Polarized IR spectra of resonance assisted hydrogen bond (RAHB) in 2-hydroxyazobenzenes

Chemical Physics 326 (2006) 458–464 www.elsevier.com/locate/chemphys

Polarized IR spectra of resonance assisted hydrogen bond (RAHB) in 2-hydroxyazobenzenes Maria Rospenk, Paulina Majewska, Boguslawa Czarnik-Matusewicz, Lucjan Sobczyk

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Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, PL-50383 Wrocław, Poland Received 7 December 2005; accepted 2 March 2006 Available online 13 March 2006

Abstract The polarized IR spectra in the region 4000–400 cm1 over the temperature range 298–370 K of liquid crystalline (LC) 4-chloro-2 0 hydroxy-4 0 -pentyloxyazobenzene (CHPAB) containing strong O–H  N RAHBs were studied. It has been established that molecules of this compound undergoes a spontaneous ordering in thin layers (10–20 lm) between the KRS-5 plates. The order degree expressed by the S parameter exceeds 0.6 for the Smectic A and crystalline phases. The best indicator of orientation is the mode at 1084 cm1 as its transition dipole moment is oriented parallel to the long axis of the molecule. A good marker is also the c(OH) band with the transition dipole moment nearly perpendicular to the long axis. The intramolecular O–H  N hydrogen bonding shows features characteristic of RAHB. The transition dipole moment of the m(OH) vibrations forms with the long axis of the molecule the angle equal to 43 ± 3° (the OH bond is oriented to this axis at the angle of 9°) that convincingly speaks in favour of a coupling between the proton and p-electron motions. Similar behaviour is manifested by a broad absorption in the finger print region that can be interpreted in terms of the modification of the potential energy shape due to the plane-to-plane intermolecular interaction and appearance of the second minimum. A marked ordering of molecules in the isotropic phase is also observed evidencing some alignment of molecules extended far beyond the monomolecular layers on the surfaces of the KRS-5 windows. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Resonance assisted hydrogen bond; Liquid crystal; Polarized IR spectra

1. Introduction As shows already rich literature [1–10] the polarized IR spectra of liquid crystals (LCs) yield interesting information from the point of view of both the degree of ordering in mesophases and characteristics of particular vibrational modes. Moreover, in the case of liquid crystals there are possibilities to prepare oriented samples, and applying the polarized light, to measure the direction of the transition dipole moment [2]. The orientation of samples can be achieved by using various methods and particularly by application of the electric or magnetic fields. In many cases the orientation of molecules can be simply achieved on

rubbed plates with a leather cloth or by covering the surfaces by active layers able to adsorb molecules with parallel or perpendicular alignment [1,2]. On the other hand, the LCs can be used as liquid matrices in which the molecules dissolved are oriented parallel to the mesomorphic units. There is a broad list of nematic liquid crystals applied as liquid matrices in studies of IR spectra [1]. Recently, we have found that liquid crystalline 4-chloro2 0 -hydroxy-4 0 -pentyloxy-azobenzene (CHPAB) undergoes spontaneous orientation perpendicular to KRS-5 windows [11,12]. In the present paper, we used this property to analyze the degree of ordering passing through the particular phases liquid crystalline phases

*

Corresponding author. Tel.: +48 71 3757237; fax: +48 71 3282348. E-mail address: [email protected] (L. Sobczyk).

0301-0104/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.03.013

Solid phase

→ 68.9 ˚C

Smectic A

→ 82.7 ˚C

Nematic

→ Isotropic phase 91.8 ˚C

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by means of polarized IR spectra successfully applied to studies of orientation of whole molecules or their groups. In our case a particular interest evoked the behaviour of intramolecular OH  N hydrogen bond. This very strong resonance assisted hydrogen bond (RAHB) revealed in an intensive very broad absorption in the finger print region [11] is illustrated in Fig. 1. This absorption is not manifested in CCl4 solution or it is very weak in the isotropic phase. The determination of the direction of the transition dipole moment of this band, induced by ordering of molecules, seemed to us important in its interpretation. Moreover, it is worth to mention that in our case the hydrogen bonds formed in the rigid part of molecules have a precise orientation. Thus, one could expect that it would be possible to get valuable information on the transition dipole moment direction that is important in analysis of RAHBs. The conception of RAHB was formulated by Gilli et al. [13] and then developed and applied in several contributions related to various hydrogen bonded systems [14–19]. These systems were analysed in connection with structural and spectroscopic properties in the review [20]. Of great importance for the problems discussed in the present paper are results presented recently for the tautomeric NH  O/ N  HO system [21]. In the cited papers such important aspects as contribution of a covalent character of RAHBs and the shape of the potential for the proton motion are analyzed. Already a long time ago the problem of the charge distribution in conjugate – chelate hydrogen bonds and its impact on the vibrational spectra were studied. For instance, it was shown [22] that the interaction dipole moment in conjugate – chelate H-bonds is not consistent with the bridge direction. Very important results are dealing with the intensity of the IR absorption bands ascribed to m(AH) vibrations in AH  B bridges. In some cases their

459

intensity is so low that it was difficult to place them properly [23]. Quantitative results were presented for phenols having a proton acceptor both unconjugated and conjugated with the benzene ring [24]. As showed Brzezinski and Zundel [25], who compared the behaviour of 2-quinuclidine carboxylic acid N-oxide and 2-pyridine carboxylic acid N-oxides, the continuous IR absorption ascribed to the high polarizable H-bonds either drops or even disappears for the second molecule. Of great importance in recognition of conjugate – chelate H-bonds are results collected by Filarowski and Koll for Schiff and Mannich bases [26]. Quantitative analysis has shown a remarkable decrease of intensity of m(OH) vibrations in Schiff bases due to the formation of the quasi-aromatic H-bond ring. Specific features of RAHBs in Schiff bases should also be related to the potential energy shape for the proton motion. As follows from papers by Filarowski and Koll [27–29] these H-bonds should be treated as low barrier hydrogen bonds (LBHB) which found in the literature a broad interest [30,31]. It was shown by using theoretical DFT analysis that in many cases of Schiff bases one should expect an asymmetric single or double minimum potential with a low barrier. Finally, it seems reasonable to mention here about the considerations of Witkowski [32,33] who has shown that for protonic vibrations the Born–Oppenheimer rule of independence of proton and electrons motions can not be fulfilled. 2. Methodology 4-Chloro-2 0 -hydroxy-4 0 -pentyloxyazobenzene (CHPAB) was synthesized and purified as previously reported [34]. Thin layers of the compound were obtained by a capillary action that allowed filling the variable temperature cell built of two KRS-5 plates that have not been coated by

Fig. 1. Infrared polarized spectra of CHPAB for two orientations of polarization (k and ?); temp. = 75 °C (Smectic phase); d = 11 lm; the reference mode at 1084 cm1 is distinguished.

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any layers facilitating the orientation process. KRS-5 is a trade name for thallium iodide bromide, a common IR crystal material. The thickness of the cell has been determined from the pattern of the interference fringes and was varying from 11 to 20 lm. Temperature of the sample was regulated by automatic controller (Graseby Specac). In measurements of the polarized IR spectra an IRKRS-5 wire grid polarizer (Graseby Specac) was used. The data were collected with step of 5° in the whole 0– 360° range. The polarization angle was regulated by using a universal electronic device of own construction. Subsequently measured spectra of the sample were rationed against background scans obtained for the polarizer in an appropriate angle. The polarized IR spectra of CHPAB were determined by using a FT-IR Nexus-Nicolet spectrometer with a resolution of 1 cm1 with 128 scans per spectrum in the frequency range 4000–400 cm1 at temperatures 25 °C (solid state), 75 °C (Smectic A phase), 85 °C (Nematic phase), and 95 °C (isotropic phase). DFT calculations (B3LYP/6-31++G(d,p)) tending towards optimized structure of molecules, vibrational transitions and the transition dipole moments direction of particular modes were performed by using Gaussian-03W programme [35]. Data processing aimed mainly on removing a fluctuating baseline, resolution of overlapping bands, and calculations of integrated absorbance were performed by the Grams 32/AI software (Galactic Ind. Corp.). 3. Results and discussion The molecular structure of 4-chloro-2 0 -hydroxy-4 0 -pentyloxyazobenzene, calculated by using the DFT method and confirmed by X-ray diffraction studies for the propyloxy derivative [36] is shown in Fig. 2. The long axis of the molecule, M, and the transition dipole moment, ~ lN , of the reference mode analysed in this paper are indicated. Important for the analysis of internal vibration geometrical parameters are the following: the molecule is planar, while the angle between the OH bond and long axis of molecule equals to 9° and the N@N bond forms with this axis the angle equal to 61°. Naturally the c(OH) vibration is nearly perpendicular to the molecule plane so that to the long axis of molecule, while the d(OH) vibration undergoes

in the plane. However, let us mention that all modes with participation of d(OH) vibrations are not neat. As show calculations, to the mode marked as d(OH) at 1625 cm1 a substantial contributions possess the d(CH) ring and ring deformation vibrations. In the case of the mode defined as m(N@N) it is resulted from m(N@N), d(CH) ring, ring deformation and d(OH) vibrations. The positions of particular modes are shown on the background of the overall spectrum for the Smectic A phase presented in Fig. 1. Naturally the m(OH) and c(OH) modes should be distinguished. The m(OH) mode is ascribed in almost 100% to the O–H stretching, while to the c(OH) mode the out-of-plane OH vibration contributes in deformation more than 90%. If the molecules possess a tendency to a perpendicular orientation with respect to the windows, as has already been suggested [11,12], the application of the infrared polarized light should allow to estimate the order degree of molecules and then the orientation of the transition dipole moment. A test for the above reasoning are measurements of integrated absorbance of bands ascribed to m(OH) vibration in the range 3300–2200 cm1 and c(OH) vibration in the range 900–750 cm1 with the transition dipole moments directed nearly perpendicular each other. In Fig. 3, we have shown dependences of these integrated absorbances on the polarization angle. From the above relations obtained for the Smectic A phase clearly results the nearly perpendicular orientation of the molecules to the cell windows. The positions of maximum and minimum of integrated absorbances do not correspond exactly to 90° and 180°. The deviation proves that the most favourable orientation of molecules forms with Ek an angle equal to ca. 15°. Identical plots (with different maximum and minimum values) were obtained for all phases. Also for isotropic phase the angle dependence of integrated absorbances takes place though with markedly weaker extrema and with considerable scattering of experimental points, as shown in Fig. 4. The order parameter, S, is usually defined as S ¼ 1=2h3 cos2 h  1i

ð1Þ

and refers to the long axis of molecules. h is the angle between the molecular long axis and a preferred orientation of molecules which in our case forms with Ek an angle c equal to 15°.

Fig. 2. The structure of CHPAB with indicated long axis of molecule, M, and the vector of the transition dipole moment, ~ lN , for the mode at 1084 cm1 expressed by arrows at the atoms participating in this mode with respect to the Cartesian x, y, z axes.

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Fig. 3. (a) Relative changes of integrated absorbance of m(OH) (s) and c(OH) (d) bands plotted versus the polarization angle (°) in the Smectic A phase (temp. = 75 °C). (b) Polar plots of these changes versus the polarization angle for the respective bands.

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Fig. 4. (a) Relative changes of integrated absorbance of m(OH) (s) and c(OH) (d) bands plotted versus the polarization angle (°) in the isotropic phase (temp. = 95 °C). (b) Polar plots of these changes versus the polarization angle for the respective bands.

The basis for the quantitative analysis of the order degree and the transition dipole moments presents Fig. 5, as follows from literature [37–40]. The experimentally estimated value is the dichroic ratio D = Ak/A? where Ak and A? are absorbances measured for parallel and perpendicular polarization of the IR beam. If the aN angle would be equal to 0° (the transition dipole moment is consistent with the long axis of a molecule) the order parameter S is expressed in the ratio: S ¼ ðD  1Þ=ðD þ 2Þ

ð2Þ

While in the cases when aN = p/2 S ¼ ð1  DÞ=ð1 þ 2DÞ

ð3Þ

For the general case there is correlation between S and aN in the form cos2 aN ¼ 1=3½2ðD  1Þ=SðD þ 2Þ þ 1

Fig. 5. Scheme of vectors of polarized IR light (Ek and E?), preferred orientation of the long axis of molecules (n), molecular long axis for individual molecule (M) and the transition dipole moment of a vibrational mode ð~ lN Þ with respect to the cell windows.

ð4Þ

The order parameter S can be defined most properly when we are able to choose a mode for which aN = 0 or p/2. The direction of the transition dipole moment can be estimated by using the Gaussian programme. In the numerical

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analysis we limited our considerations to a few selected modes indicated in Fig. 2. Their quantitative characteristics are compared in Table 1. The choice of a most convenient mode for the estimation of the S value is not a simple task because in the finger print region a broad (continuous) absorption takes place. The dichroic ratio in this region consists of the absorption coming from the particular mode and the broad component, the aN angle of which is not the same as that of the given mode. Therefore a special task was undertaken to separate the broad bands from overall absorption. During the process the Gaussian profiles have been fitted to the broad component. Widths of determined bands reached values of a few hundred wavenumbers. Fig. 6 illustrates the results of fitting that also enabled us to estimate the aN value for the transition assigned to that absorption. The detailed analysis of several modes showed that the most appropriate one is that corresponding to the frequency 1084 cm1 which comprises d(CH)r and ring deformation vibrations. This was the starting point of evaluation of the S parameters for various phases and different thickness and further estimations of aN ascribed to various modes. Obtained the S parameters based on the intensity of the band at 1084 cm1 are presented in Table 2. The values of the aN angle calculated for various modes, according to Eq. (4), are compared in Table 3. The most fascinating result is related to the behaviour of the m(OH) mode. It is clearly seen that the transition dipole moment does not coincide with the direction of OH bond. Two effects contributed to this phenomenon. The first one is connected with that the OH  N hydrogen bonds in studied molecules are not linear. The consequences of that were discussed in detail elsewhere [24,26]. The second one, as shows a rich already literature, is due to the coupling of the proton and p-electron, a feature characteristic of RAHBs [13,17,20,27]. It is well known that the intensity of m(OH) bands for the intramolecular bridges is markedly lower than that for the intermolecular ones [26]. Moreover, the intensity for RAHB is distinctly smaller than those for bridges without the coupling. The results presented in this paper we considered as of considerable importance as they confirm existence of the coupling between the proton and p-electron motions.

Table 1 Calculated (B3LYP/6-31++G(d,p)) modes and their characteristics Frequency (cm1)

Assignment

The angle between long axis and transition dipole moment (°)

Experimental frequency (cm1)

1101 3096 1671

d(CH)r + ring def. m(OH) d(OH) + ring def. + d(CH) m(N@N) + d(OH) + ring def. + d(CH)r c(OH)

0 0.6 83

1084 2900 1625

84

1414

90

838

1462 906

Fig. 6. Separation of the broad absorption (shaded area) in the finger print region; (a) Ek polarization (solid line); (b) E? polarization (dotted line); (c) the results of subtraction of the broad absorption fitted to the Gaussian profile for the two orientations.

Table 2 Calculated order parameter S for various phases based on the absorbance of the band at 1084 cm1 Thickness (lm)

Crystalline 25 °C

Smectic A 75 °C

Nematic 85 °C

Isotropic 95 °C

20 16 11

0.50 0.53 0.60

0.45 0.55 0.61

0.18 0.34 0.36

– 0.22 0.24

In our case one can estimate that the transition dipole moment of m(OH) vibrations forms with the long axis of molecule the angle equal only to 0.6°. The same concerns the broad absorption around 1200 cm1, which should be assigned to the protonic vibrations.

M. Rospenk et al. / Chemical Physics 326 (2006) 458–464 Table 3 Values of the aN angle estimated for selected modes Experimental frequencies (cm1)

Approximate assignment

aN (o)

2900 1625 1414 838 Continuum with maximum at 1250

m(OH) d(OH) m(N@N) c(OH)

43 ± 3 20 ± 4 33 ± 5 84 ± 4 42 ± 5

The fact that the vector of the transition dipole moment of the broad absorption in the finger print region is consistent with that of the m(OH) band around 2800 cm1 can imply that we are dealing with the splitting of the vibrational level. As a consequence we can suggest that this is caused by a self modification of the potential shape due to the intermolecular interaction. Both the X-ray diffraction and theoretical DFT studies [41] show that dimers possess the structure shown in Fig. 7. For Mannich bases, as follows from detailed studies [42], dimerization leads to a drastic change of the potential shape, i.e. to the formation of the polar proton transfer state. Simultaneously it was shown that the dimerization constant in solutions is very high. In contrast to Mannich bases, Schiff bases are only weakly associated [43] that most probably is due to a small increase of the dipole moment. Whenever a second potential minimum is created it is rather shallow. One can suppose that we are dealing with such a situation, i.e. in 2-hydroxy-azobenzenes the association causes appearance of the second shallow minimum. Such a situation and its consequences in IR spectra were analyzed for the O–H  O bridges [44] by assuming a stochastic model. It was possible to follow step-by-step the evolution of complicated spectra of protonic stretching vibrations consisted of a few broad subbands. It has been shown that generated IR absorption spectrum with broad subbands satisfactorily reflects the observed spectra of strongly hydrogen bonded systems from the so called critical region where the proton transfer equilibrium and a second minimum appear. In particular the analysis has shown the appearance of intense broad band at ca. 1000 cm1 exactly where the strong absorption appears in our case. Therefore, a conclusion seems to be justified that association of 2-hydroxy-azobenzenes modifies the potential for

Fig. 7. Mutual orientation of CHPAB molecules in a dimer.

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the proton motion leading to the creation of the double minimum and spectacular change of the IR spectrum. The results presented in Table 2 show that the degree of ordering in crystalline and Smectic A phases are very similar. This means that crystallization leads to a freezing of molecular arrangement in the Smectic phase that drops markedly going to the Nematic phase but for isotropic phase it is still clearly reflected. Obviously the ordering of molecules takes place first of all in the close neighbourhood of the KRS-5 surfaces. Most probably the Cl-atoms play a substantial role of anchors. Our experiments with 4-methyl derivatives show that in conditions similar to these for chloro derivative the phenomenon does not take place. For the isotropic phase the order degree cannot be only due to the formation of monomolecular adsorption layers on the cell windows. If one analyses the layers of 16 lm ˚ thickness, they contain ca. 80 molecules of ca. 21.5 A length. This means that some ordering is extended over whole the volume. Simultaneously the results obtained for various thicknesses show, in agreement with expectation, that the ordering degree falls with increasing of the cell thickness. 4. Conclusions Liquid crystalline 4-chloro-2 0 -hydroxy-4 0 -pentyloxyazobenzene (CHPAB) appeared to be a convenient compound in studies of spontaneously oriented molecules between cell window by using polarized IR spectra going from isotropic liquid to Nematic, Smectic A and crystalline phases. The molecules of CHPAB are characterized by intramolecular resonance assisted O–H  N hydrogen bonds. The infrared behaviour of them evokes a particular interest from the point of view of coupling between the proton and pelectrons motions. DFT theoretical analysis of vibrational modes enabled to choose most appropriate absorption band for the estimation of the order parameters, S. It is the band at 1084 cm1 corresponding to d(CH)r + ring deformation vibrations the transition dipole moment of which is parallel to the long axis of the molecule. It was shown that these axes are oriented to the cell windows (KRS-5) almost perpendicularly (the c angle equals to 15°). As expected, the S parameter is the highest for Smectic and crystalline phases and decreases when going to Nematic and isotropic phases. Moreover, this parameter depends on the thickness of the layer studied in the range between 11 and 20 lm. Based on the estimated S values it was possible to determine the angles (aN) between the long axis of molecules and the transition dipole moments for selected modes. It seems that most valuable result relates to the m(OH) mode for which aN = 43° proving a marked coupling of proton and p-electron motions. The same value of aN was found for broad (continuous) absorption in the finger print region at ca. 1250 cm1 suggesting that it should be ascribed to protonic stretching vibrations. The appearance of the

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broad absorption in this region can be interpreted as due to the plane-to-plane intermolecular interaction which leads to the modification of the potential for the proton motion and creation of a second shallow minimum. Acknowledgement A financial support by the Polish Ministry of Sciences and Informatics (Grant No. 3T09A07527) is acknowledged. References [1] L. Pohl, in: Liquid Crystals, in: H. Stegemeyer (Ed.), Topics in Physical Chemistry, vol. 3, Steinkopf Darmstadt, Springer, New York, 1994, p. 173. [2] T.S. Perova, J.K. Vij, A. Kocot, in: J.K. Vij (Ed.), Advances in Liquid Crystals: A Special Volume of Advances in Chemical Physics, vol. 113, 2000, p. 341. [3] M.P. Fontana, in: G.R. Luckhurst, C.A. Veracini (Eds.), The Molecular Dynamics of Liquid Crystals, Kluwer Academic Publishers, Dordrecht, 1994, p. 403. [4] K. Sakamoto, N. Ito, R. Arafune, S. Ushioda, Vib. Spectrosc. 19 (1999) 61. [5] Y. Nagasaki, T. Yoshihara, Y. Ozaki, J. Phys. Chem. B 104 (2000) 2846. [6] M.A. Czarnecki, S. Okretic, H.W. Siesler, J. Phys. Chem. B 101 (1997) 374. [7] M.A. Czarnecki, N. Katayama, M. Satoh, T. Watanabe, Y. Ozaki, J. Phys. Chem. 99 (1995) 14101. [8] S.V. Shilov, S. Okretic, H.W. Siesler, Vib. Spectrosc. 9 (1995) 57. [9] V.G. Gregoriou, J.L. Chao, H. Toriumi, R.A. Palmer, Chem. Phys. Lett. 179 (1991) 491. [10] T.I. Urano, H. Hameguchi, Chem. Phys. Lett. 195 (1992) 287. [11] J. Paja˛k, M. Rospenk, Z. Galewski, L. Sobczyk, J. Mol. Liq. 105 (2003) 53. [12] B. Czarnik-Matusewicz, J. Paja˛k, M. Rospenk, Spectrochim. Acta A 62 (2005) 157. [13] G. Gilli, F. Bellucci, V. Ferretti, V. Bertolasi, J. Am. Chem. Soc. 111 (1989) 1023. [14] V. Bertolasi, P. Gilli, V. Ferretti, G. Gilli, J. Am. Chem. Soc. 113 (1991) 4917. [15] P. Gilli, V. Bertolasi, V. Ferretti, G. Gilli, J. Am. Chem. Soc. 116 (1994) 909. [16] V. Bertolasi, P. Gilli, V. Ferretti, G. Gilli, Chem. Eur. J. 2 (1996) 925. [17] G. Gilli, P. Gilli, J. Mol. Struct. 552 (2000) 1. [18] P. Gilli, V. Bertolasi, L. Pretto, A. Lycˇka, G. Gilli, J. Am. Chem. Soc. 124 (2002) 13554. [19] P. Gilli, V. Bertolasi, L. Pretto, V. Ferretti, G. Gilli, J. Am. Chem. Soc. 126 (2004) 3845. [20] L. Sobczyk, S.J. Grabowski, T.M. Krygowski, Chem. Rev. 105 (2005) 3513.

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