chapter 3 probingmolecularaggregation in bulky...

Chapter3 Probing Aggregation Properties

Chapter 3

Probing Molecular Aggregation in Bulky PPVs,OPVs and Their Blends

NllST 93

Chapter3 Probing Aggyegation Properties

3.1. Introduction

The photophysics of conjugated polymers especially PPVs, made them

promising candidates in the optoelectronic industry soon after the discovery of

light emitting phenomena. I-6 Although there are a lot of salutary characteristics

for PPVs like color-tuning ability/-13 solubility, good processabilityl4-20 etc.,

one of the cursed problems over many versatile features of them is its lower

luminescence quantum efficiency in the solid state. This is mainly due to the

very high tendency of conjugated polymer chains towards aromatic 'It-'It

interactions, which lead to the formation of weakly emissive aggregated species

predominantly in the solid state (in films).21-30 However, in some cases these

molecular aggregates enhances the cross linking and luminescent properties,

which found applications in coating and opto-electronic industry.31,32 Recently,

Bhongale et al. have reported the enhanced emission from the fluorescent

organic nanoparticles of 1,4-di-[(E)-2-phenyl-I-propenyl]benzene (PPB) which

were prepared by the re-precipitation method.31 The fluorescent intensity was

found to be enhanced as the size of the nanoparticles increases. This

enhancement in the fluorescent emission was explained due to the nearly planar

conformation of PPB and the formation of herringbone type aggregates. Very

recently, another group also shown gel formation in the well known MEH-PPV

by the addition of large amount of alkane solvents.32They describe the gelation

phenomena as a result of the interchain cross linking via the interaction

between neighboring alkoxy chains with the alkane solvents (see scheme 3.1).

Eventhough the molecular aggregates show many promising properties as

mentioned above, most of them are lower energy non-emissive species in

which the excitation energy of the highly luminescent excimers get trapped

during the radiative emission processes. Recent studies have shown that the

intra- and inter-chain interactions playa major role in the physical origin of

molecular aggregation phenomena?3-25 In most cases, it acts as luminescent

quenching sites, therefore, control of 'It-stack induced molecular aggregation in

the polymer chain is an important task in the development of highly emissive

NIIST 94


conjugated polymers and also making the ideal polymer luminescent

optoelectronic devices in reality.

IclllOW .. lolly III<M-cllaln MEH4'PV ....... k~.~ .... _..,/ -c.-.

'~;. .

J"__ .It.olly ald...._

(Small amount<30%)

Adding nonaneI

(e)

Addong nonaneI

(l<llge( amount>40%)

InDCB

Scheme 3.1. Schematic representation ofthe physical cross linking between

MEH-PPV and alkane (nonane) solution (adoptedfrom ref 32).

Several methods have been adopted to reduce the interchain interactions,

which involves the incorporation of cis-vinylene linkages in the polymer

backbone33-36

, polymer blends 37-4°and introduction of bulky anchoring groups

41-45 etc. (explained in chapter 1 and chapter 2 in detail). The control of

molecular aggregation through inter-chain separation via bulky anchoring

groups were found to be very attractive since one can easily control the

molecular aggregation through space without disturbing the electronic/device

properties of the parent polymer. Reports of PPVs based on the bulky

anchoring groups showed additional advantages that they also enhances the

glass transition temperature (Tg) of the PPV backbone which is an important

parameter for long operating lifetime of devices.46 However, it is surprising to

notice that most of the reports on bulky PPVs have focused on the structural

parameters and so far not much effort have been paid to trace the aggregation

behavior of these emerging bulky conjugated materials. This is partially due to

NIIST 95


the poor solubility of the efficient bulky-PPYs such as adamantane and

cholesteryl rings47-50 in common organic solvents which hampered the

application of spectroscopic techniques to study the aggregation properties in

solution. Therefore, it is very important to carry out systematic spectroscopic

studies to probe the origin of the molecular aggregates in bulky conducting

polymers, which will provide valuable information for the future development

of new highly luminescent materials.

The formation of molecular aggregates can be understood by the choice

of good/poor solvents, varying the solution concentration or temperature,

thermal annealing process etc.51-54By varying the external stimuli, the

photophysical properties of the polymer chains found to alter according to the

extended and coiled conformation the polymer chains adopt. 55 This will

usually result either in a red shifted peak or an additional peak in the absorption

spectra. It was widely noticed that chains in the well-stacked regions exhibits

red shifted spectra with low luminescent quantum yield while the chains in the

poorly packed region show bluer spectra with high quantum yield.22 As already

discussed in the second chapter, the introduction of TCD-units into the PPY

backbone reduces the molecular aggregation and enhances the solution and

solid state photoluminescence characteristics. This chapter mainly deals with

the investigation of the molecular aggregation properties of the soluble TCD

substituted bulky-PPYs in combination with the well-studied poly(2-methoxy

5-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPY). A random copolymer of

MEH-PPY with 60 % of symmetrically TCD substituted polymer was chosen

for the above purpose due to its high solubility, good molecular weight and

similar photophysical properties as that of its partially soluble bulky-TCD-PPY

homo-polymer (BTCD-PPY). Structurally identical two oligo

phenylenevinylenes (OPYs) bearing either (methoxy-ethylhexyloxy) (MEH) or

TeD units were also utilized as model compounds for the aggregation studies.

Absorption and photoluminescence (PL) spectroscopic techniques were

employed as tools to trace the aggregation properties of these materials in

NIIST 96


solvents such as toluene, tetrahydrofuran (THF), THF+methanol or THF+water

combinations as well as in the solid state. Further, the effect of bulky TCD

anchoring units on the luminescence properties of PPV backbone were also

investigated by making MEH-PPV and OPV binary blends. The composition of

the MEH-PPV and OPVs were varied from 0 to 100 %. Through selective PL

excitation, both the effect of oligomer-to-polymer energy transfer and

luminescent enhancement in MEH-PPV via inter-chain separation were

investigated. Time resolved fluorescence decay measurements were also

carried out to study the luminescent decay life time of bulky polymers,

oligomers and their binary blends.

3.2. Experimental Methods

3.2.1. Materials: 1,8-Tricyclodecanemethanol (TCD) was donated by Celanese

Chemicals & Co, Fluorescence standards Rhodamine-6G and Quinine sulfate

were purchased from Aldrich Chemicals and used without further purification.

A relatively low molecular weight of poly(2-methoxy-5-(2-ethylhexyloxy))

1,4-phenylenevinylene (MEH-PPV) (Mn = 20,300 and Mw= 70,100) was

prepared for the present study and the chemical structures of the polymers and

DPVs selected for the studies is given in scheme 3.2. Synthesis and structural

details were provided in chapter 2. The symmetrically TCD-substituted

copolymer of MEH-PPV, poly[(2-methoxy-5-(2-ethylhexyloxy))-I,4

phenylenevinylene-co-(2,5-bis( I ,8-tricyclodecanemethyleneoxy)-1 ,4

phenylenevinylene] (BTCD-60) (with 60 % TCD content, Mn = 9,300 and

Mw= 23,900) was also used. It possesses high molecular weight, good

solubility in common organic solvents and the bulky BTCD-60 has

photoluminescent properties similar to that of its bulky homopolymer.

3.2.2. General Procedures: For the photo physical studies, all the solvents

were purified according to the standard procedures prior to use. The samples

were completely dissolved by ultra sonication and the homogenous solutions

thus obtained were used for further studies. The absorption and emission

NllST 97


studies were done by a Perkin-Elmer Lambda 35 UV-Visible

spectrophotometer and Spex-Fluorolog DM3000F spectrofluorometer with a

double grating 0.22-m Spex 1680 monochromator and a 450-W Xe lamp as the

excitation source using the front-face mode. For the solid state spectra,

polymers thin films (10-100 J.lM) were casted from chloroform solution on

glass substrates. The quantum yields of the PPVs and OPVs were determined

using Rhodamine 60 in water (~=0.95) and quinine sulfate in 0.1 M H2S04 (~=

0.546) as standards, respectively. The solvent induced aggregation studies

were done using methanol (MeOH) and water as nonsolvents respectively for

PPVs and OPVs and by making different solvent/nonsolvent combinations. The

absorbance or optical density (O.D) of the solutions was maintained at 0.1 for

the photo physical studies. The stock solution was prepared by dissolving the

polymer or oligomer in tetrahydrofuran (THF) and various combinations of

tetrahydrofuran and methanol/water of the samples were prepared by

maintaining the absorbance ~ 0.1 at the absorption maxima. Temperature

dependant absorbance studies were done in THF/methanol or THF/water

mixture heating and cooling were done by the help of Perkin Elmer PTP-l

Peltier cooling system. The fluorescence lifetime was measured using an IBH

FlouroCube Time-correlated picosecond single photon counting system

(TCSPC). Solutions and films were excited with a pulsed diode laser 401nm

<100 ps pulse duration with a repetition rate of 1 MHz. The detection system

consists of a microchannel plate photomultiplier (5000U-09B, Hamamatsu)

with a 38.6 ps response time coupled to a monochromator (5000M) and TCSPC

electronics (DataStation Hub including Hub-NL, NanoLED controller and

preinstalled Fluorescence Measurement and Analysis Studio (FMAS)

software). The fluorescence lifetime values were determined by deconvoluting

the data with exponential decay using DAS6 decay analysis software. The

quality of the fit has been judged by the fitting parameters such as X2<1 as well

as the visual inspection of the residuals.

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3.2.3. Synthesis of Binary Blends

The polymer-oligomer binary blends were prepared as follows. A filtered stock

solution ofMEH-PPV (2.8 mg in 10 mL) in tetrahydrofuran (THF) and BTCD

OP-Y or MEH-OPV (50 mg in 10 mL) in THF were mixed at varjous ratio (see

table 3.1) to obtain the OPV-amount as 15,30,40,55, 80,90,95 wt % in the

blend solution. The blend solution was stirred under ultrasonic for 5-10 minutes

prior to drop casting on glass substrates. The films were dried and stored at

room temperature to protect from light.

Table 3.1. Preparation ofthe polymer-oligomer binary blends

%of Volume of Volume of Amount of Amoutof

OPV OPV (mL) PPV (mL) OPV(mg) PPV(mg)

15 0.05 0.95 0.05 0.266

30 0.1 0.9 0.1 0.252

40 0.15 0.85 0.15 0.238

55 0.25 0.75 0.25 0.210

65 0.1 0.9 0.50 0.252

80 0.2 0.8 1 0.224

90 0.3 0.7 1.5 0.196

95 0.4 0.6 2 0.168

N1IST 99

Chapter3

3.3. Results and Discussion

Probing Aggregation Properties

The structures of the polymers and oligomers selected for the present

study were shown in scheme 3.2. Their detailed structural characterization and

thermal studies were given in chapter 2. In the present chapter, more effort has

been taken to understand and analyze the molecular aggregation properties of

these new classes of bulky 7t-conjugated materials.

MEH-PPV

MEH-OPV

Scheme 3.2. Structure ofthe polymers and oligomers.

3.3.1. Solvent Dependant Photophysical Properties

The photophysical properties (absorption and emission) of the polymers

were studied in solution as well as in the solid state (in film). The absorption

and emission spectra of the polymers were recorded in tetrahydrofuran (THF)

and toluene to study the influence of solvents on the polymer structure (see

figure 3.1). The absorption spectra of both polymers showed 6-7 nm red shift in

toluene compared to that in THF (see table 3.2). It is due to the fact that

aromatic hydrocarbon solvents solvate the PPV backbone in such a way to

adopt closer chain interactions via 7t-stacking which account for the req shift.22

,

NIlST 100


56The emission spectra of the polymers were recorded for 0.1 G.D (optical

density) solutions in THF and toluene. The emission spectra were almost

identical even though there is a slight difference in the absorption maxima for

the polymer solutions in THF and toluene.

5

4

- MEHPPV in THF- MEHPPV in Toluene- BTCD-60 in THF- BTCD-60 in Toluene

0.10

8 0.06§-e~ 0.04

..D-<

0.02

0.00 0350 400 450 500 550 600 650 700 750

Wavelength (run)

Figure 3.1. Absorption and emission spectra ofMEH-PPVand BTCD-60 in

solution.

The solution quantum yields were determined using Rhodamine 6G in

water as standard (<1>= 0.95) and the values were obtained in the range of 0.23

0.28 and 0.24-0.31 for MEH-PPV and BTCD-60, respectively (see table 3.2). It

can be clearly noticed that the emission quantum yield was less in toluene

compared to that of THF (see table 3.2). This may be due to the partial 7t

stacking of the polymer chains in toluene compared to that of THF, as noticed

in their absorbance spectra. Thin polymer films were caste on glass substrate

from polymer solution and the a.D. of the films were obtained as 0.1 by

adjusting the concentration of the polymer solution. The solid state absorbance

and emission spectra of films were shown in figure 3.2. The absorbance spectra

of MEH-PPV and BTCD-60 films prepared from toluene solution were almost

identical and red-shifted compared to that of those films obtained from THF

solution. It suggests that the films caste from aromatic hydrocarbon solvents

NIlST 101

Chapter3 Probing Aggregation Properties'

such as toluene posses more n-stacking compared to that of the films from

good solvent like THF. 22, 56

0.10 (a) MEHPPV (THF) 2.5(a') MEHPPV (TOL)

SO.08(b) BTCD-60 (THF)

2.0(b') BTCD-60 (TOL) ~

~ '"0'-" -<I) ><g 0.06 1.5 '-"ro .c

"€ .,...en

~ 0.04 1.0 ~<I)

,D :s~

0.02 0.5 ....:l~

0.00 0.0350 400 450 500 550 600 650 700 750

Wavelength (nm)

Figure 3.2. Absorption and emission spectra ofMEH-PPV and BTCD-60 in

film (caste from THF and Toluene solution) on glass substrate.

Among the films produced from THF solution, the absorption spectra of

BTCD-60 film is blue shifted by 15-20 nm compared to MEH-PPV films

(compare plots a and b in figure 3.2). It indicates that inter (or intra) chain

interactions (n-stacking) are relatively weak in BTCD-60 compared to MEH

PPV. It may be due to the strong steric hindrance induced by the TCD-bulky

unit. The solid state emission spectra of the polymers have shown an interesting

trend that the spectra of BTCD-60 showed almost 5-6 times enhancement in PL

intensity than that of MEH-PPV. The emission intensity of MEH-PPV films

were very low for both films caste from toluene and THF (see emission plots a

and a' in figure 3.2) compared to the BTCD-60 films. Among the two BTCD

60 films, the film obtained from THF solution showed twice the PL intensity

compared to that of films obtained toluene solution (see emission plots band b'

in figure 3.2). The comparison of absorbance and emission spectra of the films

of BTCD-60 revealed that the less n-aggregated film (plot-b) showed higher

luminescence intensity compared to that of more n-aggregated chains (plot b').

NJIST 102


Table 3.2. Photo physical, thermal and electrochemical properties ofthepolymers and oligomers

Sample In soln. (nmt In film (nmt

AAbs AEm ~FLb AAbs AEm PLlntensity(J

MEH-PPV(THF) 489 549 0.28 502 584 4.3xlO'MEH-PPV(Toluene) 495 553 0.23 504 589 2.5x105

BTCD-60(THF) 486 550 0.31 490 585 2.7xlO11

BTCD-60 (Toluene) 493 554 0.25 497 585 1.2xl06

MEH-OPV 389 433- 0.53 394 525 ~=0.27e

475BTCD-OPV 386 432- 0.54 398 525 ~=0.64e

481(a) Excitation wave length used is 480 nm for the polymers and 380 nm for theoligomers.

(b) Determined using Rhodamine 6G as standard for the polymers (A.ex=480

nm) and Quinine sulfate as the standard for the oligomers (A.ex=380 nm), the

absorbance ofsolutions were maintained as 0.1 at the excitation wavelength.

(c) Excitation wavelength is 500 nm for polymers and 390nm for the model

compounds.(d) The emission intensity ofthe polymers at the emission maxima.

(e) Determinedfrom the diffuse reflectance measurements.

It evident that the molecular aggregation is a very strong phenomenon in

PPV chains for reduction in emission characteristics in both solutions as well as

in the films caste from their solution. The presence of TCD-bulky unit

significantly hinders the 7t-stacking of polymer backbone and enhances the

solution quantum yield and PL intensity almost 5-6 times in BTCD-60

compared to that of MEH-PPV. In order to trace the aggregated species at the

excited state, the excitation spectra of the polymers were recorded at two

different concentrations in THF (10-4 and 10-5 M- l). The concentration

dependent experiment was chosen since the polymer chains are closely packed

at higher concentration which resembles the chain arrangements in the film.52

The excitation spectra of MEH-PPV and BTCD-60 were collected at three

different emission wavelengths (Aem= 550, 590, 660 nm) and shown in figure

3.3.

NlIST 103


(a) MEH-PPV in THF

-2xIO"M"1-2xJQ4M·1

Em=550nm

(b) BTCD-60 in THF

-8xIO"M-1

-8xIO"M-1

.c'00

Em=590nm ~

i:J'"""

Em=550nm

250 300 350 400 450 500 550 600 250 300 350 400 450 500 550 600 650

Wavelength (run) Wavelength (run)Figure 3.3. Excitation spectra ofMEH-PPV and BTCD-60 in THF at different

concentrations.

At lower concentration, the excitation spectra of MEH-PPV showed

maxima at 470 run with a shoulder at 340 run at all three collected emission

wavelengths. At higher concentration, the intense peak was blue-shifted by 70

nm and centered at ~ 405 run corresponding to strong inter chain interaction

and 1t-stacking in concentrated solution. Additionally, the excitation spectra

was collected at higher emission wavelengths (collection at 590 and 660 nm) of

the concentrated solution of MEH-PPV showed a new peak at 545 nm which

was assigned to the aggregated polymer chains.57 Interestingly, the BTCD-60

showed almost identical excitation spectra irrespective of the concentration of

the solution and collected emission wavelengths. It confirmed that the MEH

PPV polymer chain undergo more n-stacking at higher concentration (similar to

film) which results in quenching of the luminescence properties. From chapter

2, we have also seen that the solid state luminescence quantum yield of the

model co~pounds (MEH-OPV and BTCD-OPV) determined using diffuse

reflectance technique according to the reported procedure 58-60 was obtained as

0.64 for BTCD-OPV which was two times higher than that of MEH-OPV

(~=O.27). It clearly demonstrates that the bulky TCD unit is very efficient

structme directing unit for highly luminescent n-conjugated materials. From

the above photo physical studies, it is evident that the bulky-TCD unit increases

the inter-chain distance in the polymer backbone leading to the reduction in the

NIIST 104

Chapted Probing Aggregation Properties

molecular aggregation for enhanced luminescence. In

poly(phenylenevinylene)s, the polymer chains have very strong affinity

towards n-stacking and the resultant n-aggregates in solution or solid state

strongly quenches the luminescent intensity.29, 30 In the present investigation,

we have noticed that the introduction of bulky units enhances the solution

quantum yield as well as the solid state PL intensity for both polymers as well

as oligomers. Therefore, tracing the n-stacking properties of conjugated

polymers, especially in the case of bulky systems, it is very important to

understand their luminescent properties. There are different methods to probe

the molecular aggregation in n-conjugated systems and among them solvent

and temperature dependant aggregation studies are very important.

3.3.2. Solvent Induced n-Aggregation Studies

Solvent induced self-organization in conjugated chains is an attractive

approach, since the isolation or aggregation of polymer chains can be easily

controlled by choosing the appropriate combination of good or non-solvent (or

bad solvent).36 In good solvent the polymer chains exist as isolated chains and

subsequent addition of bad solvent forces the polymer chains to undergo

collapsed stage which is similar to that of in the film (see figure 3.4).

III Good solnllt In Hnd 1I01\'fllt

Figure 3.4. Behavior a/polymer chains in good and bad solvents

The solvent induced aggregation studies of the oligomers (MEH-OPV

and BTCD-OPV) and polymers (BTCD-60 and MEH-PPV) will provide more

insight into the effect of n-aggregation on the luminescent properties. For the

NIlST 105


solvent induced aggregation studies, the photo physical properties of polymers

were studied in THF (good solvent) and methanol (poor solvent) solvent

mixture. For the molecular aggregation studies of the model compounds

THF+water was chosen as the solvent combination. The absorption and

emission spectra of MEH-PPV and BTCD-60 in THF/methanol mixture (from

0-100 % v/v) are shown in figure 3.5.

Figure 3.5. Absorption and emission spectra ofpolymers in THF/MeOH withvarying solvent combination at 30 ce. Color change from orange to red with

the addition ofmethanol into the polymer solution (from left to rightincreasing methanol content)

7

0.12 MEH-PPV MeOH MEH-PPV MeOH0% 0010

6

0.10

I::J 5

~ 0.08 "'0OJ 4 .,....u -?:S~ 0.06 .i::'.0

1000103 'iii

0 c1l 0.04 OJ

-ex:2 C

-l1 u..

0350 400 450 500 550 600 650 520 560 600 640 680 720

0.10 BTCD-60 BTCD-60 5

MeOH MeOH~ 0.08 r

0% 4.0-0

::J

I.,....

~-?:S

OJ 0.06 3 .i::'u 'iiic ctil OJ

-e 0.04 2 C0 !()()% 1000/0 -l(Jl

.0 u..-ex: 0.02

0350 400 450 500 550 600 650 520 560 600 640 680 720

NIlST 106


The addition of methanol into the polymer solutions in THF, drastically

affect both the absorption maxima as well as luminescent intensity. The color

of the polymer solution gradually changed from orange to red with the addition

of methanol and the transition point was noticed at 40-50 % of methanol in

THF (see figure 3.5). It can be clearly noticed that MEH-PPY was more

drastically affected by the MeOH addition compared to that of BTCD-60 «see

absorption and emission spectra in figure 3.5). The quantum yields for the

different solvent/non-solvent combinations were calculated using Rhodamine

60 in water as the standard. Since the MeOH addition reflects both on the

absorption maxima and quantum yield, these two parameters can be used to

trace the extent of molecular aggregation in polymer chains.

The plots of absorption maxima (Figure 3.6a) indicate the existence of

two types of species depending upon the amount of methanol in polymer

solution. The polymer chains are free and isolated below 30-40 % of methanol

whereas they are fully aggregated above 50-60 % of methanol. The extent of

absorption maxima shift (Y-axis) revealed that the MEH-PPY chains showed

strong aggregation with 25 nm red-shift compared to that of bulky BTCD-60

which showed only 16 nm red-shift. The quantum yield plots (Figure 3.6 b)

also support the existence of the aggregated chains and the values are lower for

MEH-PPY compared to that of BTCD-60 (above 50 % methanol

concentration). It reveals that polymer chains in MEH-PPY chains experience

more n-aggregation compared to that of the bulky PPY chains in BTCD-60.

The solvent induced aggregation studies of the model compounds

(DPYs) also reflect on the molecular aggregation in absorption and emission

properties (see figure 3.7). As expected, the extent of aggregation relatively

weak in model compounds compared to their polymer counterparts since long

range of n-stack is not possible in simple OPYs compared to that of high

molecular weight polymer chains. The shift in the absorption spectra is not very

significant; however, the emission properties are affected at high amount of

non-solvent addition (figure 3.7a).

NIIST 107


- 520~ (a) /-.-......... 515Cll ......-.-E'x 510Cll J ......--.- -.~ 505 ___ MEHPPV .--"'- 1Q)

--- BTCD60/./1 ~ ~uc 500Cll V).0 /. \0 N.... 495a

1=1=lrf-.-~ -rJ).04: 490

0.35

0.30 ......-. (b)Ir '.--.--.

0.25 "." \""0 0.20

.,.Qj .\.,>= 0.15E

~:::J

0.10 -.- MEHPPV ."- ___ BTCD60 .'.,cCll:::J 0.05 '. .-.a --.--.-.'.

0.00 --.0 20 40 60 80 100

% of MeOH in the solvent mixture

Figure 3.6. Plot ofabsorption maxima and quantum yield versus the

percentage ofMeOH in the THF/MeOH mixture.

The fluorescence quantum yields of the model compounds (figure 3.7b)

almost unaffected up to 80 % of water in THF. However, at higher water

content, a significant decrease in quantum yield can be noticed for MEH-OPV

compared to that of BTCD-OPV.

25(a)

H2O (b) e.....e 0.56

",,-.,,20 e .....e-e.....e"'" ......~.0 0% .-. 0.48..... .-.......~

j"0'-' 15 v0 0.40>='Vi

~ E~ 10,s-,-MEHOPV

0.32 .2I:

H ___ BTCDOPV ro~ • ;:3

\ 0.24 CI

o 0.16400 450 500 550 600 650 0 20 40 60 80 100

Wavelength (run) % of H20 in THF+ H

20

Figure 3. 7. (a) Emission spectra ofBTCD-OPV in THF/H20 with varyingsolvent combination at 30 ce. (b) Plot quantum yield versus the percentage of

H20 in the THF/ H20 mixture

NlIST J08

ChapterJ Probing Aggregation Properties

Based on the solvent induced aggregation studies it can be concluded

that bulky TCD-unit is very efficient anchoring groups for PPV backbone (also

for OPVs) in preventing the molecular aggregation for enhanced emission

properties.

3.3.3. Temperature Dependent x-Aggregation Studies

The aggregation of the polymer chains in solution are also sensitive to

the temperature which can also be used as another stimuli for tracing the

molecular aggregation properties.36 To study the influence of temperature on

the nature and reversibility of the molecular aggregation in PPV chains, the

polymers at the aggregated stage was subjected to temperature dependant

absorption studies. There are two ways one can design the temperature

dependent studies: (i) start from the non-aggregated stage at room temperature

and force the chains to undergo aggregation and isolation by cooling/heating at

sub-ambient region36 or (ii) start from the pre-aggregated stage at room

temperature and carryout the aggregation and isolation by heating/cooling at

elevated temperatures. We adapted the second method, because it is more

relevant to the practical purpose.

For the temperature dependent studies 40 % methanol in THF solvent

combination was selected because at this stage the polymers show a transition

from the isolated to the aggregated state (see Figure 3.8a). The polymer

solution (40 % methanol in THF) were taken in a screw capped cuvette and

subjected to absorption studies by heating or cooling the solution from 30 to 80

DC. The UV-visible spectra of BTCD-60 in the heating and cooling cycles are

shown in figure 8. As the solution temperature gradually increases, the

absorption spectra found to be blue shifted corresponding to the isolation of

polymer chains from the pre-aggregated state (see figure 8a). While cooling,

the absorbance spectra were red-shifted corresponding to the aggregation of

polymer chains (see figure 8b). This experiment was found completely

reversible for many more heating/cooling cycles (see figure 3.8c) which

confirmed the reversibility of molecular aggregation in PPV chains. Similar

NIIST 109

Chapted Probing Aggregation Properties

observation was shown by MEH-PPV also; in the heating/cooling cycles (see

figure 3.8d, 3,8e, and 3.8f).

0.10,---------::----------,-----------,(a) Heating

(b) Cooling

-30'e (d) Heating - -30'e-3s'e -3s'e 0.10-4o'e -4o'e ,-...

;:l-4s'e -4s'e 0.08 o:l

so'e -so'e '-'11)

-ss'e -ss'e 0.06 C)

6O'e 6O'eco:l

-6s'e -6s'e 0.04 ~-7o'e -7o'e en-7s'e -7s'e

..D0.02 -<

-so'e -so'e0.00

-so'e (e) Cooling - -so'e 0.10-7s'e -7s'e-7o'e -7o'e 0.08 S'-6s'e -6s'e ~6O'e 6O'e 0.06 ~-ss'e -ss'e C)

so'e so'e c-4s'e -4s'e

0.04 ]...-4o'e -40'e 0

-3s'e -3S'C 0.02 15-30'e -30'C -<

0.00400 450 500 550 600 650 300 350 400 450 500 550 600 650 700

Wavelength (nm) Wavelength (run)(f) 498 ,-...BTCD-60 • MEH-PPV §• • . ~ ~. . 496• • • • ,

494 '-'

I • I I I~ , ~ 1\ .o:l

492 a\ \ I I / / I 490

.;(o:l/ S

1/ \1 \/ 11 \/\/\/\/1/488

<I)

486 C)

V ~ \co:l

~ • ~ ~ v ~ ~ 484 ..DV

...482 0• • • • • • en

480..D-<

5 10 15 20 0 5 10 15 20Cycles Cycles

0.00

300 350

,-...488 (c)§ •

'-' 486 •o:l

S 484.;<o:l 482S<I)

480C)

co:l 478..D...0

476en..D-< 474

0

S'~ 0.06

<I)C)

co:l 0.04-€oen~ 0.02

0.08

S' 0.08~'-'

8 0.06co:l

..DCi 0.04en

..D-< 0.02--~

Figure 3.8. Temperature dependent absorption spectra ofBTCD-60[(a) and

(b)) and MEH-PPV [(d) and (eJ] in the heating and cooling cycles

respectively. Plots (c)and (f) shows the reversibility ofmolecular aggregates of

BTCD-60 and MEH-PPV

The shift in the absorption maxima for the cooling and heating

experiments are plotted and shown in figure 3.9a and 3.9b, respectively. The

absorption maxima plots indicated that the MEH-PPV chains undergo 10 run

blue shift on heating while 15 run red-shift in the cooling cycle. The polymer

NIIST 110


chains in BTCD-60 followed just opposite trend in the extent of absorption

maxima shift: it showed 14 nm blue-shifts while heating and 10 nrn red-shifts

in the cooling cycle. This trend suggests that both the polymers undergo

isolation from the aggregated state while heating, however, the degree of inter

chain separation is more for BTCD-60 (14 nm) compared to MEH-PPV (10

nm). In the cooling cycle, the trend was found just opposite in which the MEH

PPV chains undergo more aggregation (15 nm) compared to that of BTCD-60

(10 nm).

30 (a) Cooling • //• .1

40 10nm /.• •;-., / /U • •0 / /

~ 50 •/ --ll-MEHPPV

l-o /.a •~ 60

1

l1.l /.0..

/. /. ____ BTCD60S

70l1.l • /.E-< I• • 15 run/ 180 • .-------80 ." .. ___ ...(b) Heating

1 10run

70 ." ~\ -ll-MEHPPV•;-., 'eu • ____ BTCD60'G 60 I \• ".l1.l \l-o

.a .",~ 50 •l1.l 1 10.. .'-........ •s 40

1l1.l • •E-< 14nm

/ 1." ~30 .. ,.476 480 484 488 492 496 500

Wavelength (run)

Figure 3. 9. Plots ofabsorption maxima versus temperature for MEH-PPV andBTCD-60 in the cooling and heating cycles.

The difference in the extent of aggregation from the isolated stage (in

cooling cycle) revealed that the bulky TCD-units in the PPV- chains strongly

hinder the polymer chain alignment and reduces the molecular aggregation.

This can be better explained by the following cartoonic representation (see

NIIST 111


figure 3.10). On heating the polymer chains undergo a conformational change

from the pre-aggregated stage to more expanded form, which upon cooling will

again undergo coiling. However, the extent of coiling in the cooling process is

completely different for MEH-PPV and the bulky PPVs. MEH-PPV undergoes

more aggregated state (than it was first) (see figure 3.10, top), while in the case

of bulky PPV it is not regaining the initial aggregated stage (see figure 3.10,

bottom). This is because the bulky TCD-units do not allow the polymer chains

to come closer due to steric effects.

MEH·PPV

Bulky PPV

Figure 3.10. Representation oftemperature dependence ofpolymer chains

The results of temperature dependant studies are in accordance with the

solvent induced aggregation studies that the inter-chain separations in the bulky

systems reduce the molecular aggregation and enhanced the luminescent

properties.

3.3.4. Polymer -Oligomer Binary Blends

The effect of bulky units on the inter-chain separation of PPV backbone

was further studied by blending MEH-PPV polymer with MEH-OPV and

BTCD-OPV through solution blending techniques. The two series of the blends

were named as MEH-blend and BTCD-blend, depending upon the types of

OPV used in the blend preparation. The weight % of the oligomer was varied

NllST 112


as 15, 30, 40, 55, 65, 80, 90, and 95 % in the blend by varying the amount of

the components. The blend solution was caste on a glass substrate in such a

way to maintain the optical density of the resultant film ~O.l.

MEH-PPV

500 nm

OPV 390 nm

••••••••••••'----->Blending

-'!"""!""- .... EnergyTransfer

(Path 2) (Path 1)

MEH-PPV!OPV Binary Blend

EcoO'lM

OPV Polymer

Figure 3.11. Polymer-oligomer binary blend and possible selective e.l:citaJionprocesses in the blend.

Two types of selective photo-physical experiments can b am ut in

the polymer-oligomer binary blend, which is explained in figure .11 (i)

Selectively excite the oligomer and transfer the e citati n n

polymer, subsequently monitor the emission of the polym r

mechanism, path-I) or (ii) directly excite the polymer hain aD!

polymer emission (path-2). In the case of path-I, the

also show self-emission, if the energy transfer proc

of the energy transfer process in such binary blends i

energy levels of the molecules. The energy level of th l~'~telISJ

and the processes of path-l and 2 are shown in fi ur

emission spectra of the MEH- and BTCD-binary in

3.12.

~T n

ChapterJ Probing Aggregation Properties

0.15"(:-:a):-----------.--------------..0.15MEH Blend (b) BTCD Blend

;;'0.12 Abs(OPV)~'-'

80.09§~0.06

CI'l.0

-< 0.03

Abs(PPV) -MEHPPV-15%-30%-40%

55%-65%

80%-90%-95%-MEHOPV

-MEHPPV-15%-30%-40%-55%-65%

80%-90%-95%-BTCDOPV

0.12 S'~

'--'0.09 Q)

(,)

§0.06 "2

oCI'l

..00.03 <t:

Ern(PPV)

(f)-MEHPPV-15%-30%-40%

55%-65%

80%-90%-95%-MEHOPV

Em(PPV)

J-~~~2~~~~~=r==~~~~0Ern(PPV) -MEHPPV 9.0

-15%-30% """'-40%) 7.5 '00-55% ......---65% 6.0 ><

80% b-90% _-95% 4.5·~-BTCDOPV E

3.0 ,s

400 500 600 700 400 500 600 700Wavelength (nm) Wavelength (nm)

Figure 3.12. Absorption spectra ofMEH-blend (a) and BTeD-blend (b).

Emission spectra ofMEH-blendfor excitation wavelengths 390 nm (c) and 500

nm (e); Emission spectra ofBTeD-blendfor excitation wavelengths 390 nm (d)

and 500 nm (f).

The absorption maxima of the OPV and PPV chains appeared 390 and

500 nm, respectively. It is very clear from the plots that the optical densities of

the PPV part increase with the increase in the MER-PPV content in the blend.

The emission spectra for 390 nm excitation (corresponds to the absorption

maximum of the oligomer) are given in figure 3.12 (c and d) for MEH and

BTeD-blends, respectively. In both blends, the emission spectra showed two

peaks at 495 and 570 run corresponding to the self-emission and polymer chain

N11ST 114


emission followed by energy transfer process, respectively. Though the pure

MEH-PPV possess small absorption at 390 run (O.D. = 0.05, see figure 3.12a

and 1l2b), but failed to show any significant luminescence at 450-550 run for

excitation at 390 run, which suggested that the pure MEH-PPV chains are not

contributing to the emission characteristics of blends in this region. The

relative intensities of these emission peaks were highly influenced by the

compositions and types of the OPVs in the blend. The emission intensity of the

peaks at 495 and 570 run were plotted with respect to their composition and

shown in figure 3.13.

(a) DPV Luminescence at 495nm • A(390nm Excitation) ~ MEI-I-Blend • e/

e -.- BTCD-Blend /e- __/ e__e·

• .1/ /." I'". .-._.-.-(b) PPY Luminescence at 570nITI ~

(390nl11 Excitation) ,.-. /f ·---------------. I• e--- -e__ /e

1-. ---ME:.Bl'OO• -e- BTCD-Blend

~(c) PPY Luminescence at 570nm ..

(500= £><''''''00) I~MEH-Blend

-e- BTCD-B1end

e_____e"'"

L;·-~::~;·1-==== --

15~

'"3 12><'-'

.c 9.-[/.)~ 6(l)......s~ 3~

0

"'~20o.--.0 16

,£ 12[/.)

§ 8

oS 4~

~ 0

8

20 40 60 80% afOPV in the blend

100

Figure 3.13. ,Plot ofPL Intensity versus % ofOPV in the blendfor MEH-blend

and BTCD-blend at different excitation wavelengths.

NIIST 115


It is very clear from the plots in figure 3.13a that the MEH-blend did not

show self-luminescence up to 80 % of the MEH-OPV content, which indicates

the strong excitation energy transfer from the OPVs to MEH-PPV chains. On

the other hand, BTCD-OPV showed a self-emission even for 30% of its

presence in the BTCD-blend. Figure 3.13b represents the luminescence

intensities of the blend at 570 nm, followed by the energy transfer process from

oligomer to polymers for 390 nm excitation. The energy transfer to the polymer

chains lead to the luminescence of the MEH-PPV at 570 nm. The increase in

the PL-intensity of MEH-PPV chain was very high for BTCD-blend series

throughout the entire composition (see figure 3.13b); it suggests that the bulky

OPV molecule showed enhanced energy transfer process compared to that of

the normal OPV molecules. The emission spectra of the blends excited at 500

nm (with respect to the absorbance of MEH-PPV, path-2) are shown in figure

3.l2e and 3.12fand the PL intensity data are plotted in figure 3.l3c. For 500

nm excitation, the blends showed emission peak at 570 nm and the intensity of

this emission peak significantly increases with the amount of OPV in the blend

(see figure 3.13c) compared to that of pure MEH-PPV film. Since the OPV

molecules do not have any luminescence behavior for excitation at 500 nm, the

enhanced emission at 570 nm in the MEH-PPV chains is the result of the

reduction of molecular aggregation in the solid state. The larger PL intensities

of BTCD-blend at the entire composition indicate that the MEH-PPV polymer

chains are separated apart very effectively by the BTCD-OPV molecules in the

blend compared to the MEH-OPVs. The well separated polymer chains in the

blend experience less n-stack induced molecular aggregation. Therefore, it is

directly evident from the selective excitation studies of the blends that the

BTCD-units are an efficient bulky group for controlling the molecular

aggregation for enhanced luminescence properties.

3.3.5. Time Resolved Fluorescence Life-time Measurements

Time resolved fluorescence decay profiles of the polymers, oligomers

and polymer-oligomer blends were carried out in solution (THF) and in the

NIlST 116


film at room temperature and are given in figure 3.14. The decay dynamics

were carried out at an excitation wavelength of 401 nm and the decays were

monitored at respective emission maxima (see table 3.3 and Figure 3.14).

10'

• MEHblend30MEHblend90BTCDblend30

BTCDblend90 103

• I

Ii• \ • MEH-PPV-Soln• 8 0 MEH-PPV-Agg

t> MEH-PPV-Film 103

oo

'"(d)

(b)

• BTCD6D-Solno BTCD6D-Agg

BTCD6D-Film

I--.-"-f---.----r-~-,-~.--......-o\- 100

6 9 12 150 3 6 9 12 15Time (ns) Time (ns)

o MEH-OPV-Solno BTC[)'()PV-Soin

MEH-OPV-Film'i1 BTCD-OPV-Film

b.

·~

··

3

100, .,.-.-.......-....-,-~..--.-,.........~

10' (c)

10' (a)

Figure 3.14. Fluorescence lifetime decay curves ofOPVs, PPVs and

their blends in solution andfilm.

The OPVs in THF show a single exponential fit and they possess similar

decay life times, 1.86 ns (MEHOPV) and 1.89 ns (BTCDOPV) indicating that

they are having similar excimer species.22• 57 The decay profiles of the MEH

OPV and BTCDOPV in film were found to be similar to that in the solution.

MEH-PPV and BTCD-60 showed a single exponential decay in THF with life

time 0.56- 0.60 ns. The aggregated polymer solution (40 % methanol in THF)

showed bi- exponential decay with two life times in the range of 0.50-0.60 (98

NllST 117


%) and 0.81-1.04 (2 %) confirming the existence of two types of luminescent

species corresponding to the aggregated and isolated chains. The polymers in

film were also showed a bi-exponential decay and their lifetimes are given in

table 3.3. The observed fast decay in the aggregated solution and in the films

may be due to the increased extent of non-radiative processes and efficient

energy transfer to the aggregated species. The decay lifetime measurements of

the polymer-oligomer binary blends were also carried out in films and they

showed bi-exponential fit (Figure 3.14d). It was clear from the fitting results

that more than 90 % fraction of components undergoes fast decay and only less

than 10 % show delayed emission.

Table 3. 3. Fluorescent decay lifetime measurements ofthe oligomers,

polymers andpolymer-oligomer blends

Sample MediumQ

Amonitor 't'l(ost 't'2 (ost(om)b (99 %) (1 %)

MEH-OPV THF 470 1.86 -BTCD-OPV THF 470 1.89 -MEH-OPV Film 525 1.70 -BTCD-OPV Film 525 2.04 -MEH-PPV THF 555 0.56 .BTCD-60 THF 555 0.60 -

MEHPPV-Agg THF/CH30H 550 0.50 1.04

BTCD60-Agg THF/CH30H 550 0.61 0.81

MEH-PPV Film 585 0.46 0.62

BTCD-60 Film 585 0.35 0.56

MEHBlend-30 Film 585 0.52 1.17

MEHBlend-90 Film 585 0.34 0.81

BTCDBlend-30 Film 575 0.32 1.33




(a) The optical density of the solutions and films were maintained at z 0.1 for

the lifetime measurements

(b) Emission wavelengths for which the decay was monitored (Aexc=401 nm).

(c) Lifetimes obtainedfrom the exponential decay fitting.

NIlST 118

Chapter3 Probing Agwegation Properties

In MEH-blend, as the percentage of OPV in the blend increases the

decay lifetime becomes fast because of the high rate of energy transfer process

from the oligomer to the polymer chain. This observation in the MEH-blend

demonstrated the complete energy transfer from the oligomer which results in

the increase in emission intensity of the polymer. However, the decay lifetime

of BTCD-blend shows no difference for the emission wavelength collected at

575 nm which corresponds to the polymer emission. But a small difference was

observed for the BTCD-blend films when collected at 495 nm which

correspond to the oligomer emission. It supports the observation for the

polymer-oligomer binary blends that in BTCD-blend two significant processes

occur (i) the self-luminescence of the oligomer and (ii) the emission of the

polymer chains as a result of the enhanced energy transfer.

It is evident that the bulky OPV sterically hinder the inter-chain

interactions preventing the molecular aggregation in the solid state. Therefore,

from the photophysical studies and decay lifetime measurements it can be

concluded that the bulky TCD- unit is an important anchoring group, which can

control the molecular aggregation by increasing the inter-chain distances in the

polymer chains as well as in the polymer-oligomer blend for the enhanced

luminescent properties.

NIIST 119


3.4. Conclusion

In this chapter, systematic spectroscopic studies was carried to probe the

origin of the molecular aggregates in bulky conducting polymers and

demonstrated the importance of the bulky TCD-anchoring group in controlling

the molecular aggregation for improved photo luminescent properties. The

absorption, excitation and emission studies of the polymers in solution and

solid state were carried out and they demonstrated the following facts: (i) Apart

from the solvent effects, the structural configuration of the polymeric backbone

plays an important role in enhancing the solid state luminescent intensity, (ii)

The solvent induced aggregation studies have proven that the addition of

methanol (poor solvent) into the polymer solutions in THF (good solvent),

drastically affect both the absorption maxima as well as luminescent intensity.

However, the comparative studies suggested that the MEH-PPV chains

experience more 1t-aggregation compared to that of the bulky PPV chains in

BTCD-60, (iii) Temperature dependant studies have demonstrated that MEH

PPV chains undergo more aggregation (15 nm) in the cooling cycle compared

to that of BTCD-60 (10 nm), (iv) The solvent and temperature dependant

studies were further confirmed with the studies done on the OPVs which

suggest that the bulky unit separates the interchain distance for the enhanced

luminescence properties, (v) Synthesis of polymer-oligomer binary blends

revealed that the bulky DPV (BTCD-DPV) increases the emission of the PPV

chain via both energy transfer and inter chain separation processes and, (vi)

Decay lifetime measurements of the binary blends further support the efficient

energy transfer from the DPY to the PPY backbone resulting in the enhanced

emission properties.

Therefore, from the present studies it is clearly evident that the bulky

substitution in the PPY or in the oligomer separates inter-chain or inter

molecular distance more efficiently leading to the improved photo-luminescent

properties which can be explored for the futuristic applications of opto

electronic devices.

N/IST 120

Chapter3

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chapter 3 probingmolecularaggregation in bulky...

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