Modeling of fluorescence line-narrowed spectra in weakly coupled dimers in the presence of excitation energy transfer

Abstract

This work describes simple analytical formulas to describe the fluorescence line-narrowed (FLN) spectra of weakly coupled chromophores in the presence of excitation energy transfer (EET). Modeling studies for dimer systems (assuming low fluence and weak coupling) show that the FLN spectra (including absorption and emission spectra) calculated for various dimers using our model are in good agreement with spectra calculated by: (i) the simple convolution method and (ii) the more rigorous treatment using the Redfield approach [T. Renger and R. A. Marcus, J. Chem. Phys. 116, 9997 (2002)]. The calculated FLN spectra in the presence of EET of all three approaches are very similar. We argue that our approach provides a simplified and computationally more efficient description of FLN spectra in the presence of EET. This method also has been applied to FLN spectra obtained for the CP47 antenna complex of Photosystem II reported by Neupane et al. [J. Am. Chem. Soc. 132, 4214 (2010)], which indicated the presence of uncorrelated EET between pigments contributing to the two lowest energy (overlapping) exciton states, each mostly localized on a single chromophore. Calculated and experimental FLN spectra for CP47 complex show very good qualitative agreement.

Fluorescence Line Narrowing and Δ-FLN Spectra in the Presence of Excitation Energy Transfer between Weakly Coupled Chromophores

“…Only when interpigment coupling is small ( V < 5 cm –1 ) will donor pigments contribute noticeably to the FLN spectra. In the models attempting to treat excitation via EET employed so far, ,− with the exception of ref , the probability of emission (or SHB) was assumed to be zero for all pigments other than the lowest-energy ones in the complex. While this is a reasonable assumption for interpigment coupling V ≫ 5 cm –1 (but then the applicability of the Förster approximation becomes questionable) and irrelevant for V = 0, it is invalid for small but nonzero interpigment couplings.…”

“…We point out that this function reflects only the dependence of the EET probability on the ZPL-ZPL energy gap, but does not take into account the probabilities of the various gaps. Importantly, for V < 10 cm –1 this is not a step function as was employed in refs and − .…”

“…EET was suggested as a possible reason for these observations without, however, the effects of EET having been quantified. The first attempt at quantitative modeling of FLN spectra in the presence of EET was reported recently . However, the authors of ref did not attempt to model the Δ-FLN spectra.…”

“…The first attempt at quantitative modeling of FLN spectra in the presence of EET was reported recently . However, the authors of ref did not attempt to model the Δ-FLN spectra. Their approach to modeling was also oversimplified in that they assumed that donor molecules do not emit light or exhibit SHB, and that they transfer energy downhill to acceptor molecules with 100% probability, independent of the wavelength of the donor or the acceptor.…”

“…While these assumptions are reasonable for moderate interpigment coupling and fast EET, they are inappropriate in the weak coupling case when EET times are comparable to fluorescence lifetimes. Additionally, the possibility of lower-energy pigments becoming higher-energy pigments and vice versa as a result of nonphotochemical spectral hole burning (NPHB) was not included in ref . Recently, we extended the NPHB model described in detail in refs − to properly include the wavelength-dependent distributions of EET rates and spectral hole burning following EET .…”