Synthesis and spectral properties of functionalized oligoarylenes containing a nitro group

The synthesis of symmetrical oligoarylenes using a reliable double Suzuki cross-coupling strategy is presented here. These novel compounds were identified by 1 H NMR, 13 C NMR, IR, and HRMS. It was found that UV-vis absorptions were in the range of 295~348 nm and fluorescence emissions were detected between 387 and 395 nm in CHCl 3 . Substituent effects on absorption and fluorescence spectra are also discussed.


Introduction
Conjugated organic oligomers and polymers have been the focus of intense research over the past few decades because of their interesting optical, electrical and optoelectrical properties.Oligo-p-phenylenes are excellent model compounds for developing a profound understanding of the spectroscopic and redox properties of polyaromatic systems. 1 These compounds have been recognized as prospective materials for opto-electronic applications (for example, organic lightemitting diodes 2 (OLEDs)). 3They exhibit non-linear optical properties, high quantum efficiency, and efficient light emission in the blue region of spectra.3c Recently, systematic theoretical optoelectronic properties research of the oligo-p-phenylenes were widely reported. 4Heteroaryls (thiophenes) are incorporated into the oligoarylenes structures, which attract much attention as potentially useful materials 5 for electronic devices such as thin-film transistors 6 and OLEDs. 7hiophene-phenylene co-oligomers crystals exhibit good performance in field-effect transistors. 8n order to influence the optical properties, introduction of functional groups such as nitro substituent into the oligoarylenes has been a valid approach. 9The nitro substituent is easily introduced and is a valuable starting point for the synthesis of more complex target molecules.For example, polyindolocarbazole and polydiindolocarbazole can be synthesized from polyaromatic systems containing nitro groups, which show good optical, electrochemical, magnetic and conductive properties. 10ncouraged by these reports, we developed the idea of introducing nitro groups and heteroaryls into oligoarylenes with the aim of tuning their optical properties.To the best of our knowledge, oligoarylenes compounds (Figure 1) containing nitro groups have not been reported so far.According to previous reports, 11 the Suzuki cross coupling reaction is a reliable protocol for the synthesis of oligoarylenes.Here, we used a double Suzuki cross coupling reaction to synthesize novel symmetrical oligoarylenes compounds (Figure 1) of general structure 3a-d and 5a-d, which might allow for the construction of electroluminescent devices in the future.The relationship between structure and spectral properties was also discussed.

Synthesis
The sequence of the reactions leading to the syntheses of compounds 3 in this study is outlined in Scheme 1.As is shown in Scheme 1, syntheses of 1,4-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)benzene 12 was started from 1,4-dibromobenzene.Firstly, 1,4-dibromobenzene reacted with n-butyllithium at -78 °C for 30 min, and then the reaction was allowed to warm to room temperature and stirred for 1 hour.When the mixture was recooled to -78 °C, trimethyl borate was added to form the desired diboronic acid.Next, the conversion of the diacid to the pinacol diester proceeded smoothly at room temperature and gave 1 in an overall yield of 15%.The synthetic route of compounds 2a-d starts with o-nitroaniline. 13Firstly, o-nitroaniline was brominated with N-bromosuccinimide (NBS) to afford 4-bromo-2-nitroaniline.Then, diazotization of 4-bromo-2-nitroaniline followed by iodination afforded the desired 4-bromo-1iodo-2-nitrobenzene which was highly selectively coupled with various phenyl boronic acids to give substituted 4-bromo-2-nitrobiphenyls (2a-d).Sequentially, compounds 3a-d were synthesized by a double Suzuki cross coupling reaction of compounds 1 and 2a-d in 44-75% yields.Reactions were carried out using 5 mol % tetrakis (triphenylphosphine) palladium(0) as catalyst and potassium carbonate (K 2 CO 3 ) as base in 1,2-dimethoxyethane (DME) and H 2 O at reflux temperature.Various substituents (R) had influence on the yield of 3a-d.The yield of 3a (R = H) was 44%.The yield was increased to 75% when R was ethyl group.It was found that the yields of 3c and 3d (changing R from H to methoxy and methylthio group) were 67% and 70%, respectively.

Spectral properties
To study the influence of the substituent, the spectral properties of compounds 3a-d and 5a-d were investigated.The UV-vis spectra of compounds 3a-d and 5a-d were recorded in chloroform (CHCl 3 ) solution using 10×10 mm quartz cells and a spectral range of 200-800 nm.The fluorescence spectra were recorded in CHCl 3 .The absorption maxima (λ abs max ), molar absorption coefficient (ε), and the emission maxima (λ em max ) of the studied compounds 3a-d and 5a-d are listed in Table 1.The maximum absorption spectra of 3a-d and 5a-d was observed in the range of 295~348 nm.When R was H and ethyl groups, the absorption maxima of compounds (3a, 3b) were 295 and 297 nm, respectively.When R was methoxy or methylthio group, the absorption maxima of compounds (3c, 3d) gave a red shift from 8 to 12 nm compared with 3a.This is due to compounds 3c and 3d having an auxochromic methoxy group and methylthio group which can shift the absorption maxima (λ abs max ).Because thiophene was introduced into oligoarylenes in the centre, the absorption maxima of compounds 5 gave a large red shift compared with compounds 3. The compounds (5b, 5c, 5d) gave a red shift from 3 to 5 nm compared with 5a.In general, electron donating groups have a bathochromic effect on the absorption and the largest bathochromic shift in λ abs max occurred when there was a methylthio group at the R position of oligoarylenes containing heteroaryls (thiophene).
Literature 16 reported that λ abs max of p-quinquephenyl (PPPPP) and 2,5-bisbiphenyl-4-ylthiophene (PPTPP) are 309 and 356 nm in CHCl 3 , respectively.The λ abs max of 3a gave blue shift of 14 nm compared with PPPPP (nitro free analogue) and 5a also generated blue shift of 13 nm compared with PPTPP (nitro free analogue).From this it can be seen that introducing nitro substituent into PPPPP and PPTPP produced blue shift of λ abs max .
The band position of fluorescence spectra of 3a-d and 5a-d was observed between 387 and 395 nm.The charge density of the substituent has a small influence on the fluorescence emission peak.According to literature 16 , λ em max of PPPPP and PPTPP are 370 and 434 nm in CHCl 3 , respectively.The λ em max of 3a gave red shift of 24 nm compared with nitro free analogue PPPPP and 5a generated blue shift of 41 nm compared with nitro free analogue PPTPP.This shows that introducing nitro substituent into PPPPP and PPTPP have influence on fluorescence spectra.The absorption spectra (3a-d, 5a-d) and emission spectra (3b, 5b) in CHCl 3 were shown in Figure 2.

Conclusions
In conclusion, we have developed a reliable protocol for the synthesis of symmetrical oligoarylenes containing nitro groups using a double Suzuki cross coupling.The maximum UVvis absorption of title compounds were between 295 and 348 nm and their maximum fluorescence emissions were at 387~395 nm in CHCl 3 .Electron donating groups have a bathochromic effect on the absorption.

Experimental Section
General Procedures.Unless stated otherwise, reactions were performed in dry, nitrogen-flushed glassware, using freshly distilled solvents.Reagent grade tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl.Reagent grade chloroform (CHCl 3 ) and ethyl acetate (EtOAc) was used without further distillation.Melting points were measured by using a XT5 melting point apparatus and are uncorrected. 1H NMR and 13 C NMR spectra were recorded on a Bruker 500MHz spectrometer using TMS as an internal reference.Chemical shift values (δ) were given in ppm.Infrared spectra were obtained on a NEXUS670 spectrophotometer using potassium bromide pellets and are reported as wave numbers (cm -1 ).

Table 1 .
The UV-vis absorption and fluorescence spectra of compounds 3 and 5