Synthesis and Tuning Optical Nonlinear Properties ... - ACS Publications

J. Phys. Chem. C 2010, 114, 15109–15115

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Synthesis and Tuning Optical Nonlinear Properties of Molecular Crystals of Benzothiadiazole Songhua Chen,†,‡ Yongjun Li,*,† Wenlong Yang,†,‡ Nan Chen,†,‡ Huibiao Liu,† and Yuliang Li*,† Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: April 8, 2010; ReVised Manuscript ReceiVed: July 20, 2010

We described the synthesis and characterization of four kinds of donor-acceptor systems of benzothiadiazole derivatives (BTDs). The crystal packing styles of BTDs can be tuned from the centrosymmetric triclinic crystal system to the noncentrosymmetric orthorhombic system. The nonlinear absorption and nonlinear refraction of BTDs were studied by using the Z-scan technique at 532 nm at 4 ns pulse. The results indicated that all of them exhibit third-order nonlinear optical properties including nonlinear absorption and refractive effects. Introduction In recent years, there has been considerable interest in the development of organic molecular crystal with spectral properties in the nonlinear optics as well as their applications in optical devices for information procession.1 The organic materials with delocalized π electrons have many advantages such as architectural flexibility, easy fabrication, ultra fast response capacity, and larger nonlinearity.2 Donor-acceptor (D-A) molecular crystals and materials have recently attracted considerable academic and technological research attention for their applications in nonlinear optics.3 Understanding the structure-function relationships that relate specifically to organic molecular materials could lead to new design concepts for producing benign, high-performance new molecular materials. Organic chemistry provides a number of advantages in the development molecular materials design and synthesis. Usually, designing proper D or A building blocks over a wide range of different functional units could rationally control nonlinear properties of the D-A materials. Moreover, their nonlinear properties could also be adjusted by appropriate manipulation of their chemical structures such as different connecters linking the D/A building blocks, distance between the D/A units, and the symmetry of the molecular.4,5 2,1,3-Benzothiadiazole is an important building block of active materials in various optoelectronic devices due to its large reduction potential and electron affinity6 and is commonly used as an electron-accepting moiety.7 Main chain conjugated polymers containing this unit have been shown to possess excellent charge transporting and electroluminescence.8 A structural study of molecular materials was a main fundamental interest in development and is expected to be widely applied on nonlinear optics.9 Still, the design and synthesis of new molecular materials with definite crystal structure and excellent nonlinear properties is a significant and ongoing challenge.10 Herein, we designed and synthesized donor-acceptor (D-A) molecules based on benzothiadiazole, where the donor parts consist of N,N-dimethylaniline units, while benzothiadiazole works as the acceptor. We investigated the influence of different * To whom correspondence should be addressed. E-mail: [email protected] and [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.

substitution groups on the structure and photophysical properties of these benzothiadiazole derivatives. Interestingly, the triazole ring is a versatile and readily installed linkage that has been used to extend the conjugation of diverse aromatic systems,11 and triazole is a good subunit for the hydrogen bond;12 we envisioned that installation of this moiety into the benzothiadiazole core could potentially improve the optical properties of benzothiadiazole derivatives and tune the crystal packing of the benzothiadiazole derivatives. Results and Discussions Synthesis. The synthesis of the benzothiadiazole derivatives (BTDs) was outlined in Scheme 1. Standard Sonogashira coupling of 113 or 213 with 4-ethynyl-N,N-dimethylaniline gave BTD-S (65% yield) and BTD-B (50% yield). Compound 3 was obtained via Sonogashira coupling from BTD-B and ethynyltrimethylsilane. Subsequently, the trimethylsilane group was removed from 3 by successive 1,3-dipolar cycloaddition14 to give BTD-C. Reaction of 2 with piperidine afforded alkylamino derivatives 4. It was then subjected to the Sonogashira coupling to give BTD-P. The synthesized compounds were characterized by 1H and 13C NMR and mass spectra. All of the BTDs could be isolated as light- and air-stable crystalline materials with a yellow to deep red color. Molecular Structures. The single crystals of BTDs for suitable analysis of single crystal were gained by slow diffusion of hexane into CH2Cl2 solutions (Figure 1). The plane angles comparisons clearly show that the angle between the benzene ring (C9-C10-C11-C12-C13-C14) and benzothiadiazole (C1-C2-C3-C4-C5-C6) in BTD-S (83.31°) is larger than that in BTD-B (4.56°) and BTD-C (5.12°). This result may be accounted for by the greater steric requirement of the bromine atom compared to the hydrogen atom. In addition, the introduction of Br atom at the 4-position must suppress more interaction than the H atom. For BTD-S, the bond lengths of C1-C7 (1.434 Å) and C8-C9 (1.440 Å) are almost the same as the typical Caromatic-Csp bond length, i.e. 1.434 Å.15 For BTD-C, the triazol ring and benzene ring in the benzothiadiazole unit are nearly coplanar with a dihedral angle of 4.33°, suggesting that an extended π-conjugated system exists in this part of the molecule.16

10.1021/jp103159b  2010 American Chemical Society Published on Web 08/18/2010

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SCHEME 1: Synthetic Route of Benzothiadiazole Derivatives

Bond length alternation in the benzene ring of N,N-dimethylaniline is a good indication for the efficiency of charge transfer conjugation from the donor to the benzothiadiazole acceptor, which could be expressed by the quinoid character (δr):17

δr ) {[(a + a′)/2 - (b + b′)/2] + [(c + c′)/2 - (b + b′)/2]}/2 (1) In benzene, the δr value equals 0 (see Figure 1a for the definition of the bonds of a, a′, b, b′, c, c′). Calculating from the X-ray crystal structure, BTD-C exhibits δr ) 0.010, while BTD-S, BTD-P, and BTD-B show higher values of 0.017, 0.018 and 0.024, respectively. This result could show the BTDs exhibit efficient intramolecular charge transfer interactions. From the packing diagram (Figure 2) of BTD-S, the most remarkable feature of the crystal structure is the existence of short S1 · · · N1 interheteroatom contacts (3.197 Å) between the two 1,2,5-thiadiazole rings with a distance of 0.702 Å. CH/π interactions from the benzothiadiazole hydrogen atom to the

benzene ring, from the CH on the benzene to the CtC bond, are also playing an important role for the creation of this headto-head crystal packing structure. In the crystal packing of BTDB, there are also shorter S1 · · · N1 (3.135 Å), N1 · · · N1 (3.004 Å) interheteroatom contacts between the two 1,2,5-thiadiazole rings. Short Br · · · Br contacts (3.519 Å) within the sum of the Van der Waals radii are also observed. The π-π stacking between the 1,2,5-thiadiazole ring and nearby N,N-dimethylaniline ascribed to the electrostatic force of the electron-rich N,Ndimethylaniline with the electron-deficient 1,2,5-thiadiazole center is also critical for the head-to-tail crystal packing of BTDB. Similarly, the shorter S1 · · · N1 (3.018 Å), N1 · · · N1 (3.051 Å) interactions between the two 1,2,5-thiadiazole rings were observed in BTD-P. For the BTD-C, the · · · N5S1 · · · N5S1 · · · N5S · · · chain formed by the shorter S1 · · · N5 interheteroatom contacts (3.168 Å) in crossing benzothiadiazoles is stabilized by intermolecular hydrogen bonds between triazole CH and S1. This chain structure resulted in two kinds of head-to-head crystal packing of the N,N-dimethylaniline (donor)-benzothiadiazole (acceptor), the angle between them is 51.27°. The packing in these two

Figure 1. The molecular structure of BTD-S (a), BTD-B (b), BTD-C (c), and BTD-P (d) with the atomic numbering scheme.

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Figure 2. Packing structures of BTD-B (a), BTD-P (b), BTD-S (c), BTD-C (d), and dipole moment of BTDs in the packing structure (e).

directions is stabilized by the hydrogen bond between the aromatic CH and the triazole N, and the dipole in these directions still remained (Figure 2d). The results indicated that introducing different substitution groups can tune the packing styles of benzothiadiazole derivatives from the centrosymmetric triclinic crystal system to the noncentrosymmetric orthorhombic system indicting an effective approach for constructing the acentral crystal system for nonlinear optical crystals. Spectroscopic Properties. The UV-vis absorbance and fluorescence spectra of BTDs in CH2Cl2 solution and thin films are shown in Figure 3. The absorption and emission data are summarized in Table 1 for comparison. Absorbance and emission data indicated the intramolecular charge transfer from the terminal amino groups to the benzothiadiazole core.18 These compounds have a narrow peak at ∼310 nm attributable to the π f π* transition of the benzothiadiazole unit.19 These BTDs molecules have their characteristic n f π* absorption bands20 which are influenced by the electronic properties of the substituents, and the results showed the peak position of 428 nm for BTD-S, 454 nm for BTD-B (due to extended π-conjugation induced by electron accepting of Br), and 478 nm for BTD-P (due to stronger electron donating of piperidine) and the absorbance of BTD-C red-shifted to 466 nm due to the extended π-conjugation between the triazol group and benzothiadiazole unit.21 Fluorescence spectra of BTDs were measured in solution (CH2Cl2) and solid state film. All of the compounds showed almost the same broad fluorescence spectra in CH2Cl2. Their

Figure 3. Optical spectra of BTDs in CH2Cl2..

fluorescence maximum wavelengths (λem) range from 642 to 683 nm with about 200 nm stocks shift. BTD-P showed a weaker photoluminescence than others, perhaps due to the electron transfer from the piperidine nitrogen atom to benzothiadiazole core. In contrast, their emission maxima in the solid state are shorter than those in solution. For example, the λem of BTD-S was blue-shifted from 642 nm in solution to 604 nm in the solid state. Intramolecular stacking of BTDs was rather distorted in the solid state due to its rigid conformation compared to that in solution.22 Electrochemical Properties. The electrochemical properties of these charge-transfer chromophores (BTDs) were studied by

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TABLE 1: Photophysical Properties of BTDs

compd BTD-S BTD-C BTD-B BTD-P

λmax /nm (log ε)a 304 312 306 312

(4.6), (4.7), (4.6), (4.5),

428 466 454 478

(4.1) (4.3) (4.2) (4.0)

Stoke’s shift film ∆λemd (nm) (nm) (nm)

λem (nm)b

Φ fc

642 662 666 683

0.002 0.05 0.01 0.001

214 196 212 205

604 628 641 662

38 34 25 21

a Measured in 1 × 10-5 M solution. b Measured in 1 × 10-5 M in CH2Cl2, the excitation of 454 nm for BTD-B, 428 nm for BTD-S, 466 nm for BTD-C (B), 478 nm for BTD-P. c Fluorescent quantum yield relative to Fluorescein in ethanol (0.97). d ∆λem equals the fluorescence maximum wavelength in solution subtract to that in the film.

TABLE 2: Electrochemical Properties of BTDs compd

E1ox (V)

BTD-S BTD-C BTD-B BTD-P

0.85 0.84 0.84 0.71

E2ox (V)

E1red (V)

0.60

-1.45 -1.35 -1.28 -1.58

E2red (V)

-1.45

cyclic voltammetry at room temperature in dichloromethane solutions (Table 2). A plausible mechanism for the electrochemical behavior of benzothiadiazole derivatives has been presented by Hirao et al.23 All the voltammograms (Figure S1, Supporting Information) show a one-electron reversible reduction wave corresponding to the benzothiadiazole acceptor moiety,24 and one irreversible oxidation wave, corresponding to the Namino in the N,N-dimethylanilino moiety. Compared with that of BTD-S, the Ered values of the benzothiadiazole moiety in BTD-B and BTD-C showed a positive shift of about 0.17 and 0.1 V, respectively. The positive shifted potential originates from the incorporation of the electron-withdrawing group. The Eox value of the Namino in BTD-B and BTD-C showed a slight negative shift (0.01 V) compared with that of BTD-S. Meanwhile, a reduction peak at -1.45 V was observed in BTD-B due to the introduction of the electron-withdrawing bromide atom. BTD-P showed different electrochemical properties, compared with that of BTD-S, the reduction potential of benzothiadiazole moiety showed negative shift (0.13 V), and the oxidation potential of Namino also shifted for 0.14 V negatively. BTD-P undergoes another irreversible one-electron oxidation (E2ox ) 0.60 V) accounting for the N in the piperidine moiety. Nonlinear Optical Properties. The third-order optical nonlinearities of BTDs were investigated by using the Z-scan technique. This method provides direct measurement of nonlinear absorption and refraction along with the sign of nonlinearity. The experimental setup is similar to that in the references.25 We measured both nonlinear absorption and refractive effects of the BTDs. The NLO absorption data obtained under the condition used in this study can be well described by eq 2,26 which describes a third-order NLO absorptive process.

T(z) )

1

√πq(z)

q(z) ) Reff 2 I(z)

∫-∞∞ ln[1 + q(z)]e-τ 1 - e-R0L R0

2

dτ

(2)

where R0 and R2 are linear and effective third-order NLO absorptive coefficients, τ is the time, and L is the optical path.

Light transmittance (T) is a function of the sample’s Z position (with respect to the focal point at Z ) 0). Figure 4 gives the nonlinear absorptive curves of BTD-B and BTD-C with 4 ns pulse duration under an open-aperture Z-scan configuration. The fitting curves were obtained on the basis of a five-level energy model that took into account the dynamic thermal effect resulting from the transient excited-state absorption. For the BTD-B, the normalized power transmittance decreased at the focus by nearly 0.6 times compared to that of the low power transmittance, indicating a reverse saturated absorber behavior. BTD-C exhibits weaker reverse saturated absorption. The nonlinear absorption coefficient R2 fitting the experimental data are summarized in Table 3. On the other hand, the nonlinear refraction Z-scan curves of BTDs were obtained by dividing closed-aperture Z-scan data and corresponding open-aperture Z-scan data. An effective thirdorder NLO refractive index n2 can be derived from the difference between normalized transmittance values at valley and peak positions (∆Tv-p), using eq 3.26

n2 )

λR0 0.812πI(1 - e-R0L)

∆Tv-p

(3)

where ∆Tv-p is the difference between normalized transmittance values at valley and peak portions, I is the peak irradiation intensity at focus, and λ is the wavelength of the laser. Figure 5 presents the typical NLO refractive date of BTD-B and BTD-C samples. The results show that BTDs have a similar negative sign for the refractive nonlinearity in the case of 4 ns pulse duration, and the valley/peak pattern of the normalized transmittance curve shows characteristic self-defocusing behavior. The effective third-order nonlinear refractive index n2 of BTD-B is -13 × 10-17 m2/W. It has been reported that a negative nonlinear refractive index is attributed to population relaxing triplet state. Refraction volumes of the triplet excited state are smaller than that of the ground state.27 The above results indicated that these BTDs exhibit the thirdorder optical nonlinearities which can be tuned by the different substitution groups. Electron-withdrawing groups on the core of BTDs are able to increase the value of the nonlinear absorption coefficient and effective third-order refractive index. However, the electron-donating group plays a role to decrease these values. Experimental Section Materials and Measurements. Most of the chemical reagents were purchased from Alfa Aesar or Aldrich Chemicals and were utilized as received unless indicated otherwise. All solvents were purified with standard procedures. Column chromatography was performed on silica gel (size 200-300 mesh). 1H and 13C NMR spectra were recorded on a Bruker ARX400 spectrometer. EI mass spectrometric measurements were performed on a SHIMADZU GCMS-QP2010 puls spectrometer. UV-vis spectra were measured on a Hitachi U-3010 spectrometer. The fluorescence spectra were measured on a Hitachi F-4500 spectrometer. Cyclic voltammetry measurements were performed with glassy carbon as the working electrode, SCE as reference, and n-Bu4NPF6 as the supporting electrolyte and at a scan rate of 100 mV/s. Nonlinear refraction and reverse stable absorption were measured by using the closed- and open-aperture Z-scan.25 A Q-switched laser was used as the light source, which provided linearly polarized 4 ns with a repetition rate of 2 Hz at 532 nm. The spatial profiles of optical pulses were nearly Gaussian

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Figure 4. Normalized open-aperture Z-scan curves of BTD-B (a) and BTD-C (b).The solid lines are the fitting curves.

TABLE 3: Values of Nonlinearity Fitting the Experimental Data samples

E (µJ)

T0 (%)

R2 (×10-10 m/W)

n2 (×10-17 m2/W)

BTD-S BTD-C BTD-B BTD-P

2.4 2.2 2.2 7.5

83 63 64 62

9.0 17.5 21 2.5

-4.0 -22 -13 -2.7

a E ) The on-axis peak energy at focus linear transmission, T0 ) linear transmission index, R2 ) nonlinear absorption coefficient, and n2 ) effective third-order refractive index.

obtained by spatial filtering. The samples were placed in quartz cells with 2 mm thickness, which were placed at the focus of a lens with a focal length of 60 cm. The laser pulses adjusted by an attenuator were separated into beams by using a splitter. The two beams were simultaneous measured by using two energy detectors (818J-09B energy probe, Newport Corporation) linked to the energy meter (model 2835-C, Newport). A personal computer was used to collect data coming from the energy meter through the RS-232C interface. General Procedure for the Synthesis of 4-(Benzo[c][1,2,5]thiadiazol-4-ylethynyl)-N,N-dimethylaniline (BTD-S). To a stirred solution of 4-ethynyl-N,N-dimethylbenzenamine (522 mg, 3.6 mmol) and compound 1 (642 mg, 3.0 mmol) in THF/(iPr)2NH (3/1, v/v) were added PdCl2(PPh3)2 (40 mg) and CuI (20 mg) under an argon flow at room temperature. The reaction mixture was then stirred for 10 h under reflux and the solvent was then evaporated under reduced pressure. The mixture was purified by SiO2 chromatography with CH2Cl2/petroleum ether (PE) (3/1, v/v) to obtain desired compound BTD-S (544 mg, 65% yield) as a yellow powder. 1H NMR (CDCl3) δ 3.02 (s, 6 H), 6.69 (d, J ) 8.8 Hz, 2 H), 7.54 (d, J ) 5.4 Hz, 2 H), 7.58 (t, J ) 8.8 Hz, 1 H),7.74 (d, J ) 7.0 Hz, 1 H), 7.93 (d, J ) 8.8 Hz, 1 H); 13C NMR (CDCl3) δ 154.9, 154.8, 150.6, 133.4, 131.8, 129.5, 120.8, 118.3, 111.8, 109.3, 97.9, 83.7, 40.3; EI-MS m/z 279 (M+).

4-((7-Bromobenzo[c][1,2,5]thiadiazol-4-yl)ethynyl)-N,Ndimethylaniline (BTD-B). To a stirred solution of 4-ethynylN,N-dimethylbenzenamine (522 mg, 3.6 mmol) and compound 2 (876 mg, 3.0 mmol) in THF/(i-Pr)2NH (3/1, v/v) were added PdCl2(PPh3)2 (40 mg) and CuI (20 mg) under argon. The reaction mixture was then stirred for 5 h under reflux and solvent was then evaporated off under reduced pressure. The product was purified by SiO2 chromatography with CH2Cl2/petroleum ether (PE) (1/1, v/v) to obtain desired compound BTD-B (535 mg, 50% yield) as a red powder. 1H NMR (CDCl3) δ 3.00 (s, 6H), 6.65 (d, J ) 8.7 Hz, 2 H), 7.51 (d, J ) 8.7 Hz, 2 H), 7.56(d, J ) 7.6 Hz, 1 H), 7.77 (d, J ) 7.6 Hz, 1 H); 13C NMR (CDCl3) δ 154.3, 153.2,150.7, 133.4, 132.2, 131.9, 117.9, 113.2, 111.8, 108.9, 99.3, 83.4, 40.3; EI-MS m/z 357 (M+). N,N-Dimethyl-4-((7-((trimethylsilyl)ethynyl)benzo[c][1,2,5]thiadiazol-4-yl)ethynyl)aniline (3). To a solution of BTD-B (178 mg, 0.50 mmol), CuI (5 mg, 0.025 mmol), and PdCl2(PPh3)2 (17 mg, 0.025 mmol) in dry THF/(i-Pr)2NH (3/1, v/v) was added (trimethylsilyl)acetylene (0.10 mL, 0.70 mmol) under an argon flow. The reaction mixture was stirred at room temperature for 4 h. The solvent was removed under reduced pressure and the resulting solid was further purified by column chromatography on silica gel with ethyl acetate/PE (1/4, v/v) to yield compound 3 as dark red solids. Yield 95%; 1H NMR (CDCl3) δ 0.34 (s, 9H), 2.96 (s, 6H), 6.62 (d, J ) 8.6 Hz, 2 H), 7.51 (d, J ) 8.5 Hz, 2 H), 7.63 (d, J ) 7.4 Hz, 1 H), 7.69 (d, J ) 7.4 Hz, 1 H); 13C NMR (CDCl3) δ 154.5, 154.4, 150.7, 133.7, 133.5, 131.3, 118.8, 115.6, 111.8, 109.0, 102.8, 100.7, 100.3, 84.3, 40.2, 0.17; EI-MS m/z 375 (M+). Elemental analysis (%) calcd for C21H21N3SiS: C 67.16, H 5.64, N 11.19. Found: C 67.45, H 5.73, N 11.06. N,N-Dimethyl-4-((7-(1-phenyl-1H-1,2,3-triazol-4yl)benzo[c][1,2,5]thiadiazol-4-yl)ethynyl)aniline (BTD-C). To the solution of compound 3 (375 mg, 1.0 mmol) in THF was added 2 mL of tetra-n-butylammonium fluoride (1 M solution in THF) at 0 °C under nitrogen atmosphere. After 10 min,

Figure 5. Normalized closed-aperture Z-scan curves of BTD-B (a) and BTD-C (b). The solid lines are the fitting curves.

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azidobenzene (119 mg, 1.0 mmol), sodium ascorbate (40 mg, 0.2 mmol) and CuSO4 (16 mg, in 5 mL of H2O) were added to the mixture. The mixture was then stirred at room temperature under nitrogen atmosphere for 6 h. The solvent was removed under reduced pressure and the resulting solid was extracted with dichloromethane. The organic layer was collected, washed with brine, and dried over anhydrous MgSO4. The crude product was further purified by column chromatography on silica gel with CH2Cl2/PE (2:1) to afford BTD-C as a red solid. 1H NMR (CDCl3) δ 3.00 (s, 6 H), 6.69 (d, J ) 8.6 Hz, 2 H), 7.49 (t, J ) 7.3 Hz, 1 H), 7.58 (m, J ) 7.6 Hz, 4 H), 7.90 (t, J ) 6.0 Hz, 3 H), 8.61 (d, J ) 7.4 Hz, 1 H), 9.21 (s, 1 H); 13C NMR (CDCl3) δ 154.9, 151.6, 150.4, 143.6, 137.0, 133.2, 132.1, 129.7, 128.7, 125.7, 122.0, 121.7, 120.5, 117.3, 111.6, 109.1, 98.7, 83.9, 40.1; EI-MS m/z 422 (M+). 4-Bromo-7-(piperidin-1-yl)benzo[c][1,2,5]thiadiazole (4). A solution of 2 (2.04 g, 7.0 mmol) in 5 mL of piperidine was heated under reflux for 10 h under nitrogen. After cooling, a reddish needle of 4 was filtrated, washed with EtOH, and dried in vacuo. The crude product was further purified by column chromatography on silica gel with CH2Cl2/PE (2:1) to afford 4 (814 mg, 2.7 mmol) as an orange solid. Yiel: 39%; 1H NMR (CDCl3) δ 1.67 (t, J ) 6.0 Hz, 2 H), 1.80 (d, J ) 4.7 Hz, 4 H), 3.45 (t, J ) 5.0 Hz, 4 H), 6.57(d, J ) 7.9 Hz, 1 H), 7.62(d, J ) 8.0 Hz, 2 H); 13C NMR (CDCl3) δ 24.5, 25.9, 51.6, 103.3, 112.1 133.1, 144.9, 149.6, 154.6; EI-MS m/z 297 (M+). Elemental analysis calcd (%) for C11H12BrN3S: C 44.30, H 4.06, N 14.09, S 10.75. Found: C 44.45, H 4.12, N 14.28, S 10.79. N,N-Dimethyl-4-((7-(piperidin-1-yl)benzo[c][1,2,5]thiadiazol-4-yl)ethynyl)aniline (BTD-P). To a stirred solution of 4-ethynyl-N,N-dimethylbenzenamine (522 mg, 3.6 mmol) and compound 4 (891 mg, 3.0 mmol) in THF/(i-Pr)2NH (3/1, v/v) were added PdCl2(PPh3)2 (20 mg) and CuI (20 mg) under an argon flow at room temperature. The reaction mixture was then stirred for 4 h under reflux, and solvent was then evaporated off under reduced pressure. The mixture was purified by SiO2 chromatography with CH2Cl2/PE (2/1, v/v) to obtain desired compound BTD-P (760 mg, 70% yield) as a red powder. 1H NMR (CDCl3) δ 1.70 (m, 2 H), 1.84 (m, 4 H), 3.00 (s, 6 H), 3.55 (m, 4 H), 6.68 (m, 3 H), 7.50 (d, J ) 8.6 Hz, 2 H), 7.64 (d, J ) 7.8 Hz, 1 H); 13C NMR (CDCl3) δ 156.5, 150.2, 149.4, 144.8, 133.8, 133.0, 111.9, 111.4, 110.3, 108.4, 94.9, 84.1, 51.5, 40.4, 26.0, 24.7; EI-MS m/z 362 (M+). Elemental analysis calcd (%) for C21H22N4S: C 69.58, H 6.12, N 15.46. Found: C 69.31, H 6.00, N 15.35. Conclusion In summary, we described the synthesis, characterization, and nonlinear optical properties of four π-conjugated D-A molecules based on benzothiadiazole. We have demonstrated the crystal packing styles of benzothiadiazole derivatives can be tuned from the centrosymmetric triclinic crystal system to the noncentrosymmetric orthorhombic system. Our results confirm that the dependence on the structure-nonlinear optical property can be tuned by the structure design of the molecule and introduction of different functional groups. The molecular systems have great potential for developing controlled nonlinear optical material. Acknowledgment. We are grateful for financial support from the National Nature Science Foundation of China (20831160507, 20721061, and 90922017) and the National Basic Research 973 Program of China. Supporting Information Available: Crystallographic information files (CIF) and cyclic voltammetric spectra. This

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