Vibrational, calorimetric and nonlinear optical studies of melaminium-bis(trichloroacetate) monohydrate molecular–ionic crystal

Optical Materials 29 (2007) 1058–1062 www.elsevier.com/locate/optmat

Vibrational, calorimetric and nonlinear optical studies of melaminium-bis(trichloroacetate) monohydrate molecular–ionic crystal S. Debrus a, M.K. Marchewka a

b,*

, M. Drozd b, H. Ratajczak

b,c

Laboratoire d’Optique des Solides, UMR CNRS 7601, Universite´ Pierre et Marie Curie (Paris 6), Campus Boucicaut, Case 80, Bureau 11-1-3 140, rue de Lourmel, 75015 Paris, France b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrocław 2, P.O. Box 937, Poland c Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland Received 2 November 2005; received in revised form 20 April 2006; accepted 21 April 2006 Available online 13 June 2006

Abstract The efficiency of second harmonic generation for melaminium bis(trichloroacetate) was estimated relatively to KDP: deff = 3.09deff (KDP). Room temperature FT IR and FT Raman spectra were recorded. Some spectral features of this new crystal are referred to corresponding one for melamine crystal as well as for other trichloroacetates. Differential scanning calorimetric measurements performed on powder sample indicate the phase transition point at approximately 276 and 239 K for heating and cooling, respectively.  2006 Elsevier B.V. All rights reserved. Keywords: Second harmonic generation; Phase transitions; Hydrogen bonded molecular–ionic crystals; Melamine; Trichloroacetates; FT Raman; FT IR

1. Introduction Some melamine (2,4,6-triamino-1,3,5-triazine) salts exhibit nonlinear optical behaviour [1,2]. In this particular phenomenon, the mutual orientation of the molecules in the solid state is important as well as their molecular structure. Trichloroacetic acid forms interesting complexes with amines and aminoacids. Rajagopal et al. [3] published the results of crystallographic studies at 105 K for L-prolinium trichloroacetate while Baran et al. [4] reported on the structure and polarised IR and Raman spectra of the solid complex betaine-trichloroacetic acid. Quite recently, the preliminary test for SHG (second harmonic generation) performed for molecular complex of 2-methyl-4-nitroaniline with trichloroacetic acid showed that this material is *

Corresponding author. Tel.: +48 71 343 5021/344 3206; fax: +48 71 344 1029. E-mail address: [email protected] (M.K. Marchewka). 0925-3467/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.04.004

a promising for NLO applications [5]. In contrary, similar test revealed no SHG light for 1:1 complex of 2-methyl-5nitroaniline with trichloroacetic acid, 1:1 complex of aniline with trichloroacetic acid (needle-shaped crystals) and 1:1 and 2:1 complexes of melamine with trichloroacetic acid [5]. It is interesting to mention here that the 1:1 complex of aniline with trichloroacetic acid existing in the form of plates, showed pronounced second harmonic generation with the determined efficiency deff = 0.7deff (KDP). This result is consistent with crystallographic studies as the complex crystallizes in noncentrosymmetric Pc space group of monoclinic system [6]. It has been shown that vibrational effects give rise to the molecular first hyperpolarisability. Therefore, it seemed to be worthwhile to characterize melaminium bis(trichloroacetate) with the help of vibrational (infrared and Raman) spectroscopy. A lot of works were performed to explain the behaviour of melamine molecule in the solid state [7–13]. Few papers with assignments of internal vibrations of melamine mole-

S. Debrus et al. / Optical Materials 29 (2007) 1058–1062

cule were already published. The fundamental frequency assignment for melamine and melamine d6 was done, almost 50 years ago in the classical paper of Jones and Orville-Thomas [8]. 2. Experimental 2.1. Preparation The starting compounds, melamine and trichloroacetic acid were used as supplied and prepared in the ratio of 1:3. The dissolved acid was added dropwise to the hot solution of melamine. The solution was cooled to room temperature and slowly evaporated during a few days till the colourless and transparent crystals appeared. The elemental analysis performed on Perkin–Elmer 2400 CHN analyzer is consistent with the 1:2 composition of melamine:trichloroacetic acid. Found: C – 17.48%; H – 2.20%; N – 17.14%. Calculated for C3H6N6 · (C2HO2Cl3)2 · H2O: C – 17.86%; H – 2.14%; N – 17.85%. 2.2. Spectroscopic measurements The vibrational measurements were carried out at room temperature. Infrared spectra were taken with a Bruker IFS-88 spectrometer in the region 4000–80 cm 1. Resolution was set up to 2 cm 1, signal/noise ratio was established by 32 scans, weak apodisation. Powder Fourier Transform Raman (FT Raman) spectra were taken with an FRA-106 attachment to the Bruker IFS-88 spectrometer equipped with Ge detector cooled to liquid nitrogen temperature. Nd3+:YAG air-cooled diode pumped laser of power approximately 200 mW was used as an exciting source. The incident laser excitation is 1064 nm. The scattered light was collected at the angle of 180 in the region

3600–80 cm 1, resolution 2 cm 1, 256 scans. Due to the poor detector response, the Raman counterparts of the infrared bands located above 3200 cm 1 are not observed in the spectrum presented in Fig. 1. The polycrystalline powders were achieved by grinding in agate mortar with pestle. Samples, as suspensions in oil, were put between KBr wafers. The powder infrared spectra were taken in Nujol and Fluorolube emulsions to eliminate the bands originating from used oils. The measured spectra are shown in Fig. 1. The wavenumbers of the bands and their relative intensities are provided in Table 1. Table 1 Wavenumbers (cm 1) and relative intensities of the bands observed in the powder infrared and Raman spectra of the melaminium bis(trichloroacetate) crystals IR 3639vs 3472w 3412s 3291ssh 3245s 3095vs 2714m

Raman

Assignment OAH stretch NH2 asym stretch NH2 sym stretch

3107vw 2708vwb

Combination tone: NH2 asym stretch – side chain out-of-plane CAN bend [8]

2673m 2252w 2190vw 2112vw 1957vw 1882vw 1829vw 1738vs 1710vs 1705vs 1681vs 1644vs 1599s 1570s 1504s 1492s

1735vw 1712vw 1681vw 1636vw 1606vw 1509vw

1458s 1449ssh 1401m 1379vs

1373vw

1370s 1342vs 1268wsh

Fig. 1. Room temperature powder FT IR and FT Raman spectrum of melaminium bis(trichloroacetate) crystal.

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1203w 1168m 1141m 1032w 1014vw 989vw 967w 943w 900wsh

1341w

C@O stretch NH2 bend NH2 bend NH2 bend COO asym stretch Exo CAN, in-phase stretch [9] Ring stretch [8] Side-chain asym CAN stretch [8] Ring: semi-circle stretch + exogenous CAN contract [9] and COO sym stretch COO sym stretch COO sym stretch Ring: semi-circle stretch + exogenous CAN contract [9] Ring: semi-circle stretch + exogenous CAN contract [9] Ring: semi-circle stretching + exogenous CAN contracting [9] Ring: Semi-circle stretch [12] Ring: semi-circle stretch + exogenous CAN contract [9] CAOH stretch Ring: semi-circle stretch [12]

1141vw

988vw 968vw 942vw

Ring breath [8] Ring breath [8] Triazine ring N, in-phase radial [12] NAH out-of-plane def Triazine ring breath [31] (continued on next page)

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S. Debrus et al. / Optical Materials 29 (2007) 1058–1062

Table 1 (continued) IR

Raman

892w 847m 829s 813msh

845w 831vw

785m 768m 754s 745ssh 701m 686s 630w 590w 573vw 554w 495vw 455vw 433w 420w 407vw 389vw

752w

686vs 581vw 561vwsh 554w 453w 431vs

391w 320w 288msh 283m 239vw 200m 188vw 163vw 114vs 103vs 83m

Assignment Ring breath [31] CCl3 asym stretch CCl3 asym stretch CAC stretch and ring-sextant out-of-plane def [12] Side-chain out-of-plane CAN bend [8] Ring-sextant out-of-plane bend [12] Ring-sextant out-of-plane bend [12] COO in-plane def Symmetric ring breath [31] Symmetric ring breath [31] NH2 twist Ring bend [8] Side-chain in-plane CAN bend [8] Side-chain in-plane CAN bend [8] NH2 twist q(COO ) or d(CCO) CCl3 sym stretch Ring: quadrant out-of-plane [12] Ring: quadrant out-of-plane [12] CCl3 sym def CCl3 asym def CCl3 rock Lattice vibration Lattice vibration Lattice vibration Lattice vibration Artifact

Abbreviations: s – strong, w – weak, v – very, sh – shoulder, b – broad, m – medium, sym – symmetric, asym – antisymmetric, rock – rocking, stretch – stretching, bend – bending, twist – twisting breath – breathing, def – deformation, contract – contracting.

2.3. Differential scanning calorimetric measurements DSC was carried out on a Perkin–Elmer DSC-7 calorimeter equipped with a CCA-7 low temperature attachment with a heating/cooling rate of 20 K/min. The sample of the mass approximately 36 mg was sealed in the aluminum caps. 2.4. Kurtz–Perry powder test SHG experiment was carried out using Kurtz–Perry powder technique described in [14]. The modified version of experimental setup was described and depicted in [15]. The calibrated samples (studied and KDP) were irradiated at 1064 nm by an Nd:YAG laser and the second harmonic beam power diffused by the powder sample (at 532 nm) was measured as a function of the fundamental beam power. 3. Assignments of the bands The bands observed in the measured region 4000– 380 cm 1 arise from the vibrations of protons in the hydrogen bonds, the internal vibrations of melaminium cations and from the vibrations of trichloroacetate anions. The

bands below 200 cm 1 in the Raman spectra arise from the lattice vibrations of the crystal. The very strong infrared band observed at 3639 cm 1 was not assigned to vibration of water molecule. This frequency is too high. Such a band may correspond to the stretching vibration of nonbonded OH group. Such a situation is possible when one acid moiety remains in molecular (nonionised) form. Therefore, the title crystal may be considered as a quasi-molecular system. Looking at Table 1 one can notice, that the position of most infrared bands and their Raman counterparts are almost the same (within the resolution limit): 1710(IR)/ 1712(R), 1681(IR)/1681(R), 1141(IR)/1141(R), 989(IR)/ 988(R), 967(IR)/968(R), 943(IR)/942(R), 847(IR)/845(R), 829(IR)/831(R), 754(IR)/752(R), 686(IR)/686(R), 554(IR)/ 554(R), 451(IR)/453(R), 433(IR)/431(R) and 389(IR)/ 391(R). This coincidence of frequency for infrared and corresponding Raman bands is an evidence for the lack of centre of inversion in the studied crystal. Here, the so-called mutual exclusion rule is not working and the g–u splitting is not observed. The striking feature in the infrared spectra (cf. Fig. 1) appears to be a very strong and broad absorption occurring in the region between 3500 and 2000 cm 1 assigned to the vibrations of very weak hydrogen bonds. 3.1. The vibrations of melaminium residues Few papers concerning the melamine molecule vibrations were already published [7–11,13,31] and constitute a good source for the assignment of observed infrared and Raman bands. According to crystallographic data, melaminium residues often form hydrogen bonds of NAH


N and NAH


O type. The complex formation is reflected especially on the symmetric stretching type of vibrations of these groups. The frequency of the band originating from this type of coupling rises from 3328 cm 1 in the melamine crystal [8] to 3412 cm 1 in the case of studied one. The difference is equal to 84 cm 1 and corresponds to well known blue-shift. Similar shift is observed in the region of NH2 bending type of vibrations. The very strong infrared bands located at 1710, 1705 and 1681 cm 1 are observed in the case of title crystal. The corresponding band for melamine crystal was observed at 1653 cm 1 [8]. Thus, the intermolecular interactions through the NH2 groups of melamine molecule cause the rising of their frequencies for bending type of motion, also. The most intense band in FT Raman spectrum is that at 686 cm 1. This band is also quite characteristic one for all melamine complexes. It is derived from the symmetric type of vibration of triazine ring. The location of this band was analysed in several crystals [16]. The complexation of melamine causes, in all cases, the rising of the frequency of analysed vibration compared to the value for melamine alone. It is suggested, that due to number of intense ionic and donor–acceptor types of interaction with environment the sym-triazine ring becomes more rigid.

S. Debrus et al. / Optical Materials 29 (2007) 1058–1062

Weak Raman band at 988 cm 1 originates from triazine ring N in-phase radial type of vibration [12]. This vibration does not couple with the substituent group and can be found in the narrow 969–992 cm 1 region. This is an excellent Raman group frequency. Such a band is present in the FT Raman spectra of all complexes obtained by authors. The Raman band at 581 cm 1 with an infrared counterpart at 590 cm 1 was assigned to the ring bending type of vibration. In the case of melamine crystal, the corresponding Raman band is observed at 582 cm 1. For other assignments of bands originating from internal vibrations of melaminium cations, see Table 1. Unfortunately, for some melamine bands the precise assignment remains an open question. 3.2. The vibrations of trichloroacetic acid and trichloroacetate anions The most important differences in infrared and Raman spectra of both the forms (neutral molecule and anion) are expected for bands derived from stretching vibrations of carbon–oxygen bonds. The previous studies of trichloroacetate species [17] were helpful in band assignments shown in Table 1. One can easily identify the bands corresponding to carboxylic acid C@O stretching mode as the very strong infrared band at 1738 cm 1 and very weak Raman band at 1735 cm 1, whereas the bands arising from the asymmetric stretching mode of carboxylate anion is observed at 1644 cm 1 as very strong infrared band with very weak Raman counterpart at 1636 cm 1. These two frequencies noticed in vibrational spectra at approximately 1700 and 1650 cm 1 can be considered as final evidence that molecular trichloroacetic acid as well as trichloroacetate ion are present in investigated compound. Both trichloroacetic acid and trichloroacetate ion give rise to the characteristic stretching vibrations of CCl3 group observed at approximately 840 cm 1 (antisymmetric, intense in the infrared spectra) and 427 cm 1 (symmetric, very intense in the Raman spectra). The observed frequencies are very similar in the case of both species. The strong infrared bands at 845 and 829 cm 1 with weak Raman counterparts at 845 and 831 cm 1 observed in the spectra of studied crystal were assigned to antisymmetric stretching type of vibrations of CCl3 group. The similar bands were observed at 839/836 cm 1 (IR/R) and 427 cm 1 (R) for the solid complex betaine-trichloroacetic acid [4]. In the case of anilinium trichloroacetate similar bands were noticed at 843/841 cm 1 (IR/R) and 830/ 827 cm 1 (IR/R) while for CCl3 symmetric stretching: 428/427 cm 1 (IR/R) [5]. For the complex of 3-nitroaniline with trichloroacetic acid the corresponding infrared bands are present at 841 cm 1 and 437 cm 1, respectively [5]. The strong shoulder observed at 745 cm 1 in the infrared spectrum of title crystal may correspond to the bending mode of ionised carboxylic group. The analogous very strong infrared band is present at 738 cm 1 in the spectrum of anilinium trichloroacetate and at 730 cm 1 in the infra-

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red spectrum of the complex 3-nitroaniline with trichloroacetic acid. According to Soliman [17] the other deformation modes of the COO group derived form trichloroacetic ions are expected at 689 cm 1 (for xCOO ) and at 453 cm 1 (qCOO ). Unfortunately, the very strong bands at 686 cm 1 corresponding to ring breath type of vibrations of melaminium cations dominate in vibrational spectra of title crystal and can mask the above mentioned band. The very weak infrared band at 455 cm 1 with weak Raman counterpart at 453 cm 1 were attributed to qCOO vibration of ionic form of trichloroacetic molecule. 3.3. Lattice vibrations The greatest differences between Raman spectra of studied crystals are observed in the range of lattice vibrations, i.e., for the wavenumbers lower than 200 cm 1. Instead of seven Raman bands – among of them four very strong – in melamine crystal, only two very strong bands at 114 and 103 cm 1 are observed in the case of melaminium bis(trichloroacetate) crystal. Additionally, a weak bands at 188 and 163 cm 1 are present. 3.4. Phase transitions Low temperature DSC measurements exhibit clearly the occurrence of phase transition of the first order at approximately 276 and 239 K for heating and cooling, respectively, accompanied with thermal effect equals to 0.25 J/g (see Fig. 2). 3.5. Second harmonic generation For the powder SHG efficiency we have obtained the following value relative to KDP: deff = 3.09deff (KDP). The quadratic dependence of the second harmonic green

Fig. 2. DSC diagram for title crystal.

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Acknowledgement

6000

This work was financially supported by Polish Ministry of Scientific Research and Information Technology (Project No. T09A 121 28).

I2ω (u.a.)

5000 4000

References 3000 2000 1000 0 0

200

400

600

800

1000 1200 1400 1600 1800 2000

I ω (u.a.) Fig. 3. The quadratic dependence of the second harmonic green light intensity (I2x) on the intensity of infrared exciting beam (Ix) for polycrystalline sample of title crystal.

light intensity (I2x) on the intensity of infrared exciting beam (Ix) for polycrystalline sample of title crystal is depicted in Fig. 3. One can compare this result with that (0.57) obtained for potassium hydrogen bis(trichloroacetate) crystal [18]. The conclusion arise that for such a big effect in the title compound, the melaminium cations should be responsible. 4. Summary Most infrared and Raman bands for melaminium cations and trichloroacetate anions corresponding to theoretical literature data were assigned. Comparison of vibrational spectra of title crystal with the spectra of melamine shows that many bands are shifted and new bands appeared. This indicates on complex formation. It is worthwhile mentioning here that the vibrational spectroscopic evidence points to the molecular–ionic complex comprising a melamine cation plus one trichloroacetic acid molecule and one trichloroacetate anion. It is very probably, that the observed phase transitions are order-disorder type and are connected with the ordering of orientationally disordered trichloroacetate anions, similarly like in the case of betaine-trichloroacetic acid [4]. Similarly to other melamine-based crystalline molecular–ionic complexes like 2,4,6-triamino-1,3,5-triazin-1, 3-ium tartrate monohydrate [1], tetrakis(2,4,6-triamino1,3,5-triazin-1-ium)bis(selenate)trihydrate [2], melaminium hippurate [5], melaminium iodate [5] and melaminium m-nitrophenolate [5], as well as to trichloroacetic acidbased complexes like 2-methyl-4-nitroaniline trichloroacetate [5] and anilinium trichloroacetate (plates) [5], the title crystal exhibits second order nonlinear optical properties. For rather pronounced efficiency, deff = 3.09deff (KDP), the melaminium cations may be responsible. These cations belong to so-called octupolar molecules widely described in literature [19–30].

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