Effect of Ultrasonication on Optical Properties and Electronic States of Conjugated Polymer MEH-PPV

CHEM. RES. CHINESE UNIVERSITIES

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2008,24(5),653--657 Article ID 1005-9040(2008)-05-653-05

Effect of Ultrasonication on Optical Properties and Electronic States of Conjugated Polymer MEH-PPV WU Meng, YANG Gui-zhong, WANG Meng, WANG Wei-zhi, WANG Min and LID Tian-xi" Key Laboratory ofMolecular Engineering ofPolymers ofMinistry ofEducation, Department ofMacromolecular Science, Laboratory ofAdvanced Materials, Fudan University, Shanghai 200433, P. R China

Abstract Poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene](MER-PPV) solutions with different concentrations were prepared in chloroform for different ultrasonication times. The ultraviolet absorption and photoluminescence(PL) spectra of the MER-PPV solutions were measured, and the electronic states of the polymer chains under different experimental conditions were studied. The results showed that the effects of ultrasonication on the dilute and concentrated solutions were different. After ultrasonication, the intensity of the absorption peak at 280 run significantly decreased, relative to the absorption peak at 500 nm for both dilute and concentrated solutions, indicating that the proportion of the two excited states in the polymer chains had changed. For dilute MER-PPV solutions, the blue-shifted absorption(at about 500 nm) and PL spectra show that ultrasonication also led to polymer chain degradation and thus shortened the effective conjugation length. For concentrated solutions, however, the peak positions of the absorption spectra remained unchanged. In addition, the effects of the solution temperatures on the optical spectra for the MER-PPV solutions were also discussed. Keywords MER-PPV; Optical property; Electronic state; Ultrasonication

1 Introduction Conjugated polymers as organic semiconductive and electroactive materials have attracted a great interest in science and technology in the past decades. The ease of processing, in combination with many desirable mechanical properties of plastics, makes conjugated polymers quite attractive for a wide variety of applications including field-effect transistorsl', light-emitting diodes'", photocells'<", and lasers-". To realize the full potential of conjugated polymers, researchers have investigated their molecular parameters, which dictate photophysical behavior, by comparing the experimental data with the theoretical models. However, it is still not very clear whether the description of electronic excitations in those materials is most appropriately formulated within a molecular(1ocalized) or semiconductor(delocalized, band-theory) picture. Kohler et al.16J studied the electronic states associated with optical excitations in the visible and ultraviolet ranges for conjugated polymers, by means of photocurrent measurement and quantum-chemical calcula-

tions. They found that the mixing of delocalized valence-band states with localized states on the molecular units produced a sequence of excited states in which positive and negative charges could be further separated at higher energies. Being an important kind of conjugated polymer, polyt Ld-phenylene vinylene)(PPV) and its derivatives are widely studied because of their excellent semiconductor and luminescent properties[7-12 J Within the class of PPVs, poly[2-methoxy-5-(2'-ethyl-hexyloxy)1,4-phenylene vinylene](MEH-PPV) exhibits characteristics that make it particularly favorable for photoelectric device fabrication. The chemical structure of MEH-PPV[shown in Scheme I(A)] is comprised of an aromatic backbone with slightly polar alkoxy side chains in the repeating units. It has eight conjugated atoms in one unit cell and is hence expected to have eight Jr-electron energy bands. As indicated in the schematic band structure in Scheme I (B)[13 J, four of those bands are occupied in the ground state( among which three Jrbands are delocalized and labeled as d.,

*Corresponding author. E-mail: [email protected] Received October 16,2007; accepted December 8,2007. Supported by the Shanghai Leading Academic Discipline Project(No. B113) and the Program for New Century Excellent Talents(NCET) in University ofChina(No.NCET-04-0355). Copyright © 2008, Jilin University. Published by Elsevier Limited. All rights reserved.

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dz, and d3, and another localized T[ band is labeled as l); and the other four bands are unoccupied(i.e., three

delocalized

bands are labeled as d, *, dz*, and d3*

T[*

and another localized T[* band is labeled as I*). Theoretical studies of the electronic structure of PPY and its derivatives, such as the band theory models[14,151 and the quantum chemical methods[6,161, are successful in accounting for the observed optical spectral features in some aspects. On the other hand, some spectroscopic data are also helpful in understanding the electronic states of PPYs. Miller et al.[171 have proved two kinds of the electronic distributions on MER-PPY main chains using the polarized ultraviolet absorption experiments: one electronic distribution has been polarized in the direction parallel to MER-PPY main chains; the other was polarized in the direction perpendicular to the chain axis. Despite a large number of photophysical studies, the nature of the excited states of PPYs still remains controversial, and there are few reports that discuss whether and how the environmental factors, such as temperature, intermolecular interactions or external forces, influence and change the distribution of the electronic and excited states in conjugated polymer chains. (A)

E(k)

(B)

=

d; +-

d,•

=-1" d\'

-------------- ---------------- 0 d,

-------t-----_d, k

Scheme 1

Chemical structure of MEH-PPV(A) and schematic electron band structure of PPVI131(B)

Ultrasonication is a commonly used method to prepare polymer solutions as well as achieve homogeneous dispersion of nanofillers in the polymer matriX[18- Z01. At the same time, however, ultrasonication als5 has strong effect on the polymer' chains at high temperatures and high pressures because of the forma-

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tion, growth, and rapid collapse of microscopic bubbles during the ultrasonic process(21]. In this study, the authors investigated the effect of ultrasonication on the optical excitations in the visible and ultraviolet ranges for MER-PPY solutions by means of ultraviolet(UY) absorption and photoluminescent(PL) spectra measurements. In addition, the effect of temperature on the optical properties of MER-PPY solutions was also discussed.

2

Experimental

MER-PPY used was purchased from Sigma-Aldrich Co. The number-averaged molecular weight of the polymer was determined to be 56000 g/mol by gel permeation chromatography(GPC) with polystyrene standard. The concentrated solutions with a concentration of 3 mg/mL and the dilute solutions with a concentration of 0.005 mg/mL were prepared by dissolving an appropriate amount of MER-PPY into chloroform. The MER-PPY solutions with different concentrations in sealed vials were then ultrasonicated at a frequency of 59 kHz and a power of 160 W for 1, 3 or 5 h in water-bath with the aid of an ultrasonic instrument(Model SK3300LR, from Kudos Co.) at room temperature. Uv-Visible absorption spectra of MER-PPY solutions were recorded on a SRIMADZU UV-3150 spectrophotometer with tungsten and deuterium lamps as light sources and 1 mm thick cuvette at room temperature. The variable temperature UY absorption measurements were performed with a double-beam spectrophotometer(JASCO Y-500) in which the temperature controller was connected to the sample holder and the temperature measured directly from solution could be controlled with an accuracy of about ±0.5 °C via a water circulation system. The PL emission spectra were measured at room temperature on a single-beam spectrophotometer(SRIMADZU RF-531Opc) with axenon lamp as the excitation source.

3

Results and Discussion

Both the dilute and concentrated MER-PPY solutions were used to investigate the effect of ultrasonication on the optical properties. After ultrasonication treatment, the concentrated(3 mg/mL) MER-PPY solution was diluted to 0.005 mg/mL for the subsequent UY absorption and PL measurements, whereas, the dilute(0.005 mg/mL) MER-PPY solution was used

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directly after ultrasonication for the measurements. The solution concentration of 0.005 mg/mL for the optical measurements is low enough to well suppress the polymer chain entanglements and thus to show intrinsic information about electric states of (A)

3

-0-0 h --lh -b-3h -<;>-

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MER-PPV molecules. The UV absorption and PL spectra of the dilute and concentrated MER-PPV solutions under ultrasonication for different times are shown in Fig. 1 and Fig.2, respectively. -0-0 h --lh -b-3 h -<;>- 5 h

(8)

5h

250 300 350 400 450 500 550 600 }Jnm

500

550 Alnm

Fig.1 Normalized UV absorption spectra(A) and PL spectra(B) of MEH-PPV/chloroform solutions(0.005 mg/mL) at room temperature under ultrasonication for 0, 1, 3 and 5 h (A) 3

-0-0 h --lh -b-3h

(8)

-<;>-

250 300 350 400 450 500 550 600 Alnm

Fig.2

500

550

5h

650 Alnm

Normalized UV absorption spectra(A) and PL spectra(B) ofMEH-PPV/chloroform solutions(diluted from 3 mg/mL to 0.005 mg/mL) at room temperature under ultrasonication for 0, 1, 3 and 5 h

In the UV absorption spectra of the dilute and concentrated MER-PPV solutions in Fig.l(A) and Fig.2(A), there are three absorption peaks(named peak 1, peak 2, and peak 3 from long to short wavelength) which are assigned to two types of excited states in MER-PPV chains!61. The first type of excited state arises from the excitations between the same types of orbits. The absorption peak 1 at about 500 nm and peak 2 at about 340 nm belong to this excited state. They are created by the electronic transition from the delocalized occupied orbitals to the delocalized unoccupied orbitals(i.e., d~d') and these excited states are polarized in the direction parallel to the polymer chains!6.15.221. The lowest excited species(peak 1) is responsible for the luminescent property of the polymer. The second type of excited state(termed by charge-transfer state) corresponds to the excitations polarized in the plane of the zr-network with strong contributions perpendicular to the chain axiS!6,17 J• There is a probability of the electron and hole separatioa by a few phenylene rings in this excited state. In contrast to the first type of excited state, the configu-

ration-interaction description of the second type of excited state involves the electronic transitions from delocalized occupied molecular orbitals to localized unoccupied levels and vice versa(i.e., d~l' or l~d"). The absorption peak 3 at about 280 nm belongs to the charge-transfer state. With respect to the intensity of peak I, the intensity of the UV absorption peak 3 decreases with increasing ultrasonication time for both dilute and concentrated MER-PPV solutions, as shown in Fig.I(A) and Fig.2(A). As peak 3 originates from the charge-transfer state and the electron and hole separation may occur on a few phenylene rings in this excited state, the decreased intensity of peak 3 relative to that of peak I indicates that the proportion -of the delocalized free electron-hole pairs decreases in MER-PPV system during ultrasonication process. Therefore, ultrasonication can significantly change the relative proportion of the two excited states of MER-PPV chains in both concentrated and dilute solutions. The position of the peak I is slightly and steadily

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CHEM. RES. CHINESE UNIVERSITIES

blue-shifted with increasing the ultrasonication time for the dilute MEH-PPV solutions, as shown in Fig.l(A), which is different from the case of concentrated solutions[Fig.2(A)]. The PL spectra of the dilute and concentrated MEH-PPV solutions excited at 500 nrn, corresponding to the UV absorption peak 1, are shown in Fig.l(B) and Fig.2(B) as a function of ultrasonication time, respectively. Being similar to the UV absorption spectra, with increasing the ultrasonication time, the maximum of PL emission spectra exhibits blue-shift for the dilute solutions[Fig.l(B)], but shows almost no changes for the concentrated MEH-PPV solutions[Fig.2(B)]. The blue shift of both UV absorption and PL spectra for the dilute MEH-PPV solutions probably indicates that the effective conjugation length of MEH-PPV chains decreases upon ultrasonication process. There are two effects that may result in the decrease in effective conjugation length of MEH-PPV: depolymerization of the polymer chains[23-25J and the changes in conformation states of polymer chains. As mentioned earlier, transient high temperature and pressure caused by collapse of microscopic bubbles in solutions may occur during the ultrasonication process. Therefore, ultrasonication may result in degradation or destruction of MEH-PPV chains to a certain extent. Although the polymer chain conformations may also be changed through the rotation of C-C single bonds upon ultrasonication, according to the molecular dynamics and the principle of energy to be the lowest, the changes of polymer conformations during ultrasonication process would be weakened after removing the external force and the polymer chains would be stabilized to their original conformation states if holding enough time. After being stored in the sealed vials for 48 h, the dilute MEH-PPV solutions were used for the UV absorption and PL spectra measurements once again, and the same results were obtained(not shown here for abbreviation). Therefore, it seems that the possible reason for the decrease of the effective conjugation length upon ultrasonication is the degradation of polymer chains instead of the changes of their conformation states. It should be noted that the peak positions of UV absorption and PL spectra remain unchanged for the concentrated MEH-PPV solutions(Fig.2), indicating that the effective conjugation length of. the polymer chains does not change much in the concentrated solu-

tions under ultrasonication. MEH-PPV chains tend to aggregate or entangle in the concentrated solution because of the stronger interchain interaction, compared with those in the dilute solutions. The results illustrate that ultrasonication has much effect on the intermolecular interaction but less effect on the intramolecular bonding force for the concentrated MEH-PPV solutions. Therefore, the aggregation or entanglement of MEH-PPV chains may be changed to some extent, but the effective conjugation length of polymer chains in the concentrated solutions remain almost unchanged during ultrasonication process. As it is known, temperature is also an important factor that influences the optical properties of conjugated polymers[26-29J. To investigate the effect of temperature on the optical properties and electronic states of MEH-PPV, the variable temperature UV absorption measurements were performed for the dilute MEH-PPV solution(O.005 mg/mL) in a temperature range of 20-60 "C, as shown in Fig.3. The intensities of the UV absorption peak 3 relative to those of peak 1 remain almost unchanged, indicating that the electronic states ofMEH-PPV chains do not change in the temperature range investigated. It can be observed that the UV absorption peak 1 blue-shifts with increasing temperature, as also reported by Traiphol and co-workers[29J. It is probably because increasing temperature can activate the rotational dynamics and disrupts the approximately coplanar configuration of the phenyl rings within each chromophore, thus decreasing the effective conjugation length of MEH-PPV chains. --<>-20 'C -<>--30 'C --.>-40 'C

1 .....,....50·C ~60'C

250

Fig.3

Normalized UV absorption spectra of MEH-PPV/ chloroform dilute solutions(O.005 mg/mL) as a function of temperature

As discussed earlier, the effective conjugation length of MEH-PPV chains has been decreased because of ultrasonication(for the dilute solutions) or high temperature. On the other hand, the relative proportion of the two excited states of MEH-PPV chains

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has changed under ultrasonication for both dilution and concentrated solutions, but does not vary at different temperatures. These indicate that the transient high temperature and pressure during the ultrasonication process result in the transformation of the excited states of the polymer as well as the degradation of polymer chains. There is no evidence that the decrease in the effective conjugation length leads to the transformation of the excited states of polymer, as illustrated by the results of the variable temperature UV absorption spectra.

4 Conclusions The influence of ultrasonication on the optical properties and the electronic states of MEH-PPV in the dilute and concentrated solutions of chloroform has been investigated. With increasing ultrasonication time, the intensity of the absorption at 280 urn decreases relative to that of the absorption peak at about 500 urn, indicating that the proportion of the delocalized free electron-hole pairs decreases and the excited state of localized excitons is in the ascendant for both dilute and concentrated solutions. At the same time, ultrasonication results in the degradation of polymer chains and shortens the effective conjugation length for the dilute MEH-PPV solutions, as indicated by the blue-shift of the UV absorption spectra at about 500 urn and PL spectra at about 555 urn. It seems that temperature has little effect on the electronic states, but induces the decrease of the effective conjugation length of MEH-PPV chains under the investigated temperature range. References [1] Gamier E, Hajlaoui R., Yassar A., et 01., Science, 1994,265(5179), 1684 [2] Burroughes 1. H., Bradley D. D. C., Brown A. R., et aI., Nature, 1990, 347(6293),539 [3] Halls J. J. M., Walsh C. A., Greenham N. C., et 01., Nature, 1995, 376(6540),498

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