Synthesis of novel benzothiazole compounds with an extended conjugated system

Novel cyano-substituted benzothiazoles (Schemes 1 and 2) were synthesized, which possess an extended conjugated system. Compound 6 consists of two benzothiazole units with bis(4-vinylphenyl)acrylonitrile as a bridging group and compound 12 was two benzothiazole units and two dodecyloxy bis(4-vinylphenyl)-acrylonitrile units. Absorption and fluorescence properties were studied for compounds 6 and 12


Introduction
Organic compounds with extended conjugated systems display unusual semiconducting and luminescent properties. 1,2][12] Benzothiazolyl derivatives have much better chemical, thermal, and photochemical stabilities than structurally similar compounds. 102-(3,5,6-Trifluoro-2-hydroxy-4-methoxyphenyl) benzothiazole with a zinc cation is used for fluorescent probe sensing. 13Bis-[2-(2hydroxyphenyl)benzothiazolate]zinc [Zn(BTZ)2] is one of the best white electroluminescent materials used in organic light-emitting diodes (OLEDs). 14Mintova et al. have synthesized nanosized zeolites by using a 2-(2'-hydroxyphenyl) benzothiazole precursor. 15Cyano-substituted poly-(phenylenevinylene) (PPV) has been used as an electron-transporting material in a highly efficient two layer light-emitting diode (LED) with PPV as a hole-transporting layer. 16These conjugated compounds which are accompanied by strong bathochromic shifts of the absorption and emission spectra showed increased electron affinity. 17,18Because of their rich electronic and photonic properties, conjugated organic molecules are ideal candidates as probe components for the structure of self-assembled monolayers (SAMs) that may find potential applications in optoelectronic devices. 19Yonezawa et al. synthesized gold nanoparticles by using a four-chained disulfide ligand and they concluded that the size of the metal core of gold nanoparticles could be finely tuned by the use of disulfide stabilizer molecules. 20The unsymmetrical azobenzene disulfide gold nanoparticles were synthesized in order to investigate the efficiency of azobenzene photoisomerization on colloidal gold surfaces. 21n addition, research on organic nanoparticles [22][23][24] receives much attention in the fields of drug delivery, 25 determinations of nucleic acids, 26 and fluorescent ion sensors. 27Organic nanoparticles are brighter, photochemically stable, and water soluble.They also have high fluorescence quantum yields and a longer fluorescence lifetime than small organic molecules. 27n continuation of our research interest on fluorescence probe sensing, self-assembled monolayers and light emitting materials, we herein describe the synthesis and characterization of conjugated benzothiazole based chromophores (Schemes 1 and 2).

Preparation
The synthesis is described in Schemes 1 and 2. Starting, 2-[2-(4-methylphenyl)ethenyl]-1,3benzothiazole 3 was synthesized according to the procedure reported in the literature. 28The compound 3 was reacted with N-bromosuccinimide in dry benzene and subsequently it was reacted with aqueous sodium cyanide in tetrahydrofuran (THF) to form a benzothiazole vinyl phenyl acetonitrile 4. Similarly, compound 3 was reacted with N-bromo-succinimide and excess of aqueous hexamethylenetetramine in chloroform and then refluxed with a mixture of glacial acetic acid and water to obtain the benzothiazole vinyl benzaldehyde 5.The elaboration of the conjugated system of 6 was performed by reacing equimolar quantities of 4 and 5 in dry THF and tert-butyl alcohol at 50 °C while a small amount of tetrabutylammonium hydroxide was slowly dropped into the mixture.Similarly, bromododecyloxy substituted benzothiazolo acrylonitrile 11 was achieved by the condensation of bromododocyloxy benzaldehyde 9 with benzothiazolo acetonitrile 4 in dry THF and tert-butyl alcohol.The bromododecyloxy substituted benzothiazolo acrylonitrile 11 was reacted with hexamethyldisilathiane and tetra-n-butyl ammonium fluoride in dry THF to give the disulfide 12.The disulfide 12 was also synthesized from the benzothiazolo acetonitrile 4 and 4-bis(12-thiododecyloxy)dibenzaldyde 10 through another route, but in this method we obtained only a 5% yield.The structure of the synthesized derivatives was established by 1 H and 13 C NMR and MS measurements.

Optical properties
The fluorescence and UV-vis spectra were measured under ambient conditions.The THF solvent was HPLC grade for absorption and fluorescence mesurements.Concentrations of compounds were about 1×10 -5 M. The maximum absorption peaks of the bis{4-[(E)-2-(benzo[d]thiazol-2yl)vinyl]phenyl} acrylonitrile 6 and disulfide 12 are at 405 nm and 385 nm, respectively (Figure 1).The absorption peak of 6 was shifted to the red, due to lengthing of a π-conjugated system.However, the absorbance of disulfide 12 was stronger than that of acrylonitrile 6, because of the more π-conjugated moiety in 12. Molar extinction coefficients (ε) of acrylonitrile 6 and disulfide 12 at peak maximum position are 1.7×10 4 and 9.6×10 4 , respectively.Bis{4-[(E)-2-(benzo[d]thiazol-2-yl)vinyl]phenyl} acrylonitrile 6 shows a fluorescence emission band at 495 nm while disulfide 12 shows an emission band at 434 nm in THF (Figure 2) at 10 -5 M concentration.The emission profile of 6 is very different from that of the disulfide 12, which may result from the presence of an extended π-conjugated length in 6.This can be explained by the electron-donor strength of the extra π-conjugated system. 29,30 dditionally, the quantum yields (Φ f ) of the two molecules vary considerably (see Table 1).In these two molecules, acrylonitrile 6 showed the highest quantum yield.This can be attributed to a shorter conjugation path in the dodecyl analogue 12 compared to the extended conjugated acrylonitrile 6.
ARKAT USA, Inc.  Peak wavelength in the Uv-vis absorption and fluorescence spectra.b ε: Molar extinction coefficients at peak maximum position of two molecules.c 9,10-diphenylanthracene was used as the reference (Φ f = 90% in cyclohexane 31 ).

Conclusions
We have synthesized novel conjugated benzothiazoles that were characterized by 1 H, 13 C NMR, and MS data.Absorption and fluorescence properties were studied for the final compounds 6 and 12. Current work within the group is exploring the synthesis of benzothiazole-based organic nanoparticles.When compared to small organic molecules, the organic nanoparticles may possess greater stability, high quantum yields, and longer fluorescence lifetime.Thus, we expect that the organic nanoparticles that will be prepared will be suitable for sensing biomolecules such as proteins, DNA, and aminothiols.

Experimental Section
General Procedures.The melting points were determined on a Fargo MP-2D Mel-Temp apparatus.Nuclear magnetic resonance spectra were recorded on a Bruker AM-400 MHz spectrometer.Mass spectra were obtained on a JEOL SX-102A spectrometer.A double-beam UV-vis spectrophotometer (Cintra 10e) obtained from GBC (Victoria, Australia) was used to measure the absorbance of compounds 6 and 12 and a fluorometer (Aminco Bowman) obtained from Thermo Spectronic (Pittsford, NY, USA) was used to collect the fluorescence spectra of compounds 6 and 12. 2-Aminothiophenol, 4-methylcinnamic acid, 1,12-dibromododecane and N-bromosuccinimide were purchased from Acros Ltd.Hexamethyldisilathiane was obtained from the Aldrich Chemical Company.