Second Order Nonlinear Optical Performances of Polymers Containing Imidazole and Benzimidazole Chromophores

Second Order Nonlinear Optical Performances of

Polymers Containing Imidazole and Benzimidazole

Chromophores

Antonio Carella,1 Roberto Centore,*1 Augusto Sirigu,1 Angela Tuzi,1 Alessia Quatela,2 Stefano Schutzmann,3

Mauro Casalboni2

1 Dipartimento di Chimica, Universita di Napoli ‘‘Federico II’’, Via Cinthia, 80126 Napoli, ItalyE-mail: [email protected]

2 Dipartimento di Fisica, Universita degli Studi di Roma ‘‘Tor Vergata’’, Via della Ricerca Scientifica, 1, 00133, Roma, Italy3 Dipartimento di Scienze e Tecnologie Chimiche, Universita degli Studi di Roma ‘‘Tor Vergata’’, Via della Ricerca Scientifica, 1,00133, Roma, Italy

Received: March 31, 2004; Revised: July 7, 2004; Accepted: July 21, 2004; DOI: 10.1002/macp.200400129

Keywords: hydrogen bonding; imidazole; nonlinear; optics; SHG; synthesis

Introduction

The synthesis of organic materials capable of competing in

terms of cost and performance with inorganics in the field

of second order nonlinear optical (NLO) applications is an

important challenge in modern materials science.[1]

The standard scheme developed to date for NLO organic

materials is based on high b, push-pull chromophores,

physically dispersed in a polymer matrix (guest-host

systems) or covalently bound to the polymer chain (homo-

polymers or crosslinked systems). The achievement of a

high degree of acentric alignment of the chromophore

units and the time stabilization of that order are two key

parameters for the actual performances of this type of NLO

organic material.[1–3]

A topic that has not yet been fully considered is the role

that hydrogen bonding can play in favoring the parallel

orientation of chromophores during poling and increasing

the time stability of the orientation after poling.[4] The

theoretical prediction of optimum hydrogen bonded struc-

tures is a complicated task in low molar mass compound

crystal engineering[5] and it is complicated even further in

the case of NLO polymers because of their frequently

amorphous nature and because of the effect of external

electric fields. However, one can reasonably expect that the

presence of both hydrogen bonding donor and acceptor

groups in the chromophore should be allowed as a

possibility. This requirement, within the general architec-

ture of push-pull chromophores for which high second

order molecular nonlinearities are reached with low

aromaticity heterocycles along the conjugation path,[6]

suggests that chromophores containing imidazole or

benzimidazole groups could be interesting candidates for

this type of investigation. In this paper we report the

synthesis and NLO behavior of some homopolymers based

on the two chromophores shown in Scheme 1.

Summary: Condensation polymers based on two differentNLO-active chromophores, namely 2-[4-[(4-N,N-dihydroxy-ethylamino)phenylazo]phenyl]-5(6)-nitrobenzimidazole(BZI) and 2-[4-(4-N,N-dihydroxyethylamino)phenylazo]-4,5-dicyanoimidazole (IMI), have been synthesized and theirbasic properties measured. The second order NLO propertiesof poled polymers were investigated by SHG procedures atdifferent temperatures (35, 65 and 85 8C) and exposure times.Resonance free d33 coefficients (l¼ 1 368 nm) took values inthe range 1.8–4.0 pm �V�1, but the time stability of theirSHG signal was fairly acceptable, particularly for the high Tg

(e.g. 188 8C) polymer for which a relaxation time (at 25 8C) of2.8 years has been determined.

BZI and IMI chromophores.

Macromol. Chem. Phys. 2004, 205, 1948–1954 DOI: 10.1002/macp.200400129 � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1948 Full Paper

Both chromophores contain good hydrogen bonding

donor and acceptor groups within the NLO active unit. In

particular, for the BZI chromophore, the chemical formula

of the 6-nitro tautomer has been reported while, in solution,

a mixture of 5-nitro and 6-nitro tautomers is expected.[7] We

note that the few reports found in the literature about

polymers based on benzimidazole-containing chromo-

phores[8] all deal with 1-alkyl-5-nitrobenzimidazole iso-

mers, which are less NLO active, as we have proved both

theoretically and experimentally,[9] and which lack the

hydrogen bonding donor group.

Starting from the two new chromophores, the polymers

in Scheme 2 were synthesized and characterized and they

will be discussed in the present paper.

Results and Discussion

Chromophores

A detailed account of the synthesis and properties of the

BZI chromophore is given elsewhere.[9] The IMI chromo-

phore was obtained in a high yield in a one step synthesis

starting from two commercially available products. Both

chromophores show good chemical stability and moder-

ately good second order molecular nonlinearity,[1–3] as

measured by the EFISH technique (see Table 1).

The drawing of the crystallographically independent unit

of IMI is reported in Figure 1.

As can be seen in Figure 1, the elongated and p con-

jugated part of the molecule, comprising the two aromatic

rings and the azo bridge, takes a planar conformation in both

the crystallographically independent molecules. Although

the crystal structure of IMI is centrosymmetric, and hence

not second order NLO active, some features of the packing

may be of interest for our discussion. In Figure 2 is shown a

molecular layer made of parallel molecules. Within the

layer, A and B molecules are bonded to each other through

strong hydrogen bonds involving N–H and N atoms of the

imidazole groups and the CN groups and OH groups of the

terminal chains (see Table 2). In addition, weak hydrogen

bonds[5b] of C–H� � �N type are also present.

The result is a tight bidimensional network of parallel H

bonded molecules. The whole three-dimensional packing

is obtained by piling up these layers along (bþ c) in a

centrosymmetric fashion through further H bonds. The

partial packing of Figure 2 suggests that with N–H and N

groups present in the chromophore, locally parallel ori-

entations of nearest to neighbor chromophore units can be

favored (through H bonding) and this may overcome, at

least in part, the tendency for a local antiparallel arrange-

ment which is favored instead by the dipole-dipole

intermolecular interactions.[2] The extension of this struc-

tural pattern to amorphous polymers containing the IMI (or

BZI) chromophore is obviously neither simple nor free

from some arbitrariness. However, we can remark on two

points. First, H bonding acceptor groups are present in the

other monomer units of the polymers (and also H bonding

donor groups in the case of PUBZI) and second, in the case

of amorphous systems, at variance with crystals, relevant

contributions of the entropy to the chemical potential are

present.

Polymers

Some thermal and analytical data of the polymers under

investigation are reported in Table 3. The phase behavior is

Scheme 1. BZI and IMI chromophores.

Scheme 2. Polymers synthesized from chromophores.

Table 1. Nonlinear optical properties of chromophores.

mba) lmaxb)

10�48 esu nm

BZIc) 940 472IMI 1 050 462

a) Measured by the EFISH technique in DMF at 1.907 mm.b) Measured in DMF.c) Datum for BZI is taken from ref.[9]

Second Order Nonlinear Optical Performances of Polymers Containing Imidazole and Benzimidazole Chromophores 1949

Macromol. Chem. Phys. 2004, 205, 1948–1954 www.mcp-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

rather simple since all the polymers are amorphous, both

when prepared and after annealing at temperatures 10 8Cabove the glass transition, as indicated by X-ray diffraction

analysis. The values of the glass transition temperatures

clearly reflect the effect of the presence of hydrogen

bonding donor/acceptor groups in the chromophore skele-

ton. In fact, a comparison between PUBZI and PE3BZI and

the polymers PU (Tg¼ 139 8C) and PE (Tg¼ 119 8C) of

ref.[10] (which differ only in the presence of benzoxazole

instead of benzimidazole) show that benzimidazole based

polymers have glass transition temperatures which are

about 40–50 8C higher. Also, it is remarkable that PUBZI

and PE3BZI are amorphous, while PU and (to a lesser

extent) the PE polymers of ref.[10] show some crystallinity.

Again, this could be related in some way to H bonding

and, in particular, to the possibility of the formation of

disordered patterns of H bonding as a result of the presence

of several H bonding sites, either in the chromophore or in

the other monomers.

The second order NLO coefficients of the poled polymer

films are reported in Table 3. From the change in the UV-

VIS spectra of the films before and after poling (Figure 3),

the order parameters of the poled films can be deter-

mined,[11–13] and the result is that they are rather high, in the

range 0.33–0.39. NLO measurements have been performed

at the fundamental wavelength of 1 368 nm which, con-

sidering the UV-VIS absorption features of the polymers

(see Figure 3), makes the data in Table 3 substantially free

from resonance enhancement. Taking this into account, the

measured NLO activity of the polymers can be considered

Figure 1. The two crystallographically independent molecules of IMI. Anisotropic displacementellipsoids are drawn at 30% probability level.

Figure 2. Layer of parallel H bonded molecules of IMI viewed down c. Strong and weakH bonds are dashed.

1950 A. Carella et al.


to be comparable with values reported to date for organic

NLO homopolymers.[4,8,14]

The time stability of the NLO activity of poled polymers

has been studied by means of an accelerated ageing test.[15]

Poled films of all polymers were kept in an oven at three

different temperatures (35 8C, 65 8C and 85 8C), and their

d33 was periodically measured (utilizing the same proce-

dure described above) over a period of 60 d. For each

oriented sample, the same thermal treatment was also

performed on a non-oriented film, and its UV-VIS spectrum

periodically recorded. These showed no change which

allowed us to exclude chemical degradation of the material

due to the thermal treatment. In Figure 4 we report, as an

example, the time behavior of the normalized d33 co-

efficient (i.e. d33(t)/d33(0)) of PE5IMI at two different

baking temperatures. For all samples, the time decay of

normalized d33 is compatible with a biexponential fit[15]

characterized by a first fast relaxation time (t1) followed by

a much slower one (t2). The latter is really related to the

thermal relaxation of the sample.[15] The t2 relaxation times

for the various polymers at the three different baking

temperatures are reported in Table 4.

Relaxation times at room temperature (25 8C) were

calculated through an Arrhenius-type fit[15–17] of the t2’s

measured at 85 8C, 65 8C and 35 8C (Equation (1) with

frequency factor A, activation energy Ea and the Boltzmann

constant k).

1

t2

¼ Ae�Ea

kT

� �ð1Þ

By plotting ln(1/t2) as a function of 1/T, the value of t2 at

room temperature can be extrapolated.[15a] The linear fit is

shown in Figure 5 for PUBZI as an example. The t2 values

extrapolated to 25 8C for all the polymers are reported in

Table 5. The data in Table 4 and 5 are noteworthy, indicating

good time stability of the NLO activity for all the polymers

and a very good one for PUBZI.

Actually, although the correlation between the extra-

polated t2’s and the glass transition temperature of the

polymers is clear (see Table 5 and Figure 6), the presence of

hydrogen bonding donor/acceptor groups in the chromo-

phore unit seems to play an additional, and in some cases

predominant, role if we consider that the glass transition

temperatures of the polymers are not particularly high and if

their behavior is compared with polymers of similar (or

even higher) Tg’s but based on chromophores lacking donor

H bonding groups, for which relaxation times far lower have

been reported.[4b,14a,17–19]

Conclusion

The polymers we have synthesized meet several of the

properties requested for materials used for second order

NLO applications.[1] In particular, they are easily prepared

in good yields from low cost, commercially available

products. They also possess good chemical and thermal

stability, good solubility and no crystallinity. Although the

NLO activity is only moderate, its time stability is high.

These features suggest that the introduction of strong H

bonding donor/acceptor groups in the chromophore can

actually improve some performances of the polymers which

are relevant to second order NLO applications.

Experimental Part

Synthesis of Chromophores

The synthesis of the BZI chromophore is given in detailelsewhere.[9] Here only analytical data are given.

2-[4-[(4-N,N-Dihydroxyethylamino)phenylazo]phenyl]-5(6)-nitrobenzimidazole (BZI)

1H NMR (DMSO-d6): d¼ 3.65 (s, 8H), 4.91 (s, 2H), 6.93 (d,2H, J¼ 8.79 Hz), 7.85 (d, 3H), 7.88 (d, 2H, J¼ 8.24 Hz), 8.18(d, 1H, J¼ 7.96 Hz), 8.39 (d, 2H, J¼ 8.24 Hz), 8.59 (s, 1H),13.73 (s, 1H).

C23H22N6O4: C 61.88, H 4.97, N 18.82; Found: C 61.69, H5.09, N 18.95.

Table 2. Hydrogen bonds for IMI. Symmetry codes: (1) xþ 1, y,z; (2) �x, �y� 1, �zþ 2; (3) �x� 1, �y� 1, �zþ 2; (4) x� 1, y,z; (5) �xþ 1, �yþ 1, �zþ 1; (6) �x, �yþ 1, �zþ 1.

D-H� � �A D� � �A <(DHA)

A degree

N4A-H� � �O2A(1) 2.729(6) 170(4)O1A-H� � �N5B(2) 2.833(6) 163(6)O2A-H� � �O1A(3) 2.716(6) 164(6)N4B-H� � �O1B(4) 2.782(6) 168(5)O1B-H� � �N7A(5) 2.926(6) 162(5)O2B-H� � �N3A(6) 3.203(7) 156(6)O2B-H� � �N5A(6) 3.516(7) 146(6)

Table 3. Thermal, analytical and NLO data of polymers.

Zinha) Tg Td

b) Tpoling d33c)

dL � g�1 8C 8C 8C pm �V�1

PUBZI 0.10 188 291 180 4.0PE3BZI 0.08 152 285 140 3.5PE3IMI 0.10 152 305 140 2.0PE5IMI 0.13 131 291 120 1.8

a) Measured at 25 8C in 0.5 g � dL�1 DMF solutions.b) Decomposition temperature taken as the temperature corre-

sponding to 5% weight loss in the thermogravimetric run (10 K �min�1, air atmosphere).

c) Fundamental harmonic at 1 368 nm.



Synthesis of 2-[4-(4-N,N-Dihydroxyethylamino)phenylazo]-4,5-dicyanoimidazole (IMI)

Commercial 2-amino-4,5-dicyanoimidazole (3 g, 22.5 mmol)was placed in a flask containing 20 mL H2O. The suspensionwas cooled to 0–5 8C in an ice water bath and 5.8 mL of 37%HCl were added. A solution obtained by dissolving NaNO2

(1.656 g, 24.5 mmol) in about 10 mL water was added drop-wise to the suspension under stirring. Stirring at low temper-ature was continued for 30 min after the addition of the nitritesolution was completed.

Separately, a water-ethanol solution containing sodiumacetate (6.350 g) and commercial bis(2-hydroxyethyl)aniline(4.078 g, 22.5 mmol) was prepared. To this, under stirring, thesuspension containing the diazonium salt was rapidly added.Immediately the color of solution turned to red and in a fewseconds a red-violet precipitate of the azo compound formed.The compound was collected by filtration and then recrys-tallized from DMF/H2O, obtaining pure IMI as a crystallinered-violet product with a yield of 81% and a melting point of230 8C.

1H NMR (DMSO-d6): d¼ 3.67 (s, 8H), 6.99 (d, 2H, J¼ 9.4Hz)),7.84 (d, 2H, J¼ 9.1 Hz).

C15H15N7O2: C 55.38, H 4.63, N 30.14; Found: C 55.42,H 4.70, N 30.26.

Synthesis of Polymers

2-Alkoxyterephthaloyl dichlorides were obtained as pre-viously described,[19] as well as dry pyridine and dry N-methyl-2-pyrrolidone. Commercial 2,4-tolylendiisocianatewas purified by vacuum distillation and stored under an inert(N2) atmosphere. All polymers were prepared by solutionpolycondensation. Equimolar amounts of chromophore (BZIor IMI) and 2,4-tolylendiisocianate or 2-alkoxyterephthaloyl-dichloride were dissolved in the solvent (NMP for polyur-ethanes, NMP/pyridine for polyesters) under stirring in an N2

atmosphere. After 4 h reaction, the mixtures were poured intomethanol/water, affording precipitation of the crude polymers.These were dissolved in DMF and reprecipitated in methanol/water (a few drops of a concentrated water solution of CaCl2were necessary in some cases for the coagulation of the pre-cipitate), recovered by filtration and dried in an oven at 80 8Cfor 2 d. Yields ranged from 67 to 74%. The 1H NMR spectra ofthe polymers were consistent with the expected structures, aswell as analytical data. The synthesis of PE3BZI is given indetail as an example.

PE3BZI

BZI (1.0 g, 2.2 mmol) and 2-propyloxyterephthaloyl dichlor-ide (0.539 g, 2.2 mmol) were dissolved in 2 mL anhydrousNMP and 0.5 mL anhydrous pyridine and kept at 120 8C understirring in an N2 atmosphere for 4 h. The solution was thenpoured dropwise into 100 mL of a methanol/H2O solution (9:1by volume). The precipitated polymer was collected by fil-tration, dissolved in 5 mL DMF and poured again into 100 mLof the methanol/H2O solution. The solid formed was collectedby filtration and dried in an oven. The yield was 67%.

Figure 3. UV-VIS spectra of polymer films before and after poling. a) PUBZI; b) PE3IMI.

Figure 4. Plot of normalized d33 vs time for PE5IMI at 35 8C(circles) and 85 8C (squares).

Table 4. Slow relaxation times of polymers.

Polymer t2

d

35 8C 65 8C 85 8C

PUBZI 850 570 380PE3BZI 260 90 60PE3IMI 320 140 90PE5IMI 190 80 40



(C34H30N6O7)n: C 64.35, H 4.76, N 13.24; Found: C 63.93,H 4.98, N 13.17.

Physical Measurements

The thermal and phase behavior of monomers and polymerswas studied by differential scanning calorimetry (Perkin-Elmer DSC-7, nitrogen atmosphere, scanning rate 10 K �min�1),temperature controlled polarizing microscopy (Zeiss micro-scope, Mettler FP5 microfurnace), TGA-DTA analysis (TAInstruments, air atmosphere, 10 K �min�1) and X-ray diffrac-tion (flat film camera, Ni filtered Cu Ka radiation). ProtonNMR spectra were recorded on a Varian XL 200 MHz spec-trometer. The polymer inherent viscosity (Zinh) at 25 8C wasmeasured with an Ubbelohde viscometer. DMF solutions wereused for all polymers.

X-Ray Analysis

Single crystals of IMI were obtained by evaporation of anethanol solution. Cell parameters were obtained through aleast-squares fit[20] to the y angles of 25 accurately centeredreflections in the range 8.368� y� 11.828 on an Enraf-NoniusMACH 3 diffractometer (Mo Ka radiation, l¼ 0.71069 A).The structure was solved by direct methods (SIR92 pro-gram[21]) and refined by the full matrix least-squares method onF2 against all independent measured reflections (SHELXLprogram of SHELX 97 package[22]). The largest peak and holein the last Fourier difference were 0.27 and�0.23 e � A�3. C, Nand O atoms were anisotropic. The H atoms of the OH and NHgroups were located in difference Fourier maps and their

coordinates were refined. The remaining H atoms were placedin calculated positions and refined by the riding model. Allcrystallographic data have been deposited with the CambridgeCrystallographic Data Centre (CCDC) under the depositionnumber CCDC 234232. These data can be obtained free ofcharge at www.ccdc.cam.ac.uk/conts/retrieving.html or from:The Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; Fax: þ44-1223/336-033; E-mail:[email protected].

Some crystal, collection and refinement data are reported inTable 6.

NLO Measurements

NLO characterization was carried out by performing SHGmeasurements on 0.1mm thick films deposited on BK7 Corningglass slides by spin-coating. For this purpose, appropriate

Figure 5. Arrhenius plot for PUBZI.

Table 5. Extrapolated t2’s at 25 8C.

Polymer t2 (25 8C)

d

PUBZI 1 040PE3BZI 370PE3IMI 440PE5IMI 280

Figure 6. Semilogarithmic plot of t2 extrapolated at 25 8Cversus the glass transition temperature of the polymer.

Table 6. Crystal, collection and refinement data for IMI.

IMI

Chemical formula C15H15N7O2

Fw 325.34T/K 293Cryst. syst. TriclinicSpace group P-1a/A 10.879(1)b/A 11.293(2)c/A 14.683(8)a/degree 68.90(3)b/degree 79.83(3)g/degree 66.95(1)V/A3 1 547.3(9)Z, Dx/(g � cm�3) 4, 1.397m/mm�1 0.099ymax/degree 27.97Data/params 7 449/451R1, wR2a) 0.0687, 0.1307

a) On F (I> 2s(I)).



amounts of the polymers were dissolved in N-methyl-2-pyrrolidone and the solutions filtered on 0.20 mm Teflon filters.The spinner was an SCS P.6204, operating at a rate of 1 000 rpmfor 30 s at room temperature. Residual solvent was removed bykeeping the films at 100 8C under a vacuum. The film thicknesswas measured with an Alphastep 200 profilometer. A brilliantQ-switched Nd:YAG laser (10 Hz repetition rate, 5 ns pulseduration, 400 mJ per pulse) followed by a Solid State RamanShifter CRS-14 was also used in the SHG experimentproviding the fundamental beam output at l¼ 1 368 nm. AMaker fringes reference experiment[23,24] was run to comparethe nonlinear optical activity of the films with that of a quartzcrystal (100) (d11¼ 0.335 pm �V�1)[25] in order to obtain thenonlinear coefficient d33 of the samples.

Orientation of dipoles was achieved by standard high-temperature high-voltage corona-poling in a nitrogen atmos-phere,[12,26] with a gold wire biased withþ 7.0 kVacross a 1 cmgap normal to the film. The HV generator was grounded at theheating stage and the poling current was carefully monitored tocontrol process efficiency. The SH signal was appropriatelycorrected depending on the sample thickness. Poling wascarried out by heating the sample at a temperature 10 8C lowerthan the Tg of the polymer with the electric field on. Thistemperature was maintained for 30 min under the electric field,after which the sample was cooled down to room temperatureat a rate of 1 8C �min�1. Upon the sample attaining room tem-perature, the poling field was cut off. The refractive index of thefilms was measured using the Brewster angle technique.[27,28]

The second-order NLO coefficient d33 was estimated on thebasis of the second harmonic signal intensity and the refractiveindices at the fundamental (1 368 nm) and double (684 nm)frequencies. In addition, absorption spectra measurements ofthe films were performed before and immediately after polingin order to independently estimate orientational order of thedipoles through the decrease in the absorption coefficients.[13]

Acknowledgements: The financial support of MIUR of Italy isacknowledged. We thank Dr. A. Fort of IPCMS-GONLO ofStrasbourg (France) for EFISH measurements and the CIMCF ofthe University of Naples ‘‘Federico II’’ for the Nonius X-rayequipment.

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