Subproject: Spatial and Spectral Control of Excitons within Quantum Dot-Molecule Assemblies via Coupled Energy Transfer Processes

This document describes a proposal to obtain a mechanistic understanding of space-separated coupled energy transfer (ET) processes within excitonic materials – specifically, assemblies of semiconductor nanocrystals (quantum dots, QDs) and organic molecules – and to use this understanding to design systems into which we control the spatial and spectral distribution of excitons within the system as a function of time. Space-separated coupled ET is energy transfer involving two or more QDs or molecular chromophores in which the number of excitons increases by multiplicative downconversion (“quantum cutting”), or decreases by upconversion (“energy pooling”). The proposed work has two main goals: Goal 1: to use new spectroscopic and theoretical approaches – specifically two-dimensional electronic spectroscopy and real-time quantum mechanics coupled to molecular dynamics – to measure and predictively model the rates of space-separated coupled energy transfer processes (up- and down-conversion processes) in solution-phase and thin-film assemblies of QDs and organic ligands, and to determine how these mechanisms depend on the chemical structure of the assembly and the mode of excitation. There exist several possible mechanisms for coupled ET processes in various materials (including rare-earth ions and organic chromophores), but none offer direct spectroscopic evidence of these mechanisms. In our studies, we will use the general mechanisms presented in the literature as a guide. Our proposed work, however, is fundamentally different than previous studies in any of the fields listed above because we will study multi-chromophore up- and down-conversion processes with a powerful and unprecedented combination of (i) rationally-designed QD-molecule assemblies (where the molecules serve as structural scaffolds, optical components, electronic couplers, and/or passivating agents), (ii) two-dimensional electronic spectroscopy, which provides direct measurements of multi-exciton states, their lifetimes, and their couplings to other states in the electronic manifold, (iii) mixed classical and quantum mechanical molecular dynamics simulations of particle self-assembly, and (iv) electronic structure and electrodynamic modeling to provide a quantitative interpretation of our experiments. Goal 2: to use our mechanistic understanding of space-separated coupled ET processes to design assembly architectures and chemistries to achieve previously unrealized coupled ET schemes, for example: the splitting of a high-energy exciton into two or more excitons of different energies, the splitting of an exciton into two excitons across a heterogeneous (organic-inorganic) interface, and the pooling of broadband absorption into a single high-energy chromophore. We expect this work to impact the development of new materials for (i) enhanced optical amplification processes for optical fiber-based communications and lasing; (ii) “optical cryptography”, where the structure of the QD-molecule assembly encodes information within the spatial arrangement of the nanoparticles, within the chemical structure of the cross-linking ligands, or within some chemical or electronic gradients that we design; and (iii) enhanced energy harvesting, carrier gener