Energy Pooling Upconversion in Organic Molecular Systems

Abstract

A combination of molecular quantum electrodynamics, perturbation theory, and ab initio calculations was used to create a computational methodology capable of estimating the rate of 3-body singlet upconversion in organic molecular assemblies. The approach was applied to quantify the conditions under which such relaxation rates, known as energy pooling, become meaningful for two test systems: stilbene-fluorescein and hexabenzocoronene-oligothiophene. Both exhibit low intra-molecular conversion but inter-molecular configurations exist in which pooling efficiency is at least 90% when placed in competition with more conventional relaxation pathways. For stilbenefluorescein, the results are consistent with data generated in an earlier experimental investigation. Exercising these model systems facilitated the development of a set of design rules for the optimization of energy pooling.2

Experimental demonstration of photon upconversion via cooperative energy pooling

“…Incorporating the experimental results above into existing analyses of the CEP15 and 2PA (ref. 32) processes suggests guidelines for optimizing the selection of sensitizer and acceptor chromophores to maximize the CEP rate ( k CEP ), as follows: The acceptor should have a large 2PA cross-section () as well as minimal FRET losses to the sensitizers, and it should exhibit minimal self-quenching in the solid state.…”

“…CEP is the process of two photoexcited sensitizer chromophores non-radiatively transferring their energy to a single higher-energy state in an acceptor chromophore. Instead of dipole–dipole coupling between the emissive states of the sensitizers and the absorbing state of the acceptor, as in the Förster resonance energy transfer process (FRET), CEP is carried out via a coupling of the emissive states of both sensitizers with the two-photon absorption (2PA) tensor of the acceptor131415. In this way the sensitizers act as photon storage centres that relax the stringent temporal and spatial constraints for achieving 2PA in the acceptor, enabling upconversion with greater efficiency and at reduced excitation intensities.…”

“…Theoretical work modelling three-body FRET processes1314 in the late 1990s laid the foundation for a quantum electrodynamical understanding of the CEP process and more recent computational work15 has highlighted the strong dependence of the CEP process on both the separation distance and relative orientations of sensitizer and acceptor chromophores. Experimental work in the 1990s by Nickoleit et al 16.…”

Electric dipole coupling in optical cavities and its implications for energy transfer, up-conversion, and pooling

“…RET, ETU and EP are all based on the same dipole coupling tensor, and donor-acceptor separation sensitivity dwells exclusively there for each process 5,26 . For the RET and ETU, the rate is proportional to the square of the coupling tensor while the EP rate is proportional to its fourth power.…”

Symmetry analysis of Raman scattering mediated by neighboring molecules

“…In recent work 5 we have developed a framework of quantum electrodynamics (QED) 6,7 to address Raman scattering influenced by a second, neighboring molecule; this is one of many situations in which such a theory of fundamental coupling is applied. [8][9][10][11][12][13][14][15][16] Our interest is not only in the modification of line intensities (beyond the familiar heterogeneous spectral broadening): we have shown that it is possible to engender additional spectra lines that are not conventionally Raman active, identifying the constraints that apply. The symmetry aspects of the latter feature are the focus of the present work.…”