We report semi-empirical calculations of modified 3,3'-diethylthiacyanine iodide (DTC1), 3,3'-diethylthiacarbocyanine iodide (DTC3) and 3,3'-diethylthiadicarbocyanine iodide (DTC5), particularly with halogen substitution at the meso position in the polymethine bridge. Primarily we investigate geometrical changes and infer spectral trends from the molecular orbital levels. The semi-empirical calculations for the unsubstituted DTC3 agree well with an ab initio Hartree-Fock result. In all molecules multiple conformations are examined. Significant changes in dipole moments are noted between the cis and trans forms. Calculated electronic spectra at the CI singles level are compared to the measured spectra with reasonable agreement. Halogenation effects show a rotation of the benzthiazole groups out of planarity. We draw conclusions about consequent effects on solubility and excited state spectral changes.