  We propose a single effective Hamiltonian to describe the low-energy
electronic structure of a series of symmetric cationic diarylmethanes, which
are all bridge-substituted derivatives of Michler's Hydrol Blue. Three-state
diabatic Hamiltonians for the dyes are calculated using four-electron
three-orbital state-averaged complete active space self-consistent field and
multi-state multi-reference perturbation theory models. The approach takes
advantage of an isolobal analogy that can be established between the orbitals
spanning the active spaces of the different substituted dyes. The solutions of
the chemical problem are expressed in a diabatic Hilbert space that is
analogous to classical resonance models. The effective Hamiltonians for all
dyes can be fit to a single functional form that depends on the mixing angle
between a bridge-charged diabatic state and a superposition representing the
canonical resonance. We find that the structure of the bridge-charged state
changes in a regular fashion across the series. The change is consistent with
an inversion of the sign of the charge carrier on the bridge, which changes
from an electron pair to a hole as the series is traversed.
