Vibrational spectral investigation of four second order nonlinear optical azobenzene-containing materials: A combination of experimental and density functional theoretical (DFT) study

Spectrochimica Acta Part A 79 (2011) 1976–1984

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Vibrational spectral investigation of four second order nonlinear optical azobenzene-containing materials: A combination of experimental and density functional theoretical (DFT) study Xianchang Li a,c,1 , Wei Li b,1 , Zhong’an Li d , Xiaodong Zhou a,∗ , Zhen Li d , Jingui Qin d , Jiming Hu a,∗∗ a

Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China College of Chemical Engineering, Wuhan Textile University, Wuhan 430073, China c School of Mathematics and Physics, Anyang Institute of Technology, Anyang 455000, China d Department of Chemistry, Hubei Key Laboratory on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China b

a r t i c l e

i n f o

Article history: Received 25 February 2011 Received in revised form 29 May 2011 Accepted 30 May 2011 Keywords: Azobenzene-containing NLO materials FT-IR FT-Raman Hyperpolarizability HOMO–LUMO gap

a b s t r a c t In this work, four-second order nonlinear optical (NLO) azobenzene-containing materials are studied in-depth by using vibrational spectra and density functional theory (DFT). The Fourier transform infrared (FT-IR) spectra and FT-Raman spectra are recorded in the range of 50–4000 and 100–3600 cm−1 , respectively. Meanwhile, the DFT computations are performed at B3LYP/6-31G (d, p) level to derive equilibrium geometry, vibrational wavenumbers and intensities, and first hyperpolarizability, and the scaled theoretical wavenumbers are also shown to be in good agreement with experimental data. The calculated results show that these four azobenzene-containing compounds are good materials and the compound with nitro substituent groups possesses a larger first molecular hyperpolarizability (ˇ) value. Moreover, the simultaneous infrared and Raman activation of R1 group and C C stretching suggest that the charge transfer interaction might occur between the R1 group and phenyl ring, and the HOMO–LUMO gap analysis also supports this viewpoint. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic second-order NLO materials, with large NLO coefficients, ultrafast response time, electrically bistable behavior, and ease of integration, have attracted much attention from numerous material scientists. Among several types of polymeric organic second-order NLO materials, azobenzene-containing polymers are fascinating materials because of the efficient photoinduced anisotropy of the azobenzene groups. These properties result in its potential applications in optical data storage, nonlinear optics, holographic memories, waveguide switches, and other photonic devices [1–8]. Recently, we have designed and synthesized a series of azobenzene-containing NLO materials, in which the size of the substituent group on the azo chromophore moieties is changed from small atoms to much larger groups such as carbazolyl group, to boost the microscopic ˇ (the first molecular hyperpolarizability) value of the chromophore moieties which is one of the major challenges encountered in the field of second-order NLO research. Based

∗ Corresponding author. ∗∗ Corresponding author. Tel.: +86 27 62258931; fax: +86 27 68754067. E-mail addresses: [email protected] (X. Zhou), [email protected] (J. Hu). 1 These authors contributed equally to this work. 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.05.103

on the obtained experimental results, we have proposed the concept of “suitable isolation group” [9–14], that is to say, for a given chromophore moiety and given linkage position, there should be a suitable isolation group present to boost its microscopic ˇ value. However, so far, we have not conducted such studies to explore the structure–property relationship in detail. Vibrational spectral studies of the molecules can provide deeper knowledge about the relationships between molecular architecture, non-linear response and hyperpolarizability, and promote the discovery of new efficient materials for technological applications. However, for a new synthesized molecule, it is difficult to describe its original and intricate vibrational spectral property. It is delighting that density functional theoretical (DFT) methods can efficiently provide the molecule structure information, such as the vibrational assignment, the distribution of the atom charge, the polarizability and hyperpolarizability, and the virtual orbital energies. Recently, vibrational spectra combined with DFT calculations have been used as an effective tool in the study of NLO active compounds by numerous literatures [15–21]. However, to our knowledge, the azobenzene-containing NLO materials, in which the substituent groups are changed from small atoms to much larger groups, have not been studied so far. To provide valuable information for designing and synthesizing new higher NLO property materials, in this study, four

X. Li et al. / Spectrochimica Acta Part A 79 (2011) 1976–1984

azobenzene-containing NLO materials are investigated by Fourier transform infrared (FT-IR) spectra and FT-Raman spectra. The detailed vibrational spectral studies are aided by DFT calculations to elucidate the assignment of the vibrational spectra and correlation between the molecular structure and NLO property by investigating the intramolecular charge transfer (ICT) interaction, the highest occupied molecular orbital (HOMO), lower unoccupied molecular orbital (LUMO) and the first order hyperpolarizability of these compounds. 2. Experimental The detailed synthetic processes of four polyurethane materials were reported in Refs. [9,10]. The FT-IR spectra in the region 50–600 cm−1 and 400–4000 cm−1 were recorded respectively using a VERTEX 70V Bruker spectrometer with a resolution of 2.0 cm−1 , and then the spectra were merged in the region 50–4000 cm−1 . The NIR-FT Raman spectra in the region 100–3600 cm−1 were measured on a Bruker RAM II FT-Raman Spectrometer using a Nd:YAG laser at 1064 nm of 10 mW output as the excitation source and with a resolution of 2.0 cm−1 . 3. Computational All the calculations were performed using the Gaussian 03 software package [22]. The optimized geometry corresponding to the minimum on the potential energy surface was first obtained by semi-empirical method with AM1 and PM3 functional, and then by DFT Becke–Lee–Young–Parr composite of exchange correlation (B3LYP) functional with the STO-3G, 3-21G, 6-31G basis set in turn. Finally, the geometry optimizations and frequency calculations were carried out at the same level by employing DFT-B3LYP hybrid functional and 6-31G (d, p) basis set. The Raman activities (Si ) calculated by the Gaussian 03 program were converted to relative Raman intensities (Ii ) using the following formula derived from the basic theory of Raman scattering.

Ii =

cm−1 ,

where v0 is the exciting wavenumber in vi is the vibrational wavenumber of the ith normal mode, h, c and k are fundamental constants and f is a suitably chosen common normalization factor for all peak intensities. The temperature T is assumed to be 298 K. The first hyperpolarizability ˇ was calculated using the DFTB3LYP functional with the 6-31G (d, p) basis set on the basis of the finite-field approach. The first hyperpolarizability is a third-rank tensor that can be described by a 3 × 3 × 3 matrix. The 27 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry. The magnitude of the first hyperpolarizability tensor can be calculated as follows: 2

Table 1 ˚ ◦ ) of the studied compounds. The selected optimized structural parameters (A,

r (1, 2) r (5, 8) r (8, 8 ) r (5 , 8 ) r (1 , 2 ) r (6 , 9)  (3, 2, 7)  (4, 5, 6)  (5, 8, 8 )  (5 , 8 , 8)  (5 , 6 , 7 )  (3 , 2 , 7 )  (4 , 5 , 6 )

a

b

c

d

1.47 1.41 1.26 1.39 1.38 1.49 121.85 119.70 114.36 116.44 119.19 116.83 118.52

1.80 1.42 1.27 1.40 1.39 1.49 121.24 119.84 114.30 116.20 119.08 116.97 118.58

1.80 1.42 1.27 1.40 1.38 1.49 121.19 119.76 114.03 116.51 118.82 116.89 118.68

1.79 1.42 1.27 1.40 1.38 1.49 121.37 119.77 113.99 116.54 118.84 117.05 118.70

are plotted in Fig. 2. The selected bond lengths and angles of these compounds obtained by calculations are listed in Table 1. The calculated results show that all optimized structures are stable. It is well-known that the azobenzene can exist in two configurations, the trans and cis form. The trans form is generally more stable than the cis forms, and the cis–trans interconversion can be affected by light and heat. For example, when exposed to light of a certain wavelength, the stable trans form can be photoisomerized to the cis form. As seen from Fig. 2, it is clear that the azobenzene exist in trans form in isolated molecules of a, b, c and d compounds, and the substituent groups are not in a plane with the azobenzene except the nitro substituent groups. As shown in Table 1, the bonds length of r(5, 8), r(8, 8 ), r(5 , 8 ) in a compound is 0.01 A˚ shorter than that in b, c and d compounds, because of the R1 substituent groups which are generally used as the acceptor of the NLO active material are changed from nitro to sulfonyl. However, the R2 substituent groups which are used to reduce the intermolecular electrostatic interactions of the polar chromophore moieties affect the azobenzene structure slightly when it changed from phenyl (Ph) to carbazolyl (Cz). 4.2. First hyperpolarizability

f (v0 − vi )4 Si vi [1 − exp(−hc vi /kT )]

2

1977

2 1/2

ˇ = (ˇx + ˇy + ˇz ) ˇx = ˇxxx + ˇxyy + ˇxzz ˇy = ˇyyy + ˇxxy + ˇyzz ˇz = ˇzzz + ˇxxz + ˇyyz

4. Results and discussions 4.1. Optimized geometry The sketched structures of the studied compounds are depicted in Fig. 1, and the optimized molecular structures of the isolated a, b, c and d calculated by DFT methods at B3LYP/6-31G (d, p) level

The ˇ values of a, b, c and d compounds which are calculated by the DFT-B3LYP functional with the 6-31G (d, p) basis set are given in Table 2. A large value of the first hyperpolarizability is the prerequisite to behave as a good NLO material, and the important parameters influencing ˇ generally are (i) donor–acceptor system, (ii) nature of substituents, (iii) conjugated ␲ system and (iv) the influence of planarity. The coplanar of phenyl-nitro group and stronger accepted property of the nitro groups should increase the ˇ value of a compound. The calculated results well illustrate this point. From the results, it can be easily concluded that the ˇ value of a compound with the nitro groups is larger than that of b, c and d compounds with the sulfonyl groups. Moreover, it also Table 2 The first hyperpolarizabilities of the studied compounds. a ˇxxx (a.u.) ˇxyy (a.u.) ˇxzz (a.u.) ˇyyy (a.u.) ˇxxy (a.u.) ˇyzz (a.u.) ˇzzz (a.u.) ˇxxz (a.u.) ˇyyz (a.u.) ˇtot (a.u.) ˇtot (10−30 esu)

22300 −495 −14 −407 −14 −27 −99 1230 −110 21820 118.5

b −12100 282 38 −186 315 78 128 −302 70 11783 101.8

c

d

11800 780 −153 −3720 2130 148 −400 −255 148 12521 108.1

12900 −316 −180 −3660 393 206 −160 −101 −70 12780 110.4

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Fig. 1. The sketched structures and atom numbers of the studied compounds a, b, c, and d.

can be observed that the isolation spacer group (R2 ) slightly affects the ˇ value. The results also show that the compounds studied in our research are good NLO materials due to that their ˇ values are about 400 times that of urea (about 0.3728 × 10−30 esu). 4.3. Vibrational spectral analysis The measured FT-IR and FT-Raman spectra of a, b, c and d compounds are stacked in Figs. 3 and 4 respectively. The comparisons of calculated and measured IR and Raman spectra are shown in Fig. 5. For simulation, a spectral uniform scaling factor of 0.9726 [23] is used to offset the systematic errors caused by basis set incompleteness, neglect of electron correlation and vibrational anharmonicity. The calculated spectra have been plotted using pure Lorentizian band shape with a bandwidth of 10 cm−1 . Vibrational

spectral assignments performed based on the theoretically predicted by density functional B3LYP/6-31G (d, p) methods, as well as the full description of the observed and calculated IR and Raman wavenumbers are shown in Table 3. The calculated and experimental wavenumbers match with each other except for the stretching mode of C–H and O–H bonds, and assignments of wavenumbers for different functional groups are discussed below. 4.3.1. Phenyl ring vibrations There are 3 and 4 phenyl rings in the compound a, b and c, d, respectively. The C C stretching modes of these phenyl rings are overlapped at around 1590 cm−1 . This peak is very strong in the measured IR and Raman spectra, and its wavenumber and relative intensity are in very fair agreement with the theoretical prediction. The very weak bands found in the 3000–3100 cm−1 region are

Table 3 The calculated and experimental vibrational wavenumbers and proposed assignments. Assignments

b

c

d

Calculations

Experiments

Calculations

Experiments

Calculations

Experiments

Wavenumber (cm−1 )

Raman (cm−1 )

IR (cm−1 )

Wavenumber (cm−1 )

Raman (cm−1 )

IR (cm−1 )

Wavenumber (cm−1 )

Raman (cm−1 )

3698 3681 3099 – 2927 – 1596 1524 1490 – 1443 1425 1401 1384 – 1325 – 1280 – 1237 1188 1148 1135 – 1088 1021 860 822 780 753 736 701 679 672 – 635 591 522 503 – – 329 260 132 –

– – 3069 – – – 1587 – – – 1437 1421 1400 1378 – 1335 – 1291 – 1240 1191 1155 1129 – 1097 1001 868 819 776 753 733 698 – 665 653 626 588 531 505 – – 316 225 129 –

– 3421 – 2967 2923 2860 1597 1518 1490 – 1435 – – 1378 – 1335 – 1291 – 1240 1191 1167 1128 – 1094 1001 860 819 776 753 733 701 685 – – 631 586 533 507 – – 322 231 121 –

3691 3685 3100 3080 2927 – 1599 – 1497 1456 1446 1425 1396 1387 1353 – 1314 1282 1260 1253 1188 – 1143 1092 – 1023 849 811 762 – 733 702 687 – – 627 575 545 503 460 – 327 – 151 –

– – 3074 – 2931 2869 1587 – – 1450 1439 1422 1399 1385 1346 – 1319 1291 1255 1240 1191 1157 1140 1120 – 1007 860 816 771 753 740 700 687 670 655 629 594 556 505 460 338 321 255 141 111

– 3441 – 2956 2923 2864 1592 – 1490 1467 1426 – – 1381 1348 – 1317 1292 1272 1241 1188 1155 1144 1121 – 1005 852 810 776 – 739 706 685 – 655 639 594 559 508 467 – 322 231 – –

3691 3678 3089 3088 2927 – 1599 – 1490 1456 1440 1422 1396 1384 1348 – 1324 1284 1263 1234 1188 – 1146 1092 – 1023 847 811 761 – – – 697 657 – 632 591 551 505 440 336 – 242 – –

– 3471 3068 2962 2920 2876 1587 – – 1456 1437 1423 1399 1384 1348 – 1319 – 1255 1240 1191 1155 1140 1120 – 1003 860 821 766 748 740 702 695 660 645 622 589 556 518 – 338 326 238 144 111

IR (cm−1 ) – 3474 3441 – 2964 2923 2864 1592 – 1493 1461 1437 – – 1386 1346 – 1324 1280 1256 1236 1189 1155 1141 1121 – 999 852 814 768 741 – 706 690 – 645 631 585 556 510 475 – 320 234 – –

Calculations

Experiments

Wavenumber (cm−1 )

Raman (cm−1 )

IR (cm−1 )

3692 3593 3096 3088 2926 – 1599 – 1496 1453 – 1426 – 1378 1350 – 1320 1278 1256 1230 1188 1169 1141 1083 – 1004 843 805 760 – 720 – 693 665 655 627 585 551 – 450 – – 239 – –

– – 3064 – 2918 – 1587 – – – 1442 1421 – 1376 1351 – – – 1257 1237 1194 1158 – 1114 – 1007 852 816 – 745 733 707 691 670 655 629 600 – – 455 – – – – –

– 3424 – 2965 2922 2864 1591 – 1490 1467 – 1426 – 1378 1348 – 1316 1280 1253 1233 1191 1172 1144 1116 – 1001 846 803 768 749 733 706 685 – 653 636 575 540 – – – – 231 – –

X. Li et al. / Spectrochimica Acta Part A 79 (2011) 1976–1984

O–H O–H C–H(Ph) C–H(CH3 ) C–H(CH2 ) C–H(CH2 ) C C asym O N O ıCH2 ıCH2 N N, ıCH2 N N, ıC–H ıC–H ıC–H asym O S O sym O N O ıC–H ıC–H ıC–H ıC–H sym C–N, ıC–H ıC–H ıC–H sym O S O C–NO2 C–O def (Ph ring) C–H C–H C–H C–H def (Ph ring), C–H def (Ph ring) def (Ph ring) def (Ph ring) C–H def (Ph ring) C–H C–N, C–H C–N, C–C C–N, C–C C–N, O–H O–H Framework vibration Framework vibration

a

, stretching; def, deformation; ı, in-plane bending; , out-of-plane bending; , twist; asym, asymmetrical; sym, symmetrical. 1979

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Fig. 2. The optimized structure of the studied compounds a, b, c, and d.

Fig. 3. (a and b) Are measured FT-IR spectra of the studied compounds a, b, c, and d. The LT magenta and LT yellow range are the vibration of donor and acceptor groups respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Fig. 4. (a–c) Are measured Raman spectra of the studied compounds a, b, c, and d. The LT magenta and LT yellow range are the vibrations of donor and acceptor groups respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

related to the C–H stretching mode of the phenyl rings in the measured Raman spectra. This mode is submerged by the broad band of the O–H stretching mode which is found at around 3440 cm−1 in the measured IR spectra. The strong band found at around 1380 cm−1 in the measured Raman spectra is mainly contributed by the in-plane C–H bending mode of the phenyl rings. This band is also probably contributed by the phenyl skeleton and N N groups vibrations according to our theoretical prediction and some previous studies [24,25]. In b, c and d compounds, it is also probably contributed by the asymmetrical bending of methyl groups. The very weak bands which are found in the 700–820 and 600–700 cm−1 region are mainly contributed by out-of-plane C–H bending and deformation of the phenyl rings, respectively.

4.3.2. Azo groups vibrations The N N stretching mode is always found at around 1440 cm−1 , for the measured IR and Raman spectra of a, b, c and d compound, there are two strong bands found at around 1420 and 1440 cm−1 that are related to the N N stretching mode, and the intensity of the band at around 1420 cm−1 found in the b, c and d compound is stronger than 1440 cm−1 mostly because the R1 substituent groups are changed from nitro groups to sulfonyl groups. This vibrational mode is reproduced with the strongest band in the theoretical Raman spectra of the b, c and d compound. The band found at around 1191 cm−1 is contributed by symmetrical C–N stretching and in-plane C–H bending of the phenyl rings, although the C–H bending contribution is very little. This mode is a mid-strong band

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Fig. 5. Comparison of the calculated and experimental spectra of compounds a, b, c, and d. (a and b) Are IR and Raman spectra respectively. The navy lines are experimental spectra and the purple lines are calculated spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

in Raman spectra, however it is very weak in IR spectra, and it is well reproduced by the DFT calculations. 4.3.3. Nitro groups vibrations The asymmetric stretching of Ph–NO2 compound generally gives a band in the range 1485–1570 cm−1 and the symmetric one occurs in the region 1320–1370 cm−1 . For compound a, a strong band observed in the measured IR spectrum at 1518 cm−1 is assigned to NO2 asymmetric stretching mode, and this mode is not found in the measured Raman spectrum. The strongest band observed at 1335 cm−1 in the measured IR and Raman spectrum is attributed to the symmetric NO2 stretching. Its strongest intensity is mostly due to conjugation with the phenyl ring. Furthermore, we can easily deduce that the strong band found at around 1097 cm−1 is attributed to the C–NO2 stretching mode, because it disappears in b, c and d compounds, and appears at 1088 cm−1 by the theoretical prediction. 4.3.4. Sulfonyl group vibrations The sulfonyl groups also have two stretching vibrational modes, symmetric and asymmetric stretching modes, which are generally found in the 1100–1150 and 1300–1400 cm−1 region, respectively. For b, c and d compounds, the strongest band which is found at around 1120 cm−1 in the measured Raman spectra is assigned to the symmetric SO2 stretching mode, and it is also a strong band in the measured IR spectra. The calculated relative intensity of this mode is in fair agreement with experimental results although the calculated wavenumber is reproduced at around 1090 cm−1 . Another strong band found at 1348 cm−1 in the measured IR spectra is assigned to the asymmetric SO2 stretching mode which is a bit weak in the measured Raman spectra. 4.3.5. Vibrational contribution to NLO activity The potential application of a, b, c and d compounds in the field of nonlinear optics demands the investigation of the contribution of IR and Raman active vibrational modes to the hyperpolarizability enhancement. A detailed vibrational analysis such as comparison of the Raman and IR spectra can provide valuable information about intramolecular interaction. For example, in most cases, even in the

absence of inversion symmetry, the strongest band in the Raman spectrum is weak in the IR spectrum, but the intramolecular charge transfer from the donor to accepter group through a single-double bond conjugated path can induce large variations of both the molecular dipole moment and the molecular polarizability, enhancing IR and Raman activity at the same time. The simultaneous strong Raman and infrared activity of some normal modes are the prerequisite for attaining large ˇ value [15], which has been reported as a prerequisite for a good NLO material. In the case of compounds studied in our research, the bands in the light (LT) magenta and LT yellow range from IR spectra (Fig. 3) which are attributed to C C and R1 groups stretching modes respectively have their counterparts in Raman spectra (Fig. 4), and their relative intensities in IR and Raman spectra are comparable. The C C stretching vibrations spreads over the whole  conjugated path with relevant vibrational amplitudes from almost all the constituent parts of the molecule. These vibrations favor the intramolecular charge transfer from the donor to the acceptor and give rise to a large vibration of the dipole moment, thus gaining a strong infrared activity. Furthermore, the simultaneous IR and Raman activation of the R1 groups also provide evidence for the charge transfer interaction between the donors and the acceptor groups through the  system. 4.4. HOMO–LUMO gap As discussed above, simultaneous activation of the Raman and IR bands show that the electron might transfer through the conjugated frame work from the electronic donor to electronic acceptor groups. The analysis of the wave function indicates that the electron absorption corresponds to the transition from the ground to the first excited state and is mainly described by one-electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied orbital (LUMO). The HOMO and LUMO energy gap for the four compounds are calculated at B3LYP/6-31G (d, p) level, and sketched by Gaussian view software in Fig. 6. The eigen values of LUMO and HOMO and their energy gap reflect the chemical activity of the molecule. LUMO as an electronic acceptor represents the ability to obtain an electron from the donor. As

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Fig. 6. The HOMO–LUMO gaps of the studied compounds a, b, c, and d.

can be seen easily from Fig. 6, the electron density of the HOMO is larger than LUMO, which might lead to intramolecular charge transfer from phenyl ring to R1 groups.

5. Conclusion The vibrational spectra of four self-assembly second order nonlinear optical azobenzene-containing materials are examined by FT-IR, FT-Raman spectra, and the assignments are proposed with the DFT calculation at B3LYP 6-31G (d, p) basis level. The results show that the calculated vibrational wavenumbers excellently agree with the experimental data. The calculated first molecular hyperpolarizability results show that these four azobenzenecontaining compounds are good materials and the compound with the nitro substituent groups possesses a larger ˇ value, however, the R2 substituent groups affect the ˇ value slightly. Furthermore, simultaneous infrared and Raman activation of nitro (sulfonyl) groups and C C stretching show that the intramolecular charge transfer might occur between R1 groups and phenyl ring, and the HOMO–LUMO gap analysis also supports this viewpoint.

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