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Abstract
We report terahertz electromagnetic wave emission by optical rectification from hydrogen-bonded single molecular crystal 4-nitro-2,5-bis(phenylethynyl)aniline designed to be polar via the hydrogen bonding between nitroaniline cores. The terahertz emission efficiency is comparable to the representative inorganic terahertz emitter ZnTe. We show terahertz emission characteristics, optical spectrum, and theoretical molecular orbital calculations. Another three kinds of nitroaniline-based organic molecules are revealed to form polar crystal structure, and they have large hyperpolarizabilities and have potential for terahertz photonics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Materials with broken inversion symmetry are characterized by finite nonlinear optical susceptibilities (χ(2) for the second order) [1], leading to broadband electromagnetic emission at terahertz (THz) frequencies under illumination by femtosecond laser pulses [2]. THz emission is caused by optical rectification (OR) effect, and it is commonly used as a light source of terahertz time-domain spectroscopies and terahertz imaging [3]. Prototypical terahertz nonlinear optical crystals are inorganic semiconductor ZnTe and ferroelectrics LiNbO3 [4], and organic dimethylamino-4-N-methylstilbazolium tosylate (DAST) [5–7]. Recently, a single molecular hydrogen-bonded crystal 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile (OH1) was demonstrated to be an efficient THz emitter [8]. The large electro-optic coefficient of OH1 (r = 52 pm/V at 1.05 eV) is larger than those of the donor-acceptor-type DAST (r = 47 pm/V at 1.55 eV) [7] and ZnTe (r41 = 4 pm/V at 1.96 eV) [6]. Hydrogen-bonded organic molecular crystals have high potential for terahertz emitter by their high molecular hyperpolarizability, controllability of bandgap energy, lightweight, mechanical flexibility, manufacturing costs, and environmental compatibility [9–11].
Among polar molecules, nitroaniline has a high potential to realize larger ${\chi ^{(2 )}}$ [12]. The nitro group (-NO2) and the amino group (-NH2) sit in the opposite side of the benzene ring in the nitroaniline molecule. It has large dipole moment of 11 to 16 Debye (2.29-3.33 Å) at the ground state [13]. This value is comparable to that of OH1 (2.15 Å) [11] formed with a donor (phenolic OH) and an acceptor cyanomethylidene (C = C(CN)2) [14]. In molecular crystals, polar molecules generally stack in a staggered manner, leading to the vanishing of macroscopic polarization. However, the intermolecular hydrogen bonding between the nitro and the amino groups can align the molecules in polar manner, as exemplified in 2-methyl-4-nitroaniline (MNA) and in its derivatives, resulting in large ${\chi ^{(2 )}}$ values [15] and efficient THz emission [16]. The nitroaniline structure with large hyperpolarizability also helps realize this large nonlinear optical response. Thus, the polar molecular crystals containing nitroaniline with broad base-selectivity and yield, will be major candidates for future nonlinear optical crystals.
In this study, we show that four types of molecular crystals composed of nitroaniline cores form polar structures. Among them, we show the THz electromagnetic wave emission at high efficiency from 4-nitro-2,5-bis(phenylethynyl)aniline (di-Ph-ethynyl-NA). The susceptibility ${\chi ^{(2 )}}\; $is found to be comparable to that of ZnTe. We found other three kinds of nitroaniline-based derivatives (4-nitro-2,5-diphenylaniline, 4-nitro-2,6-diphenylaniline, and 4-nitro-2-(phenylethynyl)aniline) form non-centrosymmetric crystal structure. We discuss the potential of these nitro-aniline-based molecular crystals for the THz photonics applications, by using extensive optical spectroscopy and molecular orbital calculations.
2. Results and discussions
This section is organized as follows. Firstly, we describe the structure of the isolated molecules (Sec. 2.1). Following the analysis of crystal structure (Sec. 2.1), the details of the optical spectra of a single crystal sample will be discussed (Sec. 2.3). Then we proceed to the features of THz radiation (Sec. 2.4, 2.5), and compare the nonlinear optical susceptibilities deduced from experiments and theory (Sec. 2.6). Finally, we compare polar crystals of nitroaniline-based derivatives (Sec. 2.7).
2.1. Molecular structure
Figure 1(a) and 1(b) show the molecular structure of di-Ph-ethynyl-NA. The molecule forms a flat skeleton, which is advantageous for the delocalization of π-orbitals to induce large χ(2). The optimized atomic positions are listed in Supplement 1. Note that deformation of the molecule in the crystal was negligibly small, which is reflected in the carbon atoms aligning almost in the same plane. The nitroaniline and substituents (phenylethynyl substituents at 2-5 positions) were found to stay almost in the same plane [xy-plane in Fig. 1(b)].
Fig. 1. (a) Molecular structure, and (b) optimized geometry of di-Ph-ethynyl-NA. (c),(d) Perspective and plane view of the crystal structure. Labels for each molecule (1-4) are same as Table 1. (e) Schematics and photograph of a di-Ph-ethynyl-NA single crystal.
Table 1. Cosines of four molecules on each axis directions in unit cell. See Fig. 1(c) for the molecule numbering (1-4).
2.2. Crystal structure
From the X-ray diffraction analysis, it is found that the di-Ph-ethynyl-NA crystallizes in the non-centrosymmetric space group P43 (Fig. 1(c-e)). There exist four molecules in the unit cell, and the nitroaniline core forms one-dimensional hydrogen-bonded chains along the c-axis (O-N distance of 3.043(3) Å, dotted lines in Fig. 1(c, d)). The crystal is stable in dried air at room temperature. The single crystal has a rectangular pole shape, with the typical size of 200 × 200 × 2000 µm along the a, b, and c axes, respectively (Fig. 1(e)). The longest axis corresponds to the polar c-axis.
The molecular arrangements in the crystal is a crucial factor to realize large χ(2) in organics [17,18]. We found the crystal parameters a = 8.8694(5) Å, c = 21.2163(5) Å, V = 1669.01(19) Å3, R1 (I > 2σ(I)) = 0.0263, wR2 (all data) = 0.0709, and goodness of fit (GOF) = 1.060 (Fig. 1(c,d)), with the cosine projection of molecules on the a, b, and c axes in the unit cell listed in Table 1. All the molecules show cosine of 0.5854 along the c-axis, leading to the large nonlinear susceptibility.
2.3. Optical spectrum
The experimental linear optical spectra are shown in Fig. 2. We fitted the normal reflectance R spectrum (Fig. 2(a)) above 1.0 eV by using a Lorentz oscillator model. The complex dielectric constant $\tilde{\epsilon}(\omega )$, refractive index $\tilde{n}(\omega )$, and $R(\omega )\; $are expressed by
Fig. 2. Optical spectra for infrared to near-ultraviolet range. (a) Polarized reflectance (R) spectra, and (b) transmittance (T) spectra. Inset in (b) shows the absorption coefficient. (c) Refractive index spectra. Dotted lines are fitting curves by the Sellmeier's relation. Dashed lines are group refractive index.
Table 2. Fitting parameters for reflectance spectrum analysis. ${\mathrm{\epsilon}_\infty } = 2.02\ \textrm{in}\ E\parallel c,\; \textrm{and}\ {\mathrm{\epsilon}_\infty } = 2.49\ \textrm{in}\; E\parallel a$.
The transmittance (T) spectrum shows an absorption edge at 2.5 eV (Fig. 2(b)). From the absorption coefficient (α) (inset in Fig. 2(b)), we get ${\alpha _a} = 7.0\; \textrm{c}{\textrm{m}^{ - 1}}$ and ${\alpha _c} = 8.0\; \textrm{c}{\textrm{m}^{ - 1}}$ at 1.55 eV. Here the penetration depth of 1.55 eV laser pulse (∼1 mm) is much larger than the sample thickness. Note that the absorption below 1.0 eV should be ascribed to the harmonics of molecular vibrations. This molecular crystal is transparent between 1.0 and 2.4 eV.
2.4. THz emission spectroscopy
The femtosecond laser pulses delivered from a mode-locked Ti:sapphire laser (1.55 eV, 100 fs duration at 80 MHz) were focused on the ac-plane of a sample crystal at normal incidence (spot size ∼10 µm). The crystallographic a, b, and c axes are set parallel to the experimental Y, Z, and X axes, respectively. We detected the emitted THz waves with X-axis polarization in the transmission geometry (along the Z-axis) by using a photoswitching dipole antenna [3]. The laser power was set to 10 mW except for the incident power dependence. All optical experiments were performed at room temperature in dried air. The time-trace of the THz wave and its amplitude were compared with those from a reference ZnTe(110) crystal (thickness ${d_{\textrm{ZnTe}}} = 200{\; }\mathrm{\mu}\textrm{m}$) in ${E_{\textrm{laser}}}\parallel Y\parallel [{1\bar{1}0} ]$ and ${E_{\textrm{THz}}}\parallel X\parallel [{001} ]$. Although we used this optical polarization for precise estimation of nonlinear optical susceptibility, note that the amplitude can be optimized by azimuthal angle of the ZnTe(110) crystal in ${E_{\textrm{laser}}}\parallel {E_{\textrm{THz}}}\parallel X$ configuration. The maximum in ${E_{\textrm{laser}}}\parallel {E_{\textrm{THz}}}\parallel X$ should become $2/\sqrt 3 \approx 1.15$ times of that in our optical polarizations [19].
Figure 3(a) shows a prototypical THz waveform radiated from the di-Ph-ethynyl-NA single crystal (thickness ${d_\textrm{s}} = 220{\; }\mathrm{\mu}\textrm{m}$). We found that the magnitude of the THz wave is comparable to that from a reference ZnTe with the similar thickness (${d_{\textrm{ZnTe}}} = 200{\; }\mathrm{\mu}\textrm{m}$). Since the power spectrum is found to be rather broad (Fig. 3(b)), and the absorption coefficient at 1.55 eV is rather small, the THz waves should be generated by the in-gap optical rectification (OR). The arrival time difference of the THz waves between the sample and the ZnTe reference ($\mathrm{\Delta }t\; $ = 0.8 ps) is due to the difference in the optical depth $({n_\textrm{g}^{\textrm{ZnTe}}{d_{\textrm{ZnTe}}} - n_\textrm{g}^{\textrm{s},{\; }c}{d_\textrm{s}}} )/{c_0} = 0.85\;\textrm{ps}$, where the group refractive indices of the ZnTe and the sample are $n_\textrm{g}^{\textrm{ZnTe}} = 3.0$ (ref. 20) and $n_\textrm{g}^{\textrm{s},{\; }c} = 1.57$ at 1.55 eV, respectively, and$\; {c_0}$ is the speed of light in vacuum.
Fig. 3. THz emission from the 220-µm-thick ac-plane di-Ph-ethynyl-NA crystal and a reference ZnTe(110) crystal. (a) THz waveforms for optical polarizations Elaser || ETHz || X || c for the sample and Elaser ⊥ETHz || X || [001] for ZnTe, and (b) absolute amplitude spectra deduced by Fourier transformation.
From experimentally obtained spectral information, we expect the coherence length lc ∼ 3 mm at 1.0 THz (the difference of refractive indexes n(1.55 eV) - n(ω→0) ∼ 0.05). It is comparable to lc of ZnTe (∼3 mm) [20]. The penetration length of pump light was calculated as 1/${\alpha _{\textrm{laser}}}$∼ 1 mm at 1.55 eV. The amplitude of the terahertz wave is expected to increase linearly up to the thickness of ∼1 mm if the absorption for terahertz waves is weak. Since the obtained crystal had length along the a- and b-axes directions only 220 µm, it was difficult to measure the refractive index and absorption coefficient spectrum in terahertz frequencies. Growth of large crystal and precise elucidation of phase matching conditions are future works.
2.5. Light polarization and laser power dependences
The laser polarization and power dependences of the THz emission also support that the in-gap OR is the predominant optical process in di-Ph-ethynyl-NA. Figure 4(a) shows the schematics of the polarimetry measurement (see Materials and methods for details). The point group P43 is associated with the $\chi _{ijk}^{(2 )}$ tensor of the form
Fig. 4. (a) Schematics of the polarimetry measurement. θ: laser light polarization angle. ϕ: polarization angle of terahertz wave. (b) Incident laser polarization dependence, and (c) power dependence of the absolute THz amplitude $I_c^{1/2}$.
Table 3. Parameters for THz emission$.$ The emitted THz wave polarization ${E_{\textrm{THz}}}\parallel c$. THz amplitude ${E_{\textrm{THz}}}$ is relative value to 200-µm-thick (110) ZnTe with ${E_{\textrm{laser}}}\parallel [{1\bar{1}0} ]$, ${E_{\textrm{THz}}}\parallel [{001} ]$.
2.6. Estimation of the nonlinear optical susceptibilities
To compare the experimentally observed ${\chi ^{(2 )}}$ with that from theory, we calculated the molecular orbitals (Fig. 5(a)) and transition dipole moments between their energy levels (Table 4). |µave| is absolute value of transition dipole moment averaged in the unit cell.
Fig. 5. Molecular orbitals of di-Ph-ethynyl-NA. (a) Molecular orbitals involved in the optical transitions in visible to near-UV range. L: LUMO, H: HOMO. (b) Density difference plot and transition dipole moment of optical transitions with large oscillator strength. (c) Molar extinction coefficient spectrum (solid line) and oscillator strength (vertical bars).
Table 4. Ground to excited state transition electric dipole moments (a.u.). δS = x2+ y2+ z2. Transition dipole moment µtra = (δS)1/2. Units are in a.u. (atomic units, 1 a.u. = 2.541746230211 Debye). Dipole moment of the HOMO is (x, y, z) = (0.9611, -2.3637, 2.5724). |µave| is transition dipole moments averaged in the four molecules in the unit cell.
We found four predominant optical transitions with the oscillator strength (f) larger than 0.3 (transition index #1, #2, #4, #7). For HOMO-2 (MO86) to LUMO+4 (MO93) states, the molecular orbitals are mostly composed of the π-electron cloud, while HOMO-4 (MO84) and HOMO-3 (MO85) states of the σ-electrons. The optical transitions with large f appear between these π-orbitals in general; (#1) HOMO→LUMO [95.0%], (#2) HOMO→LUMO+1 [67.6%], (#4) HOMO-1→LUMO [65.4%], and (#7) HOMO-2→LUMO [88.3%] (Table 5). It is also seen that the molecular orbitals with the larger density of states at the nitroaniline structure contribute to these optical transitions. The spectrum of molecular extinction coefficient ${\epsilon _{\textrm{mol}}}$ has broad peak structures around the four peaks in 2.5-4.5 eV (Fig. 5(c)). While molecular orbital calculation tends to overestimate the corresponding transition energies in the crystal state, the single molecular spectrum is consistent with the optical spectrum (Fig. 2).
Table 5. Excitation energies, oscillator strengths of excitations, and contributions of molecular orbitals to the transitions with large oscillator strength. Molecular orbitals are plotted in Fig. 5.
By comparing the fitting parameters for the linear optical spectra (Table 2) with the energy levels of molecular orbitals (Table 4), we assign the resonances in the $E\parallel c$ polarization at 2.86 eV (i = 2) and 4.33 eV (i = 3) to be HOMO→LUMO (#2 in Fig. 5(c)) and HOMO-2→LUMO (#7), respectively. Also, the resonances for the $E \bot c$ polarization at 2.51 eV (i = 4) and 3.51 eV (i = 5) should be assigned to HOMO→LUMO (#1) and HOMO-1→LUMO (#4), respectively, since #1 and #4 have strong transition moments along the nitroaniline structure (y-axis in Fig. 1(b)).
The ${\chi ^{(2 )}}$ tensor can be estimated from molecular arrangements in the unit cell and the hyperpolarizability of an isolated molecule [21] (Table 6). The second order hyperpolarizability ${\beta _{ijk}}$ is found to be large in the xy-plane in Fig. 1(b), since the molecular skeleton lies in this plane (with ${\beta _{xxx}} = 28.847 \times {10^{ - 30}}\; \textrm{esu}$, and ${\beta _{yyy}} = 6.031 \times {10^{ - 30}}\; \textrm{esu}$). The nonlinear optical susceptibility ${\chi ^{(2 )}}\; $was calculated by following Ref. [22]. Here we defined effective hyperpolarizability $\beta _{ijk}^{(2 ),\textrm{eff}}$ as the averaged value of ${\beta _{ijk}}$ for molecules in unit cell:
Table 6. Output of the Gaussian 16. Hyperpolarizability βijk(0;0,0). Unit of βijk is in (10−30 esu) of an isolated single molecule. Directions of axes are same as those in Fig. 1(b). All the components are non-zero since there are no cancelation effects from molecular arrangements.
In contrast, the $\chi _{caa}^{(2 ),\textrm{exp}}$ is found to be ∼10% of $\chi _{caa}^{(2 ),\textrm{calc}}$. This discrepancy suggests that large anti-phase nonlinear optical effects from the lattice system may be contributing to $\chi _{caa}^{(2 ),\textrm{exp}}$ [17]. The ${\chi ^{(2 )}}$ can have its origin in acoustic and optical phonon resonances ($\chi _a^{(2 )}$ and $\chi _o^{(2 )}$) in addition to the electronic transitions ($\chi _e^{(2 )}$); ${\chi ^{(2 )}}$=$\chi _a^{(2 )} + \chi _l^{(2 )} + \chi _e^{(2 )}$. In the THz emission, $\chi _a^{(2 )}$ can be neglected since upper limit of acoustic phonons are in the sub-THz frequency range. Here ${\chi ^{(2 ),\textrm{calc}}}$ includes only the $\chi _e^{(2 )}$contribution, however the $\chi _l^{(2 )}$ can also appear and interfere with $\chi _e^{(2 )}\; $depending on the incident laser polarization.
2.7. Polar crystals with different side chains
In addition to di-Ph-ethynyl-NA, three kinds of nitroaniline-based derivatives (4-nitro-2,5-diphenylaniline, 4-nitro-2,6-diphenylaniline, and 4-nitro-2-(phenylethynyl)aniline) were found to form polar crystals (Supplement 1). From molecular orbital calculations and crystal structure, we found that the 4-nitro-2,6-diphenylaniline has large $\beta _{ccc}^{(2 )} = 24.15 \times {10^{ - 30}}\; \textrm{esu}$. However, this molecular crystal gradually degraded in air due to evaporation of the solvent molecules (acetone or hexane). Crystal structures and molecular orientations of the three molecular crystals are detailed in Supplement 1.
4-Nitro-2,5-diphenylaniline molecules has cosine of 0.3592 on the polar b-axis in its crystal. This value is smaller than the case of di-Ph-ethynyl-NA (0.5854 on the c-axis). It suggests that the longer side chains make the nitroaniline structure aligned straightly. 4-Nitro-2,6-diphenylaniline has two phenyl side chains in the 2,6-positions on the nitroaniline core, and four molecules in the unit cell had almost same orientation (cosine of 0.9996 along the c-axis). This suggests that nitroaniline-based molecules with same substituents on 2,6-positions can have large $\beta _{ijk}^{(2 ),\textrm{eff}}$. In the molecule with only one side chain [4-nitro-2-(phenylethynyl)aniline], molecules oriented in various directions in its crystal. This fact indicates that two same side chains are needed to arrange molecular orientation. The nitroaniline-based molecular crystals have a high degree of freedom on substituent type, number, and positions. Our results on four molecular crystals suggest molecular design strategies for terahertz nonlinear optical crystals.
3. Summary
We found that the hydrogen-bonded polar crystal of 4-nitro-2,5-bis(phenylethynyl)aniline shows highly-efficient THz electromagnetic wave emission by the in-gap optical rectification process. The polar crystal structure was effectively stabilized by the careful design of the hydrogen-bonding between the nitroaniline cores, which can be an efficient route to realize functional organic molecular crystals. We evaluated the nonlinear optical susceptibilities both experimentally and theoretically. We conclude that the molecules with the nitro and amino substitutes can have large nonlinear optical susceptibilities with the advantage of high flexibility of its substituents.
4. Materials and methods
4.1 Synthesis and crystal growth
di-Ph-ethynyl-NA and its derivatives were synthesized via the Sonogashira or Suzuki–Miyaura cross-coupling reactions of halogen-substituted 4-nitroanilines (Supplement 1). For the X-ray crystallographic analysis, the sample was recrystallized from acetone-hexane solution. The single crystals for the optical experiments were prepared by a slow evaporation method.
4.2 Reflection and transmission spectroscopies
Reflectance (R) and transmittance (T) spectra in the infrared to visible range (0.07 eV–4.96 eV) were measured by the Fourier-transform infrared- and grating-spectrometers. The refractive index (n) and extinction coefficients (κ) were deduced from the R spectra by using the Kramers-Kronig transformation [23]. The absorption coefficient (α) was evaluated for one crystal (thickness d = 120 µm) through the relation $\alpha \; ={-} \ln \left( {\frac{T}{{1 - R}}} \right)/d.$ The group refractive index $({{n_\textrm{g}}} )$ is given by ${n_\textrm{g}} = n - \frac{{\textrm{d}n}}{{\textrm{d}\lambda }}$, where λ is the wavelength.
4.3 Polarimetry of the THz emission
The polarimetry of the THz emission was evaluated with a λ/2 plate for the incident laser and a pair of wire-grid polarizers (WG1 and WG2) for the THz waves [24]. We set the WG1 and WG2 to pass X-axis polarized THz waves, except for the polarimetry. In the polarization dependence, we set the rotation angle of WG1 to be +45° or -45° with respect to the X-axis. In this case, the vector components EX and EY of terahertz electric field ETHz can be expressed as follows: EX = E+45° + E-45° and EY = E+45° − E-45°, here E+45° (E-45°) is the signal obtained with the WG1 angle at the angle of +45° (−45°).
4.4 First principles calculations
The nonlinear optical susceptibility χ(2) with its origin in the electronic process was evaluated by considering the hyperpolarizability tensor $({{\beta_{ijk}}} )\; $and molecular arrangement in the crystal [17]. We conducted molecular orbital calculation with the optimized structures by using the base function of B3LYP/6-31G(d) in Gaussian 16 [25].
Funding
Precursory Research for Embryonic Science and Technology (JPMJPR17I3); RIKEN-AIST Quantum Innovation Fund 2019; Japan Society for the Promotion of Science (15H02196, 17H02914, 18K14155, 19H00906, 20K15198).
Acknowledgements
We thank Y. Tokura for his thoughtful comments. Molecular orbital calculations were performed in the Supercomputer System in the Advanced Center for Computing and Communication (ACCC) of RIKEN. We thank Dr Takemichi Nakamura of the Molecular Structure Characterization Unit, RIKEN Center for Sustainable Resource Science (CSRS), for high-resolution mass spectrometry (HR-MS) measurements.
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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