crossover from tunneling to hopping

It was reported in Science [1] of observing directly the crossover in energy transfer from single step tunneling to multi-step hopping in a donor-bridge-acceptor molecular structure (DBA). The electrons carry energy from D to A by passing through the B. This passage can be via tunneling or hopping, depending on the bridge length.

Triplet energy transfer (TT), a key process in molecular and organic electronics, generally occurs by either strongly distance-dependent single-step tunneling or weakly distance-dependent multistep hopping. We have synthesized a series of p-stacked molecules consisting of a benzophenone donor, one to three fluorene bridges, and a naphthalene acceptor, and studied the rate of TT from benzophenone to naphthalene across the fluorene bridge using femtosecond transient absorption
spectroscopy. We show that the dominant TT mechanism switches from tunneling to wire-like hopping between bridge lengths 1 and 2. The crossover observed for TT can be determined by direct observation of the bridge-occupied state.

[1] SCIENCE VOL 328 18 JUNE 2010