  We show that molecular aggregation can strongly influence the nonradiative
decay (NRD) lifetime of an electronic excitation. As a demonstrative example,
we consider a transition-dipole-dipole-interacting dimer whose monomers have
harmonic potential energy surfaces (PESs). Depending on the position of the NRD
channel ($q_{\rm nr}$), we find that the NRD lifetime ($\tau_{\rm nr}^{\rm
dim}$) can exhibit a completely different dependence on the
intermolecular-interaction strength. We observe that (i) for $q_{\rm nr}$ near
the Franck-Condon region, $\tau_{\rm nr}^{\rm dim}$ increases with the
interaction strength; (ii) for $q_{\rm nr}$ near the minimum of the monomer
excited PES, the intermolecular interaction has little influence on $\tau_{\rm
nr}^{\rm dim}$; (iii) for $q_{\rm nr}$ near the classical turning point of the
monomer nuclear dynamics, on the other side of the minimum, $\tau_{\rm nr}^{\rm
dim}$ decreases with the interaction strength. Our findings suggest design
principles for molecular systems where a specific fluorescence quantum yield is
desired.
