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Synthesis, characterization and optical properties of a new heterocycle-based chromophore
Optical Materials 30 (2007) 423–426 www.elsevier.com/locate/optmat
Synthesis, characterization and optical properties of a new heterocycle-based chromophore Yun-Xing Yan a,d,*, Yuan-Hong Sun b, Lei Tian c, Hai-Hua Fan d, He-Zhou Wang d, Chuan-Kui Wang b, Yu-Peng Tian c, Xu-Tang Tao a, Min-Hua Jiang a a
d
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China b Department of Physics, Shandong Normal University, Jinan 250014, PR China c Department of Chemistry, Anhui University, Longhe Road, Hefei 230039, PR China State Key Laboratory of Optoelectronic Materials and Technologies/School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China Received 12 July 2006; received in revised form 26 October 2006; accepted 23 November 2006 Available online 20 February 2007
Abstract A new heterocycle-based two-photon absorption chromophore, 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)styryl]pyridine (DIYSP), has been synthesized and characterized. The molecule possesses an A–p–A 0 structure. The p-deficient heteroaromatic ring (4,5-diphenyl1H-imidazole) is used as an acceptor (A), and the pyridine ring is used as another acceptor (A 0 ). Pumped with 740 nm laser excitation, DIYSP had a two-photon absorption cross-section 41 GM (1 GM = 1 · 1050 cm4 s photon1) and a two-photon excited fluorescence 511 nm in DMF, respectively. A microstructure using DIYSP as an initiator has been fabricated under irradiation of 80 fs, 80 MHz Ti:sapphire femtosecond laser at 800 nm. The possible mechanism of photopolymerization is discussed. 2007 Elsevier B.V. All rights reserved. Keywords: Synthesis; Heterocycle; Two-photon absorption; Initiator; DIYSP
1. Introduction Two-photon absorption (TPA) holds a great potential for a number of industrial and medical applications such as optical power limiting [1,2], two-photon upconversion lasing [3,4], two-photon fluorescence excitation microscopy [5,6], three-dimensional (3D) optical data storage and microfabrication [7,8]. Although 3D microfabrication has been illustrated using the two-photon-initiated polymerization of resins incorporating conventional ultravioletabsorbing initiators, the TPA cross-sections (r) of these initiators are typically very small, and as a result they exhibit low two-photon sensitivity. Therefore, it is important to establish efficient methods for the synthesis of novel initia*
Corresponding author. Address: State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China. E-mail address: [email protected] (Y.-X. Yan). 0925-3467/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.11.073
tors. In recent years, the optimization of molecular TPA has largely focused on one-dimensional dipolar or quadrupolar structures, and octupolar structures. And the corresponding efficiently two-photon photopolymerization initiators had been reported [9,10]. Recent studies showed that the heterocycle-based two-photon absorbing chromophores exhibit large TPA cross-sections [11,12]. However, as two-photon photopolymerization initiators, they were seldom reported. In this paper, we present the synthesis and the TPA properties of a new heterocycle-based organic chromophore, 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)styryl]pyridine (DIYSP). It shows strong one- and two-photon excited fluorescence. The two-photon initiating polymerization microfabrication experiment has been carried out using DIYSP as an initiator, and the possible interpretation is discussed by the charge-transfer process when the laser irradiation is applied.
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Y.-X. Yan et al. / Optical Materials 30 (2007) 423–426
2. Experimental 2.1. Instruments The 1H NMR spectra were recorded on a Mercury-Plus 300 spectrometer. Elemental analyses were performed by a Perkin–Elmer 2400 elemental analyzer. The melting points and decomposition temperatures were determined on a Netzsch DSC 204 and a Netzsch TG 209 thermogravimetric analyzer at a heating rate of 10 C min1 under dinitrogen atmosphere, respectively. The spectral resolution and accuracy of the DSC 204 thermal analyzer are ±0.1 C and ±1.0%, respectively. Mass spectra were measured on a LCMS-2010A liquid chromatograph mass spectrometer. UV–visible–near-IR spectra were measured on a UV-3150 spectrophotometer. The one-photon fluorescence spectra and quantum yields (U) were measured on a Hitachi F-4500 fluorescence spectrophotometer. Coumarin307 (whose U is assumed to be 56% in methanol) was used as the reference [13]. The concentration is the same as that of the linear absorption spectra. The two-photon induced fluorescence spectra were observed with a single sweep streak camera equipped with a polychromator as a recorder (Hamamatsu Model C1587). A mode-locked Nd: YAG laser (PC2143, pulse duration of 25 ps) was used to pump up the OPG (Optical Parameter Generator) system operating at 10 Hz, tuned from 420 to 10,000 nm. The average power density is 2.1 GW/cm2. The excitation wavelength for DIYSP is 740 nm. The concentrations are 1 · 103 mol/L, respectively.
C21H15BrN2: C 67.21, H 4.03, N 7.47%; found C 66.94, H 3.97, N 7.36%. 1H NMR (DMSO-d6, 300 MHz) [ppm] d = 8.11 (d, J = 8.4 Hz, 2H), 8.05 (s, 1H), 7.71 (d, J = 8.4 Hz, 2H), 7.48 (d, J = 3.3 Hz, 4H), 7.37 (d, J = 3.3 Hz, 6H). ESI–MS: m/z calcd for C21H15BrN2 374.04, found 374.15. 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)styryl]pyridine (DIYSP). BPDI (1.87 g, 5 mmol), tri-o-tolylphosphine (0.65 g, 2.15 mmol), 4-vinylpyridine (1.16 mL, 10.75 mmol), palladium(II) acetate (0.06 g, 0.27 mmol), and redistilled triethylamine (100 mL) under dinitrogen, were added to a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and a dinitrogen input tube. The reaction mixture was refluxed in an oil bath under dinitrogen. An orange product was obtained after heating and stirring for 24 h. Then the solvent was removed under reduced pressure and the residue was dissolved in dichloromethane, washed with distilled water, and dried with anhydrous magnesium sulfate. The organic layer was filtered and concentrated. The resulting solution was chromatographed on silica gel using ethyl acetate/petroleum ether (1:1) as eluent. The yellow– green powder of compound DIYSP (mp 301.8 C) was obtained in a yield of 55%. The enthalpy is 126.2 mJ/mg. Anal. calcd for C28H21N3: C 84.18, H 5.30, N 10.52%; found C 83.87, H 5.32, N 10.48%. 1H NMR (DMSO-d6, 300 MHz) [ppm] d = 12.72 (s, 1H), 8.54 (d, J = 5.1 Hz, 2H), 8.11 (d, J = 8.1 Hz, 2H), 7.76 (d, J = 8.1 Hz, 2H), 7.44 (m, 14H). ESI–MS, m/z calcd for C28H21N3 399.17, found 399.25. 3. Results and discussion 3.1. One- and two-photon fluorescence
2.2. Synthesis The synthetic route for the preparation of DIYSP is shown in Scheme 1. 2-(4-Bromophenyl)-4,5-diphenyl-1H-imidazole (BPDI). A mixture of benzil (1.05 g, 5 mmol), 4-bromobenzaldehyde (0.925 g, 5 mmol), ammonium acetate (2.5 g), and acetic acid (25 ml) was refluxed for 12 h, and then cooled to room temperature. After adding concentrated hydrochloric acid (10 ml), the mixture was left for 1 h, filtered, and washed with water. The filtrate was treated with 30% aqueous sodium hydroxide solution (20 ml) and gave pale-yellow precipitate. The crude product was purified by column chromatography on silica gel using ethyl acetate/petroleum ether (1:15) as eluent. Pale-yellow powders of BPDI were then obtained in 86% yield. Anal. calcd for
O O
a
N Br
b
N N
NH
NH
BPDI
DIYSP
Scheme 1. Synthesis of DIYSP: (a) NH4OAc/HOAc/4-bromobenzaldehyde reflux; (b) 4-vinylpyridine/tri-o-tolylphosphine/palladium(II) acetate/triethylamine, reflux.
The photophysical properties of the compound DIYSP are summarised in Table 1. The linear absorption, onephoton fluorescence, and two-photon fluorescence spectra in DMF are shown in Fig. 1. The linear absorption spectra were measured in solvents of different polarity at a concentration of 1 · 105 mol/L, in which the solvent influence has been excluded. Fig. 1 shows that the solutions of compound DIYSP are completely transparent above 500 nm. The one-photon fluorescence spectra were measured with the same concentrations as those of the linear absorption spectra. The excitation wavelength is 385 nm. As shown in Table 1, the absorption maxima are not significantly different, while the one- and two-photon fluorescence maxima are slightly red-shifted, and the fluorescent lifetimes are lengthened upon increasing the polarity of the aprotic solvent for compound DIYSP. This can be attributed to the fact that the excited states may possess higher polarity than that of the ground state, since the solvatochromism is associated with the lowering of energy levels. By increasing dipole–dipole interaction between the solute and solvent, the energy level can be lowered greatly [14,15]. In the case of ethanol, the fluorescence maximum is slightly red-shifted compared to those in other aprotic solvents. The possibility of forming hydrogen bonds between
Y.-X. Yan et al. / Optical Materials 30 (2007) 423–426
425
Table 1 The data of absorption, one- and two-photon fluorescence with solvent effects of DIYSP Solvents
kð1aÞ max (nm)
eres (104)
Þ kð1f max (nm)
Dm (cm1)
s (ns)
U (%)
kð2Þ max (nm)
DMF Ethyl acetate Ethanol THF CHCl3 Toluene
368 357 360 365 355 357
3.39 3.20 3.50 3.38 2.70 2.43
507 490 510 490 484 479
7450 7603 8170 6989 7508 7134
3.16
59 78 54 72 78 90
511
2.53 2.03
494 487
ð1f Þ ð2Þ kð1aÞ max , kmax , and kmax are one-photon absorption, one-photon fluorescence, and two-photon fluorescence maxima peak, respectively. U is one-photon fluorescence quantum yield determined using coumarin 307 as the standard [13]. s is two-photon fluorescence lifetime. Dm is Stokes’ shift. eres is the corresponding molar absorption coefficient.
0.6
Absorbance
Relative intensity (a.u.)
(c)
0.5
(b)
0.4
(a) 0.3
0.2
0.1
0.0 300
400
500
600
700
800
900
where the subscripts s and r refer to the sample and the reference material, and the terms c and n are the concentration and refractive index of the sample solution, respectively. U is the fluorescence quantum yield, F is two-photon excited fluorescence integral intensity, and rr is the TPA cross-section of the reference molecule. The excitation wavelength of DIYSP is 740 nm. The concentration is 1 · 103 mol/L. The TPA r for Rhodamine 6G is 21 GM at 740 nm [19]. The TPA r of DIYSP is 41 GM. However, it should be noted that the optimal excitation wavelength for the TPA should be slightly less than two times of the wavelength for the linear absorption maxima [20]. A significant increase in the TPA r value is expected at the optimal excitation wavelength for TPA.
Wavelength (nm) Fig. 1. Absorption and fluorescence spectra of DIYSP in DMF: linear absorption (a), one- (b) and two-photon (c) fluorescence spectra.
the solvent and solute molecules will further lower the excitation energy [16]. By comparing Table 1 and Fig. 1, we can see that the two-photon fluorescence maxima of compound DIYSP have no obvious shift compared with the corresponding one-photon fluorescence maxima in the same solvent. Although the two-photon fluorescence spectra are measured at high concentration compared to the one-photon fluorescence spectra, the Stokes’ shift is large enough to make re-absorption effect negligible [17]. As shown in Fig. 1, there is no obvious overlap between the blue side of the one-photon induced fluorescence band and the red side of the linear absorption band.
3.2. The TPA cross-section The TPA r of DIYSP was determined by comparing its two-photon induced fluorescence with the two-photon fluorescence excitation r of Rhodamine 6G (at a concentration of 5 · 104 mol/L in methanol) according to the following equation [18] rs ¼ rr
Ur cr nr F s U c n Fr
3.3. Two-photon photopolymerization A microstructure was made according to the reported method [10]. The oligomer is SR349. The 80 fs, 80 MHz mode-locked Ti:sapphire laser is used for two-photon microfabrication. The 800 nm lasing source is tightly focused via an objective lens (·40, NA = 0.65), and the focal point is focused on the sample film on the xy-step monitorized stage controlled by computer. The pulse energy after being focused by the objective lens is 300 mW. The lattice fabricated is observed through a microscope (Olympus BX-51) and the photograph is illustrated in Fig. 2. In order to understand the photopolymerization mechanism, a theoretical investigation was made. We optimized the molecular equilibrium geometry using the 6-31G basis set together with the hybrid density functional theory (DFT/B3LYP) coded in GAUSSIAN package for the molecule DIYSP. The results show that the first excited state is the charge-transfer (CT) state with the excited energy k = 413 nm. When the molecule is irradiated by 800 nm laser beam, it can be expected that the molecule will simultaneously absorb two photons and is excited to the first excited state (the CT state). For a better understanding of the charge-transfer process, we have plotted the charge density difference between the ground and the CT states for DIYSP in gas phase (see Fig. 3), which is visualized by use of MOLEKEL program [21]. From Fig. 3, one can see that upon the excitation, charges are mainly trans-
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rescent lifetimes and large TPA cross-section. The experimental results confirm that DIYSP is an operative twophoton photopolymerization initiator. A two-dimensional microstructure has been fabricated by using DIYSP as a two-photon photopolymerization initiator and the possible mechanism of photopolymerization is discussed by a theoretical investigation. Acknowledgements This work was supported by NSFC (No. 20531070, 50323006 and 50402018), and Doctorate Foundation of the State Education Ministry of China (No. 20040445001). References
Fig. 2. Optical micrograph of the lattice fabricated via two-photon polymerization using DIYSP as an initiator.
Fig. 3. Density difference between the charge-transfer and ground states of DIYSP in gas phase. Areas with chickenwire and dots represent the electron gain and lose, respectively, upon the excitation.
ferred from the donor side to the acceptor side. In the CT state, there are more electrons at the acceptor side, indicating that the molecule could be given away its electron to its surrounding to initiate the photopolymerization reaction. However, whether the photoinduced electron-transfer reaction can be energetically feasible needs to be further investigated theoretically. 4. Conclusions A new heterocycle-based two-photon photopolymerization initiator of DIYSP has been synthesized and characterized. DIYSP shows excellent thermal stability, onephoton fluorescence quantum yields, long two-photon fluo-
[1] P.A. Fleitz, M.C. Brant, R.L. Sutherland, F.P. Strohkendl, J.R. Larson, L.R. Dalton, SPIE Proc. 91 (1998) 3472. [2] J.E. Ehrlich, X.L. Wu, L.-Y.S. Lee, Z.-Y. Hu, H. Roeckel, S.R. Marder, J. Perry, Opt. Lett. 22 (1997) 1843. [3] J.D. Bhawalkar, G.S. He, P.N. Prasad, Rep. Prog. Phys. 59 (1996) 1041. [4] C.F. Zhao, G.S. He, J.D. Bhawalkar, C.K. Park, P.N. Prasad, Chem. Mater. 7 (1995) 1979. [5] W. Denk, J.H. Strickler, W.W. Webb, Science 248 (1990) 73. [6] R.H. Ko¨hler, J. Cao, W.R. Zipfel, W.W. Webb, M.R. Hansen, Science 276 (1997) 2039. [7] B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Keikal, S.M. Kuebler, I.Y.S. Lee, D.M. Maughon, J. Qin, H. Ro¨ckel, M. Rumi, X.L. Wu, S.R. Marder, J.W. Perry, Nature 398 (1999) 51. [8] D.A. Parthenopoulos, P.M. Rentzepis, Science 245 (1989) 843. [9] Y.-X. Yan, X.-T. Tao, Y.-H. Sun, C.-K. Wang, G.-B. Xu, W.-T. Yu, H.-P. Zhao, J.-X. Yang, X.-Q. Yu, Y.-Z. Wu, X. Zhao, M.-H. Jiang, New. J. Chem. 29 (2005) 479. [10] Y.-X. Yan, X.-T. Tao, Y.-H. Sun, C.-K. Wang, G.-B. Xu, J.-X. Yang, Y. Ren, X. Zhao, Y.-Z. Wu, X.-Q. Yu, M.-H. Jiang, J. Mater. Chem. 14 (2004) 2995. [11] A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante, G.A. Pagani, D. Pedron, R. Signorini, Org. Lett. 4 (2002) 1495. [12] Z.-L. Huang, H. Lei, N. Li, Z.-R. Qiu, H.-Z. Wang, J.-D. Guo, Y. Luo, Z.-P. Zhong, X.-F. Liu, Z.-H. Zhou, J. Mater. Chem. 13 (2003) 708. [13] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991. [14] P. Fromherz, J. Phys. Chem. 99 (1995) 7188. [15] U. Narang, C.F. Zhao, J.D. Bhawalkar, F.V. Bright, P.N. Prasad, J. Phys. Chem. 100 (1996) 4521. ˚ gren, J. Chem. Phys. 114 (2001) [16] C.-K. Wang, P. Macak, Y. Luo, H. A 9813. [17] G.S. He, L. Yuan, Y. Cui, M. Li, P.N. Prasad, J. Appl. Phys. 81 (1997) 2529. [18] Z.-Q. Liu, Q. Fang, D. Wang, D.-X. Cao, G. Xue, W.-T. Yu, H. Lei, Chem. Eur. J. 9 (2003) 5074. [19] M.A. Albota, C. Xu, W.W. Webb, Appl. Opt. 37 (1998) 7352. [20] C. Xu, R.M. Williams, W. Zipfel, W.W. Webb, Bioimaging 4 (1996) 198. [21] MOLEKEL, References in
Synthesis, characterization and optical properties of a new heterocycle-based chromophore Yun-Xing Yan a,d,*, Yuan-Hong Sun b, Lei Tian c, Hai-Hua Fan d, He-Zhou Wang d, Chuan-Kui Wang b, Yu-Peng Tian c, Xu-Tang Tao a, Min-Hua Jiang a a
d
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China b Department of Physics, Shandong Normal University, Jinan 250014, PR China c Department of Chemistry, Anhui University, Longhe Road, Hefei 230039, PR China State Key Laboratory of Optoelectronic Materials and Technologies/School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China Received 12 July 2006; received in revised form 26 October 2006; accepted 23 November 2006 Available online 20 February 2007
Abstract A new heterocycle-based two-photon absorption chromophore, 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)styryl]pyridine (DIYSP), has been synthesized and characterized. The molecule possesses an A–p–A 0 structure. The p-deficient heteroaromatic ring (4,5-diphenyl1H-imidazole) is used as an acceptor (A), and the pyridine ring is used as another acceptor (A 0 ). Pumped with 740 nm laser excitation, DIYSP had a two-photon absorption cross-section 41 GM (1 GM = 1 · 1050 cm4 s photon1) and a two-photon excited fluorescence 511 nm in DMF, respectively. A microstructure using DIYSP as an initiator has been fabricated under irradiation of 80 fs, 80 MHz Ti:sapphire femtosecond laser at 800 nm. The possible mechanism of photopolymerization is discussed. 2007 Elsevier B.V. All rights reserved. Keywords: Synthesis; Heterocycle; Two-photon absorption; Initiator; DIYSP
1. Introduction Two-photon absorption (TPA) holds a great potential for a number of industrial and medical applications such as optical power limiting [1,2], two-photon upconversion lasing [3,4], two-photon fluorescence excitation microscopy [5,6], three-dimensional (3D) optical data storage and microfabrication [7,8]. Although 3D microfabrication has been illustrated using the two-photon-initiated polymerization of resins incorporating conventional ultravioletabsorbing initiators, the TPA cross-sections (r) of these initiators are typically very small, and as a result they exhibit low two-photon sensitivity. Therefore, it is important to establish efficient methods for the synthesis of novel initia*
Corresponding author. Address: State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China. E-mail address: [email protected] (Y.-X. Yan). 0925-3467/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.11.073
tors. In recent years, the optimization of molecular TPA has largely focused on one-dimensional dipolar or quadrupolar structures, and octupolar structures. And the corresponding efficiently two-photon photopolymerization initiators had been reported [9,10]. Recent studies showed that the heterocycle-based two-photon absorbing chromophores exhibit large TPA cross-sections [11,12]. However, as two-photon photopolymerization initiators, they were seldom reported. In this paper, we present the synthesis and the TPA properties of a new heterocycle-based organic chromophore, 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)styryl]pyridine (DIYSP). It shows strong one- and two-photon excited fluorescence. The two-photon initiating polymerization microfabrication experiment has been carried out using DIYSP as an initiator, and the possible interpretation is discussed by the charge-transfer process when the laser irradiation is applied.
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Y.-X. Yan et al. / Optical Materials 30 (2007) 423–426
2. Experimental 2.1. Instruments The 1H NMR spectra were recorded on a Mercury-Plus 300 spectrometer. Elemental analyses were performed by a Perkin–Elmer 2400 elemental analyzer. The melting points and decomposition temperatures were determined on a Netzsch DSC 204 and a Netzsch TG 209 thermogravimetric analyzer at a heating rate of 10 C min1 under dinitrogen atmosphere, respectively. The spectral resolution and accuracy of the DSC 204 thermal analyzer are ±0.1 C and ±1.0%, respectively. Mass spectra were measured on a LCMS-2010A liquid chromatograph mass spectrometer. UV–visible–near-IR spectra were measured on a UV-3150 spectrophotometer. The one-photon fluorescence spectra and quantum yields (U) were measured on a Hitachi F-4500 fluorescence spectrophotometer. Coumarin307 (whose U is assumed to be 56% in methanol) was used as the reference [13]. The concentration is the same as that of the linear absorption spectra. The two-photon induced fluorescence spectra were observed with a single sweep streak camera equipped with a polychromator as a recorder (Hamamatsu Model C1587). A mode-locked Nd: YAG laser (PC2143, pulse duration of 25 ps) was used to pump up the OPG (Optical Parameter Generator) system operating at 10 Hz, tuned from 420 to 10,000 nm. The average power density is 2.1 GW/cm2. The excitation wavelength for DIYSP is 740 nm. The concentrations are 1 · 103 mol/L, respectively.
C21H15BrN2: C 67.21, H 4.03, N 7.47%; found C 66.94, H 3.97, N 7.36%. 1H NMR (DMSO-d6, 300 MHz) [ppm] d = 8.11 (d, J = 8.4 Hz, 2H), 8.05 (s, 1H), 7.71 (d, J = 8.4 Hz, 2H), 7.48 (d, J = 3.3 Hz, 4H), 7.37 (d, J = 3.3 Hz, 6H). ESI–MS: m/z calcd for C21H15BrN2 374.04, found 374.15. 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)styryl]pyridine (DIYSP). BPDI (1.87 g, 5 mmol), tri-o-tolylphosphine (0.65 g, 2.15 mmol), 4-vinylpyridine (1.16 mL, 10.75 mmol), palladium(II) acetate (0.06 g, 0.27 mmol), and redistilled triethylamine (100 mL) under dinitrogen, were added to a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and a dinitrogen input tube. The reaction mixture was refluxed in an oil bath under dinitrogen. An orange product was obtained after heating and stirring for 24 h. Then the solvent was removed under reduced pressure and the residue was dissolved in dichloromethane, washed with distilled water, and dried with anhydrous magnesium sulfate. The organic layer was filtered and concentrated. The resulting solution was chromatographed on silica gel using ethyl acetate/petroleum ether (1:1) as eluent. The yellow– green powder of compound DIYSP (mp 301.8 C) was obtained in a yield of 55%. The enthalpy is 126.2 mJ/mg. Anal. calcd for C28H21N3: C 84.18, H 5.30, N 10.52%; found C 83.87, H 5.32, N 10.48%. 1H NMR (DMSO-d6, 300 MHz) [ppm] d = 12.72 (s, 1H), 8.54 (d, J = 5.1 Hz, 2H), 8.11 (d, J = 8.1 Hz, 2H), 7.76 (d, J = 8.1 Hz, 2H), 7.44 (m, 14H). ESI–MS, m/z calcd for C28H21N3 399.17, found 399.25. 3. Results and discussion 3.1. One- and two-photon fluorescence
2.2. Synthesis The synthetic route for the preparation of DIYSP is shown in Scheme 1. 2-(4-Bromophenyl)-4,5-diphenyl-1H-imidazole (BPDI). A mixture of benzil (1.05 g, 5 mmol), 4-bromobenzaldehyde (0.925 g, 5 mmol), ammonium acetate (2.5 g), and acetic acid (25 ml) was refluxed for 12 h, and then cooled to room temperature. After adding concentrated hydrochloric acid (10 ml), the mixture was left for 1 h, filtered, and washed with water. The filtrate was treated with 30% aqueous sodium hydroxide solution (20 ml) and gave pale-yellow precipitate. The crude product was purified by column chromatography on silica gel using ethyl acetate/petroleum ether (1:15) as eluent. Pale-yellow powders of BPDI were then obtained in 86% yield. Anal. calcd for
O O
a
N Br
b
N N
NH
NH
BPDI
DIYSP
Scheme 1. Synthesis of DIYSP: (a) NH4OAc/HOAc/4-bromobenzaldehyde reflux; (b) 4-vinylpyridine/tri-o-tolylphosphine/palladium(II) acetate/triethylamine, reflux.
The photophysical properties of the compound DIYSP are summarised in Table 1. The linear absorption, onephoton fluorescence, and two-photon fluorescence spectra in DMF are shown in Fig. 1. The linear absorption spectra were measured in solvents of different polarity at a concentration of 1 · 105 mol/L, in which the solvent influence has been excluded. Fig. 1 shows that the solutions of compound DIYSP are completely transparent above 500 nm. The one-photon fluorescence spectra were measured with the same concentrations as those of the linear absorption spectra. The excitation wavelength is 385 nm. As shown in Table 1, the absorption maxima are not significantly different, while the one- and two-photon fluorescence maxima are slightly red-shifted, and the fluorescent lifetimes are lengthened upon increasing the polarity of the aprotic solvent for compound DIYSP. This can be attributed to the fact that the excited states may possess higher polarity than that of the ground state, since the solvatochromism is associated with the lowering of energy levels. By increasing dipole–dipole interaction between the solute and solvent, the energy level can be lowered greatly [14,15]. In the case of ethanol, the fluorescence maximum is slightly red-shifted compared to those in other aprotic solvents. The possibility of forming hydrogen bonds between
Y.-X. Yan et al. / Optical Materials 30 (2007) 423–426
425
Table 1 The data of absorption, one- and two-photon fluorescence with solvent effects of DIYSP Solvents
kð1aÞ max (nm)
eres (104)
Þ kð1f max (nm)
Dm (cm1)
s (ns)
U (%)
kð2Þ max (nm)
DMF Ethyl acetate Ethanol THF CHCl3 Toluene
368 357 360 365 355 357
3.39 3.20 3.50 3.38 2.70 2.43
507 490 510 490 484 479
7450 7603 8170 6989 7508 7134
3.16
59 78 54 72 78 90
511
2.53 2.03
494 487
ð1f Þ ð2Þ kð1aÞ max , kmax , and kmax are one-photon absorption, one-photon fluorescence, and two-photon fluorescence maxima peak, respectively. U is one-photon fluorescence quantum yield determined using coumarin 307 as the standard [13]. s is two-photon fluorescence lifetime. Dm is Stokes’ shift. eres is the corresponding molar absorption coefficient.
0.6
Absorbance
Relative intensity (a.u.)
(c)
0.5
(b)
0.4
(a) 0.3
0.2
0.1
0.0 300
400
500
600
700
800
900
where the subscripts s and r refer to the sample and the reference material, and the terms c and n are the concentration and refractive index of the sample solution, respectively. U is the fluorescence quantum yield, F is two-photon excited fluorescence integral intensity, and rr is the TPA cross-section of the reference molecule. The excitation wavelength of DIYSP is 740 nm. The concentration is 1 · 103 mol/L. The TPA r for Rhodamine 6G is 21 GM at 740 nm [19]. The TPA r of DIYSP is 41 GM. However, it should be noted that the optimal excitation wavelength for the TPA should be slightly less than two times of the wavelength for the linear absorption maxima [20]. A significant increase in the TPA r value is expected at the optimal excitation wavelength for TPA.
Wavelength (nm) Fig. 1. Absorption and fluorescence spectra of DIYSP in DMF: linear absorption (a), one- (b) and two-photon (c) fluorescence spectra.
the solvent and solute molecules will further lower the excitation energy [16]. By comparing Table 1 and Fig. 1, we can see that the two-photon fluorescence maxima of compound DIYSP have no obvious shift compared with the corresponding one-photon fluorescence maxima in the same solvent. Although the two-photon fluorescence spectra are measured at high concentration compared to the one-photon fluorescence spectra, the Stokes’ shift is large enough to make re-absorption effect negligible [17]. As shown in Fig. 1, there is no obvious overlap between the blue side of the one-photon induced fluorescence band and the red side of the linear absorption band.
3.2. The TPA cross-section The TPA r of DIYSP was determined by comparing its two-photon induced fluorescence with the two-photon fluorescence excitation r of Rhodamine 6G (at a concentration of 5 · 104 mol/L in methanol) according to the following equation [18] rs ¼ rr
Ur cr nr F s U c n Fr
3.3. Two-photon photopolymerization A microstructure was made according to the reported method [10]. The oligomer is SR349. The 80 fs, 80 MHz mode-locked Ti:sapphire laser is used for two-photon microfabrication. The 800 nm lasing source is tightly focused via an objective lens (·40, NA = 0.65), and the focal point is focused on the sample film on the xy-step monitorized stage controlled by computer. The pulse energy after being focused by the objective lens is 300 mW. The lattice fabricated is observed through a microscope (Olympus BX-51) and the photograph is illustrated in Fig. 2. In order to understand the photopolymerization mechanism, a theoretical investigation was made. We optimized the molecular equilibrium geometry using the 6-31G basis set together with the hybrid density functional theory (DFT/B3LYP) coded in GAUSSIAN package for the molecule DIYSP. The results show that the first excited state is the charge-transfer (CT) state with the excited energy k = 413 nm. When the molecule is irradiated by 800 nm laser beam, it can be expected that the molecule will simultaneously absorb two photons and is excited to the first excited state (the CT state). For a better understanding of the charge-transfer process, we have plotted the charge density difference between the ground and the CT states for DIYSP in gas phase (see Fig. 3), which is visualized by use of MOLEKEL program [21]. From Fig. 3, one can see that upon the excitation, charges are mainly trans-
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rescent lifetimes and large TPA cross-section. The experimental results confirm that DIYSP is an operative twophoton photopolymerization initiator. A two-dimensional microstructure has been fabricated by using DIYSP as a two-photon photopolymerization initiator and the possible mechanism of photopolymerization is discussed by a theoretical investigation. Acknowledgements This work was supported by NSFC (No. 20531070, 50323006 and 50402018), and Doctorate Foundation of the State Education Ministry of China (No. 20040445001). References
Fig. 2. Optical micrograph of the lattice fabricated via two-photon polymerization using DIYSP as an initiator.
Fig. 3. Density difference between the charge-transfer and ground states of DIYSP in gas phase. Areas with chickenwire and dots represent the electron gain and lose, respectively, upon the excitation.
ferred from the donor side to the acceptor side. In the CT state, there are more electrons at the acceptor side, indicating that the molecule could be given away its electron to its surrounding to initiate the photopolymerization reaction. However, whether the photoinduced electron-transfer reaction can be energetically feasible needs to be further investigated theoretically. 4. Conclusions A new heterocycle-based two-photon photopolymerization initiator of DIYSP has been synthesized and characterized. DIYSP shows excellent thermal stability, onephoton fluorescence quantum yields, long two-photon fluo-
[1] P.A. Fleitz, M.C. Brant, R.L. Sutherland, F.P. Strohkendl, J.R. Larson, L.R. Dalton, SPIE Proc. 91 (1998) 3472. [2] J.E. Ehrlich, X.L. Wu, L.-Y.S. Lee, Z.-Y. Hu, H. Roeckel, S.R. Marder, J. Perry, Opt. Lett. 22 (1997) 1843. [3] J.D. Bhawalkar, G.S. He, P.N. Prasad, Rep. Prog. Phys. 59 (1996) 1041. [4] C.F. Zhao, G.S. He, J.D. Bhawalkar, C.K. Park, P.N. Prasad, Chem. Mater. 7 (1995) 1979. [5] W. Denk, J.H. Strickler, W.W. Webb, Science 248 (1990) 73. [6] R.H. Ko¨hler, J. Cao, W.R. Zipfel, W.W. Webb, M.R. Hansen, Science 276 (1997) 2039. [7] B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Keikal, S.M. Kuebler, I.Y.S. Lee, D.M. Maughon, J. Qin, H. Ro¨ckel, M. Rumi, X.L. Wu, S.R. Marder, J.W. Perry, Nature 398 (1999) 51. [8] D.A. Parthenopoulos, P.M. Rentzepis, Science 245 (1989) 843. [9] Y.-X. Yan, X.-T. Tao, Y.-H. Sun, C.-K. Wang, G.-B. Xu, W.-T. Yu, H.-P. Zhao, J.-X. Yang, X.-Q. Yu, Y.-Z. Wu, X. Zhao, M.-H. Jiang, New. J. Chem. 29 (2005) 479. [10] Y.-X. Yan, X.-T. Tao, Y.-H. Sun, C.-K. Wang, G.-B. Xu, J.-X. Yang, Y. Ren, X. Zhao, Y.-Z. Wu, X.-Q. Yu, M.-H. Jiang, J. Mater. Chem. 14 (2004) 2995. [11] A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante, G.A. Pagani, D. Pedron, R. Signorini, Org. Lett. 4 (2002) 1495. [12] Z.-L. Huang, H. Lei, N. Li, Z.-R. Qiu, H.-Z. Wang, J.-D. Guo, Y. Luo, Z.-P. Zhong, X.-F. Liu, Z.-H. Zhou, J. Mater. Chem. 13 (2003) 708. [13] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991. [14] P. Fromherz, J. Phys. Chem. 99 (1995) 7188. [15] U. Narang, C.F. Zhao, J.D. Bhawalkar, F.V. Bright, P.N. Prasad, J. Phys. Chem. 100 (1996) 4521. ˚ gren, J. Chem. Phys. 114 (2001) [16] C.-K. Wang, P. Macak, Y. Luo, H. A 9813. [17] G.S. He, L. Yuan, Y. Cui, M. Li, P.N. Prasad, J. Appl. Phys. 81 (1997) 2529. [18] Z.-Q. Liu, Q. Fang, D. Wang, D.-X. Cao, G. Xue, W.-T. Yu, H. Lei, Chem. Eur. J. 9 (2003) 5074. [19] M.A. Albota, C. Xu, W.W. Webb, Appl. Opt. 37 (1998) 7352. [20] C. Xu, R.M. Williams, W. Zipfel, W.W. Webb, Bioimaging 4 (1996) 198. [21] MOLEKEL, References in