J. Phys. Chem. B 2008, 112, 4545-4551
4545
Nonlinear Optical Chromophores with Pyrrole Moieties as the Conjugated Bridge: Enhanced NLO Effects and Interesting Optical Behavior Qianqian Li,† Changgui Lu,‡ Jing Zhu,‡ Enqin Fu,† Cheng Zhong,† Suyue Li,† Yiping Cui,*,‡ Jingui Qin,*,† and Zhen Li*,† Department of Chemistry, Hubei Key Laboratory on Organic and Polymeric Opto-Electronic Materials, Wuhan UniVersity, Wuhan 430072, China, and Department of Electronic Engineering, Southeast UniVersity, Nanjing 210096, China ReceiVed: August 27, 2007; In Final Form: February 9, 2008
A series of nonlinear optical (NLO) chromophores were successfully prepared, in which pyrrole moieties were the conjugated bridge. In comparison with their analogues containing furan or thiophene groups as the bridge, these chromophores demonstrated similar or enhanced NLO effects (up to 3.3 times) and interesting optical behavior. While the acceptor groups were malononitrile (Mal), 3-phenyl-5-isoxazolone (Iso), and 1,3diethylthiobarbituric acid (Bar), the chromophores exhibited much blue-shifted maximum absorption wavelengths (λmax) (up to 36 nm); however, the λmax of the chromophore containing tricyanovinyldihydrofuran (TCF) as acceptor became much longer than that of the analogue (up to 75 nm).
Introduction The second-order nonlinear optical (NLO) chromophores are the key constructing blocks for electrooptic (EO) materials. To realize the huge potential applications of polymeric EO materials as active media in high-speed broad-band waveguides for optical switches, optical sensors, and information processors, NLO chromophores should exhibit good properties such as high thermal and chemical stability, large nonlinearity, and good transparency, as well as easy syntheses. Thanks to the great efforts of scientists, many NLO chromophores (with the structure of donor, π-conjugated bridge, and acceptor) were designed to meet the above criteria. However, it is still a difficult task to achieve all of these requirements simultaneously, since there are experimentally observed trade-offs, such as the nonlinearitythermal stability trade-off and the nonlinearity-transparency trade-off.1-5 According to synthetic and theoretical studies, the usage of easily delocalized five-membered heteroaromatic rings (usually furan and thiophene moieties) instead of the benzene ring resulted in an enhanced molecular hyperpolarizability of donor-acceptor compounds.6 However, strangely, the pyrrole group was seldom applied to construct NLO chromophores as conjugated bridges, possibly due to its unstability. We are interested in developing new NLO chromophores and have successfully prepared some series of chromophores to defeat the trade-offs by using a combined conjugation bridge, special electron acceptors, or a special donor.7 So far, all the obtained results indicated that the final properties of the chromophores were heavily dependent on the cooperation of each part of the whole molecule but not some special part. Recently, there have been some exciting results concerning pyrrole moieties in different research fields, including photochemistry and two-photon absorption materials.8 However, there were no systemic reports on the NLO chromophores with * To whom correspondence should be addressed. Phone: 86-2762254108. Fax: 86-27-68756757. E-mail:
[email protected]. † Wuhan University. ‡ Southeast University.
pyrrole moieties as the conjugated bridge. These points prompted us to design and synthesize a series of pyrrole-containing NLO chromophores with different electron donors and acceptors (Schemes 1 and 2). In comparison with their analogues with furan or thiophene as the conjugated bridge, the pyrrolecontaining chromophores demonstrated enhanced NLO effects (up to 3.3 times) and much blue-shifted maximum absorption (up to 36 nm) (except those with TCF as electron acceptor), as well as good thermal stability. Also, on the basis of more than 10 model compounds, for the first time, we proposed the following: the conjugated bridge could be regarded as electron donor in some cases and the generally considered donor might be an assistant donor. This idea might shed light on the optimum design of new NLO chromophores. Herein, we report the syntheses, characterization, and NLO properties of these new pyrrole-containing NLO chromophores. Experimental Section Materials and Instrumentation. Tricyanovinyldihydrofuran (TCF) was prepared by following the procedure reported in the literature.9 All other reagents were used as received. 1H and 13C NMR and spectra were measured on a Varian Mercury300 spectrometer using tetramethylsilane (TMS; δ ) 0 ppm) as internal standard. The Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer-2 spectrometer in the region 3000-400 cm-1. UV-visible spectra were obtained using a Schimadzu UV-2550 spectrometer. EI-MS spectra were recorded with a Finnigan Prace mass spectrometer. Elemental analyses were performed by a Carlo Erba-1106 microelemental analyzer. Thermal analysis was performed on Netzsch STA449C thermal analyzer at a heating rate of 10 °C/min in argon at a flow rate of 50 cm3/min for thermogravimetric analysis (TGA). The thermometer for measurement of the melting point was uncorrected. The second-order nonlinear hyperpolarizability of these chromophores was determined by hyper-Rayleigh scattering in chloroform using the fundamental excitation wavelength of 1064 nm, and in the same solvent,
10.1021/jp0768322 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008
4546 J. Phys. Chem. B, Vol. 112, No. 15, 2008 SCHEME 1
SCHEME 2
the known hyperpolarizability of p-nitroaniline (p-NA) was used as the external reference. General Procedure for the Synthesis of Chromophores Ia-d and IIa-d. The prepared aldehyde (1 equiv) and the acceptor (1 equiv) were dissolved in absolute ethanol, and then 2-3 drops of piperidine were added as the catalyst. The mixture was refluxed for 2-4 h, and then the solvent was removed. The solid was purified through a silica chromatography column. Chromophore Ia: compound 5 (237 mg, 0.52 mmol), malononitrile (52 mg, 0.78 mmol); red solid (108 mg, 41.2%). Mp ) 212-215 °C. IR (thin film), ν (cm-1): 2219 (-CN). 1H NMR (CDCl3) [δ (ppm)]: 7.83 (d, 1H, J ) 4.5 Hz, -CHd), 7.36-7.25 (m, 11H, ArH), 7.19 (d, 1H, J ) 16.2 Hz, -CHd CH-), 7.11-7.04 (m, 5H, ArH), 7.00-6.95 (m, 4H, ArH), 6.85 (d, 1H, J ) 4.2 Hz, ArH), 6.73 (d, 1H, J ) 15.6 Hz, -CHd CH-), 5.31 (s, 2H, -CH2-). 13C NMR (DMSO-d6) [δ (ppm)]: 156.7, 148.6, 147.3, 144.6, 143.1, 138.5, 135.4, 130.5, 130.4, 129.5, 129.1, 128.8, 128.2, 126.5, 125.3, 124.5, 122.6, 121.3, 117.2, 116.7, 113.9, 112.9, 46.2. MS (EI), m/z [M+]: 502.4; calcd, 502.2. Anal. Calcd for C35H26N4: C, 83.64; H, 5.21; N, 11.15. Found: C, 83.36; H, 5.20; N, 11.29.
Li et al. Chromophore Ib: compound 5 (250 mg, 0.55 mmol), 3-phenyl-5-isoxazolone (93 mg, 0.58 mmol); dark solid (123 mg, 37.4%). Mp ) 232-234 °C. IR (thin film), ν (cm-1): 1726 (-CdO). 1H NMR (CDCl3) [δ (ppm)]: 8.89 (d, 1H, J ) 4.2 Hz, -CHd), 7.46 (d, 1H, J ) 7.2 Hz, ArH), 7.34-7.26 (m, 6H, ArH), 7.21 (d, 4H, J ) 8.4 Hz, ArH), 7.12-7.05 (m, 7H, ArH + -CHdCH-), 7.01-6.91 (m, 6H, ArH), 6.83 (d, 1H, J ) 15.6 Hz, -CHdCH-), 5.27 (s, 2H, -CH2-). 13C NMR (DMSO-d6) [δ (ppm)]: 170.7, 164.4, 149.0, 148.0, 147.1, 138.0, 137.6, 132.7, 131.5, 131.0, 130.4, 130.2, 129.8, 129.6, 128.8, 128.3, 126.3, 125.5, 124.7, 122.2, 113.7, 113.4, 103.3, 46.7. MS (EI), m/z [M+]: 597.4; calcd, 597.2. Anal. Calcd for C41H31N3O2: C, 82.39; H, 5.23; N, 7.03. Found: C, 82.61; H, 5.23; N, 7.01. Chromophore Ic: compound 5 (246 mg, 0.54 mmol), 1,3diethylthiobarbituric acid (119 mg, 0.60 mmol); dark solid (168 mg, 48.7%). Mp ) 226-230 °C. IR (thin film), ν (cm-1): 1651 (-CdO). 1H NMR (CDCl3) [δ (ppm)]: 8.91 (d, 1H, J ) 4.5 Hz, -CHd), 8.39 (br, 1H, ArH), 7.35-7.26 (m, 10H, ArH), 7.13-7.06 (m, 8H, ArH + -CHdCH-), 7.00 (d, 2H, J ) 8.7 Hz, ArH), 6.95 (d, 1H, J ) 7.5 Hz, ArH), 6.86 (d, 1H, J ) 15.6 Hz, -CHdCH-), 5.50 (s, 2H, -CH2-), 4.64-4.51 (m, 4H, -CH2-), 1.35-1.22 (m, 6H, -CH3). 13C NMR (CDCl3) [δ (ppm)]: 207.3, 178.6, 162.5, 149.4, 147.7, 147.1, 138.2, 137.2, 136.4, 134.1, 132.3, 131.1, 129.7, 129.4, 128.5, 126.2, 125.5, 124.2, 122.2, 112.8, 112.5, 106.4, 47.1, 44.1, 43.5, 12.7. MS (EI), m/z [M+]: 636.7; calcd, 636.3. Anal. Calcd for C40H36N4O2S: C, 75.44; H, 5.70; N, 8.80. Found: C, 75.38; H, 5.92; N, 8.54. Chromophore Id: compound 5 (266 mg, 0.58 mmol), TCF (140 mg, 0.70 mmol); dark green solid (20 mg, 5.4%). Mp ) 247-249 °C. IR (thin film), ν (cm-1): 2225 (-CN). 1H NMR (CDCl3) [δ (ppm)]: 7.68 (d, 1H, J ) 15.5 Hz, -CHd), 7.387.18 (m, 11H, ArH), 7.11-7.06 (m, 7H, ArH), 6.98 (d, 2H, J ) 8.7 Hz, ArH), 6.88 (d, 1H, J ) 4.8 Hz, ArH), 6.83 (d, 1H, J ) 16.2 Hz, -CHdCH-), 6.39 (d, 1H, J ) 15.3 Hz, -CHd CH-), 5.34 (s, 2H, -CH2-), 1.48 (s, 6H, -CH3). MS (EI), m/z [M+]: 635.7; calcd, 635.3. Anal. Calcd for C43H33N5O: C, 81.24; H, 5.23; N, 11.02. Found: C, 81.39; H, 5.20; N, 11.19. Chromophore IIa: compound 8 (300 mg, 0.84 mmol), malononitrile (96 mg, 1.45 mmol); dark purple solid (250 mg, 73.5%). Mp ) 213-215 °C. IR (thin film), ν (cm-1): 2214 (-CN). 1H NMR (CDCl3) [δ (ppm)]: 7.82 (d, 1H, J ) 5.1 Hz, -CHd), 7.38-7.25 (m, 5H, ArH), 7.20-7.15 (m, 2H, ArH + -CHdCH-), 6.97 (d, 2H, J ) 7.2 Hz, ArH), 6.82 (d, 1H, J ) 5.1 Hz, ArH), 6.63-6.58 (m, 3H, ArH + -CHdCH-), 5.28 (s, 2H, -CH2-), 3.41-3.34 (m, 4H, -CH2-), 1,16 (t, 6H, J ) 4.2 Hz, -CH3). 13C NMR (CDCl3) [δ (ppm)]: 148.8, 145.0, 140.3, 136.9, 136.0, 129.6, 129.1, 128.5, 128.2, 125.7, 123.0, 122.6, 117.1, 116.1, 111.7, 111.6, 108.6, 46.7, 44.7, 12.8. MS (EI), m/z [M+]: 406.7; calcd, 406.2. Anal. Calcd for C27H26N4: C, 79.77; H, 6.45; N, 13.78. Found: C, 79.83; H, 6.11; N, 14.09. Chromophore IIb: compound 8 (300 mg, 0.84 mmol), 3-phenyl-5-isoxazolone (177 mg, 1.10 mmol); dark solid (280 mg, 66.6%). Mp ) 276-277 °C. IR (thin film), ν (cm-1): 1700 (-CdO). 1H NMR (CDCl3) [δ (ppm)]: 8.91 (d, 1H, J ) 5.4 Hz, -CHd), 7.48-7.43 (m, 2H, ArH), 7.36-7.31 (m, 5H, ArH), 7.26 (d, 1H, J ) 6.6 Hz, ArH), 7.22-7.14 (m, 3H, ArH + -CHdCH-), 6.91 (d, 4H, J ) 5.1 Hz, ArH), 6.71 (d, 1H, J ) 15.3 Hz, -CHdCH-), 6.61 (d, 2H, J ) 8.7 Hz, ArH), 5.25 (s, 2H, -CH2-), 3.41-3.34 (m, 4H, -CH2-), 1.17 (t, 6H, J ) 6.6 Hz, -CH3). 13C NMR (CDCl3) [δ (ppm)]: 171.5, 166.4, 149.0, 147.4, 138.0, 136.5, 131.3, 131.0, 130.1, 129.5,
Nonlinear Optical Pyrrole-Containing Chromophores
J. Phys. Chem. B, Vol. 112, No. 15, 2008 4547
TABLE 1: Characterization Data of Chromophores chromophore
λmax/nma
Ia Ib Ic Id IIa IIb IIc IId IIIa IIIb IIIc IIId IVa IVb IVc IVd VIa VIb VIc VIIa
501 (6.81 × 10 ) 543 (8.17 × 104) 563 (1.03 × 105) 661 (1.02 × 105) 523 (7.27 × 104) 566 (8.91 × 104) 583 (1.14 × 105) 697 (1.25 × 105) 521 553 577 628 523 552 574 620 4
λmax/nmb
Eeg/eVc
βHRSd
β0e
Td/°Cf
497 546 562 666 524 574 594 709 507 551 567 600 509 547 559 591 552g 604g 630g 554g
2.47 (2.49) 2.28 (2.27) 2.20 (2.20) 1.87 (1.86) 2.37 (2.36) 2.19 (2.16) 2.12 (2.08) 1.77 (1.74) 2.37 (2.44) 2.24 (2.25) 2.14 (2.18) 1.97 (2.06) 2.37 (2.43) 2.24 (2.26) 2.16 (2.21) 1.99 (2.09) (2.24) (2.05) (1.96) (2.23)
554 1271 1660 1718 1594 1847 1104 2789
52 32 125 546 45 185 158 1130
359 270 255 323 308 277 227 288
648
166
728 77f 300f 254f 62f
172 34f 101f 75f 27f
a Tested in chloroform, and the value of molar absorptivity (, cm-1‚mol-1‚L-1) is given in parentheses. b Tested in DMSO. c Calculated from their UV-vis spectra tested in chloroform, while those calculated from the spectra tested in DMSO are given in parentheses. d β values (in unit of 10-30 esu) measured by the hyper-Rayleigh scattering (HRS) technique in methylene chloride using the fundamental excitation wavelength of 1064 nm. e Dispersion-corrected β values calculated by using an approximate two-level model. g The 5% weight loss temperature (TGA). f These data were obtained from refs 14a,c.
129.4, 129.3, 129.1, 128.7, 128.4, 125.9, 123.1, 112.4, 111.7, 108.8, 104.5, 46.9, 44.7, 12.8. MS (EI), m/z [M+]: 502.8; calcd, 502.3. Anal. Calcd for C33H31N3O2: C, 78.86; H, 6.42; N, 8.36. Found: C, 78.91; H, 5.96; N, 8.43. Chromophore IIc: compound 8 (206 mg, 0.57 mmol), 1,3diethylthiobarbituric acid (114 mg, 0.57 mmol); green solid (110 mg, 35.4%). Mp ) 210-213 °C. IR (thin film), ν (cm-1): 1644 (-CdO). 1H NMR (CDCl3) [δ (ppm)]: 8.94 (d, 1H, J ) 4.5 Hz, -CHd), 8.33 (br, 1H, ArH), 7.36-7.27 (m, 6H, ArH + -CHdCH-), 7.12 (d, 2H, J ) 7.2 Hz, ArH), 6.93 (d, 1H, J ) 4.5 Hz, ArH), 6.75 (d, 1H, J ) 15.3 Hz, -CHdCH-), 6.62 (d, 2H, J ) 8.1 Hz, ArH), 5.49 (s, 2H, -CH2-), 4.62-4.54 (m, 4H, -CH2-), 3.43-3.37 (m, 4H, -CH2-), 1.35-1.26 (m, 6H, -CH3), 1.21 (t, 6H, J ) 6.6 Hz, -CH3). 13C NMR (CDCl3) [δ (ppm)]: 178.5, 162.7, 159.5, 149.5, 149.2, 139.1, 137.1, 136.5, 132.5, 131.7, 129.6, 129.4, 128.3, 126.3, 122.9, 112.9, 111.6, 109.0, 105.1, 46.9, 44.8, 44.1, 43.5, 12.9. MS (EI), m/z [M+]: 540.5; calcd, 540.3. Anal. Calcd for C32H36N4O2S: C, 71.08; H, 6.71; N, 10.36. Found: C, 71.42; H, 6.53; N, 10.48. Chromophore IId: compound 8 (450 mg, 1.26 mmol), TCF (260 mg, 1.3 mmol); green solid (245 mg, 36.2%). Mp ) 285287 °C. IR (thin film), ν (cm-1): 2230 (-CN). 1H NMR (CDCl3) [δ (ppm)]: 7.71 (d, 1H, J ) 15.6 Hz, -CHdCH-), 7.39-7.21 (m, 6H, ArH + -CHdCH-), 7.10 (d, 2H, J ) 6.6 Hz, ArH), 6.88 (d, 1H, J ) 4.2 Hz, ArH), 6.74 (d, 1H, J ) 16.2 Hz, -CHdCH-), 6.62 (d, 2H, J ) 8.7 Hz, ArH), 6.32 (d, 1H, J ) 14.7 Hz, -CHdCH-), 5.34 (s, 2H, -CH2-), 3.433.36 (m, 4H, -CH2-), 1.48 (s, 6H, -CH3), 1.18 (t, 6H, J ) 7.5 Hz, -CH3). MS (EI), m/z [M+]: 539.6; calcd, 539.3. Anal. Calcd for C35H33N5O: C, 77.89; H, 6.16; N, 12.98. Found: C, 78.30; H, 6.33; N, 12.97. Results and Discussion Synthesis. The vinylpyrrole-bridged chromophores (Ia-d and IIa-d) were prepared according to Schemes 1 and 2 (detailed synthetic procedure presented in the Experimental Section and Supporting Information). 2-Formyl-N-benzylpyrrole (2) was easily yielded from the substituted reaction between 2-formylpyrrole (1) and benzyl chloride in DMF at the presence of potassium
carbonate as the base, similar to our other cases reported previously.10 The Wittig reaction of the obtained 2-formyl-Nbenzylpyrrole (2) with (4-(diphenylamino)benzyl)triphenylphosphonium bromide (3) gave the corresponding triphenylamine-vinylpyrrole (4). The followed Vilsmeier reaction of 4 yielded the aldehyde 5, which was readily converted to chromophores Ia-d by Knoevenagel condensations with different acceptor moieties under basic conditions (in ethanol with piperidine catalyst). The Wittig reaction gave mixtures of Z and E isomers, and the aldehyde was also obtained as mixture. But after the Knoevenagel condensations, the final obtained chromophores were totally E isomers. This should be due to the linkage of the strong acceptors, and the similar cases were reported previously.11 Following nearly the same procedure (Scheme 2), aniline-vinylthiophene (7) was prepared. Similarly, after the Knoevenagel condensations, the obtained chromophores (IIa-d) were no longer mixtures but E isomers. All the compounds were well characterized, and the spectral data are shown in the Experimental Section, with the FT-IR and 1H NMR spectra demonstrated in the Supporting Information. As shown in Schemes 1 and 2, 2-formylpyrrole (1) was first reacted with benzyl chloride to substitute the active hydrogen atoms on the nitrogen atoms, and this reaction went easily. Some other alkyl groups could be easily introduced to the pyrrole ring according to the same procedure, especially those containing the functional groups at the other end of the alkyl groups, such as hydroxyl groups, alklyl groups, etc., which could make the resultant chromophores become easily bonded to the polymeric system.12 Also, 2-formyl-N-benzylpyrrole (2) could react with other phosphonium salts instead of (4-(diphenylamino)benzyl)triphenylphosphonium bromide (3) and (4-(diethylamino)benzyl)triphenylphosphonium iodide (6), to give other starting materials for other donor end-capped NLO chromophores. There were other advantages for our pyrrole-containing chromophores, and the related study is currently under way in our laboratory. Optical Properties. All the chromophores were soluble in common organic solvents, such as acetone, chloroform, THF, DMF, and DMSO. The UV-vis absorption maxima (λmax) and the transition energy (Eeg) for the new chromophores in
4548 J. Phys. Chem. B, Vol. 112, No. 15, 2008 CHART 1: Structures of Chromophores III-VII
chloroform and DMSO are summarized in Table 1. The values of β were measured in methylene chloride by hyper-Rayleigh scattering (HRS), with the known hyperpolarizability of pnitroaniline (p-NA) in the same solvent as an external reference.13 The dispersion-corrected β0 values were estimated by using an approximate two-level model. For comparison, we listed those data of chromophores III-VII (some data in Table S1),14 with their structures shown in Chart 1. It was easily seen that, in comparison with their analogues, chromophores Id and IId demonstrated better NLO effects, which should be ascribed to the special electronic properties of the pyrrole moieties used here. As the UV-vis spectra were sensitive to the electronic property, and concerned much with the optical transparency of the chromphores, we explore the advantages of pyrrole moieties in detail from the UV-vis spectra. As shown in Table 1, in comparison with chromophores IIIa-c and IVa-c, chromophores Ia-c exhibited much blueshifted maximum absorptions. For example, the λmax of Ia was about 22 nm shorter than that of its analogue, chromophore IVa, in their diluted chloroform solutions (∆Eeg ) 0.10 eV). However, the situation became totally different when TCF acted as the acceptor: Id demonstrated much longer λmax, about 33 and 41 nm red-shifted, respectively (∆Eeg ) 0.10 and 0.12 eV), compared to those of IIId and IVd. And just with the same donor and π-conjugated bridge, the λmax of Id was 98 nm longer than that of Ic (∆Eeg ) 0.33 eV). These two points were very strange, not coinciding with the trend present in the cases of Ia-c. Similar phenomena were also observed in chromophores IIa-d and their analogues, and the difference of the λmax of IIc,d was even larger (114 nm, ∆Eeg ) 0.35 eV). Also, to compare the λmax of the same chromophore (one of I-IV) in its solutions of chloroform and DMSO, there were smaller difference in I and II than in III and IV. For example, the λmax of Ia in chloroform was only 4 nm larger than that in DMSO (∆Eeg ) 0.02 eV); however, the λmax of IIIa in chloroform was 14 nm larger than that in DMSO (∆Eeg ) 0.07 eV). To see this abnormal behavior unique in the chloroform and DMSO solutions or not, we tested their UV-vis spectra in different solvents (all the curves shown in the Supporting Information), with their λmax summarized in Table S2. In all the solvents, the difference of the λmax of the same chromophore in chromophores I and II was not as big as those in chromophores III and IV. And in chromophores I and II, while the acceptor was changed from Mal to Iso, to Bar, and then to TCF, the difference of the λmax of chromophores with the same donor and π-conjugated bridge varied from about 40 to 20 nm and then to 100 nm (Chart 2). For example, the λmax of Id was about 100 nm longer than that of Ic (∆Eeg ) 0.33 eV), which was
Li et al. about 20 nm longer than that of Ib (∆Eeg ) 0.08 eV). This phenomenon was very interesting and should surely be concerned with the pyrrole moieties. Thus, we further prepared compounds VIII (Chart 3, with detailed synthetic procedure presented in the Supporting Information), just removing the donor part in I and II, to study their UV-vis absorption properties. As shown in Table 2, the above-mentioned rule was still present, and from Mal to TCF as the acceptors, the difference of the λmax of chromophores VIII was about 40 to 20 and to 60 nm (Chart S1, ∆Eeg ) 0.31, 0.14, and 0.33 eV) and not as 40 to 20 and to 100 nm in chromophores I and II. To find some clues for the above questions, model compounds IX and X (Chart 4) were also synthesized. This time, it seemed that, from Mal to TCF as the acceptors, the difference of the λmax of chromophores IX and X was about 40 to 20 and to 30 nm. In comparison with VIII, IX and X demonstrated much blue-shifted λmax, as larger as about 40 nm (about 70 nm while TCF as acceptor) (Table 2). This trend was against that found in chromophores I and II. If we focused our eyes only on the chromophores VIII-X, it might be reasonable that VIIIa-d exhibited larger λmax than IX and X, with the same acceptor: pyrrole moieties possessed a more abundant electronic cloud than thiophene and furan, proved by their higher reacting activity in electrophilic substituted reactions.10 Then the intramolecular charge-transferring extent in chromophores VIII should be much higher than that in their analogues, IX and X. This point directly led to their longer λmax. Also, the electric dipole moment of pyrrole was much different from those of thiophene and furan, with its direction from the heteroatom to the five-membered ring (Chart 5),15 indicating that the big π system comprised the electronic properties of n electrons, derived from the nitrogen atom, to a larger degree. Then we came back to chromophores Ia-c and IIa-c: after the triphenylamino or aniline groups were used as donors, chromophores demonstrated blue-shifted λmax, in comparison with their analogues. These results indicated that the pyrrole group was not a good π-conjugated bridge as thiophene and furan moieties and could not transfer the electronic cloud from the donor to acceptor efficiently to benefit the delocalization of the whole chromophore. While the acceptor was changed from Mal to Iso and then to Bar, the similar difference of the λmax (40 and 20 nm) in Ia-c, IIa-c, and VIIIa-c demonstrated that the difference of the intramolecular charge-transferring extent was similar upon acceptor changing. Then we could assume in Ia-c and IIa-c the actual donor was pyrrole, the intramolecular charge-transferring process almost took place between the pyrrole moieties and the acceptor, and the triphenylamino or aniline groups were just assistant donors, which increased the electronic cloud density of the pyrrole groups to enhance the intramolecular charge-transferring extent. As to Id and IId, TCF was a so strong acceptor, making the electron density of the pyrrole ring decrease dramatically, that the chargetransferring process between the pyrrole ring and the triphenylamino or aniline groups became apparent (just as in chromophores III-VI); thus, here, the pyrrole moieties were not donors any more but real conjugated bridges. Then their abundant electron density and good aromaticity provided a good explanation for the longer λmax of Id and IId, in comparison with their analogues. This case also accounted for the much higher β values of Id and IId. Next, we came back to the NLO properties of the obtained pyrrole-containing chromophores. As shown in Table 1, chromophores I and II demonstrated better NLO effects, in comparison with their analogues with different π-conjugated
Nonlinear Optical Pyrrole-Containing Chromophores
J. Phys. Chem. B, Vol. 112, No. 15, 2008 4549
CHART 2: Difference of the λmax of Chromophores with the Same Donor and π-Conjugated Bridge but Different Acceptor in the Same Solvent
CHART 3: Structures of Chromophores VIII
CHART 4: Structures of Chromophores IX and X
TABLE 2: Characterization Data of Chromophores
CHART 5: Dipole Moment of Pyrrole, Thiophene, and Furan
chromophore VIIIa VIIIb VIIIc VIIId IXa IXb IXc IXd Xa Xb Xc Xd a
CH2Cl2 (nm)
chloroform (nm)
DMSO (nm)
Eega (eV)
380 419 439 498 343 380 397 432 347 384 399 432
380 419 440 498 344 379 399 435 348 384 399 433
386 425 444 514 339 386 403 436 350 391 401 437
3.26 2.95 2.81 2.48 3.60 3.26 3.10 2.84 3.56 3.22 3.10 2.86
Calculated from their UV-vis spectra tested in chloroform.
bridges (furan or thiophene). These phenomena were reasonable, according to the above discussion (concerning the special electronic property of pyrrole moieties). In the pyrrole ring, the n electrons on the nitrogen atom contributed to the big π system to a large extent, much more than the cases in thiophene and
furan (partially proved by the different directions of the electric dipole moment). This special point should benefit their abundant electron density, which directly led to the enhanced NLO effects of chromophores I and II in comparison with their analogues, since the abundant electronic cloud at the ortho-position to nitrogen atom would benefit the delocalization efficiency of chromophores in a large degree, to contribute much to their higher β values (similar as previously found in the indole-based chromophore system).7 In the case of chromophores Id and IId, since TCF is one of the strongest acceptors reported so far, the pyrrole moieties acted as the real π-conjugated bridge as discussed above, and then the intramolcular chargetransfer extent was enhanced dramatically, so their tested β values were much higher than other chromophores with other acceptors.
4550 J. Phys. Chem. B, Vol. 112, No. 15, 2008 Since aniline groups were better electronic donor than triphenylamine moieties, it was also reasonable that chromophores II exhibited similar or higher β0 values than chromophores I with the same electronic acceptor. However, the changing trend of their β0 values was different. The big different point was that the β0 value of IIb was greater than that of IIc, while Ic demonstrated a much higher β0 value than Ib. This should be due to the difference of the electronic donor used and indicated that the final properties of the chromophores were heavily dependent on the cooperation of each part of the whole molecule. However, all of the above discussion still lacked direct experimental proof, and further study was needed for a full interpretation. It should be pointed out that the second-order nonlinear hyperpolarizability of all the chromophores was determined by hyper-Rayleigh scattering in chloroform using the fundamental excitation wavelength of 1064 nm. Thus, the calculated β0 values of the chromophores based on an approximate two-level model were not so accurate since the chromophores exhibited significant absorption at the wavelength of 532 nm.16 However, the special optical properties of the chromophores reported here were very interesting, and the NLO effects of Id and IId were still much larger than their analogues. All these obtained results demonstrated that the pyrrole moieties were good building blocks to construct NLO chromophores and might lead to good chromophores for pratical applications. Thermal Stability. The thermal stabilities of chromophores were evaluated by thermal gravimetric analysis (TGA) under nitrogen, with a heating rate of 10 °C/min. The temperatures for 5% weight loss of each chromophores are summarized in Table 1. Nearly all the chromophores demonstrated relatively high Td temperatures (higher than 250 °C). This property would benefit the practical applications of these chromophores. Chromophores Ic and IIc, containing barbituric acid (Bar) acceptor, showed a relatively lower temperature, which might be due to the instability of Bar moieties. Compounds Ia and IIa with the dicyanovinyl acceptor were more thermally stable than chromophores with other electron acceptors, exhibiting a higher temperature (over 300 °C). Similar phenomena were observed in our previous cases.7d Summary In summary, in this work, we have successfully developed a series of new NLO chromophores, I and II, containing pyrrole moieties as the conjugated bridge. Thanks to the special electronic properties of pyrrole group, the preliminary results demonstrated that, in comparison with their analogues, I and II possessed larger nonlinearities, interesting optical properties, and good thermal stabilities. The concept of “assistant donor” might be useful in designing other new NLO chromophores. Further research for a deeper insight into the observed phenomena is still under way in our laboratories. Acknowledgment. We are grateful to the National Science Foundation of China (Grant Nos. 20402011, 20674059), the National Basic Research “973” Program, and Hubei Province for financial support and the National Science Foundation for a Distinguished Young Scholars Award of China (Grant No. 60125513). Supporting Information Available: Preparation and characterization details for all the chromophores and related compounds, UV-vis spectra of their solutions in different solvents, FT-IR and 1H NMR spectra, structures of the analogue
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