APPLICATION OF AROMATICITY INDICES AS MOLECULAR DESCRIPTORS FOR PREDICTION OF OPTICAL PROPERTIES OF 9,10-ANTHRAQUINONE DERIVATIVES IN ETHANOL SOLUTION

Optical properties of 60 anthraquinone derivatives in ethanol solution were characterized using QSPR multivariance linear regression (MLR). Application of parameter set comprising molecular descriptors and aromaticity indices allowed for highly accurate prediction of lambda(max) values of considered anthraquinone dyes. Due to the nature of chromophore the main predictive power comes from hardness what obviously is related to pi -> pi* excitation and consequently the LUMO and HOMO energies. However, due to the fact that substituent effect is much more pronounced on pi-electron delocalization in central quinonoid ring rather than in vicinal aromatic ones, the aromaticity indices describing properties of this ring can be effectively used in MLR procedure. The highest predictive power of the final models was found for three parameters regression equation comprising hardness, NICS(1)(zz) and normalized Pozharsky geometry based index. The values of standard deviation were equal to 12.6 nm and square of adjusted correlation coefficient reached 0.97. This confirmed validity of the hypothesis that in cases in which molecular orbitals responsible for UV-Vis excitation belong, at least partly, to delocalized ring orbitals the UV-Vis imposed pi -> pi* excitations can be related to the extent of pi-electron delocalization expressed in term of geometric and magnetic indices.