Electronic coupling mechanisms and characteristics for optically nonlinear photoactive nanomaterials

In a range of nanophotonic energy harvesting materials, resonance energy transfer (RET) is the mechanism for the intermolecular and intramolecular transfer of electronic excitation following the absorption of ultraviolet/visible radiation. In the nonlinear intensity regime, suitably designed materials can exhibit two quite different types of mechanism for channeling the excitation energy to an acceptor that is optically transparent at the input frequency. Both mechanisms are associated with two-photon optical excitation - of either a single donor, or a pair of donor chromophores, located close to the acceptor. In the former case the mechanism is two-photon resonance energy transfer, initiated by two-photon absorption at a donor, and followed by RET directly to the acceptor. The probability for fulfilling the initial conditions for this mechanism (for the donors to exhibit two-photon absorption) is enhanced at high levels of optical input. In the latter twin-donor mechanism, following initial one-photon excitations of two electronically distinct donors, energy pooling results in a collective channeling of their energy to an acceptor chromophore. This mechanism also becomes effective under high intensity conditions due to the enhanced probability of exciting donor chromophores within close proximity of each other and the acceptor. In this paper we describe the detailed balance of factors that determines the favored mechanism for these forms of optical nonlinearity, especially electronic factors. Attention is focused on dendrimeric nanostar materials with a propensity for optical nonlinearity.