Synthesis and characterization of four novel 1,3-azole based push-pull heterocyclic systems

Benz[X]azole derivatives are interesting compounds due to their diverse biological activities and interesting optical properties. The benzothiazole, benzimidazole and benzoxazole heterocycles are heteroaromatic electron deficient moieties that act as both acceptor groups and  -conjugated spacers with auxiliary electron withdrawing ability. Moreover, benzimidazole derivatives offers the possibility of substitution on the nitrogen atom for further tuning of their optical and electronic properties. Recently we have reported the synthesis and evaluation of the electronic, thermal and optical properties of a large number of series of benz[X]azole derivatives functionalized with different π-spacers having in mind their application as optical chemosensors, nonlinear optical and photochromic materials, and emissive organic components for OLEDs. In continuation of the work developed in our research group, we report in this work the synthesis, the characterization and the evaluation of the optical properties of four novel 1-(4-thiophene-2-yl)phenyl)- 1H -pyrrole derivatives functionalized with different benz[X]azole moieties (benzothiazole, benzimidazole and benzoxazole). The results showed that the optical properties could be readily tuned by changing the electronic nature of the azole ring, or even by introduction of a strong acceptor group.


Introduction
Push-pull π-conjugated systems are molecules end-capped with an electron donor group (D)   and an electron acceptor group (A) interacting through a -system.This molecular arrangement allows for intramolecular charge-transfer (ICT), which favors a new low energy molecular orbital, with electrons easily excited by visible light.Therefore, push-pull molecules are generally colored and referred to as charge-transfer chromophores.Organic CT chromophores are usually of easy synthesis, have well-defined and easily modified structures, and readily tunable properties, like HOMO and LUMO levels, position of the longest-wavelength absorption maxima, and dipole moment, by modification of the electron donor or acceptor moieties, alteration of the spacer, and by varying the overall chromophore arrangement. 1 The heteroaromatic thiazole, imidazole, and oxazole moieties are electron deficient groups that act as both acceptor and -conjugated spacers with auxiliary electron withdrawing ability.
These moieties when fused with a phenyl ring (benzothiazole, benzimidazole, and benzoxazole) represent a way of extending the π-conjugation of the system. 2,3Moreover, the imidazole moiety offers the possibility of substitution on the nitrogen atom for further tuning of their optical and electronic properties.
Benz[X]azole derivatives are interesting compounds due to their diverse biological activities 4 and interesting optical properties. 2 Our research group has reported the synthesis and evaluation of the electronic, thermal and optical properties of several series of benz[X]azole derivatives functionalized with different πspacers having in mind their application as optical chemosensors, nonlinear optical and photochromic materials, and emissive organic components for OLEDs. 2 In this communication, and as an extension of the work developed by this research group, we report the synthesis, the characterization and the evaluation of the optical properties of four novel 1-(4-thiophene-2-yl)phenyl)-1H-pyrrole derivatives functionalized with different benz[X]azole moieties (benzothiazole, benzimidazole and benzoxazole).

Methods and Materials
NMR spectra were obtained on a Brucker Avance III 400 at an operating frequency of 400 MHz for 1 H NMR using the solvent peak as internal reference at 25 ºC (δ relative to TMS).All chemical shifts are given in Hz.Assignments were made by comparison of chemical shift, peak multiplicities and J values.IR spectra were determined on a BOMEM MB 104 spectrophotometer.UV-Visible absorption spectra (200-800 nm) were obtained using a Shimadzu UV/2501PC spectrophotometer.Fluorescence spectra were collected using a FluoroMax-4 spectrofluorometer.Luminescence quantum yields were measured in comparison with ethanol solution of 9,10-diphenylanthracene (ΦF = 0.95) 5 as standard.All melting points were measured on a Gallenkamp melting point apparatus and are uncorrected.

Synthesis
Procedure for the synthesis of benzothiazole derivative 2a: 6 (1 equiv.)and o-aminobenzenethiol (1 equiv.)were heated in DMSO (1 mL/mmol) at 120ºC with stirring for 7 h.The reaction was followed by TLC.When the reaction was complete, the reaction mixture was allowed to cool, poured into water and extracted with ethyl acetate (3 x 50 mL).The organic layer was dried with magnesium sulphate and evaporated under reduced pressure.The crude residue was submitted to silica gel column chromatography using mixtures of diethyl ether and light petroleum of increasing polarity.The fractions containing the purified product were collected and evaporated under vacuum.General procedure for the synthesis of benzimidazole derivatives 2b-c: ) and sodium bissulphite (1.3 equiv.)were stirred in ethanol (5mL) for 4 h at room temperature.o-Phenylenediamine or 4-nitro-o-phenylenediamine (1 equiv.)dissolved in DMF (5 mL) was added to the mixture and heated to 80 ºC for another 4 h.When the reaction was complete, the reaction mixture was allowed to cool, poured into water and extracted with ethyl acetate (3 x 50 mL).The organic layer was dried with magnesium sulphate and evaporated under reduced pressure.The crude residue was submitted to silica gel column chromatography using mixtures of diethyl ether and light petroleum of increasing polarity.The fractions containing the purified product were collected and evaporated under vacuum.General procedure for the synthesis of benzoxazole derivative 2d: 5-(4-(1H-Pyrrol-1-yl)phenyl)thiophene-2-carbaldehyde 1 (1 equiv.)and 2-amino-5methylphenol (1.2 equiv) were refluxed in ethanol (5 mL) for 7 h.After evaporating the solvent, lead tetracetate (3 equiv) in DMSO (5mL) was added, and the mixture left stirring at room temperature.The reaction was followed by TLC.When the reaction was complete, the reaction mixture was allowed to cool, poured into water and extracted with ethyl acetate (3 x 50 mL).
The organic layer was dried with magnesium sulphate and evaporated under reduced pressure.
The crude residue was submitted to silica gel column chromatography using mixtures of diethyl ether and light petroleum of increasing polarity.The fractions containing the purified product were collected and evaporated under vacuum.

Synthesis
A series of heterocyclic chromophores were designed and synthesized in order to study the effect of different benz[X]azole acceptor moieties in their optical properties.All designed πconjugated systems are based on a 1-(4-thiophene-2-yl)phenyl)-1H-pyrrole system which acts simultaneously as electron donor group and π-spacer.
The preparation of the aldehyde precursor 1 has been previously reports by our research group elsewhere. 6The final push-pull chromophores were achieved by cyclization between aldehyde 1 and the appropriate functionalized phenyl derivatives (Scheme 1) in fair yields (14-23 %).
The heterocyclic compounds 2 were characterized by standard spectroscopic techniques.The disappearance of the IV band and 1 H NMR signal corresponding to the -CHO group was observed, as well as the presence of new 1 H NMR signals attributed to the phenyl ring fused to the 1,3-azole heterocycle.

Optical Study
The electronic absorption spectra of benz[X]azoles 2 in ethanol solutions (10 -4 M) show intense lowest energy charge-transfer absorption bands in the UV-visible region between 360-380 nm.
The results showed that the position of these bands depends on the electronic nature of the heteroaromatic 1,3-azole linked to the thiophene heterocycle (Table 1, Figure 1).
The benzimidazole derivative 2c exhibit the longer wavelength of maxima absorption (λ = 380 nm), indicating the stronger ICT, due to the additional electron withdrawing effect of the -NO2 substituent, that is responsible for the bathochromic shift of 19 nm relative to the analogous benzimidazole 2b.In this series of compounds the second longer wavelength of maxima absorption was found at λ = 373 nm for the benzothiazole derivative 2a, that can be attributed to the bathochromic effect of the sulphur.The benzimidazole 2b and the benzoxazole with a methyl substituent 2d, show very similar ICT, exhibited by the maxima of the absorption bands at 361 and 360 nm, respectively.Benz[X]azoles 2 were excited at the wavelength of maximum absorption, at room temperature, in order to study their fluorescence properties (Figure 1).The relative fluorescence quantum yields were determined using a solution of 9,10-diphenylanthracene in ethanol as fluorescence standard (ΦF = 0.95). 5In general, the synthesized chromophores show strong emissive properties, with relative quantum fluorescence yields ranging from 0.68 to 0.92.Benzimidazole 2c exhibited the lowest relative quantum fluorescence yield, at 0.21, that can be attributed to the existence of low-lying n→π* transitions due to the -NO2 group. 7
A study of the optical properties of the prepared chromophores showed that the UV-vis absorption and emission properties could be readily tuned by changing the electronic nature of
. It is noticeable bathochromic shifts in the benzothiazole derivative 2a due to the presence of sulphur, and in benzimidazole 2c when compared to chromophore 2b due to the introduction of a strong acceptor group like -NO2.The emission spectra followed the same trend as absorption.The prepared chromophores showed high relative quantum fluorescence yields (F = 0.68-0.92), the exception being chromophore 2c (F = 0.21) due to functionalization of the benzimidazole ring with the -NO2 group, which is a well-known quencher of fluorescence.Due to their optical properties and push-pull character, π-conjugated heterocyclic systems 2 can have potential application as nonlinear optical second harmonic generators (SHG), two-photon absorption (TPA) fluorophores or luminescent organic components for OLEDs.

Table 1 .
UV-visible absorption and emission data for benz[X]azoles 2, in ethanol.
Figure 1.Normalized UV-visible absorption (full line) and emission (double line) data for benz[X]azoles 2a-d, in ethanol.