Fundamental Insight and Solvent Dependent Photophysical Properties of Poly(2-Methoxy 5-(2- Ethylhexyloxy)-1,4-Phenylenevinylene) (MEH-PPV)

Enormous researches have been performed in the field Light Emitting Polymers (LEPs) since the discovery of Poly(p-phenylene Vinylene) (PPV) [1]. The LEP represents a novel class of materials, which combine the optoelectronic properties of semiconductors along with the mechanical processability of polymers [2]. LEPs have been potentially used in optoelectronic applications due to their high absorption coefficient and photoluminescence efficiency along with flexibility, less toxicity and low cost [3]. Among the LEPs, the study of PPV as an active material in optoelectronic devices such as Light Emitting Diodes [4,5], LightEmitting Electrochemical Cell [6], Organic Solar Cells [7] and plastic lasers [8] has been an active area of research. So far, PPV and its derivatives remain the most prominent polymer for PLED applications due to their semiconducting nature, luminescent properties, ease of synthesis and better environmental stability [9,10]. Within the classes of PPVs, poly(2-methoxy 5-(2’-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) is particularly beneficial for device fabrication as the LUMO level (5eV) matches well with the work function of Indium Tin oxide Crimson Publishers Wings to the Research Research Article

The advantageous polymer, MEH-PPV is generally prepared by two novel approaches i) Wessiling route which involves the treatment of dialkylsulfonium salts that yields high molecular weight polymer. ii) Gilch route, employs the treatment of α-α'-dihalop-xylenes with potassium tert-butoxide in the organic solvent [13].
Here, the Gilch route process with slight modification was adopted for the synthesis of MEH-PPV to avoid high temperature processing involved in the Wessiling method.
Potassssium tert Butoxide (t-BuOK) and benzyl bromide were obtained from TCI chemicals. All the required solvents were obtained from Emplura@ (MERK). To the reaction vessel, mixture of 5 equivalent weight of 30% HBr and 30mL acetic acid was added dropwise. This reaction mixture was reflux at 80 ℃ for 48 hours in the N2 atmosphere.

Methodology
The product was diluted with chloroform followed by extraction with cold water. The organic layer was separated, dried over anhydrous magnesium sulphate and filtered. Later purification by recrystallization through n-Hexane afforded ~11g of pale white color substance which is MEHDBMB labelled as Monomer -II

Polymerization of 1,4-bis(bromomethyl)-2-((2-ethylhexyl)
oxy)-5-methoxybenzene (MEHDBMB): Polymerization of the MEHDBMD (300gm) was carried out in 30mL THF solution with 2g t-BuOK as the catalyst. To this reaction mixture, 60mg of Benzyl bromide (chain stopper) was added to prevent gelation. The reaction mixture was refluxed at 68 ℃ for 3 hours in a N2 atmosphere. The reaction product was precipitated by the addition of 40ml methanol. The precipitate was filtered and washed with distilled water and methanol. Later it was dried in a vacuum oven for 12 hours. It is to be noted that the monomer -2 (MEHDBMD) which was white in color became deep red after polymerization, was associated with the long-conjugated structure of PPV. Thus, the polymerization of MEHDBMB yielded 180mg MEH-PPV. The reaction scheme is represented in Figure 1.

Elemental and morphological analysis of MEH-PPV
The morphological and element analysis of MEH-PPV was obtained using SEM and EDAX as shown in Figure 4

Influence of solvent on the photophysical properties of MEH-PPV
The knowledge of solvation ability is significantly important aspect during multi-layer optoelectronic device fabrications.

Nature of solvents affects macromolecular conformation and
hence the optoelectronic properties of MEH-PPV. Therefore, the optoelectronic properties of MEH-PPV in various aromatic and non-aromatic solvents was studied. The aromatic solvents (Toluene and Xylene) and non-aromatic solvents (Tetrahydrofuran (THF) and Chloroform) were obtained from Emplura MERK and used as received.

RDMS.000886. 16(3).2022
Initially, a stock solution of 1mg mL -1 concentration was prepared by dissolving prescribe quantity of MEH-PPV in a particular solvent and stirring for it 12 hours in inert atmosphere at 45 ℃.

A. Absorbance and steady-state photoluminescence spectroscopy
The normalized absorption and emission spectra of diluted MEH-PPV solutions (10 -2 mg mL -1 ) in toluene, xylene, THF and chloroform are shown in Figure 5. length accounted for high-energy photon absorption (330nm) [17]. The steady state PL spectra produces a dominant peak and a shoulder attribute to monomeric and excimeric states of MEH-PPV.
The emission peak at lower wavelength (~550nm) arises from single-chain excitons (intrachain excitation). This dominant peak in the emission spectrum is a characteristic of the PPV backbone that arises from the radiative relaxation of excited π-electrons to the ground state. From the literatures, researchers had proven that photoexcitation in conjugated polymers produced mainly by intrachain singlet excitons. However, the peak at longer wavelength (~595nm) is associated with interchain interactive excimers which are related to aggregation of polymer chains in the solution [18].  respectively which is in good agreement with the previous reports (Eg value of MEH-PPV ~2.3eV) [20]. The acceptable reason behind slight decrease Eg in case of aromatic solvent is that the excitation energy will migrate to longer conjugation lengths before radiative relaxation. This is to be referred to as energy migration through the inhomogeneous density of states.  Huang-Rhys factor (S) is obtained by calculating the related strength of the 0-1 peak intensity compared to 0-0 peak intensity: S = I0-1/I0-0 which could be related to the effective conjugation length of the conjugated polymer [22]. According to the empirical function proposed by Yu et al. [23] for the PPV, the relationship between the Huang-Rhys factor S and the conjugation length are empirically chosen to be 3.2 and 38 respectively [23]. Table 1 reveals a greater conformational disorder exists in non-aromatic solvent than aromatic solvent. This phenomenon is attributed to the solvation ability of various groups in the polymer structure.

C. Effect of solvent on optical parameters (refractive index, dielectric constant, optical conductivity and electrical conductivity)
It is well-known that the photophysical behaviour such as shape, intensity, absorbance maxima and optical parameters of a material depends on the nature of the environment and solvent-solute interaction [27].
The Refractive index is related to the reflectance R and extinction coefficient k.
in which α is the absorption coefficient given by , where d is the thickness (path length of 10mm) and A is the absorbance value.
Dielectric Constant is a complex value (ε'+iε") comprising of a real and imaginary part.  with the change in solvent [30]. The value of n is ~2.6 in all solvents which is close to the reported value [30,31]. The polarizability of any material under the applied electric field is examined by dielectric constant [28]. It can be inferred from Figure 9    To analyze the quality of thin films casted using various solvents, contact angle measurements were performed. Figure 12 displays the contact angle measurements for MEH-PPV thin films.

E. Relative quantum yield measurement for MEH-PPV in various solvents
Fluorescence quantum yield (Φf) is the ratio of number of photons emitted to the number of photons absorbed. The relative fluorescence quantum yield measurement was performance for MEH-PPV in comparison fluorescein. The procedure and formula for estimation of relative quantum efficiency is described elsewhere [33]. The ϕf of pristine MEH-PPV material in THF and toluene were identified as 16% and 24% respectively. MEH-PPV in THF exhibits coiled structure which creates torsional defects along the backbone leading to lower photoluminescence (PL) quantum yield. Rather in the case of aromatic solvents (toluene) MEH-PPV exhibit significant π-electron interaction between chromophores on the same chain which is beneficial for efficient charge transfer and higher photoluminescence quantum efficiency.