The Q-band of aluminum phthalocyanine chloride (AlPcCl) shows experimentally distinct splitting in the aqueous solution. Time-dependent density functional theory calculations with inclusion of polarizable continuum model have been performed to reveal the origin of this splitting. It is found that the spectral splitting caused by the formation of dimers and interaction with two water molecules are much smaller than what was predicted by previous semi-empirical study. Our calculations indicate that the experimentally observed large Q-band splitting can be explained by considering the geometrical distortions that are associated with changes of bonds between carbon and nitrogen atoms. (c) 2007 Elsevier B.V. All rights reserved.