We present quantum Monte Carlo calculations of the effective rotational constant B of several cromophore molecules embedded in He clusters, as a function of the cluster size. The simulation results, in close agreement with available experimental data, offer insight into the relationship between structural and dynamical properties and the onset of superfluidity. Furthermore they reliably extend our knowledge of doped clusters to sizes where the assignment of measured spectral lines has not been possible. The convergence to the asymptotic B value in large He droplets reveals unexpected trends: it is slow (fast) for heavy (light) dopant molecules. This reverses the presently accepted scenario, with implications on models of correlation between the impurity and the quantum solvent, currently adopted to characterize the dynamics of a rotor in a superfluid host.
