AM1, PM3, and PM5 calculations of the absorption maxima of basic organic dyes

The absorption maxima, λmax, of various organic dyes such as indigo, azobenzene, phenylamine, hydrazone, anthraquinone, naphthoquinone, and malachite green were calculated using the AM1, PM3, and PM5 semiempirical molecular orbital theories with the configuration interaction singles (CIS) and random phase approximation (RPA) approaches. The calculated λmax were then compared with the values obtained by CNDO/S, INDO/S, ab initio CIS, and time-dependent density functional theory (TD-DFT). We found that the λmax values calculated by AM1, PM3, and PM5 were in good correlation with the observed λmax values. When B3LYP/cc-pVDZ optimized geometries were used, the square of the correlation coefficients between the calculated and observed λmax, rλ2, at the AM1-RPA, PM3-RPA, and PM5-RPA levels were 0.891, 0.897, and 0.927, respectively. In particular, rλ2 at PM5-RPA//B3LYP/cc-pVDZ was the largest among those obtained from all the other calculations including TD/B3LYP/cc-pVDZ//B3LYP/cc-pVDZ (rλ2=0.827). Accordingly, the standard deviation of the difference between observed and calculated λmax by the linear regression function at PM5-RPA//B3LYP/cc-pVDZ was the smallest. It was therefore concluded that this method was the most promising for the prediction of λmax of various dyes among the computational methods studied here. When AM1 optimized geometries were used, rλ2 at the AM1-RPA, PM3-RPA, and PM5-RPA levels were 0.822, 0.841, and 0.901, respectively, and they were also comparable to that at TD/B3LYP/cc-pVDZ//B3LYP/cc-pVDZ. Therefore, although some calibration efforts may be needed for AM1 geometries, PM5-RPA(CIS)//AM1 may be a second candidate available for the prediction of the absorption maxima of dyes, especially in the case of emphasizing computational cost.