Optical absorption measurements and quantum-chemical simulations on 1H-pyrazolo[3,4-b]quinoline derivatives

We present the results of experimental studies and quantum chemical simulation (PM3 and AM1 methods) on a new synthesized pyrazolo[3,4-b]quinoline derivatives: 1,4-diphenyl-3-methyl-, 3,4-diphenyl-1-methyl-, 1,3,4-triphenyl-, 1,3-dimethyl-4-phenyl- and 1,3,4-trimethyl-1H-pyrazolo[3,4-b]quinoline. The quantum chemical analysis reveals similarity in the absorption spectra of 1H-pyrazolo[3,4-b]quinoline and 1,3,4-trimethyl-1H-pyrazolo[3,4-b]quinoline which are characterized by four strong absorption bands in the spectral range 200-500 nm. A substitution of the methyl groups by phenyl groups causes substantial changes of the absorption spectra mainly in the spectral range 240-370 nm. We attribute these differences to additional molecular double bounding segments C=C of the substituted phenyl groups i.e., to pi-->pi* transitions. The comparison of measured and calculated absorption spectra by quantum-chemical AM1 method manifests rather good agreement for all compounds in the part regarding the spectral positions of the first oscillator (absorption threshold). At the same time, the measured spectra reveal the considerable broadening practically of all absorption bands. We attribute these discrepancies to a strong solvent effect. By analyzing the charge density distribution we have shown that the most strongest absorption bands in the spectral region 200-270 nm are related with double bonding segments C=N (n-->pi* transition). (C) 2003 Elsevier B.V. All rights reserved.