Electronic states of naphthazarin and related compounds. UV-VIS linear dichroism spectroscopy and quantum chemical model calculations

The four 1,4-naphthoquinones, 1,4-naphthoquinone, 2,3-dichloro-1,4-naphthoquinone, 5,8-dihydroxy-1,4-naphthoquinone (naphthazarin) and 2,3-dichloro5,8-dihydroxy-1,4-naphthoquinone have been investigated by UTI-VIS linear dichroism spectroscopy in uniaxially stretched polyethylene sheets. Experimental results are compared: with theoretical predictions using the ZINDO/S and LCOAO quantum chemical models. Excellent agreement with observed data is obtained. The observed shift of bands due to chlorine substitution is also satisfactorily described using a new set of chlorine parameters in the ZINDO/S model. Analyses of the substituent effects are used in the assignment of transitions for naphthoquinone and naphthazarin for which conclusive LD results are missing, The combined results lead to a determination of transition moment directions for the lowest (observed) electronic transitions.