Exciton reactions in ultrathin molecular wires, filaments and pores: A case study of kinetics and self-ordering in low dimensions

Abstract

When are molecular wires thin enough to show one-dimensional exciton kinetics? Cylindrical naphthalene wires ( 5-5000 nm radius) show a definite one- to three-dimensional transition (about 25 nm for triplet excitons at 4 K; 40 nm at 77 K). Nuclear channel pore membranes (polycarbonate) serve as templates and calibrators. The triplet exciton migration (multiple hopping) length is 50-100 molecules. The closest neighbor distribution is used as an order criterion. Steady-state simulations of the reactions:A+A-->0 and A+A-->A in low-dimensional media give non-random reactant distributions, e.g. Wigner-like rather than Poissonian spacings in one dimension. Effectively, a dynamic, quasi-ordered superlattice of excitations is created (excitation grating with a 5 nm spacing). Experimental verification involves a new technique: excitation time modulation. Porous glass (Vycor) is shown to have an effective one-dimensional channel topology. Naphthalene powder and isotopic alloys also show nonrandom steady-state exciton distributions.