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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.
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