Polycyclic aromatic compounds are the important compounds in scientific research and practical(applications.)Generally speaking

they have a bigger conjugate system.Almost atoms of them are in one plane.So they are good electroluminescence materials.The study of spectra for these substances has become a(focus)in recent scientific researches.A theoretical study on fluorescence spectra of four polycyclic aromatic(compounds)is given.Their geometric configurations have been optimized by semi-empirical method RHF/PM3.The structure analysis indicates that double C=C bond length in four compounds is longer than normal double bond and shorter than normal C—C bond

which shows that their electrons partly shift.From the analysis of the torsion angles

it is found that all benzene cycles of four compounds are in one plane

which

to some extent

enhances the compounds' stability.For all optimal configurations

we have calculated the IR spectra and there is no imaginary frequency in vibrational analyses

which indicates that the opitimized geometries are basically reasonable.On this basis

the electronic spectra have been calculated by CIS method.All calculated results are basically consistent with experimental values.