Optical absorption of methoxy and carboethoxy derivatives of 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline.

Paper deals with experimental investigations and quantum chemical calculations of the optical absorption spectra of methoxy and carboethoxy 1,3-diphenyl derivatives of the pyrazoloquinoline ([PQ]): 6-methoxy-1,3-dyphenil-[PQ], 6-methoxy-1,3-(p-methoxyphenyl)-[PQ], 6-methoxy-1-(p-methoxyphenyl)-[PQ] and 6-carboethoxy-1,3-diphenyl-[PQ]. The quantum chemical calculations are performed by means of the semiempirical quantum chemical methods (AM1 or PM3) applied to: (a) the equilibrium molecular conformation in vacuo T=0 K; (b) the molecular dynamic (MD) trajectory T=300 K which includes the dynamics of a certain molecular fragment (moiety) only (fragmental MD simulations); or (c) the MD trajectory obtained for most general case within the total MD simulations at T=300 K. The results of these calculations are compared with the measured spectra of the optical absorption. The quantum chemical simulations show that the dynamics of the methoxy or carboethoxy groups practically does not influence the absorption spectrum whereas the strongest its modification 300<lambda< or =360 nm is found to be related with dynamics of phenyl(II) [Ph2] or phenyl(II)-methoxy [Ph2-MeO] moieties which are characterized by large libration amplitudes. At the same time, the total MD reproduces evidently much better the halfwidth of most absorption bands compared to ones observed in the measured spectra. Comparing the measured and calculated spectral positions of the absorption threshold the quantum chemical method PM3 gives the best agreement for all compounds.