From pirazoloquinolines to annulated azulene dyes: UV–VIS spectroscopy and quantum chemical study

Paper reports UV–Vis absorption and photoluminescence spectra of 6-R derivatives (R=CH3, O–CH3, C(C6H5)3, C6H5–N–C10H7) of 4-(2-chlorophenyl)-1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline, belonging to pyrazoloquinoline (PQ) family, likewise its regioisomeric products 10-R derivatives of 6-phenyl-6H-5,6,7-triazadibenzo[f,h]naphtho[3,2,1-cd]azulene representing cyclized seven-membered annulated azulene (AA) dyes. Cyclization of PQs into AAs is accompanied by a significant red shift of the first optical absorption band. This finding agrees with the results of quantum-chemical calculations performed by means of the semiempirical method PM3. As the solvent polarity rises all the dyes exhibit a blue shift of the first absorption band and a red shift of the fluorescence band. Such opposite trends in solvatochromic behavior have been reproduced within the semiempirical calculations in combination with the Lippert–Mataga dielectric polarization model. Depending on solvent polarity AA dyes emit light in the green, green–yellow or orange range of the visible spectrum what may be of interest for potential luminescent or electroluminescent applications.