Energy migration in molecular assemblies: the characterisation and differentiation of two-photon mechanisms

Abstract

Energy migration between chromophores plays a prominent role in a range of energy harvesting assemblies. Recent advances in the design and production of light-harvesting polymers have led to the synthesis of novel two-photon absorbing dendrimers. To construct increasingly efficient multifunctional macromolecules of this type, understanding the inherent optical processes and disentangling them has become imperative. This paper explores the fundamental processes by means of which energy transfers from a donor chromophore to an acceptor through two-photon absorption from an input laser beam. It is determined that three distinct classes of mechanism can operate: (i) two-photon absorption by individual chromophores is followed by transfer of the energy to an acceptor group; (ii) a singly excited chromophore is excited to a virtual state by the additional absorption of a photon from the pump radiation field, coupled with resonance energy transfer to the acceptor, or; (iii) two-photon excitation of the acceptor results from acquisition of one quantum of energy from a singly excited neighbour group and another from the throughput radiation. These mechanisms may compete and, in certain cases, lead to manifestations of quantum interference. Generally, the most favoured mechanism is determined by a balance of factors and constraints. Principal amongst the latter are the choice of wavelength (connected with the possibility of exploiting certain electronic resonances, whilst judiciously avoiding others) and the precise chromophore architecture (taking account of geometric factors concerned with the relative orientation of transition moments). As the relative importance of each mechanism determines the key nanophotonic characteristics of the assembly, the principles and results reported here afford the means for expediting highly efficient two-photon energy migration.

Two-Photon Absorbing Dendrimers and Their Properties—An Overview

“…Theoretical studies of the energy migration processes in dendrimers after two-photon excitation, and thus of their light-harvesting properties, were carried out with the objective of clarifying the different mechanisms of energy transfer that are involved in these multichromophoric molecules [ 54 , 55 ]. Experimentally, dendrimers of the type 5-G n ( Figure 3 ), with two or four donor TPA chromophores at the periphery playing the role of antennas and a Nile Red acceptor at the core, were studied for their light-harvesting properties [ 56 , 57 ].…”