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  • 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

  • Chapter3 Probing Aggregation Properties

    conjugated polymers and also making the ideal polymer luminescent

    optoelectronic devices in reality.

    IclllOW .. lolly III

  • Chapter3 Probing Aggregation Properties

    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

  • Chapter3 Probing Aggregation Properties

    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 andMw= 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

  • Chapter3 Probing Aggregation Properties

    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

  • Chapter3 Probing Aggregation Properties

    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.

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