An extended Förster theory (EFT), previously derived (L. B.-Å. Johansson et al. J. Chem. Phys., 1996,105) has theoretically been adapted and used in simulations of donor-acceptor energy transfer (DAET), which is a process often referred to as FRET. It was shown that the classical Förster theory is only valid in the initial part of the fluorescence decay. In this thesis an EFT is derived and outlined for electronic energy transport between two fluorescent molecules which are chemically identical, but photophysically non-identical. The energy migration within such asymmetric pairs is partially reversible and therefore referred to as partial donor-donor energy migration (PDDEM). The previously derived model of PDDEM (S. V. Kalinin et al. Spectrochim Acta Part A, 2002,58) is an approximation of the EFT. In particular, the EFT accounts for the time-dependent reorientations as well as the distance that influence the rate of electronic energy migration. The reorientation of the fluorophores transition dipole moments has been simulated using Brownian dynamics. As a result, the related “k2-problem” has been solved. The EFT of PDDEM has also been studied regarding the effect of PDDEM on experimental observables e.g. quantum yield of fluorescence and steady-state anisotropies

A kinetic model of electronic energy migration within pairs of photophysically non-identical fluorophores has been developed. The model applies to fluorescent groups that exhibit different photophysical and spectral properties when attached to different positions in a macromolecule. The energy migration within such asymmetric pairs is partially reversible, which leads to the case of partial donor-donor energy migration (PDDEM). The model of PDDEM is an extension of the recently developed donor-donor energy migration model (DDEM, F. Bergström et al, PNAS 96 (1999) 12477), and applies to quantitative measurements of energy migration rates and distances within macromolecules. One important distinction from the DDEM model is that the distances can be obtained from fluorescence lifetime measurements. A model of fluorescence depolarisation in the presence of PDDEM is also presented. To experimentally test the PDDEM approach, different model systems were studied. The model was applied to measure distances between rhodamine and fluorescein groups within on-purpose synthesised molecules that were solubilised in lipid bilayers. Moreover, distances were measured between BODIPY groups in mutant forms of the plasminogen activator inhibitor of type 2 (PAI-2). Measurements of both the fluorescence intensity decays and the time-resolved depolarisation were performed. The obtained distances were in good agreement with independent determinations. Finally, the PDDEM within pairs of donors is considered, for which both donors exhibit a nonexponential fluorescence decay. In this case it turns out that the fluorescence relaxation of a coupled system contains distance information even if the photophysics of the donors is identical. It is also demonstrated that the choice of relaxation model has a negligible effect on the obtained distances. The latter conclusion holds also for the case of donor-acceptor energy transfer.