Poly(phenylene ethynylene thienylene ethynylene)

ttp tolyterpyridine

UV Ultra-violet

Chapter 1 Introduction to Fluorescent Chemosensors

1.1 Introduction to Sensors

A sensor is defined by the Oxford English Dictionary as “a device that detects or measures a physical property and records, indicates or otherwise responds to it” A sensor achieves this goal by responding to an external stimulus and converting it into a signal which can be measured or recorded.1-3 The stimulus could be physical quantities (such as

length, weight, temperature, pressure etc.), chemical analytes or reactions, or biological

components in the real world.1-3 Sensors are everywhere in our life We have at least five sensors of our own: nose, tongue, ear, eye and skin Smoke alarms, thermometers, barometers, radar, pH paper or pH meter are also very common sensors

Generally, a sensor contains three elements: a receptor element, a signal transducer and a read-out mechanism (see Figure 1-1) The recognition element is known as the receptor which is the key component of the sensor device that interacts directly with the analyte The receptor should have the ability to discriminate the stimulus of interest, avoiding interferences from the environment The transducer converts the energy from

the stimulus interaction with the receptor to another form, suitable for readout The out domain is the part responsible for reporting the recognition event after processing the signal from the transducer

Transducer Receptor

Figure 1-1 Schematic illustration of a sensor

Sensors can be classified in several different ways based on the stimulus or the mechanism of the transduction.1-3 There are three main types of sensors based on the stimulus: physical sensors, chemical sensors and biosensors A chemical sensor is a device that qualitatively or quantitatively detects the presence of specific chemical substances, a class of chemicals or a specific chemical reaction Each type can have many sub-classes Chemical sensors have been developed for cations, anions, neutral molecules, acids, vapors, volatile organics and many more Ideally, a chemosensor is a chemical sensor based on one molecule A wide variety of transduction mechanisms exist including electrochemical (such as potentiometric, voltammetric), optical (such as fluorescent or chromophoric), thermal and so on

Signal

Signal processor Read-out

Recognition

The ideal performance of a sensor device for a real application is determined by the following factors:1 selectivity, sensitivity, detection limit, reversibility, response and recovery time and lifetime Different applications may have different requirements for these factors

Selectivity is the ability to discriminate between different targeted species It is the most important characteristic for most sensor applications Interference with similar molecules is a common problem It is very difficult to find a non-biological sensor that will respond to only one analyte In some cases, it is sufficient to respond to a class of similar analytes Usually, the receptor is responsible for achieving the selectivity

Sensitivity is defined as the ratio between the change in signal and the change in analyte concentration.1 This determines the ability of the sensor device to discriminate accurately and precisely between small difference in a single molecule concentration The detection limit is the lowest concentration of analyte the sensor can detect The requirement for it changes with the application It could be in the range of the millimolar

to nanomolar and always poses a challenge for the sensor design

Most sensor applications require continuous monitoring, so reversibility of the sensor process becomes important The response time and recovery time ideally will be as short

as possible for convenient use In addition, the lifetime of a sensor device is usually determined by the stability of the material and long lifetime is also required for most sensor devices Like any other analytical instrument, the precision, accuracy and reproducibility of the experiment results are also expected from the sensor device.1

1.2 Fluorescent Chemosensors

Fluorescence is the radiative relaxation of excited electrons from the excited state to the ground state by emission of photons.5 Chemosensors based on fluorescence signal changes are commonly referred to as fluorescent chemosensors.4 Fluorescent chemosensors are usually made up of three components: a receptor, a fluorophore and a spacer to link them together (Figure 1-2.) These three components are not exactly the same as the three components shown in Figure 1-1 The read-out of a fluorescent sensor can be either a change in the fluorescence intensity, a shift in the emission wavelength, or

a change in the fluorescence lifetime In this dissertation, we will focus our discussion on those sensors designed to respond with a change in fluorescence emission intensity

Fluorophore Spacer Receptor

Figure 1-2 Schematic illustrate for fluorescent chemosensor

Fluorescent chemosensors are gaining increased attention due to their high sensitivity and ease of measurement.6-10 There are several mechanisms of fluorescence sensing.6-8 Among them, photoinduced electron transfer (PET) and electronic energy transfer (EET) have been extensively studied and widely used in the design of small molecule sensors with fluorescence intensity changes as the signal read-out.6-8

1.2.1 Photoinduced Electron Transfer

PET based fluorescent sensors can be classified into two categories: fluorescence

“turn-off” or fluorescence “turn-on” In the cases of “turn-off” sensors, the receptors take

part in the photophysical process either directly or indirectly For indirect quenching the only role of the receptor is to recognize the analytes and hold the analytes close to the fluorophore When the energy level of the receptor/analyte pair LUMO is between the energy levels of the fluorophore HOMO and LUMO, a non-radiative path, such as PET quenching (Figure 1-3), leads to dissipation of the excitation energy and a quenching of the fluorescence of the chemosensor

Figure 1-3 Orbital energy diagrams for fluorescent “turn-off” PET sensors before and

after binding analyte: (a) fluorescence emission; (b) forward electron transfer; (c)

backward electron transfer processes

For fluorescence “turn-on” sensors, the receptors usually contain a relatively high energy non-bonding electron pair In the absence of analytes, this electron pair quenches the emission by rapid intramolecular electron transfer from the receptor to the excited fluorophore, as shown in Figure 1-4 When this electron pair coordinates to electron deficient analytes in solution, the energy of the HOMO of the receptor is lowered This decreases the driving force for the PET process and can turn on the fluorescence of the chromophore

Strongly fluorescent Weakly fluorescent

HOMO

a

b

E HOMO

c hv

LUMO LUMO

E

HOMO

HOMO

Excited fluorophore with

Weakly fluorescent Strongly fluorescent

Figure 1-4 Orbital energy diagrams for fluorescence “turn-on” PET sensors before and

after binding analyte: (a) forward electron transfer; (b) backward electron transfer; (c) fluorescence emission processes

There is a fundamental difference between the “turn-on” and “turn-off” mechanisms involving PET process takes place either before or after the analyte binding

In the first case the analyte stops the PET process and in the later case PET is created by the binding event In both cases, the relative electronic potential of the HOMO and LUMO are critical in the design of a selective system

1.2.2 Electronic Energy Transfer

Electronic energy transfer (EET) is another mechanism that can be involved in fluorescence quenching There are two kinds of EET mechanisms: the through bond double electron exchange (Dexter) energy transfer or the dipole-dipole coupling (Förster) energy transfer.5,11 In organic fluorophore systems, usually the Dexter energy transfer dominates as shown in Figure 1-5 In this case, the fluorophore returns to the ground state through a nonradiative decay Dexter energy transfer requires close contact between the donor and the acceptor This would involve direct orbital overlap This type of

fluorescence quenching requires matching in energy levels between the donor and acceptor orbitals

Figure 1-5 Orbital energy diagrams for double exchange transfer between the excited

fluorophore to the analyte bound by receptor followed by analyte return to the ground state by nonradiative decay

The Förster energy transfer mechanism involves the long range coupling of dipoles, allowing for an exchange of excitation energy through space, i.e without a path

of direct orbital overlap. 5,11 In this case, overlap of the emission and absorption spectrum

is the key factor and there is no dependence on conjugation These two mechanisms of energy transfer are differentiated primarily in their dependence on the distance between the donor and acceptor sites The rate of Dexter energy transfer5,11 is proportional to exp(-

R DA ) and the rate of Förster energy transfer5,11 is proportional to 1/ , as shown

qualitatively in Figure 1-6 Here R

Excited sta te

fluorophore

Ground state analyte

Ground state fluorophore

Excited state analyte

LUMO

Figure 1-6 Plot of log(kET) versus distance, r, for both Dexter (solid line) and Föster

(dotted line) energy transfer mechanism, absent any criteria other than distance kET

donates the rate of energy transfer This figure was based on the equations in references 5 and 11

1.3 Conjugated Polymers as Fluorescence Chemosensors

Small molecules have been extensively studied as fluorescence sensors in the past several decades.6-10 The most commonly used fluorophores are aromatic compounds, especially anthracene.12 Conjugated polymer chemosensors have been used with great success in the past decade as the fluorophore for detection of a range of analytes from biomolecules to explosives in the past decade as the fluorophore.13-18 Swager and co-workers13 first demonstrated that conjugated polymers, or molecular wires, have several advantages over small molecules for sensing applications The enhancements were associated with electronic communication between the receptors along the polymer backbone.13 The polymer structure also provide for facile structural modification and good processability

Wires convey signals or energy in one direction The concept of a molecular wire seeks

to do the same job except on a molecular scale The concept has been discussed for about

prepared.19,20 In conjugated polymers, each carbon atom along the backbone has sp or sp 2 hybridization The remaining p orbitals, with one unpaired π electron each, form an

overlapping series of π bonds (Figure 1-7) Since the orbitals of successive carbon atoms along the backbone overlap, the π electrons are delocalized along the entire polymer backbone This electronic delocalization provides a pathway for electron or hole mobility along the backbone of individual polymer chains

Figure 1-7 π-conjugation in conjugated polymers

Recently, there has been renewed interest in developing conjugated polymer molecular wires for their applications in organic light emitting diodes,21-25 photovoltaics,26-29 actuators,30,31 batteries,32,33 and field-effect transistors.34,36 This wide range of applications is the result of the unique metallic and semiconducting properties inherent to many of these conjugated polymer systems An interesting recent extension in the use of polymer materials has been their application to fluorescent sensors.13-18 Numerous studies have been made on the properties and applications of conjugated polymers such as

poly(p-phenylenevinylene) (PPV),39-41 poly(p-phenyleneethylene) (PPE)21-25 and polythiophene (PT)42,43 (Figure 1-8) Several groups have investigated the structure- property relationships of poly [p-(phenyleneethynylene)-alt-(thienyleneethynylene)

(PPETE).44,45 The PPETEs have been found to be highly luminescent and also very easy

to process as a material They deviate slightly from the PPE rigid-rod structure due to the

inclusion of the five member thiophene ring, but still have a relatively high degree of π electron delocalization

Figure 1-8 Basic structures of several fluorescent conjugated polymers

Conjugated polymer sensors have been classified based on various transduction principles including conductometry, potentiometry, chemical field-effect transistor (CHEMFET) and fluorescence as discussed in section 1.2.1.12,13,46,47 Conjugated polymers can act as any one of the three elements in a chemosensory system: receptor, transducer or signal read-out Sometimes a conjugated polymer can act as a combination

of two or all three elements with careful molecular design

The combination of the sensitivity of fluorescence and unique electronic properties of conjugated polymers provides new opportunities for sensory system development One type of conjugated polymer fluorescent sensor is based on the conformational change of conjugated backbone driven by interaction with the analyte For instance, the fluorescence of poly[3-oligo(oxyethylene)-4-methythiophene] (Figure 1-9a) changes in the presence of alkali metals In this case, both the intensity of fluorescence and the emission wavelength changed as a function of the concentration of alkali metal cations.48

S S

C

O C

H3

n O m

a

N

n OR

O

R

n R

R=CON(C8H17)2

c

Figure 1-9 Structures of several conjugated polymers as fluorescent sensors (a)

poly[3-oligo(oxyethylene)-4-methythiophene] from reference 48; phenylenevinylene) from Wang and Wasielewski’s (reference 49); (c) para poly(phenyleneethynylene) with cyclophane as the receptor from Zhou and Swager’s paper (reference 15)

(b)poly(bipyridyl-Another early type of conjugated polymer sensor involved the introduction of molecular recognition units directly into the conjugation of the polymer backbone An elegant example of this was the polymer synthesized by Wang and Wasielewski.49(Figure 1-9b) A bipyridyl group with a dihedral angle of 20° was introduced into the polymer backbone It was forced into a planar configuration after chelation to certain transition metals such as Mn2+, Zn2+ and Pd2+ The result was an increased conjugation length The conformational change was monitored by changes in both the UV-Vis and fluorescence spectroscopy

Polymer synthetic strategies offer many more options for sensor application Another excellent approach is to assemble the receptor pendants onto the conjugated polymer

backbone.13,14 This concept was first advanced and demonstrated by Swager’s group.13,14

They assembled cyclophane-based receptors onto a para-poly(phenyleneethynylene)

backbone (Figure 1-9c) and this polymer showed a 65-fold fluorescence enhancement in sensitivity to paraquat, compared to a model small molecule fluorescent chemosensor with one receptor This signal amplification creates a conjugated polymer chemosensor that is significantly more sensitive to analytes than its single molecule counterparts

In this case the receptors are connected by the conjugated polymer-‘molecular wire’

as illustrated in Figure 1-10.13,14 Analyte binding produces energy trap sites for the fluorescent excitons Due to facile energy migration, in which excitons travel along the conjugated polymer backbone, the emission intensity of the conjugated polymers decreases dramatically Enhanced sensitivity for polymer based fluorescent sensors relative to small molecule sensors was achieved due to the fact that any one bound receptor would turn off the whole polymer

Figure 1-10 Schematic representation of molecular wire approach to sensory signal

amplification This figure was based on the “molecular wire” concept advanced by

Swager et al in reference 13 and reference 14

A nalyte Analyte

1.4 Fluorescence “Turn-off” Sensors for Transition Metals

The toxicity of certain metal ions has been a persistent cause of environmental concern Thirteen transition metal ions are listed as “priority pollutants” by the Environmental Protection Agency (EPA).50 These metals include chromium, manganese, cobalt, copper, zinc, molybdenum, silver, mercury, cadmium, lead and nickel, in different oxidation states There is an increasing need for the development of portable polymer based sensor devices for these toxic metal cations, although a number of laboratory-based instruments are available, such as atomic emission or atomic absorption

Lewis bases such as oligopyridyl ligands are known to coordinate a large number

of transition metal ions.51 There are only a limited number of reports involving incorporating oligopyridyl ligands into conjugated polymers as chemosensors.49, 52 Jones group has successfully designed and synthesized a series of new chemosensory polymers,

poly[p-(phenyleneethynylene)-alt-(thienyleneethynylene)] (PPETE) with different

oligopyridine pendant groups as receptors for transition metals (Figure 1-11).53 This polymer contains two primary absorbance peaks at 338 nm and 354 nm; emission peak at

508 nm These systems take advantage of the strong conjugation and luminescent properties of the polyarylene ethynylene backbone When combined with the multi-dentate Lewis base coordinating ability of oligopyridines, the system yields a highly

effective transition metal chemosensor Among all the polymers,

tolylterpyridine-poly[p-(phenyleneethynylene)-alt-(thienyleneethynylene) (ttp-PPETE) showed the highest sensitivity of fluorescence quenching towards cations.53 It was observed that 5% of the emission of this polymer in THF solution was quenched by Ni2+ at nanomolar concentration

Figure 1-11 Structure of PPETE polymers with oligo-pyridine pedants

To quantify the emission quenching relative to the concentration of the cations, a Stern-Volmer analysis was carried out in the ttp-PPETE system Based on this analysis, Jones group has also advanced a new mathematical model to explain the enhanced quenching of our polymer system by cations.54 On the basis of this model, it was demonstrated that different energy-transfer mechanisms exist as a function of the chemical identity of the analytes For instance, the fluorescence quenching of ttp-PPETE

in the presence of Ni2+ was best modeled through a Dexter type energy transfer mechanism while the fluorescence quenching of ttp-PPETE in the presence of Co2+, however, was best modeled through a Förster energy transfer mechanism Through careful tuning of the receptor interactions with both the polymer backbone and the analyte, it may therefore be possible to tune the selectivity of the system This model helped to achieve a better understanding of energy-transfer processes in conjugated polymer sensors and also the design of new, more sensitive, chemosensory materials

Exploration was also carried out on the different percent loading of the receptor tolylterpyridine (ttp) onto the PPETE polymer backbone.55, 56

1.5 Fluorescence “Turn-on” Sensors

Most literature6-10 reports use fluorescence quenching as the readout mechanism for the sensor response as already mentioned in the previous sections Few involve a fluorescence “turn-on” response, especially when targeting cation analytes.12,57,58 The greatest advantage of fluorescence “turn-on” sensors, compared to related to “turn-off” sensors, is the ease of measuring low concentration contrast against a “dark” background This reduces the likelihood of false positive signals and increases the overall sensitivity

as demonstrated by some previous studies. 57,58

1.5.1 Small Molecules as Fluorescence “Turn-on” Sensors

Photoinduced electron transfer (PET) sensors are an important family of chemosensors,6-10,12 as discussed in section 1.2.1 A number of small molecule fluorescence turn-on sensors have been developed based on the PET mechanism Several groups, including the group of Czarnik,41 de Silva,6,9 and Fabbrizzi59,60 have established the fundamental mode of action for this class of sensors The phenomenon (Figure 1-4)

of fluorescence “turn-on” by coordination between the cations and the receptor is called chelation-enhancement fluorescence (CHEF).12 In most cases anthracene or other aromatic compounds are used as the fluorophores There are many literatures in the using amino groups as the receptors However, some other receptors such thiourea group

or boronic acid were also developed

The de Silva group has several examples of the PET fluorescence “turn-on” sensors

for cations and several representative sensors are shown in Figure 1-12(a-c).6,9,61-63Sensor a contains a dialkyl amino group connected to the anthracene fluorophore This

was used as a proton signaling system and was the first PET fluorescence “turn-on” sensor synthesized in de Silva’s group.61 A rational outgrowth from this system was

sensor b In this case an azacrown ether was used in this case as the receptor for the

purpose of sensing alkali cations It was found that chelation of K+ by the receptor induced fluorescence enhancement by a factor of forty-seven in methanol.62 Two PET active receptors linked to one fluorophore was also investigated by the de Silva group (sensor c in Figure 1-12).63 This compound showed a higher sensitivity than the monoamino receptor since the receptor concentration was doubled

Fundamental research on metal cation sensing based on the PET mechanism in

Czarnik’s group started with sensor d in Figure 1-12.64 A strong fluorescence enhancement of 1000 fold was achieved for this compound in CH3CN, when saturated with ZnCl2 It was found that the fluorescence of the anthracence was quenched via photoinduced electron transfer (PET) from nitrogen in the receptor to the fluorophore The fluorescence was revived by the addition of Zn2+ The phenomenon was referred to chelation-enhanced fluorescence (CHEF) due to the termination of the PET process between the amino receptor and the fluorophore with binding the cation