We present a model of excited-state dynamics in dimerized molecular crystals such as α-perylene. The essential element of the exciton-phonon interaction in this case is the modulation of resonance integrals by the relative motion of the molecules engaging in excitation transfer. Eigenstates are found for a substantial part of the model Hamiltonian and a small remainder is treated perturbatively. The eigenstates are identified with the excimer and Y states found in α-perylene, and a fit of experimental data is performed on this basis. Experimental quantities considered include Stokes shifts, Y-state lifetime, and fluorescence linewidths.