Date of Award

2026-08-01

Degree Name

Master of Science

Department

Physics

Advisor(s)

Tunna Baruah

Abstract

Excitation energy transfer (EET) is a fundamental process in photosynthetic systems and molecular photonic devices, where the efficient transport of electronic excitation between donor and acceptor chromophores determines the overall performance of the system. The electronic coupling V_DA between the transition densities of the two fragments is the central quantity governing this process in the weak-coupling (Förster) regime, and its accurate numerical evaluation is the main objective of this work. Two independent computational implementations of the Coulomb interaction between transition densities were developed and applied to a set of five anthracene-BODIPY donor-acceptor dyads, studied under two geometric configurations: one preserving the covalent linker connecting the two chromophores (Model II), and one with the linker removed while keeping the fragment positions fixed (Model I). The first implementation, adopts a supermolecular perspective in which the complete donor-acceptor system is treated as a single quantum entity and the coupling is extracted from a constrained excitation calculation within density functional theory. The second treats the donor and acceptor fragments independently in separate self-consistent calculations and evaluates the coupling through an explicit numerical Coulomb integration between the transition densities of the two fragments, using a real-space Poisson solver and a mesh translation procedure that allows the coupling to be evaluated at multiple intermolecular separations from a single set of calculations. The two implementations produce coupling values that are mutually consistent and in qualitative agreement with high-level DFT/MRCI reference data across the full set of systems. The results show that the covalent linker connecting the two chromophores acts primarily as a structural spacer rather than as an electronic mediator, and that the anthracene donor has a consistently larger excitation energy than all BODIPY acceptors, establishing the energetic driving force for unidirectional energy transfer. The methodology was further extended to the coherent regime, where the coupling values were used to propagate excitation in a three-site anthracene chain and reveal the interplay between nearest-neighbor and second-neighbor interactions at different intermolecular distances. The asymptotic distance dependence of the coupling was also verified, confirming the expected R^-3 behavior predicted by the dipole-dipole limit at large separations. The present work provides a validated computational framework for the evaluation of electronic couplings in donor-acceptor systems relevant to EET. The electronic coupling evaluated here constitutes one of the two key ingredients of the Förster energy transfer rate; its combination with the spectral overlap integral, which requires the full optical spectra of each fragment, represents the natural continuation of this work toward a complete first-principles description of the energy transfer rate.

Language

en

Provenance

Received from ProQuest

File Size

97 p.

File Format

application/pdf

Rights Holder

Irving Alejandro Lopez Ruiz

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