Electrochemical Synthesis of the Copolymer Poly ( 2-methoxy-5-bromo-p-phenylenevinylene ) / ( 2 , 5-dicyano-p-phenylenevinylene ) ( MB-PPV / DCN-PPV ) : Tuning Properties

Descreve-se um método eletroquímico para a preparação do copolímero poli(2-bromo-5-metoxip-fenilenovinileno/2,5-diciano-p-fenilenovinileno) (cop-MB-PPV/DCN-PPV). Este copolímero foi proposto por apresentar uma estrutura D-A, produzida pela presença do grupo elétron-doador metoxi e do grupo elétron-retirador diciano. O copolímero foi sintetizado eletroquimicamente por redução catódica dos materiais de partida convenientes dissolvidos em DMF/LiClO4 usando um poço de mercúrio como eletrodo de trabalho. O copolímero foi caracterizado por medidas de espectroscopia no infravermelho (IR), no UV-Vis e de fluorescência (FL) e por medidas de voltametria cíclica (CV). Todos os resultados foram comparados com o homopolímero MB-PPV. A análise dos resultados indica que o cop-MB-PPV/DCN-PPV foi obtido através da formação de blocos contendo unidades de DCN-PPV ligadas por blocos contendo unidades de MB-PPV. A caracterização da estrutura eletrônica, obtida por meio de medidas de CV, UV-Vis e FL mostraram que a banda de condução é mais estabilizada no copolímero em comparação ao homopolímero. Além disso, os espectros de FL indicaram que a emissão de luz do cop-MB-PPV/DCN-PPV em solução de clorofórmio ocorre por meio da formação de excímeros.


Introduction
2][3] Due to massive research efforts there has been significant progress in the field of organic solar cells and within about 25 years higher efficiencies and lifetimes were achieved and power conversion efficiencies (PCEs) higher than 10% have been reported. 4Nevertheless, before commercialization, additional improvements, particularly in the power conversion efficiency, are required.One of the reasons for the low efficiency of the PSCs is the lack of acceptor polymers which present properties such as high electron affinity, high electron mobility, and higher sunlight absorption.6][7][8][9] One of the most useful methods to obtain a low band gap polymer is the production of a copolymerized donor-acceptor (D-A) structure. 10In these types of structure the proper combination of donor (D) and acceptor (A) units in the copolymer gives the possibility of tuning the electronic energy levels and produce a broadening in the photoactive material absorption.1][12] Another important feature that must be taken into account concerning the production of organic semiconductors suitable for PSC application is the possibility of molecular ordering, since higher aggregation states could result in higher mobility.4][15] This class of polymer has presented some of the highest mobility values assigned to the remarkable aggregation properties of the DPP moieties. 15sed on what has been discussed above the present paper describes the electrochemical synthesis of the copolymer poly(2-methoxy-5-bromo-p-phenylenevinylene)/ (2,5-dicyano-p-phenylenevinylene) (cop-MB-PPV/DCN-PPV) as shown in Scheme 1.This copolymer was proposed due to the fact that it contains the electron-donating methoxy-phenylene units (the MB-PPV units) as the donating group (D) and the electron-withdrawing dicyanophenylene units (the DCN-PPV units) as the acceptor (A) group.In this way, the MB-PPV/DCN-PPV copolymer was electrochemically synthetized by the cathodic reduction of a mixture of compounds 1 and 2 (as shown in Scheme 1) dissolved in a DMF/LiClO 4 electrolytic solution using a mercury pool as working electrode.This approach allows the production of a soluble fraction of the copolymer.This soluble copolymer was characterized by infrared (IR), UV-Vis and fluorescence emission spectroscopies.The electronic characteristics of the films were measured by cyclic voltammetry.All the results were compared to the MB-PPV homopolymer.

Experimental
Solvents and electrolyte N,N-dimethylformamide (DMF) (Vetec 99.8%) was heated under reflux over copper sulfate for 36 h before distillation under reduced pressure.It was stored over freshly baked 4 Å molecular sieves.Lithium perchlorate (LiClO 4 ) (Merck, 99%) was held in a dessicator with phosphorus pentoxide overnight before use.All other chemicals were commercial grade and used as received.
spectra were obtained by transmission on a Hitachi U-321 spectrophotometer.Emission spectra were recorded on an Ocean Optics HR-4000fluorimeter.Electrochemical characterizations were carried out using a Palm Sens Potentiostat/Galvanostat.

Starting material
The MB-PPV starting material (compound 1 from Scheme 1) was obtained as described below: In a three neck round-bottom flask, 46.7 mmol of 2,5-dimethylanisole and 47 mL of CCl 4 were mixed.In round-bottom flask a reflux condenser and a dropping funnel with 0.2 mol of previously dried Br 2 were connected.This mix was heated with constant stirring and lighted by an IR incandescent lamp of 500 W. The Br 2 in dropping funnel was added dropwise in flask, and a new drop was only added after decolorization of the solution.After finished reaction, the mix was cooled overnight.The solid obtained was filtered and recrystallized in CHCl 3 .The DCN-PPV starting material (compound 2 from Scheme 1) was obtained as described by Garcia et al. 16

Electropolymerization
The electrochemical synthesis of the MB-PPV/DCN-PPV copolymer was carried out on a stirred mercury pool working electrode, using Ag/AgCl as reference electrode and graphite sheet as counter electrode.Starting materials were used on a mass proportion of 7:3 of compound 1 and 2, respectively, from a total mass of 0.6 g of starting material dissolved in 50 mL of distilled DMF and 0.1 mol L -1 of LiClO 4 .Syntheses were performed using potentiostatic methods by passing a total charge of 4.4 F mol -1 .Using this approach the copolymerization was completed after 5 hours.It was also produced the homopolymer MB-PPV using a similar procedure as described above, but in the absence of compound 2. Purification of the copolymer and the homopolymer occurred by water addition followed by filtration of the solid polymers and Soxhlet extraction of the soluble fraction with CHCl 3 .Obtained copolymer and homopolymer were submitted to FTIR, UV-Vis and emission fluorescence spectroscopies.

Films of MB-PPV and cop-MB-PPV/DCN-PPV
were obtained by drop casting method.Soluble portion of separated materials dissolved on CHCl 3 were dropped over indium tin oxide (ITO) transparent substrate and the solvent was evaporated.Cyclic voltammetry measurements were performed on an H format electrochemical cell using ACN/LiClO 4 as a solvent/electrolyte system.A graphite sheet was used as the counter electrode and Ag/AgCl as reference electrode.N 2 was bubbled through the system, shortly before measurements, in order to remove any dissolved O 2 .

Electrochemical polymerization
The cop-MB-PPV/DCN-PPV was synthesized by application of -2.0V (vs.Ag/AgCl) potential and the MB-PPV by application of -1.8V (vs.Ag/AgCl) potential.The more negative potential used to synthesize the copolymer was necessary because it was demonstrated that the formation of the DCN-PPV occurs in less negative potential values in relation to MB-PPV. 16For this reason the preparation of copolymer MB-PPV/DCN-PPV was held through the application of a more negative potential value in order to increase the amount of MB-PPV units in the copolymeric chain since the formation of DCN-PPV would be predominant in less negative potentials.The polymer formation was accompanied using achronoamperometric measurement technique and the current profiles as a function of time for the electrochemical formation of the two polymers are shown in Figure 1.
The behavior of the current with the elapsed time shows interesting information about the electropolymerization process, for example, in the case of the MB-PPV formation (Figure 1a) the current value remains practically constant during the time in which the starting compound is present in the reaction medium.This kind of profile indicates that the rate of the polymer formation is the same while the charge transfer process is possible.This homogeneity on electropolymerization kinetics is possible due to mechanical agitation of the solution during the process.In the case of the preparation of cop-MB-DCN-PPV (Figure 1b) the current profile as a function of time shows that the current value is greater at the beginning of the process and decreases with increasing time.It is observed that the current profile form three levels with values of ca.35, 28 and 25 mA, respectively.
This behavior indicates that the rate of polymerization is higher at the beginning and decreases during the polymer formation.This suggests that the copolymer formation occurs mainly by the preferential formation of blocks of DCN-PPV due to the fact that the overpotential for electropolymerization of the DCN-PPV is lower than for the MB-PPV. 16,17[18]

Spectroscopic analysis
The electrochemical polymerization of the MB-PPV homopolymer and the cop-MB-PPV/DCN-PPV over the mercury pool allows the production of a soluble and an insoluble fraction.The IR spectra of MB-PPV and cop-MB-PPV/DCN-PPV insoluble fraction are shown in Figure 2.
Both the soluble and insoluble fractions of the cop-MB-PPV/DCN-PPV presents typical C≡N band at 2222 cm -1 , although it should be noted that the ratio of the intensity of this band related to the other characteristic bands of the copolymer (for example in ca.1594, 1490 and 1379 cm -1 ) is smaller for the spectrum of the soluble fraction than for the spectrum of the insoluble fraction.Furthermore, only the insoluble fraction of this copolymer presents the typical DCN-PPV band at 505 cm -1 . 16These two observations can be an indication that, in condition of preparation used in this study, the insoluble fraction of the cop-MB-PPV/DCN-PPV contains more units of DCN-PPV than the soluble fraction.][18][19] The observations described above corroborate the discussions of the behavior of the current profile observed during the electrochemical production of the cop-MB-PPV/ DCN-PPV where the higher current value at the beginning of the reaction was assigned to the preferential formation of DCN-PPV blocks.Table 1 shows the band assignments   for the principal absorptions in the FTIR spectra of the studied polymers. 16he UV-Vis spectra of solutions of the soluble fraction of MB-PPV and cop-MB-PPV/DCN-PPV prepared in chloroform are shown in Figure 3a and 3b, respectively.Both spectra present a main absorption band around 320 nm and a shoulder at 395 nm.According to Brédas et al. 19 the transition in ca.320 nm could be assigned to the localized transitions l→π* and π→l* (where l represents localized levels and π represents delocalized levels).On the other hand the shoulder at 395 nm could be assigned to the strongly delocalized π→π* electronic transition.
The predominance of non-delocalized transitions is an indication that the polymers formed have a short average chain length.In fact, the method of electropolymerization in Hg pool produces the conjugated polymer in one step, and for this reason, the major chains formed remain in the insoluble form.However, the addition of O-CH 3 (methoxy) group increases the solubility, as can be seen when comparing polymers DCN-PPV and PPV obtained by Péres et al. 20,21 with the polymers obtained in this work.
Fluorescence emissions were also investigated in order to get information about the electronic structure of polymers as well as data about the process of radiation emission.Figure 4 shows fluorescence emission spectra of the soluble fraction in chloroform solution of and cop-MB-PPV/DCN-PPV, both containing 3.2 mg L -1 .The emission of the polymers was evaluated at four different wavelengths of excitation, 350, 400, 450 and 500 nm.It may be noted in Figure 4 that the emission spectra for the first three wavelengths of excitation present similar format with a small displacement of the emission maximum to the red region as excitation radiation is also shifted to red.This behavior is a strong indication that the emission process should occur from entities like aggregates or excimers formed by strong interactions between the polymer molecules.These entities have a tendency to emit at longer wavelengths than the isolated molecules.
The excitation spectra obtained for both polymers also indicate that the maximum of excitation does not match the maximum absorption of the UV-Vis spectrum (Figure not shown).These characteristics are typical of exciton emissions, which could be formed between chains, such as excimer and aggregates.Excimers are groups of two or more segments of molecules with a coplanar arrangement having a repulsive interaction in the ground state, which  becomes attractive when one of these molecules is excited.Aggregates are arrangements of molecules that form entities through attractive interactions in both the ground state and exited state. 18The formation of aggregates or excimers requires basically the same conditions.In the case of the excimer, the distance between the two parallel units must be between 3 to 4 Å to enable a π orbital overlap.For the formation of aggregates the space between the forming units must be smaller than in the case of excimer in order to provide a sufficiently large interaction to form a stable association in the ground state. 18,19Thus it can be inferred that the observed emission behavior of the polymers evaluated in this paper occurs due to the formation of excimers since the excitation spectra for both polymers does not match the absorption of the UV-Vis spectra.

Cyclic voltammetry of films
The cyclic voltammograms of the films of the soluble fraction of the polymers obtained in this work are shown in Figure 5.These measurements were performed by cycling the potential from 0.0 V (vs.Ag/AgCl) toward positive potentials until 1.5 V (vs.Ag/AgCl) in the limit range allowed by the solvent/electrolyte system used.In this case no current increase was observed concerning the oxidation of the MB-PPV or the cop-MB-PPV/DCN-PPV.
On the other hand, toward negative potential values it was observed that the cop-MB-PPV/DCN-PPV reduction process starts at less negative values in comparison with MB-PPV reduction process.The use of electrochemical data to estimate parameters of the electronic structure requires that the charge transfer process be reversible, which is not true for the processes noted above. 22,23However it can be observed that the reduction to cop-MB-PPV/DCN-PPV begins to flow through the system in less negative potentials compared to MB-PPV.These results indicate that the copolymer presents a more stable conduction band than MB-PPV, which is consistent with the presence of DCN-PPV blocks formed during electropolymerization process, as already shown by IR results.This observation indicated that the cop-MB-PPV/DCN-PPV formed at the Hg pool presented a D-A characteristic, and for this reason is a good candidate to be used as active layer in photovoltaic devices.

Conclusions
The electrochemical method has been effective for preparing soluble PPV-like polymer and copolymer, containing units with -O-CH 3 groups and/or -CN groups attached to the aromatic ring.The analysis of electrochemical measurements and infrared spectroscopy indicated that the copolymer cop-MB-PPV/DCN-PPV was obtained through the formation of blocks containing DCN-PPV units linked by blocks containing units of MB-PPV.The characterization of the electronic structure of the obtained polymers, performed by cyclic voltammetry measurements, UV-Vis and fluorescence spectroscopy allowed verifying that the conduction band of the copolymer is more stabilized than the conducting band of the homopolymer MB-PPV.Furthermore, the measurements of emission and excitation spectra indicated that the process of light emission in the case of dilute solutions of the copolymer prepared in chloroform occurs by means of the formation of excimers.The production of the cop-MB-PPV/DCN-PPV opens the possibility of using the electrochemical methods to produce interesting materials as PPV-like polymers with good properties to be used in photovoltaic devices with improved processability due to the increase of solubility caused by the presence of the -O-CH 3 moieties.

Figure 1 .
Figure 1.Profile of the current as a function of time for the potentiostaticelectropolymerization of a) MB-PPV (-1.8 V vs. Ag/AgCl) and b) cop-MB-PPV/DCN-PPV (-2.0 V vs. Ag/AgCl) in an electrolytic solution of 0.1 mol L -1 LiClO 4 in DMF using a mercury pool as working electrode.

Figure 5 .
Figure 5. Cyclic voltammograms of the polymer films obtained with the soluble fraction of -- --MB-PPV (-1.8 V vs. Ag/AgCl) and -- -cop-MB-PPV/DCN-PPV (-2.0 V vs. Ag/AgCl) at 80 mg L -1 .The films were obtained by drop casting of the polymeric solution in CHCl 3 over ITO subtract, scan rate = 50 mV s -1 .The black dash line shows the voltametric profile of the bare ITO surface on an ACN/LiClO4 0.1 mol L-1 solution used as solvent/electrolyte system in all the voltametric characterization.

Table 1 .
Assignments for the main IR bands observed for MB-PPV and cop-MB-PPV/DCNPPV at soluble and insoluble fraction a o-o-p: out-of-plane.