The anomalous Stark effect of single terrylene molecules in p-terphenyl crystals

Single terrylene molecules embedded in a p-terphenyl crystal behave like well-defined optical switches. Recent measurements of the shift of their optical lines under an applied electric field (Stark effect) have revealed surprising anomalies: unexpectedly large and widely spread differences of polarizability between the ground and excited states, even in crystals of excellent quality. Some molecules even had a greater polarizability in the ground than in the excited state, We propose to explain these results by a symmetry breaking in the supermolecule (including the terrylene molecule and the first shells of p-terphenyl neighbours), leading to two distorted conformations with opposite dipole moments, Tunnelling between these conformations gives rise to a quadratic Stark effect with a large effective polarizability. The model is internally consistent, and provides reasonable orders of magnitude for the tunnel barrier. We argue that this tunnelling involves correlated movements of groups of atoms, 1 nm apart, and therefore the system is particularly sensitive to small residual strains in otherwise perfect crystals. (C) 2000 Elsevier Science B.V. All rights reserved.