Solid-state nuclear magnetic resonance study of relaxation processes in MEH-PPV

A. C. Bloise,1 E. R. deAzevedo,1 R. F. Cossiello,2 R. F. Bianchi,1 D. T. Balogh,1 R. M. Faria,1

T. D. Z. Atvars,2 and T. J. Bonagamba1,*1Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, CEP: 13560-970, São Carlos - SP, Brazil

2Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, CEP: 13084-971, Campinas - SP, BrazilsReceived 12 October 2004; revised manuscript received 8 February 2005; published 23 May 2005d

Solid-state nuclear magnetic resonance methods were used to study molecular dynamics of MEH-PPV atdifferent frequency ranges varying from 1 Hz to 100 MHz. The results showed that in the 213 to 323 Ktemperature range, the motion in the polymer backbone is predominantly slowsHz–kHzd involving small anglelibrations, which occurs with a distribution of correlation times. In the side chain, two motional regimes wereidentified: Intermediate regime motions1–50 kHzd for all chemical groups and, additionally, fast rotations,100 MHzd for the terminal CH3 group. A correlation between the motional parameters and the photolumi-nescent behaviors as a function of temperature was observed and is discussed.

DOI: 10.1103/PhysRevB.71.174202 PACS numberssd: 36.20.Ey

Despite the success of polyf2-methoxy,5-s2’-ethyl-hexyloxyd-p-phenylenevinyleneg sMEH-PPVd as an elec-troluminescent active polymer,1,2 the knowledge of themolecular chain dynamics and its effects on the electrolumi-nescent properties is still deficient. Recent studies haveshown that polymer conformation and chain dynamics areespecially important for the luminescent properties of conju-gated polymers.3,4 It is also known that thermal-induced mor-phological changes can affect the photophysics, mainly forprocesses which involve interchain species.5,6 For instance, itis known that thermally induced torsional motion shortensthe conjugation length, and consequently induces changes inthe polymer photoluminescencesPLd.7 Moreover, it was alsoobserved that a substantial narrowing of the photolumines-cence band of MEH-PPV occurs below 220 K, although nofurther explanation was given.8

Modern solid-state nuclear magnetic resonancesNMRdmethods achieved great success in studying dynamics andstructural properties of organic materials.9 Using differentNMR methods, segmental dynamics and conformation canbe studied at a molecular level and very specific information,such as correlation times, reorientation angles, and their re-spective distributions, can be obtained in a broad range offrequencies. Despite the great development of advancedNMR for studying molecular dynamics, there exist so faronly few applications of such modern solid-state NMR tech-niques for investigating dynamics and structure of conjugatepolymers.10,11 In this work, a systematic study of theMEH-PPV chain dynamics using conventional and newadvanced solid-state NMR methods is presented. Slows1 Hz–1 kHzd, intermediates10 kHz–100 kHzd, and fastdynamicss,100 MHzd were probed by different NMR tech-niques.

Molecular motions occurring with rates of the order ofLarmor frequency s,100 MHzd can be probed usingT1-relaxation measurements.12 Dynamic processes occurringwith rates between 1 to 100 kHz are accessible by one-dimensional s1Dd-DIPSHIFT experiments. Such experi-ments, performed under magic-angle spinningsMASd, pro-vide a measurement of the13C–1H magnetic dipolar

coupling for each chemical group.13 This is done by measur-ing the dependence of the signal amplitude with the evolu-tion period st1d, used for codifying the13C-1H dipolarcoupling,13 which produces a typical curve that depends onthe strength of the averaged dipolar coupling,kndipl. Sincemotions with correlation times shorter than,10 ms averagethe dipolar coupling between1H and13C, from the measuredkndipl it is possible to distinguish rigid from mobile segmentsand estimate the amplitude of the molecular rotation. Mo-lecular order parameters,S, for each chemical group can alsobe obtained as the ratio between this averaged dipolar cou-pling and its respective rigid-lattice value,S=kndip/ndip

rigidl.Assuming wobbling in a cone model for CH carbons, theseorder parameters can be converted into the amplitude of mo-tions of the CH bond vector given by the opening angle ofthe cone according toS2=fcosus1+cosud /2g2, whereu isthe cone semiangle.14 Slow dynamicss1 to 1000 Hzd canbe studied by the centerband-only detection of exchangesCODEXd method.15,16 In summary, the experiment detectsthe signal reduction due to changes in the orientation-dependent chemical-shift frequencies, which results fromsegmental reorientations that take place during a longs,msto sd mixing time stmd. The tm dependence of the ratio be-tween each NMR line intensity,Sstm,Ntrd, and the respectiveline intensity obtained from a control experiment,S0s0,Ntrd,sthat does not encode any molecular motiond provides thetwo-time correlation function of the slow molecularmotions.15,16Other information that is taken from this experi-ment is the amplitude of the motion, which is determinedfrom the dependence of the normalized line intensities,Estm,Ntrd=Sstm,Ntrd /S0s0,Ntrd, with the period that the spinsystem has evolved under the chemical shift anisotropysCSAd, Ntr.

15,16 These modern methods are particularly at-tractive, because model free information about the moleculardynamics can be obtained.

MEH-PPV with M̄n=86.000 g/mol was purchased fromAldrich Chemical Company Inc. and studied as received.Films for photoluminescencesPLd experiments were castfrom a toluene solutionsthickness,800 mmd. Afterward,films were dried under a dynamic vacuum. NMR experi-

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ments were performed using a 9.4 T VARIAN INOVA spec-trometer in a 7 mm VT-MAS double-resonance probe headequipped with spinning speed control.

Figure 1sad shows the repeat unit and the13C CP/MASspectrum of MEH-PPV at 323 K with the corresponding lineassignments. Figure 1sbd shows the corresponding13C spin-lattice relaxation ratessT1

−1d as function of temperature. Itcan be observed thatT1

−1 is temperature independent for allthe carbons, except for the three methyl carbonss9, 15, and17d, where a slight decrease in temperature is observed. Thisindicates that the CH3 groups execute molecular movementsin the frequency scale of the order of Larmor frequencys100MHzd, and the estimated activation energies were about 0.1eV, which is typical for CH3 free rotations, as observed inRef. 12, where the deuterated methyl groups in differentmolecules were studied using2H T1 relaxation measure-ments and line shape analysis.

Figure 2sad shows the temperature dependence of themolecular order parameters for CH and CH3 groups inMEH-PPV measured using 1D-DIPSHIFT experiments. Themain chain carbons have temperature-independent order pa-rameters of approximately 1.0. The order parameter formethoxy carbonss9d is also temperature independent and theobtainedS value is 0.33±0.02, which shows that the CH3group motions correspond to axial rotations, occurringaround its C3-axis.13 For the CH carbons11d in the sidegroup, the order parameter remains equal to,1 below 230 Kand decreases monotonically with temperature, indicating again of mobility. CH3 carbons in the side groups15 and 17dalso exhibit aS decay as a function of temperature and, in

this case, it is smaller than 0.33. This is a consequence of thefact that these CH3 groups located in the side chain followthe motion of the side groups in addition to the axial rota-tions. Another interesting issue is the temperature depen-dence of dipolar coupling for CH2 groups along the sidechain. Because the dipolar coupling for CH2 carbon 10,ndip

C10,was found to be temperature independent, the order param-eters were calculated using the dipolar coupling of this groupas a reference,S=kndip/ndip

C10l. The temperature dependence ofthe order parameter for all side chain CH2 groups is shown inFig. 2sbd. As it can be observed, overall the order parametersincrease as a function of the proximity to the main chain.Additionally, the order parameters for side chain carbonss12, 13, 14, and 16d decrease as a function of temperature.For CH side group carbons11d, the average amplitude of themotions was estimated as,u.=s14±5d° ,s25±5d° ,s37±3d°, and s42±3d° at T=243, 263, 300, and 323 K,respectively, indicating the progressive increase of motionalamplitudes as a function of temperature.

Figure 3sad shows the two-dimensional13C s2Dd MAS-exchange spectrum of MEH-PPV. Cross peaks linking the

FIG. 1. sad 13C CP/MAS NMR spectra of MEH-PPVs* indi-cates the spinning side bandsd. sbd Temperature dependence of the13C spin-lattice relaxation ratessT1

−1d for all the carbons ofMEH-PPV.

FIG. 2. sad Plot of the molecular order parameters as a functionof temperature for CH and CH3 groups in MEH-PPV.sbd Same asin sad for CH2 groups.scd MEH-PPV PL spectra at several tempera-turessfrom 40 to 410 Kd. The inset shows the correlation betweenthe differential normalized PL intensities and order parameters atseveral temperatures.

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sidebands of Carbons 1, 4, 3, 6, and 9 are clearly observed,showing that these groups experience dynamics in theslow motion regime.17 This is confirmed by the series of13C CODEX spectra presented in Fig. 3sbd, which were ac-quired at 300 K, withtm=200 ms,Ntr =667ms. Exchange,Sstm,Ntrd, reference,S0s0,Ntrd, and pure-exchange,sS0−Sd,CODEX spectra are shown. NosS0−Sd intensity is observedin the pure-exchange CODEX spectrum for the 0–50 ppmregion, indicating the absence of slow motions of the sidegroups. In contrast, slow moving groups in the polymerbackbone are directly detected as a significantsS0−Sd inten-sity for the phenylene and methoxy carbons. Together withthe 1D-DIPSHIFT data, these results indicate that, while themain chain in MEH-PPV experiences slow dynamicsstc

,100 msd at room temperature, the side groups move fasterstc,100 msd.

Figure 4sad shows the13C CODEX normalized intensities,Estm,Ntrd, as a function of the mixing timetmsNtr =667msdfor backbone Carbons 1, 4, 3, 6, and 9, as well as for thesidegroupssthe line intensity was obtained by integrating thespectrum in the 0–50 ppm ranged at 300 K. TheEstm,Ntrdintensity for the side group carbons remains approximatelyzero for all mixing times, confirming the absence of the slowmotions in these molecular segments. In contrast, the back-bone carbons have significant exchange intensity. TheEstm,Ntrd versustm curves for the 1, 3, 4, and 6 phenylenecarbons are identical, while OCH3 carbon shows a differentplateau value. This is attributed to differences in the CSA,

which is smaller for OCH3 as compared to phenyl ringcarbons.18 TheEstm,Ntrd versustm curves were adjusted withthe stretched exponential correlation functionEstm,Ntrd= fmf1−exps−stm/t0dbdg,19 where fm represents the fractionof molecular segments moving in the millisecond to secondtime scale andb is associated to the width of the correlationtime distribution. The mean correlation time,ktcl, is calcu-lated fromt0 andb according toktcl=b−1t0Gs1/bd and areshown in Fig. 4sad. The smallb values<0.4d, accounts for arelatively large distribution of correlation timessabout 3 de-cadesd and suggests a significant dynamic heterogeneity forthe polymer backbone at this temperature. The motional am-plitudes involved in such processes were obtained by mea-suring theEstm,Ntrd versusNtr curves. Figure 4sbd showssuch curves for the 3,6 phenylene carbons. The simulationwas performed considering average motional amplitudes be-tween 18° and 32°, Fig. 4sbd reproduces the experimentaldata. The observed slow backbone motions might be inter-preted as small angle rotations, and their amplitudes reflectthe width of the rotational barrier distribution around theplanar configuration.20,21These slow torsional motions of themain chain may also play a role in the electroluminescenceand PL of the polymers. Because the time scale of the elec-tronic processes involved in the PL is at least six orders ofmagnitude faster than the time scale of the slow processesobserved here, the average reorientation angle ofs25±7d°

measured in the CODEX experiments can be interpreted asthe degree of chain disorder along the main chain. In Ref. 21,the backbone motion of PPV was investigated and a degreeof motional heterogeneity that is substantially smaller than inthe case of MEH-PPV was foundsb for PPV main chainmotion is,0.8 compared to,0.4 in the case of MEH-PPVd.This might be related to the presence of the long side group

FIG. 3. sad 13C 2D-MAS exchange spectrum performed under5.4 kHz. sbd Top: Reference CODEX spectrum; middle: CODEXspectrum; bottom: Pure-exchange CODEX spectrum.s* indicatesthe spinning side bandsd.

FIG. 4. sad Mixing time dependence of the normalized CODEXintensities.sbd Evolution time,Ntr, dependence of the normalizedCODEX intensity.

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in MEH-PPV, which not only suppresses some motionalmodessfor example ring flipsd, but also introduces changesin the microenvironment that makes the system more hetero-geneous.

Figure 2scd shows MEH-PPV PL spectra at several tem-peratures, from 40 to 410 K. As can be seen, there are threeregimes in this series of spectra. From 40 to,220 K, thespectral characteristics remain unchanged; between,220and 320 K width and spectral intensity remain mostly con-stant and a spectral blueshift is observed; above 320 K, be-sides the blueshift, the width and spectral intensity increasecontinuously. Theoretical calculations and experimental re-sults have shown that chain disorder is an important mecha-nism for blueshifting the PL spectrum of conjugatedpolymers.3,20,22,23 Thus, the presence of small angle mainchain motions at 300 K suggests that this blueshift of the PLbands is associated with the conformation disorder intro-duced by the small angle motion in the main chain as ob-served from CODEX experiments. The spectral changesstarting at 220 K can be attributed to the onset of theb-relaxation processes24 involving thermal activation oflateral segments of the polymer chain, increasing the confor-mational disorder. At temperatures above 320 K, the signifi-cant intensity increase and the blueshift in the PL spectraobserved are associated with the onset of the glass transition

processsmeasured by differential scanning calorimetry–notshownd, which involves dissociation of interchain species.The dissociation of interchain producing intrachain excitonsexplains the simultaneous increase in emission intensity andspectral blueshift. In fact, an increase can be also observed ofthe plateau of theEstm,Ntrd versustm curves for temperatureshigher than 300 Ksnot shownd. This is an indication thatthere is an increase in the amplitude of the main chain mo-tion as a function of temperature, characteristic of a glasstransition process.

Analyzing the behavior of the order parameters for sidechain carbonss11, 12, 13, 14, and 16d shown in Figs. 2sadand 2sbd, one can observe that it is about 1 at temperatureslower than,230 K and decreases for higher temperatures.Because the decrease of the order parameter is strictly relatedto the increase of the amplitude of the polymer chain motion,the correlation between the PL changes and the onset of sidegroup motion is apparent.8,25 Therefore, the modifications inthe PL spectra, which occur just above 220 K, might becorrelated to the change in the microenvironment due to theonset of the side group motion, which produces regionswhere interchain interactions are less effective than in thefrozen state. This shows that the secondary relaxation occur-ring in side chain is directly related with the processes thataffect the photophysics of MEH-PPV.

*Author to whom correspondence should be addressed; E-mail:tito@if.sc.usp.br

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