Halo-organic heterocyclic compounds are described, which have the formulas Ring A is an aromatic ring bound to at least two halogen atoms, at least one of which is iodine or bromine. Also described are polymethine dyes based on these heterocyclic compounds, and dendrimeric compounds and conjugates of such polymethine dyes. The polymethine dyes are characterized by enhanced properties, e.g., brightness, photostability, sensitivity and/or selective affinity that make them useful to target cancer cells, pathogenic microorganisms, and/or other biological materials, in applications such as photodynamic therapy, photodynamic antimicrobial chemotherapy (PACT), cancer treatment, selective removal or attachment of biological materials, antimicrobial coating materials, and other diagnostic, theranostic, spectrum shifting, deposition/growth, and analytic applications.