Synthesis and Luminescence Properties of Nine Novel Carbazolyl Diacylhydrazone Schiff-bases

Nine novel carbazolyl diacylhydrazone Schiff-bases were synthesized by alkylation, F-C acylation and condensation reactions starting from carbazole and hydrazide. The title Schiff-bases were characterized by 1H NMR, MS, IR and elemental analysis. The synthetic conditions were optimized, and the best yield of the title Schiff-bases was up to 92.3 %. The relationships between the luminescence properties and the structures of the title Schiff-bases were studied. The results showed that the introduction of the Naphthalene-2-yloxy could form great plane conjugate structure to improve their luminescence properties.


INTRODUCTION
As a peculiar Schiff-base, acylhydrazone have received great attention for its strong p-π conjugated system, various coordination forms, strong coordination abilities, good biological activity, [1][2][3][4][5] catalytic activity 6 and fluorescence property 7,8 in recent years.The strong conjugated system and high intramolecular electron transfer efficiency of carbazole make it have potential application prospect in electroluminescence (EL) and photoluminescence (PL). 9Carbazole derivatives represent one of the most widespread class of holetransporting materials with strong absorption in the ultraviolet spectral region and blue-light emission. 10The conjugated π bonds, existed both in acylhydrazone and carbazole, possess a strong ability in energy transfer.However, carbazole has never been used to synthesize the diacylhydrazone Schiff-bases.In this study, we developed nine novel carbazolyl diacylhydrazone Schiff-bases by alkylation, F-C acylation and condensation reactions starting from carbazole and hydrazide.The hydrogen atom of carbonyl and carbazolyl were replaced by several different groups 11 to clarify the influence of the substituents on the luminescence properties of the Schiff-bases (Scheme 1).Scheme 1. Synthesis route of carbazolyl diacylhydrazone Schiff-bases.

General Synthetic Procedure of N-alkyl Carbazole (1a-c)
Carbazole (1.67 g, 10 mmol), tetrabutyl ammonium bromide (0.32 g, 1 mmol) and potassium hydroxide (2.24 g, 40 mmol) dissolved in acetone (20 mL) were added to a 100 mL three-necked flask, the mixture solution was stirred for 40 minutes.Alkyl bromide (12 mmol) was added dropwise to the solution with constant stirring.The mixture was refluxed for 1 h.The reaction mixture was then poured into ice water (300 mL) with vigorous stirring to obtain a great deal of deposit.The mixture was filtered, washed with water and the filter cake obtained was then recrystallized from alcohol and water to give N-alkyl carbazole (1a-1c). 12-ethylcarbazole (1a) was synthesised from bromoethane and carbazole.A whitish needle crystals product (9.2 mmol, 92 %).m.p 67.0-68.

General Synthetic Procedure of 3,6-Diacetyl-9-alkyl Carbazole (2a-c)
Aluminium chloride powder (1.10 g, 8 mmol) was dissolved partly in dichloromethane (10 mL) in a 100 mL three-necked flask with constant stirring until the solution became orange-yellow.To the solution acetyl chloride (1 mL, 10 mmol) in dichloromethane and compound 1 (2 mmol) in dichloromethane was added dropwise to it in an ice bath.The reaction was carried out in the ice bath for 2 h and further stirred at room temperature for 48h.The reaction mixture was then poured into ice water (400 mL) with vigorous stirring to get a pale-yellow precipitate.The crude products were filtered, washed with water, dried, and then recrystallized from alcohol to get compound 2. 13 3,6-diacetyl-9-ethylcarbazole (2a) was synthesised from acetyl chloride and 1a.A yellow solid (1.80

Synthetic Procedure of Ethyl β-naphthyloxyacetate
Ethyl bromoacetate (5.00 g, 30 mmol) , β-naphthol (4.30 g, 30 mmol) in acetone (15 mL) and sodium carbonate anhydrous (6.20 g, 45 mmol) were added to a 100 mL single aperture flask with constant stirring.The solution was refluxed for 14 h under an anhydrous condition.After standing overnight, the mixture was filtered, washed with cold water (50 mL) several times, neutralized to pH 7-8 with 10 % NaOH, and extracted with ether (40 mL) two times.The ether layer collected was dried by sodium sulfate (4.00 g), and then filtered after standing for 48 h.Then the filtrate was evaporated to remove the ether.The product was not characterized and applied directly to the next step reaction. 14

Synthetic Procedure of Methyl p-Nitrobenzoate
p-Nitrobenzoic acid (1.67 g, 10 mmol) and methanol (10 mmol) were added to a 50 mL single aperture flask.To the solution concentrated sulphuric acid (2 mL) was then added dropwise slowly.The mixture was refluxed for 12 h and poured into ice water, getting lots of white solid.The coarse product was filtered, recrystallized from 50 % alcohol to get a white product, applied directly to the next step reaction. 14

General Synthetic Procedure of the Title Schiff-bases (L1− L9)
The synthesis of Schiff-bases was performed according to the Scheme 1. Compound 2 (1 mmol) and the hydrazide 3 (2.2 mol) were dissolved in ethanol (10 mL) in a 100 mL three-necked flask, and then glacial acetic acid (2 mL) was added dropwise to it.The solution was heated to reflux for 2 h, getting lots of solid.The reaction mixture was then filtered taking the advantage of heat, washed with hot ethanol, and dried to get the title Schiff-base L1−L9.
Because the glacial acetic acid can enhance the electropositivity of the carbonyl carbon, promote the breakdown of the carbazolyl diacylhydrazone Schiffbases, and reduce the nucleophilic ability of the amino, the condensation reactions were sensitive to the acid catalyst (glacial acetic acid).
In order to investigate the influence of the acid catalyst on the yields of the title Schiff-bases, the condensation reactions of 3,6-diacetyl-9-alkylcarbazole with hydrazide were carried out in the presence of glacial acetic acid or not.The experimental data were listed in Table 1.It is seen from the table 1 that the yields of the title Schiff-bases were no more than 15 % in the absence of glacial acetic acid.However, the yields of L4−L9 were all about 90 % in the presence of glacial acetic acid.The slightly lower yields of L1−L3 may be due to the steric hindrance of naphthalene-2-yloxy.The results indicated that the addition of the glacial acetic acid and the steric hindrance from the substituents in benzene ring had remarkable influence on the yields of the title Schiff-bases, and the electronic effect of the substituents had little influence on them.
In order to optimize the synthetic conditions of the title Schiff-bases, the influences of the reaction time and the volumn fraction of glacial acetic acid on the yield of L5 were investigated.The experimental data were listed in Table 2.
Based on the optimization experimental data in Table 2, it was concluded that the optimal synthesis conditions of the compound L5 via the condensation of benzoylhydrazine with 3, 6-diacetyl-9-butylcarbazole were that the volumn fraction of glacial acetic acid was 20 % and the reaction time was 2 h, the yield of the compound L5 was up to 92.3 % under this reaction condition.

Effect of Substituents on the Properties of the Title Schiff-bases
All the title Schiff-bases showed the luminescence properties, however, their fluorescent intensities were quite different.In order to evaluate the relationships between the luminescence properties and the structures of the title Schiff-bases, the UV spectra and fluorescence emission spectra of the title Schiff-bases were measured.
The UV spectra data of the title Schiff-bases (L1-L9) were listed in Table 3 and the UV spectra were shown in Figure 1 and Figure 2. The UV absorption bands of L1-L9 appeared at λ a , 1 and λ a , 2 were assigned to the л-π* and n-π* transition respectively.
The fluorescence emission spectra of the title Schiff-bases (L1-L9) were excitated by their respective maximum UV absorption peak wavelengths.
The fluorescence emission spectra of the compounds L1-L3 were shown in Figure 3.It was found that the fluorescence emission intensities of the compounds L1 and L2 were strong due to the introduction of naphthalene-2-yloxy which could improve the structural planarity and conjugation effect of the compounds L1 and L2.However, when a butyl group was bonded to the carbazole ring, such as the compound L3, the fluorescence emission intensity was observed to be much weaker relative to that of the compounds L1 and L2 because the increase of alkyl carbon numbers in the carbazole ring resulted in the reduction of the planarity of the whole molecular structure.In addition, the title Schiff-bases with benzene ring, such as the compounds L4-L9, showed particularly weak fluorescence because of their poor conjugated structure.b) The yields of L1-L9 in glacial acetic acid.

Table 1 .
The yields of the title Schiff-bases

Table 2 .
The yields of the Schiff-base L5 (a)The volume fraction of glacial acetic acid.

Table 3 .
The UV spectra data of the title Schiff-bases L1-L9