Synthesis and Characterization of Novel 1, 4-Bis (carbazolyl) benzene

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Synthesis and Characterization of Novel 1,4-Bis(carbazolyl)benzene Derivatives with Blue-Violet Two-Photon-Excited Fluorescence Baodong Zhao,† Xiaoqin Jia,† Jiqiang Liu,† Xiaoyu Ma,† Huiqing Zhang,† Xiaoning Wang,‡ and Tao Wang*,† †

State Key Laboratory of Chemical Resource Engineering, College of Science, Beijing University of Chemical Technology, Beijing, 100029, People’s Republic of China ‡ College of Material Engineering, Beijing Institute of Fashion Technology, Beijing 100019, People’s Republic of China S Supporting Information *

ABSTRACT: Novel fluorescent two-photon absorption molecules containing bis(carbazolyl)benzene as the central unit and phenylethynyl moieties as peripheral groups (Cz-Ps) are synthesized and characterized. In these molecules, two carbazolyl moieties are linked with benzene at the 9-position and synthesized by a concise process involving nucleophilic substitution between the cyclopentadienyliron complexes of dichloroarenes and phenylethynyl carbazole, followed by photolysis. The optimal structures of Cz-Ps reveal that two carbazolyl rings linked by a benzene ring are not planar. This feature prevents the electron conjugation of the molecule from extending throughout the whole molecule and allows Cz-Ps to realize blue-violet emissions and high fluorescence quantum yield. With increasing number of conjugated phenylacetylene structures in Cz-Ps, the maximal absorption and emission peaks were red-shifted. The quadrupolar compound DMoCz-P shows strong one-photon and twophoton activities. The resulting molecules are also thermally stable with high glass transition temperatures due to the rigid bicarbazole central unit. This work has demonstrated that using bis(carbazolyl)benzene as central building block could enhance two-photon absorption (TPA) performance and also provides a possible general synthetic strategy for a series of bis(carbazolyl)benzene derivatives, which would advance the understanding of the rational design of new organic optical materials.

1. INTRODUCTION Materials with two-photon-excited fluorescence (TPEF) are of great significance for applications in three-dimensional (3D) optical memory and data storage, 3D optical imaging, 3D lithographic microfabrication, photodynamic therapy, and so on.1−5 Materials that emit at wavelengths between the ranges of 500 and 700 nm are fairly common, but few show efficient blueviolet TPEF.6 To realize blue-violet or violet emission, the π conjugation of organic molecules must be strictly confined. Prior works have revealed the presence of efficient blue TPEE in both broad and intense TPA bands in symmetric A−π−A and D−π−D and asymmetric A−π−A′, D−π−D′, and D−π−A model molecules. Such confinement can cause co-instantaneous decreases in fluorescent quantum yield and charge-transporting capability.7−11Therefore, to achieve a blue-violet or violet emission TPEF, appropriate substitutions and the length of the conjugated linker must be considered during the molecular design of relevant materials. Carbazole and its derivatives are widely used as organic materials due to their photorefractive, photoconductive, holetransporting, and light-emitting properities.12−15 3,6-Functionalized carbazole derivatives were extensively studied because carbazole can be easily functionalized by electrophilic aromatic substitution at its 3,6-positions with high electron density.16 The nitrogen atom of the carbazole moiety can also be functionalized by alkylation or arylation reaction. Arylated carbazoles upon which a phenyl or a naphthyl group is attached were found to have excellent thermal stability and good electrooptical properties.17 Transition-metal-catalyzed arylation © XXXX American Chemical Society

of carbazole with aryl halides is one of the most efficient and powerful methods for the synthesis of N-arylcabazole derivatives.18−21 However, from an ecological perspective, the current methods present several limitations because the transformations often use expensive transition metal catalysts, such as palladium, rhodium, nickel, and cobalt complexes. In our recent studies, an improved process for the preparation of arylated carbazoles has been found. This process involves a nucleophilic substitution between the cyclopentadienyliron complexes of chloroarenes and carbazole or hydroxylcarbazole, followed by photolysis of cyclopentadienyliron complexes of arylated carbazoles. The process combines two steps in good yields and is cost-effective.22,23 In the design of the π-conjugated compounds, phenylacetylene is an important unsaturated backbone linkage. The linkage is rigid, has large π-electron delocalization, and is sterically less demanding.24−26 These advantageous characteristics are the key elements in the design of organic molecules for optical applications.27−29 Some phenylethynylcarbazole derivatives have been designed as two-photon polymerization initiators and highly efficient fluorescent sensors of explosive peroxide.30−32 However, in these studies, most of the reported molecules are using monocarbazole as a center building block. Little study on phenylethynylcarbazole derivatives using 1,4Received: November 26, 2015 Revised: January 17, 2016 Accepted: January 27, 2016

A

DOI: 10.1021/acs.iecr.5b04501 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Chart 1. Structure of 1,4-Bis(carbazolyl)benzene Derivatives Studied in This Work

Scheme 1. Synthetic Route for Novel 1,4-Bis(carbazolyl)benzene Derivatives

phenylethynyl moieties as well as increase the glass transition block and thermal stability.35 The compounds obtained were characterized in detail, and their linear and nonlinear optical properties were investigated experimentally and compared with their nonphenylethynyl bonding analog 1,4-bis(carbazolyl)benzene (DCz-P).

bis(carbazolyl)benzene as a center building block has been reported. Two-photon polymerization is a three-dimensional microfabrication method that typically uses a near-infrared femtosecond laser source to generate polymeric structures.33 In the present work, dipolar and quadrupolar 1,4-bis(carbazolyl)benzene molecules were designed as photoinitiator and were intended to be applied in two-photon polymerization. Both a large two-photon absorption cross section and a high yield of photoinitiation are critical for the overall performance of a given photoinitiator.34 Another important, but often overlooked, factor is the thermal stability of the photoinitiator during the application process, such as 3D lithography. In these molecules, use of the carbon−carbon triple bond is expected to adjust the absorption and emission wavelengths and improve the TPA cross-section of the resultant molecules. Use of 1,4bis(carbazolyl)benzene as the central building block is further expected to provide a plurality of active positions for bonding

2. EXPERIMENTAL SECTION 2.1. Synthetic Procedures. Full synthetic details and characterization data for all compounds synthesized in this work are provided in the Supporting Information. 2.2. Linear Characterization. UV−vis absorption spectra were recorded on a UV-5200 (UNICO) UV−vis spectrophotometer. The photolysis reactions of the products were carried out with a EDR-100V-300W halogen lamp (Beijing Institute of Opto Electronics). One-photon excited fluorescence (OPEF) spectra were recorded at a concentration of 1 × 10−6 mol/L in dichloroB


Article

Industrial & Engineering Chemistry Research methane on an F-4500 (Hitachi High-Technologies Corporation) fluorescence spectrophotometer. The fluorescence quantum yields were obtained with quinine sulfate dehydrate (1 × 10−6 mol/L) in 0.1 M sulfuric acid aqueous solution as a reference standard (Φref = 0.54) by eq 1,36 Φ=

A ref n2F Anref 2 Fref

Φref

(1)

where Φ is the quantum yield, n is the refractive index, A is the absorbance of solution at the exciting wavelength, and F is the integrated area under the emission spectrum. 2.3. Two-Photon Absorption Measurements. The twophoton absorption cross section of the compounds has been measured with the two-photon-induced fluorescence method by using the femtosecond laser pulses as described.37 Twophoton excited fluorescence (TPEF) spectra were recorded on a SD2000 spectrometer (Ocean Optical) with excitation with a femtosecond laser (Tsunami, Spectra-Physics). This laser provided pulses of 100 fs of duration at a repetition rate of 80 MHz and was tunable in the wavelength range of 720−880 nm. The laser beam was focused into a quartz cell of 1 cm path length by using a 5 cm focal-length lens. The excitation intensity was 8.2 GW/cm2, and the laser spot diameter is 2 mm. To calculate the TPEF cross sections, rhodamine B in methanol solution (1 × 10−4 mol/L) was utilized as references for the calculation. For the products of DMoCz-P, a THF solution (1 × 10−4 mol/L) was prepared to calculate the TPEF cross sections and 1 × 10−3 mol/L for MMoCz-P. All the samples and standards were tested under the same experimental conditions. The TPEF cross sections were calculated by eq 2, where c and n were the concentration and refractive index of the samples and reference, and F was the integral of the TPEF spectrum. The two photon cross section σTPA was then calculated by eq 3, where Φ was the fluorescence quantum yield of the sample as stated in eq 1.

σTPEF =

cref nref F σref cnFref

(2)

σTPA =

σTPEF Φ

(3)

Figure 1. 1H NMR spectra of DMoCzFc during photolysis under halogen lamp (λ ≥ 370 nm, DMSO-d6 as the solvent, M = 4.2 × 10−3 mol/L, I = 10 mW/cm2).

Supporting Information). The characteristic peak of cyclopentadiene at 5.25 ppm gradually disappeared under irradiation, which demonstrates the occurrence of decomplexation in the cationic cyclopentadienyliron complexes. Changes in the region from 6 to 11 ppm indicate that Ar−H peaks of the liberated aryl-substituted carbazoles gradually substitute those of the cyclopentadienyliron complexes CzFcs. On the basis of the 1H NMR spectra obtained, these newly generated aryl-substituted carbazoles have no changes under irradiation, which indicates their good photostability. Ferrocene formation was observed by a new peak appearing at 4.17 ppm; this peak is attributed to complexation of liberated cyclopentadiene and ferrous ions. On the basis of the 1H NMR spectra of Cz-Ps, the photodemetalation rates of CzFcs were investigated by calculating characteristic peak area changes in cyclopentadienyl at about 5.20 ppm with respect to irradiation time. The calculation results are shown in Figure 2. CzFcs exhibited rapid photodemetalation, and the photodemetalation rates of CzFcs showed the order DMoCzFc > MMoCzFc. After photolysis, pure arylated carbazoles were successfully obtained, as determined by IR, MS, and NMR. All of the compounds showed 1H NMR signals for the different kinds of protons at their respective positions. All of the data on the compounds confirm that the desired structures were correctly

3. RESULTS AND DISCUSSION 3.1. Synthesis. The structures of 1,4-bis(carbazolyl)benzene derivatives (Cz-Ps) studied in this work are shown in Chart 1, and the synthetic routes are depicted in Scheme 1. Phenylethynylcarbazole derivatives (MMoCz and DMoCz) were prepared through the Pd-catalyzed cross-coupling reactions of bromo-substituted carbazole and phenyl/methoxylphenyl acetylene. The cyclopentadienyliron complexes MMoCzFc and DMoCzFc were then prepared through the SNAr reaction of Fc-2Cl with compounds MMoCz and DMoCz, respectively. Novel 1,4-bis(carbazolyl)benzene derivatives MMoCz-P and DMoCz-P were obtained by the photolysis of MMoCzFc and DMoCzFc, respectively. Photolysis is an important step in preparing Cz-Ps. 1H NMR spectroscopy was used to observe the demetalation process of the cyclopentadienyliron complexes, including MMoCzFc and DMoCzFc (CzFcs). Figure 1 shows changes in the 1H NMR spectra of the compound DMoCzFc with increasing irradiation time (demetalation process of MMoCzFc; see Figure S1 in the

Figure 2. Photodemetalation conversions of MMoCzFc and DMoCzFc. C


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Industrial & Engineering Chemistry Research

Figure 3. UV−vis spectra (a) and one-photon fluorescence spectra (b) of Cz-Ps in dichloromethane at a concentration of 1 × 10−6 mol/L.

Table 1. UV−Visible Absorption Spectra Data of Cz-Ps in Dichloromethane with a Concentration of 1 × 10−6 mol/L compd

λmax,1 (nm)

εmax,1 (104 M−1·cm−1)

λmax,2 (nm)

εmax,2 (104 M−1·cm−1)

DMoCz-P MMoCz-P DCz-P

267.6 252.4 240.2

6.0 4.9 6.2

316.3 302.4 292.6

10.0 7.0 3.0

synthesized. The 1H NMR spectra of the Cz-Ps are shown in Figure S2 in the Supporting Information. 3.2. One-Photon Spectra. The one-photon absorption spectra of Cz-Ps and 1,4-bis(carbazolyl)benzene (DCz-P) in dichloromethane are shown in Figure 3a, and the corresponding data are presented in Table 1. Cz-Ps exhibited strong and wide absorptions from 280 to 360 nm because of π−π* transitions in these conjugated systems. With increasing number of conjugated phenylacetylene structures and degree of π delocalization in Cz-Ps, the maximal absorption peaks were red-shifted. The order of absorption maxima was DMoCz-P > MMoCz-P > DCz-P. The fluorescence emission and excitation spectra of Cz-Ps in dichloromethane solution are shown in Figure 3b, and the corresponding data are listed in Table 2. Cz-Ps showed blue-

Figure 4. Optimized structures of compound DMoCz-P by DFT method.

were demonstrated in Tables S1 and S2 in the Supporting Information.) The optimal structures of Cz-Ps reveal that two carbazolyl rings linked by a benzene ring are not planar. This feature prevents the electron conjugation of the molecule from extending throughout the whole molecule and allows Cz-Ps to realize blue-violet emissions. 3.3. Two-Photon Excited Fluorescence (TPEF) Spectra. As shown in the UV−vis spectra of the compounds (Figure 3a), no linear absorption above the 500 nm range may be observed, which suggests that if the sample produces fluorescence under light irradiation above 700 nm, the samples must show twophoton absorption effects. In this study, TPEF spectra were measured using excitation wavelengths ranging from 720 to 760 nm. The corresponding two-photon excitation spectra of the compounds in THF under different excitation laser wavelengths are shown in Figure 5. Figure 5 shows that all compounds exhibit bluish violet upconversion fluorescence and present the strongest emissions when excited by a 720 nm laser light. As the excitation wavelength increases from 720 to 790 nm, the strength of upconversion fluorescence decreases, likely because of a decrease in absorption of the compounds from 360 nm to some longer wavelength. TPA cross sections were measured using the two-photon induced fluorescence technique and calculated with eqs 2 and 3. The calculation outcomes of the TPA cross sections of the compounds under different excitation laser wavelengths are shown in Table 3. The corresponding TPA cross sections of the compounds in THF at excitation wavelengths ranging from 720

Table 2. Data of One-Photon Fluorescence Spectra of Cz-Ps compd

Exmax (nm)

Emmax (nm)

Stokes shift (nm)

quantum yield Φ

DMoCz-P MMoCz-P DCz-P

316 302 296

396 369 350

80 67 54

0.63 0.27 0.16

violet fluorescence emission from 350 to 450 nm. With increasing numbers of conjugated phenylacetylene structures and the degree of π delocalization in Cz-Ps, the maximal emission peaks were red-shifted. The maximum emissions of MMoCz-P and DMoCz-P were about 369 and 396 nm, respectively. The fluorescence quantum efficiency of Cz-Ps was tested in THF solution at a concentration of 1 × 10−6 mol/L, and the relevant data are listed in Table 2. This table clearly shows that the quadrupolar molecule DMoCz-P exhibits higher fluorescence quantum yields than the dipolar molecule MMoCz-P. To gain insights into the geometries of Cz-Ps, quantum chemical calculations were performed and optimized geometric structures were obtained. As an example, the structures of compound DMoCz-P are shown in Figure 4. (The optimized structure of MMoCz-P was shown in Figure S3, and data of selected bond lengths, angles, and dihedral angles of Cz-Ps D


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Figure 5. Two-photon excited fluorescence spectra of Cz-Ps in tetrahydrofuran under different excited laser wavelengths and laser intensity of 8.2 GW/cm2: (a) DMoCz-P; (b) MMoCz-P.

Table 3. Two-Photon Absorption Data of Cz-Ps under Different Excited Laser Wavelength compd

wavelength (nm)

λmax (nm)

σ (GM)

σTPA (MW)

DMoCz-P

720 730 740 750 760 720 730 740 750

395.2 395.7 395.2 396.4 395.7 383.4 390.5 394.1 395.3

448.1 417.9 348.0 222.0 94.5 66.6 20.2 12.3 11.4

0.48 0.45 0.37 0.23 0.10 0.10 0.03 0.02 0.02

MMoCz-P

to 790 nm are shown in Figure 6. This figure reveals that the TPA cross sections of the compounds follow the order σDMoCz‑P

Figure 7. Energy level and electron density distribution of frontier molecular orbitals of Cz-Ps.

density distribution of the HOMO and LUMO of the Cz-Ps, and more detail related information was summarized in Table S3 in the Supporting Information. In the LUMO of these compounds, electrons are mainly localized in the benzene ring system and nitrogen atoms from the center building block. In the HOMO of these compounds, electrons are localized throughout the whole molecule, to be exact, mainly localized in the branches. With increasing number of conjugated phenylacetylene structures in Cz-Ps, the energy level gap between LUMO and HOMO was reduced accordingly. Therefore, DMoCz-P has better electron charge transfer ability than MMoCz-P due to more branches connected to the center building block, which also may be attributed to its large TPA cross sections. 3.4. Thermal Properties. To explore the thermal properties of the Cz-Ps, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed at a scanning rate of 10 °C/min under flowing nitrogen. The TGA curves in Figure 8 reveal that all of the synthesized compounds possess high decomposition temperatures. The decomposition temperatures Td0.1 (corresponding to 10% weight loss) of the Cz-Ps were observed at 429 °C (DMoCz-P) and 454 °C (MMoCz). High glass transition temperatures (Tg) of 176 °C (DMoCz-P) and 159 °C (MMoCz-P) were also obtained via DSC. Such high Tg values could be attributed to the use of 1,4bis(carbazolyl)benzene as a central building block and the presence of N-phenyl groups.

Figure 6. TPA cross sections of the compounds in THF at excitation wavelengths ranging from 720 to 790 nm.

≫ σMMpCz‑P at the same excitation wavelength. This finding suggests that as increases in the conjugated structure are achieved, two-photon absorption is significantly enhanced; here, the electron-donating functionality (of the methoxyl group) exhibits some function in two-photon absorption. TPA measurements show that TPA cross sections decrease with increasing excitation wavelength; in particular, the maximum values of the TPA cross sections of all two compounds occur at the shortest wavelengths of the excitation region, i.e., 720 nm. The molecular weight normalized σTPA/ MW values of DMoCz-P and MMoCz-P are 0.48 and 0.10 GM· g−1·mol, respectively. The electron density distribution of the frontier molecular orbital was analyzed to gain insights into the charge transfer process of these compounds. Figure 7 shows the electron E


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Figure 8. Thermogravimetric analysis (TGA) and differential scanning colorimetric (DSC) curves (inset) of Cz-Ps: (a) DMoCz-P; (b) MMoCz-P. (3) Hu, D.; Hu, Y.; Huang, W.; Zhang, Q. Two-photon induced data storage in hydrogen bonded supramolecular azopolymers. Opt. Commun. 2012, 285, 4941−4945. (4) Zhao, T.; Yu, K.; Li, L.; Zhang, T.; Guan, Z.; Gao, N.; Yuan, P.; Li, S.; Yao, S. Q.; Xu, Q. H.; Xu, G. Q. Gold nanorod enhanced twophoton excitation fluorescence of photosensitizers for two-photon imaging and photodynamic therapy. ACS Appl. Mater. Interfaces 2014, 6, 2700−2708. (5) Schmitt, J.; Heitz, V.; Sour, A.; Bolze, F.; Ftouni, H.; Nicoud, J. F.; Flamigni, L.; Ventura, B. Diketopyrrolopyrrole-porphyrin conjugates with high two-photon absorption and singlet oxygen generation for two-photon photodynamic therapy. Angew. Chem., Int. Ed. 2015, 54, 169−173. (6) Hu, Z. J.; Sun, P. P.; Li, L.; Tian, Y. P.; Yang, J. X.; Wu, J. Y.; Zhou, H. P.; Tao, L. M.; Wang, C. K.; Li, M.; Cheng, G. H.; Tang, H. H.; Tao, X. T.; Jiang, M. H. Two novel π-conjugated carbazole derivatives with blue two-photon-excited fluorescence. Chem. Phys. 2009, 355, 91−98. (7) Zhang, X.; Yu, X.; Sun, Y.; Xu, H.; Feng, Y.; Huang, B.; Tao, X.; Jiang, M. Synthesis, structure and nonlinear optical properties of two new one and two-branch two-photon polymerization initiators. Chem. Phys. 2006, 328, 103−110. (8) Zhang, Y.; Lai, S. L.; Tong, Q. X.; Lo, M. F.; Ng, T. W.; Chan, M. Y.; Wen, Z. C.; He, J.; Jeff, K..S.; Tang, X. L.; Liu, W. M.; Ko, C. C.; Wang, P. F.; Lee, C. S. High efficiency nondoped deep-blue organic light emitting devices based on imidazole-π-triphenylamine derivatives. Chem. Mater. 2012, 24, 61−70. (9) Ye, J.; Chen, Z.; Fung, M.-K.; Zheng, C.; Ou, X.; Zhang, X.; Yuan, Y.; Lee, C. S. Carbazole/sulfone hybrid D-π-A-structured bipolar fluorophores for high-efficiency blue-violet electroluminescence. Chem. Mater. 2013, 25, 2630−2637. (10) Linton, K. E.; Fisher, A. L.; Pearson, C.; Fox, M. A.; Palsson, L. O.; Bryce, M. R.; Petty, M. C. Colour tuning of blue electroluminescence using bipolar carbazole-oxadiazole molecules in singleactive-layer organic light emitting devices (OLEDs). J. Mater. Chem. 2012, 22, 11816−11825. (11) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. Design of efficient thermally activated delayed fluorescence materials for pure blue organic light emitting diodes. J. Am. Chem. Soc. 2012, 134, 14706−14709. (12) He, P.; Wang, H.; Liu, S.; Shi, J.; Wang, G.; Gong, M. Effect of different alkyl groups at the N-position on the luminescence of carbazole-based β-diketonate Europium(III) complexes. J. Phys. Chem. A 2009, 113, 12885−12890. (13) Tomkeviciene, A.; Grazulevicius, J. V.; Kazlauskas, K.; Gruodis, A.; Jursenas, S.; Ke, T. H.; Wu, C. C. Impact of linking topology on the properties of carbazole trimers and dimers. J. Phys. Chem. C 2011, 115, 4887−4897. (14) Jiang, W.; Duan, L.; Qiao, J.; Dong, G.; Zhang, D.; Wang, L.; Qiu, Y. Novel carbazole/pyridine-based host material for solutionprocessed blue phosphorescent organic light-emitting devices. Dyes Pigm. 2012, 92, 891−896. (15) Shakir, M.; Noor, I.; Khan, M. S.; Al-Resayes, S. I.; Khan, A. A.; Baig, U. Electrical conductivity, isothermal stability, and ammonia-

4. CONCLUSIONS Dipolar molecule (MMoCz-P) and quadrupolar molecule (DMoCz-P) that can emit bluish violet up-conversion fluorescence had been designed and synthesized by a simple method. This study found that using 1,4-bis(carbazolyl)benzene as a center building block not only provides a plurality of active position for bonding phenylethynyl moieties that benefit the TPA performance but also increases the glass transition block and thermal stability. Among the products obtained, DMoCz-P possessed high fluorescence quantum efficiencies and large TPA cross sections, which indicates that increasing the dimensionality of molecules, such as certain branched systems, is a good approach to exhibit enhanced twophoton absorption cross sections.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04501. Details of synthesis, characterization, measurements, data of selected bond lengths, angles, and dihedral angles of optimized structure, and energy level and electron density distribution of frontier molecular orbitals (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-010-64445350. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for financial support of National Natural Science Foundation of China (Project Grant No. 21176016) and the Fundamental Research Funds for the Central Universities (Grant YS1406). We also are thankful for calculations support from Beijing University of Chemical Technology CHEMCLOUDCOMPUTING Platform.

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Synthesis and Characterization of Novel 1, 4-Bis (carbazolyl) benzene

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