Science in China Series B: Chemistry © 2008

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Terahertz time-domain spectroscopic investigation on quinones GE Min, ZHAO HongWei, WANG WenFeng†, YU XiaoHan & LI WenXin Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China

Well resolved far-infrared spectra of 1,4-benzoquinone, 1,4-naphthoquinone, and 9, 10-anthraquinonme in polycrystalline form have been measured with terahertz time domain spectroscopy at room temperature. The characterizations of power absorption and index of refraction in the frequency range 0.3 －2.0 THz are presented. Theoretical calculation is applied to assist the analysis and assignment of individual THz absorption spectra of the p-quinones with semiempirical AM1, Hartree-Fock (HF), and density functional theory (DFT) method. Observed THz responses are assigned to the translational and torsional vibrations of p-quinone dimer held together by weak hydrogen bonds. quinones, terahertz time domain spectroscopy, DFT

1 Introduction Interests in quinone and its derivatives arise from the chemical properties and biological importance. They play significant roles in the dye industry, medicinal chemistry, and other fields[1,2]. They are parent compounds for a large palette of dyes and so are the most important starting materials in their production. The conjugated structure of quinones is also common in numerous natural products associated with antifungal, antibacterial, antiviral, and antitumour activities[3]. More importantly, the planar aromatic ring of quinones means they intercalate easily to adjacent base pair planes of DNA double helical structures through non covalent interaction. Small molecules with similar structure that target specific sites along a DNA helix have become a － subject of considerable interest[4 7]. They serve as analogues in studies of protein-nucleic acid recognition, provide site-specific reagent for molecular biology, and yield rationales for new drug design. Terahertz time-domain spectroscopy (THz-TDS) is becoming an attractive tool to investigate non covalent interaction in the far infrared region. Its unique characteristics, such as high brightness, stability, single-cycle pulse generation, and coherent time-gated detection,

make it a significant complementary technique to Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. In recent years, THz-TDS has been successfully applied to examine a wide variety of biological samples, such as benzoic acid and its derivatives[8], retinal isomers[9], glucose[10] and pentose[11], DNA and bovine serum albumin[12], and nucleobases of DNA[13,14]. These researches are the foundations for recent studies on bimolecules with THz-TDS and also point towards potential future applications, such as chemical recognition, time-resolved structural studies, and diagnostics. In this work, we measured three para-quinones, namely 1,4-benzoquinone, 1,4-naphthoquinone, and 9,10-anthraquinone, in the frequency range from 0.3 THz to 2.0 THz with THz-TDS at room temperature. The frequency-dependent power absorption, index of refraction, and complex dielectric function of these quinones were presented. Characteristics of those small molecules in the THz range will provide a new point of view to understand their structural properties and lay the Received April 24, 2007; accepted June 25, 2007 doi: 10.1007/s11426-008-0004-9 † Corresponding author (email: [email protected]) Supported by the National Natural Science Foundation of China (Grant No. 10675158) and the major project of the Shanghai Municipal Commission of Science and Technology (Grant No. 06dj14008)

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foundations for future studies on weak interaction between drugs and DNA/proteins. Theoretical calculation with semiempirical AM1, Hartree-Fock (HF), and density functional theory (DFT) was carried out to analyze the vibrational frequencies of p-quinones. Observed THz responses were attributed to intermolecular vibrational modes mediated by hydrogen bonds.

2 Materials and methods 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ): Acros, 9, 10-anthraquinonme (AQ): Fluka. All samples were in powder phase with 99% purity and used without further purification. Samples were mixed with polyethylene (PE) powder at weight ratio 1:1 and made into pellets with thickness about 1.4 mm by applying an approximate pressure of 22 MPa with an oil press. PE is nearly transparent under 2.0 THz. All crystals were crushed well before making the pellets in order to ensure that the particles were of sub-micron size and that the observed features were not a result of Rayleigh and/or Mie scattering. The grain sizes are about 10 μm which were measured by a laser diffraction particle size analyzer (OASISDRY, SYMPATEC GmbH, Germany). The THz-TDS apparatus (Figure 1) used in our experiments has been discussed in detail elsewhere[11,15]. In brief, a model-locked Ti: sapphire laser provides pulses of 100 fs duration at a center wavelength of 805 nm with average power of 700 mW and a repetition rate of 80 MHz. THz radiation was generated by illuminating a biased GaAs photoconductive antenna and detected by a 2 mm thick ZnTe crystal via electro-optical sampling. The variable delay stage that provides the time delay

between the THz pulse and the probe pulse is scanned over a distance of 4 mm. The available bandwidth of the spectrometer is 0.3－2.0 THz and the spectral resolution is better than 40 GHz. The THz beam was purged with dry nitrogen in order to minimize absorption by residual water vapor in the beam path. The temperature of the whole box is (293±0.5) K and the humidity is controlled below 4% during data collection. Quantum chemical calculations with full geometry optimizations and frequency analysis of the optimized structures were performed using the GAUSSIAN 03[16] program package. Minimum energy structures were found, which were confirmed by vibrational analyses. The atom labeling and visualization of the normal modes were found on the file readable by Gaussview molecular modeling software[17].

3 Results and discussion Figure 2 shows the frequency-dependent power absorption coefficient α and the index of refraction n of BQ, NQ, and AQ obtained in the range of 0.3－2.0 THz at room temperature. The solid line represents the power absorption, while the dash line represents the refractive index. The collective data below 0.3 THz are not considered due to interference between reflections of the probe pulse inside the sample pellets. All obtained spectra show distinctive peaks and continuous absorption that increase with frequency. In detail, BQ shows a distinct peak at 1.21 THz. NQ has a clear absorption band at about 1.75 THz. AQ displays three sharp peaks respectively at 0.96 THz, 1.07 THz, and 1.87 THz. The presences of all the absorption features were confirmed

Figure 1 Schematic of experimental THz-TDS.

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by the observation of changes in the refractive index (dash lines) shown in Figure 2. Generally, the characteristics of spectrum observed by using the traditional IR and FTIR correspond to the molecular structure and the mode of different atomic bonds. For solid without heavy atom, the absorption in far infrared region is mainly caused by intermolecular vibration, including (1) intermolecular action, such as hydrogen bond vibration, and (2) lattice vibration[18] which means the atoms in a crystal are not locked into a rigid pattern but can oscillate around their average position. Previous researches on benzoic acid[8], 2, 2′-biphenol and 4, 4′-biphenol[20], and poly-crystalline saccharides[11,21] showed the low frequency discrete vibrational features, which were mainly associated with collective vibrational modes mediated by hydrogen bonds. According to Adamowicz and coworkers’ theoretical calculations[19], p-quinones can form dimers held together by the weak hydrogen bonds of C—H…O. Based on those previous reports, we infer that the low-frequency vibrational modes of p-quinones also originated from collective vibrations related to H-bonds.

Figure 2 The absorption coefficient α and the index of refraction n of BQ, NQ, and AQ measured with THz-TDS between 0.3 and 2.0 THz at room temperature. (a) BQ; (b) NQ; (c) AQ. Solid line: α; dash line: n.

In order to reveal the relationship between structures and vibrational modes of the samples, theoretical calculations were carried out with Gaussian 03 program on dimer held together by H-bonds. Recently, quantum 356

chemical calculations were used to explain the THzTDS spectrum by several groups and obtained some sat－ isfactory results fortunately[17 20]. Figure 3 shows the calculated frequencies between 500 cm-1 and 3500 cm-1 with AM1 (dash dot line), HF/6-31G (dot line), and B3LYP/6-311G (dash line) together with the experimental results measured with Nicolet Avater-360 spectrometer. In the FTIR spectra, all samples show a characteristic stretching band of C=O around 1670 cm-1 and a band of C=C around 1587 cm−1. The DFT method is proven to be a better method in predicting mid-IR frequencies compared with AM1 and HF method. However, whether it is suitable for the low frequency in THz range is really a crucial problem. For p-quinones dimer, the predicted THz frequencies in the range of 0.3－2.0 THz with AM1, HF/6-31G and B3LYP/6-311G are presented in Table 1. To account for higher electronic correlations and vibrational anharmonicities, these frequencies in Table 1 are scaled down by 0.96, which is the recommended scaling factor for HF/6-31G and B3LYP/6-311G level of theory[22]. In this experimental frequency range, theoretical results with AM1 are unrealistic descriptions of the spectroscopic properties of p-quinones while calculations with HF and DFT provide more satisfactory simulations. With the aid of Gaussian View 3.09, the low-frequency modes of BQ, NQ, and AQ were assigned based on the calculations with DFT method (Table 1). The peak of BQ at 1.21 THz originates from the cogwheel mode. The strong absorption of NQ at 1.75 THz is due to the stretching of the whole dimer linked by H-bonds, and the other band at 1.45 THz is assigned to the tilting mode. For AQ, the sharp 0.96 THz absorption is caused by the stretching of the whole molecule linked by H-bonds, the nearby 1.07 THz band came from tilting mode, and the clear peak at 1.87 THz is assigned to the butterfly mode. The predicted modes of three p-quinones are similar and can be concluded to the collective vibrations involving the weak hydrogen bonds in p-quinones dimers. In spite of about 10% error compared with the experimental data for quinone dimer, the spectrum by theoretical simulation is well reproduced. The difference between calculation and experiment is due to the limitations of the theoretical method and the complicated real status. The measurement is affected by many factors, such as the crystalline state, the process of the samples preparation, temperature, ambient humidity, etc. Most

GE Min et al. Sci China Ser B-Chem | Apr. 2008 | vol. 51 | no. 4 | 354-358

Figure 3 Calculated and experimental results of BQ (a), NQ (b), and AQ (c) dimer between 500 cm-1 and 3500 cm-1 with AM1 (dash dot line), HF/6-31G (dot line), and B3LYP/6-311G (dash line). Table 1 Calculated and experimental vibrational frequencies for p-quinones dimer Calculated frequency (THz, dimer) Experimental frequency (THz) DFTb) AM1 HFa) BQ 1.37 1.2 1.23 1.21 NQ 0.93 1.30 1.43 1.45 1.53 1.65 1.75 AQ 0.52 0.93 0.87 0.96 0.85 1.16 1.05 1.07 1.30 1.59 1.69 1.87 a) HF/6-31G, scale factor: 0.96; b) DFT: B3LYP/6-311G. GE Min et al. Sci China Ser B-Chem | Apr. 2008 | vol. 51 | no. 4 | 354-358

Assignments (Based on DFT) Cogwheel Tilting H-bonds stretch H-bonds shearing Tilting Butterfly

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important of all is the experimental data indicating the collective results. There exist intermolecular force, such as hydrogen bonds, van der Waals forces, and other weak interactions, among the molecules. For example, according to Adamowicz and coworkers’ studies[19], BQ dimer has another configuration: a stacked complex stabilized by the dispersion interaction. However, in our calculations mentioned above, this stacked complex has not been considered. In further theoretical calculation, the structural parameter obtained from X-ray crystallography should be taken into account as the initialization, and the forces in multi-molecules network should also be considered. 1

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4 Conclusions In this paper, we report the measurements of THz characterizations of BQ, NQ, and AQ by THz-TDS at room temperature. The distinct absorption features of samples in the frequency range between 0.3 and 2.0 THz are presented. Semi-empirical AM1, HF, and DFT were used to interpret the relations. The results with DFT produce better simulation with the experimental data than AM1 and HF. With the help of Gaussian View, the observed spectroscopic data of p-quinones are assigned to collective vibrations mediated by hydrogen bonds. 14

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