Semiempirical-FF calculations were performed for the second-order hyperpolarizabilities ( 7 ) of a series of polycyclic aromatics to examine the structure-optical-propertyrelationship. First, ( y ) of the unsubstituted PAHs were presented. For linear polyacenes, ( y ) and total resonance energy were found to be increased while the HOMO-LUMO gap and resonance energy per electron decreased as the number of six-membered rings increased. For nonlinear PAHs, the chain topology should be taken into consideration. An empirical formula was given to relate the computed ( 7 ) and linear polarizabilities (a). ( y ) of mono-and di-substituted PAHs were then presented to examine the substituent effects on ( 7 ) . Q
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