Conjugated polymers display remarkable optoelectronic properties that emerge from the strong coupling between delocalized pi-electrons, leading to highly mobile excited states. This is a very useful property from a light-harvesting perspective. We showed previously that a promising and environmentally attractive approach to formation of an ultrafast light-harvesting antenna is to use aqueous ionic assembly of oppositely charged conjugated polyelectrolytes (CPEs). However, to rationally design the excited-state transfer efficiency, the basic physical chemistry of inter CPE assembly must be elucidated. We demonstrate that the delocalization of pi-electrons and the orientation of ionic sidechains relative to the conjugation plane together dictate the complexation kinetics and thermodynamics, and thus the resulting structure. We find that the enthalpy of complexation is substantially larger than previously observed with nonconjugated polyelectrolytes. We further show that the reaction enthalpy changes sign with increasing temperature. We argue that the interaction between the water solvation shell and the polarizable aromatic backbone is key to understanding both the activation energy and the reaction enthalpy.