Demet Gülen

THEORY OF POLARIZED FLUORESCENCE FROM MOLECULAR PAIRS: FÖRSTER TRANSFER AT LARGE ELECTRONIC COUPLING

Polarization properties of the fluorescence from a pair of identical molecules coupled electronically are examined on the basis of a stochastic Liouville equation formalism developed in 1979 by Rahman, Knox and Kenkre. The time development of polarization is calculated for random ensembles of rigid molecule pairs under initial conditions that represent either selective excitation or broad‐band coherent excitation. We hold that the applicability of the Forster mechanism is not limited to cases of weak coupling, and we indicate the rationale and a method for observing it in cases involving large interaction between transition dipoles.

The quantitative relationship between structure and polarized spectroscopy in the FMO complex of Prosthecochloris aestuarii: refining experiments and simulations

New absorption, linear dichroism (LD) and circular dichroism (CD) measurements at low temperatures on the Fenna—Matthews—Olson complex from Prosthecochloris aestuarii are presented. Furthermore, the anisotropy of fluorescence excitation spectra is measured and used to determine absolute LD spectra, i.e. corrected for the degree of orientation of the sample. In contrast to previous studies, this allows comparison of not only the shape but also the amplitude of the measured spectra with that calculated by means of an exciton model. In the exciton model, the point-dipole approximation is used and the calculations are based on the trimeric structure of the complex. An improved description of the absorption and LD spectra by means of the exciton model is obtained by simply using the same site energies and coupling strengths that were given by Louwe et al. (1997, J Phys Chem B 101: 11280–11287) and including three broadening mechanisms, which proved to be essential: Inhomogeneous broadening in a Monte Carlo approach, homogeneous broadening by using the homogeneous line shape determined by fluorescence line-narrowing measurements [Wendling et al. (2000) J Phys Chem B 104: 5825–5831] and lifetime broadening. An even better description is obtained when the parameters are optimized by a global fit of the absorption, LD and CD spectra. New site energies and coupling strengths are estimated. The amplitude of the LD spectrum is described quite well. The shape of the CD spectrum is modelled in a satisfactory way but its size can only be simulated by using a rather large value for the index of refraction of the medium surrounding the chromophores. It is shown that the estimated coupling strengths are compatible with the value of the dipole strength of bacteriochlorophyll a, when using the empty-cavity model for the local-field correction factor.

Optical effects of sodium dodecyl sulfate treatment of the isolated light harvesting complex of higher plants

The light-harvesting complex (LHC) of higher plants isolated using Triton X-100 has been studied during its transformation into a monomeric form known as CPII. The change was accomplished by gradually increasing the concentration of the detergent, sodium dodecyl sulfate (SDS). Changes in the red spectral region of the absorption, circular dichroism (CD), and linear dichroism spectra occurring during this treatment have been observed at room temperature. According to a current hypothesis the main features of the visible region absorption and CD spectra of CPII can be explained reasonably successfully in terms of an exciton coupling among its chlorophyll (Chl) b molecules. We suggest that the spectral differences between the isolated LHC and the CPII may be understood basically in terms of an exciton coupling between the Chl b core of a given CPII unit and at least one of the Chlas of either the same or the adjacent CPII. We propose that this Chl a-Chl b coupling existing in LHC disappears upon segregation into CPII, probably as a result of a detergent-related overall rotation of the strongly coupled Chl b core which changes the relative orientations of the two types of pigments and thus the nature of their coupling.

Chlorophyll transition dipole moment orientations and pathways for flow of excitation energy among the chlorophylls of the major plant antenna, LHCII.

Abstract. We have attempted in this work an assignment of the Qy dipole moment orientations for all the chlorophylls in the major plant antenna, light-harvesting complex II (LHCII). Information that has recently become available through a structural model of the LHCII, site-directed mutagenesis, and spectroscopy of both LHCII and CP29 has been evaluated to model the electronic excited state structure in the presence of chlorophyll-chlorophyll and chlorophyll-protein interactions. An assignment has been obtained which satisfactorily reproduces the polarized linear absorption characteristics. The assignment proposed has also been found to be adequate in reproducing the time scales of the energy transfer processes. The pathways for the flow of excitation energy among the chlorophylls of the complex have been suggested in the context of identity and orientation assignments.

Electronic excited states and excitation transfer kinetics in the Fenna-Matthews-Olson protein of the photosynthetic bacterium Prosthecochloris aestuarii at low temperatures

Abstract The molecular structure-function relationship of the Fenna-Matthews-Olson light-harvesting complex of the photosynthetic green bacterium Prosthecochloris aestuarii has been investigated. It has been assumed that the electronic excited states responsible for the function (transfer of electronic excitation energy) result from the dipole-dipole interactions between the bacteriochlorophyll molecules bound to the polypeptide chain of the complex at a specific three-dimensional geometry. The molecular structure-electronic excited states relationship has been addressed on the basis of simultaneous simulations of several spectroscopic observations. Current electronic excited state models for the Fenna-Matthews-Olson complex have generally been based on obtaining an optimal match between the information contents of the optical steady-state spectra and the bacteriochlorophyll organization. Recent kinetic and spectral information gathered from ultrafast time-resolved measurements have not yet been used effectively for further refinement of the excited state models and for quantification of the relation between the excited states and the energy transfer processes. In this study, we have searched for a model that not only can explain the key features of several steady-state spectra but also the temporal and spectral evolution observed in a recent absorption difference experiment and we have discussed the implications of this model for equilibration of the electronic excitation energy in systems at low temperatures.

Effect of inhomogeneous broadening on the fluorescence anisotropy of a square-symmetric molecule

Abstract Inhomogeneous broadening has the potential of introducing a new picosecond-range quasi-relaxation rate in an electronically twofold-degenerate system such as a molecule whose symmetry is square (D 4h ). The physics of the process is the mutual cancellation of oscillatory contributions whose periods are determined by the random energy splittings. The effect occurs only for quite specific conditions of excitation and line broadening, and may be useful in the interpretation of existing experiments on magnesium tetraphenyl porphyrin.

Optical response of lorentzian nanoshells in the quasistatic limit.

Recently, plexcitonic systems consisting of a plasmonic nanoshell or a core covered by an excitonic shell are engineered. Such systems hold promise for tunable nanophotonic devices for imaging, chemical sensing, and resonance energy transfer. Their plasmonic response is grasped well, while understanding of their excitonic response remains to be improved. To this end, we have developed a methodology in which the functionalities of the dispersive properties of the spherical shell and the nanoenvironment in tuning the optical response are clearly separated. Using this methodology, we have studied the response of the Lorentzian/excitonic nanoshells with optically inactive core and embedding medium and compared it with the well-known properties of the Drude/plasmonics nanoshells. Contrary to Drude nanoshells exhibiting a resonance pair red-shifted with respect to the bulk, Lorentzian nanoshells are identified by a resonance pair blue-shifted with respect to the in-solution excitonic resonance. While the Drude red-shifting is more effective at increasing dielectric constants (core, shell, and embedding medium), the Lorentzian blue-shifting is governed by the excitonic strength and is suppressed at increasing dielectric constants. The implications of the results for manipulating the optical response of plexcitonic systems are briefly discussed.

Significance of the Excitonic Intensity Borrowing in the J-/H-aggregates of Bacteriochlorophylls/Chlorophylls

A quantitative analysis of the excitonic intensity borrowing for the J-/H-aggregates of the bacteriochlorophylls/chlorophylls (BChls/Chls) in specific, and of porphyrins in general, is presented. The analysis is based on the argument that the mixing between the two energetically well-separated bands, such as the Q and B bands of BChls/Chls, should be considered important if the aggregated system possesses an excitonic superstate. A remarkably simple explanation of the significance of the excitonic intensity borrowing is given: superhyperchromism is manifested by the mediation of interband coupling between the superstates in␣the two well-separated bands of such aggregates. A comprehensive discussion on the significance of superhyperchromism and on its size-dependence is provided in connection with its effects on the absorption spectra of the BChl/Chl J- and H-aggregates.

Theory of Picosecond-Laser-Induced Fluorescence from Highly Excited Complexes with Small Numbers of Chromophores

The problem of singlet excitation kinetics and dynamics, especially at high excitation intensities, among a small number of chromophores of a given system has been addressed. A specific scheme for the kinetics is suggested and applied to CPII, a small chlorophyll (Chl)a/b antenna complex the fluorescence lifetime of which has been reported to be independent of excitation intensity over a wide intensity range of picosecond pulses. We have modeled the kinetics from the point of view that Chla molecules in CPII are Förster coupled so that a second excitation received by the group of Chlas either creates a state with two localized excitons or raises the first one to a doubly excited state. The data on CPII can be understood on the basis of a kinetic model that does not exclude exciton annihilation during the excitation pulse. The implied annihilation rate is consistent with our theoretical estimates of that rate obtained by applying excitation transfer theory to pairs of molecules both initially excited.

Optical response of Lorentzian nanospheres in the quasistatic limit

Recently, plexcitonic systems consisting of a plasmonic nanoshell or a core covered by an excitonic shell are engineered. Such systems hold promise for tunable nanophotonic devices for imaging, chemical sensing, and resonance energy transfer. Their plasmonic response is grasped well, while understanding of their excitonic response remains to be improved. To this end, we have developed a methodology in which the functionalities of the dispersive properties of the spherical shell and the nanoenvironment in tuning the optical response are clearly separated. Using this methodology, we have studied the response of the Lorentzian/excitonic nanoshells with optically inactive core and embedding medium and compared it with the well-known properties of the Drude/plasmonics nanoshells. Contrary to Drude nanoshells exhibiting a resonance pair red-shifted with respect to the bulk, Lorentzian nanoshells are identified by a resonance pair blue-shifted with respect to the in-solution excitonic resonance. While the Drude ...