Synthesis of new fluorescent compounds from 5-nitro-1 H - indazole

Nucleophilic substitution of hydrogen followed by intramolecular electrophilic aromatic substitution in nitro drevitavies of indazole has been used as a key step in the one pot synthesis of new fluorescent heterocyclic compounds 3 H -pyrazolo[4,3-a ]acridin-11-carbonitriles.


Introduction
Fluorescence is used as an analytical tool to determine the concentrations of various species, either neutral or ionic.When the analyte is fluorescent, direct determination is possible; otherwise, a variety of indirect methods using derivatization, formation of a fluorescent complex or fluorescence quenching have been developed.2][3] Fluorescence is also a powerful tool for investigating the structure and dynamics of matter or living systems at a molecular or supramolecular level.5][6][7] The latter can be intrinsic or introduced on purpose.][10] Fluorescent heterocyclic compounds are of interest as functional materials in many disciplines such as emitters for electroluminescence devices, 11 molecular probes for biochemical research, 12 in traditional textile and polymer fields, 13 whitening agents 14 and photo conducting materials. 159][20][21] Now we describe here the syntheses of new fluorescent heterocyclic compounds from nitro derivatives of indazole by this method and evaluation of their spectroscopic properties.

Scheme 1
The structural assignments of compounds 3a-d were based on the analytical and spectral data.For example, in the 1 H NMR spectrum of 3a, there are the signals at δ 4.07 and 4.28 ppm assignable to protons of methoxy and methyl group and the doublet of doublet signal at δ 7.53 ppm (J 9.2 Hz and J' 2.4 Hz), the doublet signals at δ 7.61 (d, J 2.4 Hz), δ 7.90 (d, J 9.6 Hz), δ 8.07 (d, J 9.6 Hz), δ 8.35 (d, J 9.2 Hz) ppm and singlet signal at δ 9.15 ppm attributed to six protons of aromatic rings.Moreover, the FT-IR spectrum of 3a in KBr showed the absorption band at 2240 cm -1 corresponding to cyanide group.All this evidence plus the 13 C NMR spectrum, molecular ion peak at m/z 288 and microanalytical data strongly support the tetracyclic structure of compound 3a.

Scheme 2
The fluorescence absorption and emission spectra of compounds 3a-d were recorded at the concentration of 10 -5 and 10 -6 M in chloroform.Figure 1 and Figure 2 show the visible absorption and emission spectra of compounds 3a-d.The λabs, values of extinction coefficient (ε), λex, λem and fluorescence quantum yield (ΦF) data are presented in Table 1.Values of extinction coefficient (ε) were are determined as the slope of the plot of absorbance vs concentration.The fluorescence quantum yields (ΦF) of compounds 3a-d were determined via comparison methods, using fluorescein as a standard sample in 0.1 M NaOH and MeOH solution. 23Also the fluorescence absorption and emission spectra of compound 3a were measured in different solvents (Figure 3 and Figure 4).As it is demonstrated in these figures, the fluorescence absorption and emission spectra of 3a in polar solvents exhibit solvatochromic red shift with the increasing solvent polarity.Solvent effects shift the emission to lower energy owing to stabilization of the excited state by the polar solvent molecules (Table 2).This type of behavior is observed for most of the dyes.For example, in Table 2, one can see that in the absorption spectrum for 3a, λabs shifts from 369 to 397 nm, and in the emission spectrum, λem shifts from 447 to 456 nm as the solvent is changed from n-hexane to methanol.

Conclusions
We have presented a new, facile, efficient and useful protocol for the synthesis of new derivatives of pyrazolo[4,3-a]acridin which have fluorescent properties and research into their possible applications is in progress.For example, thsese compounds can be used as new molecular probes for biochemical research with hydrolyzing the cyanide group to corresponded carboxylic acid and linking the latter compounds to biologically important molecules such as carbohydrates, lipids, proteins and nucleic Acids

Experimental Section
General.Melting points were recorded on an Electrothermaltype-9100 melting-point apparatus.
The IR spectra were obtained on a Tensor27 spectrometer and only noteworthy absorptions are listed.The 13 C NMR (100 MHz) and the 1 H NMR (400 MHz) spectra were recorded on a Bruker Avance DRX-400 Fouriertransformer spectrometer.Chemical shifts are reported in ppm downfield from TMS as internal standard; coupling constant J are given in Hz.The mass spectra were recorded on a Varian Mat, CH-7 at 70 eV.Elemental analysis was performed on a Thermo Finnigan Flash EA microanalyzer.Absorption spectra were recorded on Varian 50-bio UV-Visible spectrophotometer.Fluorescence spectra were recorded using Varian Cary Eclipse spectrofluorophotometer. UV-vis and fluorescence scans were recorded from 350 to 700 nm.All measurements were carried out at room temperature.Compounds 1a, b 24 were obtained according to the published methods.Other reagents were commercially available.
General procedure for the synthesis of 3a

Table 1 .
Photophysical data for absorption (abs) and emission (em) of 3a-d in chloroform a Wavelengths of maximum absorbance.b Extinction coefficient.c Wavelengths of fluorescence excitation.d Wavelengths of fluorescence emission.e Fluorescence quantum yield.