Organic Electronics 8 (2007) 57–62

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Crosslinkable organic glasses with quadratic nonlinearoptical activity

Antonio Carella a, Roberto Centore a,*, Loıc Mager b, Alberto Barsella b,Alain Fort b

a Dipartimento di Chimica, Universita degli Studi di Napoli ‘‘Federico II’’, Via Cinthia, 80126 Napoli, Italyb IPCMS, UMR CNRS-ULP 7504, 23 Rue du Loess, BP 43, 67034 Strasbourg Cedex, France

Received 6 September 2006; received in revised form 27 October 2006; accepted 29 October 2006Available online 21 November 2006

Abstract

In this paper, the use of a pure benzimidazole based dimethacrylic push–pull chromophore has been investigated for thepreparation of thin films exhibiting quadratic nonlinear optical (NLO) properties. To stabilize these NLO properties, thechromophore orientation has been frozen in a noncentrosymmetric arrangement by cross-linking the material using ther-mal polymerization induced by suitable initiators. The films present a very stable quadratic optical activity up toT = 152 �C with SHG coefficient d33 of 14 pm/V measured at 1.9 lm.� 2006 Elsevier B.V. All rights reserved.

PACS: 82.35.Ej

Keywords: NLO; Benzimidazole; Cross-linked polymer; SHG

1. Introduction

In the last decade, huge efforts have beendeployed to optimize the nonlinear optical (NLO)efficiencies of organic materials functionalised withdoped or grafted push–pull chromophores [1–4].In this context, relatively simple formalisms havebeen developed as guidelines for the synthesis ofpush–pull molecules possessing large quadratichyperpolarizabilities. In the same way, the optimisa-tion of the geometric structure has led to a better

1566-1199/$ - see front matter � 2006 Elsevier B.V. All rights reserved

doi:10.1016/j.orgel.2006.10.008

* Corresponding author. Tel.: +39 081 674450; fax: +39 081674090.

E-mail address: roberto.centore@unina.it (R. Centore).

efficiency of the poling process by limiting the cen-trosymmetric chromophore–chromophore interac-tions. These improvements have led to theelaboration of polymeric materials with high secondorder nonlinear optical coefficients (r33, d33) [5,6].However, the temporal stability of such organicNLO materials is still an important challenge. Therelaxation of the orientation of the chromophores,inducing the loss of the NLO properties of the poly-mer, has generally to be improved when looking forapplications.

In this work, we have developed an originalapproach based on a pure crosslinkable NLO chro-mophore to elaborate a new organic materialdesigned for quadratic optical properties. This

.

58 A. Carella et al. / Organic Electronics 8 (2007) 57–62

method maximizes the concentration of the NLO-active functions as no NLO-inactive binder isrequired. Actually, the fully functionalised NLOpolymers strategy also limits the amount of binder.But, in general, grafting the chromophore to thepolymer chain is achieved in the preliminary syn-thetic step before any orientation process. As a con-sequence, the covalent binding of the chromophoresto the chain limits their orientational mobility andcan therefore restrain the orientation rate and extent[7].

The lifetime of the chromophore orientation, akey feature for the elaboration of efficient materials,mainly depends on the efficiency of freezing chro-mophore movements in the matrix. For polymericmaterials, this can be roughly correlated to the glasstransition temperature Tg, a long lifetime being gen-erally associated with high Tg polymers [8–13]. Forcross-linked materials [14–17], chromophore motionis limited compared with doped materials and there-fore a compromise has to be found between polingand relaxation efficiencies.

The precursor of our material is higher anorganic push–pull chromophore functionalised withtwo crosslinkable functions (see Fig. 1). This mole-cule, called DIMET, is the only monomer used toobtain the final three-dimensional organic network(organic glass). Thus, one can expect a high effi-ciency coming from the high chromophore concen-tration and from the high mobility of smallmolecules before cross-linking, the long lifetimeoriginating from the high cross-linked nature ofthe final material.

Fig. 1. Molecular scheme of DIMET (a), benzoyl peroxide (b)and t-butyl peroxide (c).

We report on the synthesis of this molecule, onthe preparation of the NLO material and on someof its specific properties.

2. Results and discussion

The material is obtained from the azo-benzimi-dazolic chromophore functionalised with two cros-slinkable methacrylate groups (see Fig. 1). Exceptfor the radical initiator incorporated in very smallquantities (2% wt), the unique component of thematerial is the chromophore itself. This differs fromcomparable approaches reported in the literature[14–17] as the present formulation of the materialdoes not include any significant amount of NLO-inactive binder.

Our approach avoids the problems of solubility,segregation or inhomogeneities frequently observedin doped polymers. It is based on the observationthat, even if DIMET chromophore can be crystal-line at room temperature, it also presents a largeovercooling effect for the liquid phase (melting pointat 102 �C) preserving the amorphous state over daysat room temperature and even after a two hourannealing at 60 �C.

The amorphous structure at room temperature ofrecently melted DIMET samples has been provedby X-ray diffraction and differential scanning calo-rimetry (DSC) measurements. The same behaviourhas been observed for samples obtained by meltingchromophore powders and for samples obtained byevaporation of chloroform DIMET solution. DSCmeasurements also show that the glassy amorphouschromophore phase has a 37 �C glass transitiontemperature and that, without initiator, no poly-merization occurs up to 250 �C; the degradationtemperature, Tdec = 300 �C, was determined bythermogravimetric analysis.

The amorphous structure and the chemicalhomogeneity of the precursor material originatethe very good optical quality of the final films whichis of great importance for optical applications.

The absorption spectrum of DIMET, measuredin chloroform, shows a maximum at kmax = 435 nmand an absorption cut-off at 575 nm. The quadraticnonlinear property of DIMET in chloroform solu-tion has been determined by electric field inducedsecond harmonic generation (EFISHG) technique[18] performed at 1907 nm. In these conditions,the resonance enhancement is limited and there isno re-absorption of the second harmonic signal.The value of 660 · 10�48 esu for the product

A. Carella et al. / Organic Electronics 8 (2007) 57–62 59

l Æ b(2x), (l is the ground state permanent dipolemoment and b the quadratic hyperpolarizability ofthe molecule at the second harmonic frequency),has been deduced from the experimentalmeasurements.

Two different formulations were elaborated, theironly difference being in the radical initiator. The firstone (BP) contains benzoyl peroxide while the secondone (tBP) relies on t-butyl peroxide. The two initia-tors have a different working temperature. DSCanalysis performed on films of chromophores andthermal initiators, in the proper amounts, indicatedthat decomposition of the latter starts at 85 �C forthe BP containing films and at 115 �C for tBP films.

Thin amorphous films were deposited on glasssubstrates by spin coating a DMF solution ofDIMET with 2% wt. of thermal initiator. The filmshave been submitted to poling-polymerization pro-cedures with a first common step at 60 �C for30 min, followed by a further step at 110 �C forthe BP films and 150 �C for the tBP ones, maintain-ing an electric corona potential of 6 kV throughoutthe entire process. After the polymerization process,the films were no more soluble in DMF.

A preliminary study of poling efficiency as afunction of temperature was performed on a filmof pure DIMET (i.e. without initiator). As shownin Fig. 2, the second harmonic generation signal(SHG) is stable when the electric field is applied at30 �C. When the temperature exceeds Tg the signalsharply decreases and finally almost totally vanishesat 60 �C, despite of the presence of the electric field.

Such behaviour could be ascribed to the enhance-ment of the material electrical conductivity beyond

Fig. 2. Normalized SHG signal at four different temperaturevalues for an unpolymerized sample during poling with a coronaneedle at 4.5 kV.

Tg. This phenomenon limits the amount of chargeswhich can be accumulated on the film surface dur-ing the poling process, leading to a lower effectiveelectric field and to a poor degree of orientation [19].

When the polymerization is enabled, the electri-cal conductivity of the system decreases as thedegree of polymerization increases and, in thisway, it is possible to orient the chromophores evenat higher temperatures as those used here (110 �Cor 150 �C). Therefore, it is more appropriate to referto the present situation as a simultaneous poling-polymerization process.

The stability of second order NLO activity in thecross-linked films was checked by applying a tem-perature ramp (1 �C/min) and measuring the inten-sity of SHG signals as a function of temperature.The behaviour of the samples containing the twodifferent initiators is shown in Fig. 3.

The depoling temperature Tdep is defined as thetemperature for which the second harmonic genera-tion signal is half the maximum. Fig. 3 shows thatfor the BP doped material Tdep = 116 �C and for

Fig. 3. NLO stability of the film containing BP (up) and tBP(down); a ramp of temperature (1 �C/min) was applied and thedecay of the SHG signal was observed.

60 A. Carella et al. / Organic Electronics 8 (2007) 57–62

tBP doped Tdep = 152 �C, which indicates its higherstability. This arises from a higher degree of cross-linking reached for tBP films, improving the freez-ing of the chromophore orientation. In fact, onpolymerizing a film at constant temperature, thesystem keeps reacting until the mobility of the mol-ecules involved is sufficiently high. However, as thepolymerization goes on, the system hardens moreand more, leading to a reduced mobility of the mol-ecules and rate of reaction. In order to have a higherdegree of cross-linking and therefore a higher rigid-ity of the final material, it is then better to start andperform the polymerization at higher temperature.It’s noteworthy that the stability of second orderNLO activity of our material is higher as comparedwith other cross-linked systems based on similarapproaches [16,17].

The NLO activities of the two different kinds offilms, measured by SHG experiments, are compara-ble and we measured a d33 coefficient of 11 pm/Vfor BP and 14 pm/V for tBP. It is worth noticing thatin a previous work on polycondensates (polyestersand polyurethanes) containing the same chromo-phore covalently attached to the polymer chain asside pendant, d33 coefficients close to 2 pm/V weremeasured at k = 1.536 lm [20]. By choosing a differ-ent approach, i.e. orienting chromophores while theyare not yet linked to the polymer, it is possible toreach a higher NLO activity of the material as a con-sequence of a better efficiency of the poling process.

3. Experimental part

3.1. N,N-bis (2-methacryloxyethyl)-aniline

N,N-bis-(2-hydroxyethyl)-aniline (10 g, 0.055mol) was solved in 10 mL pyridine (0.122 mol).Methacrylic anhydride (23.125 g, 0.15 mol) wasslowly added to the solution cooled in an ice-waterbath. N,N-dimethylaminopyridine had been previ-ously added in catalytic amount. The system was leftunder stirring in the dark for the first 30 min in the ice-water bath and subsequently at room temperature for24 h. The organic phase was then extracted twice withchloroform (2 · 100 mL chloroform), washed withwater and dried over Na2SO4. The solvent was thenremoved at reduced pressure, obtaining a yellow oil.Yield 65%.

1H-NMR (CDCl3, d, ppm): 1.926 (s, 6H); 3.684(t, 4H, J = 6.4 Hz); 4.319 (t, 4H, J = 6.4 Hz);5.563 (s, 2H); 6.083 (s, 2H); 6.720–6.815 (m, 3H);7.196–7.258 (m, 2H).

3.2. 1-Ethyl-2-(4-{[4-Bis(2-methacryloxyethyl)

amino]phenylazo}-6-nitro- benzimidazole)

(DIMET)

2-(4-aminophenyl)-1-ethyl-6-nitrobenzimidazole(3 g, 10.6 mmol), prepared as previously described[21], was placed in a flask containing 15 mL of waterand 2.7 mL 37% HCl; the suspension was cooled to 0–5 �C in an ice-water bath. A solution obtained dis-solving NaNO2 (0.797 g, 11.55 mmol) in 5 mL ofwater was added dropwise to the suspension understirring. Stirring at low temperature was continuedfor 30 min after the addition of nitrite solution.

Separately, a water–ethanol solution containingsodium acetate (2.970 g, 0.036 mol) and N,N-bis-(2-methacryloyloxyethyl)aniline (3.237 g, 10.6 mmol)was prepared. To this solution, cooled in an ice bathand under stirring, the suspension containing the dia-zonium salt was rapidly added. Immediately the colorof solution turned to red and in few seconds, a red pre-cipitated of the azo compound was formed. This com-pound was collected by filtration and then purified byliquid column chromatography using silica gel as sta-tionary phase and chloroform as eluent. Yield 55%.

Mp: 102 �C.1H-NMR (CDCl3, d, ppm): 1.563 (t, 3H,

J = 7.2 Hz); 1.940 (s, 6H); 3.815 (t, 4 H,J = 5.6 Hz); 4.369–4.444 (m, 6H); 5.602 (s, 2H);6.110 (s, 2H); 6.112 (d, 2H, J = 8.8 Hz); 7.853–7.944(m, 5H); 8.026 (d, 2H, J = 8.4 Hz); 8.256 (dd, 1H,J1 = 9 Hz. J2 = 2.2 Hz); 8.415 (d, 1H, J = 2.2 Hz).

Elemental analysis calcd. for C33H34N6O6: C64.91; H 5.61; N 13.76%. Found: C 64.70; H 5.60;N 13.85%.

4. Physico-chemical characterization

The thermal behaviour of the compounds wasstudied by differential scanning calorimetry (Per-kin–Elmer DSC-7, nitrogen atmosphere, scanningrate 10 K/min), temperature controlled polarizingmicroscopy (Zeiss microscope, Mettler FP5 micro-furnace) and thermo-gravimetric analysis (TAinstruments, air, 10 K/min). 1H-NMR spectra wererecorded with Varian XL 200 spectrometer. X-raydiffraction patterns were recorded on flat film cam-era, using Ni filtered Cu Ka radiation.

5. Processing

In a typical procedure a 15 wt% solution contain-ing DIMET and the thermal initiator (chromo-

A. Carella et al. / Organic Electronics 8 (2007) 57–62 61

phore/initiator molar ratio, 98/2) in DMF was spin-coated over a glass substrate (2.5 · 2.5 cm2) at500 rpm for 10 s and subsequently at 1000 rpm for60 s. Prior to use, the glass was cleaned by treatmentwith a 5 wt% basic detergent solution in an ultra-sonic bath for 30 min, rinsed with distilled waterand ethanol and finally treated in an ultrasonic bathfor 30 min in methanol and chloroform, respec-tively. Two different formulations were used to pre-pare films, differing for the thermal initiatorsemployed. The two thermal initiators employedwere benzoyl peroxide (BP) and t-butyl peroxide(tBP). The typical thickness of the films was foundto be around 350 nm.

6. Poling and cross-linking

Films for SHG measurements were poled with atungsten wire corona electrode (U = 30 lm) duringcross-linking. Films obtained from the two differ-ent formulations were preliminary poled in N2

atmosphere at 60 �C with an applied voltage of6 kV for 30 min; then, the films with A formulationwere polymerized at 110 �C for 4 h under the elec-tric field and in N2 atmosphere for the first 2 h,while for films with B formulation the polymeriza-tion temperature was 150 �C. The final commonstep for the two different films was the slow coolingdown at room temperature still under electric field.Finally, the field was removed and the samplesrecovered. The cross-linked films prepared withthis procedure are insoluble in chloroform andDMF.

7. Second harmonic generation measurements

The optical nonlinearities of the material aremeasured using second harmonic generation setup.The pump beam is generated by a Nd:YAG laseremitting 10 ns pulses at 1.064 lm with a 10 Hz rep-etition rate. Wavelength conversion to 1.9 lm isachieved using a high pressure H2 Raman cell.The beam is then focalized on the sample and theSHG signal at 950 nm is measured using a photo-multiplier equipped by a narrow-band interferomet-ric filter. The setup is used for both measurement ofthe d33 by the Maker fringes method and for themonitoring of the evolution of the SHG efficiency.Measurements were calibrated relative to a quartzcrystal slab: the experimental value ofdQuartz

11 ¼ 1:2� 10�9 esu at 1.06 lm was extrapolatedto 1.1 · 10�9 esu at 1.907 lm.

8. Conclusion

We have presented the results of the quadraticNLO activity of thin films obtained by thermopoly-merization of a pure chromophore functionalizedwith di-methacrylate function. The stability andthe efficiency of the NLO properties are comparablewith those observed for various doped or function-alized polymers described in the literature. How-ever, in our case, as the material precursor isbased on a single component with a weak crystalli-zation tendency, segregation and inhomogeneitiesdo not appear and the final material presents a highoptical quality. These results demonstrate the valid-ity of our approach. While the d33 efficienciesobtained are moderate, we expect further improve-ments by modifying the molecular properties toincrease the hyperpolarizability as well as to limitthe material conductivity during the poling step.

Acknowledgement

MIUR of Italy (PRIN 2004) is acknowledged forfinancial support of this research.

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