Energetics and Scattering of Mixed Exciton–Photon States in Organic Crystals

Johnson, Carey K.; Small, Gerald J.

doi:10.1016/b978-0-12-227206-6.50007-7

Cited by 5 publications

(5 citation statements)

References 112 publications

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“…It was shown that the L−E patterns and their associated permanent dipole moments are important for interpretation of electric field effects on the B850 absorption band. Such patterns as well as the corresponding wave functions are required for calculation of phonon-induced interexciton-level relaxation rates, − a firm understanding of exciton delocalization or extendedness, and superradiance from single or ensembles of complexes. , The superradiance enhancement is defined as the radiative dipole strength of the exciton level divided by that of the isolated chromophore. Monshouwer et al measured an enhancement of 2.8 for the A (B870) level of an ensemble of LH2 complexes at 4.2 K. Since this level is essentially forbidden for perfect C n symmetry, the superradiance is the result of energy disorder which also leads to clump localization of the A level.…”

Section: Discussionmentioning

confidence: 99%

Symmetry-Based Analysis of the Effects of Random Energy Disorder on the Excitonic Level Structure of Cyclic Arrays: Application to Photosynthetic Antenna Complexes

and

Small

1998

J. Phys. Chem. B

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We extend the work of Wu and Small who introduced symmetry-adapted basis defect patterns (BDPs) for systematic analysis of the effects of random static energy disorder (diagonal or off-diagonal) on the excitonic level structure of cyclic C n arrays of chromophores. Examples include the B850 and B875 rings of bacteriochlorophyll (BChl) molecules associated with the LH2 and LH1 antenna complexes of purple bacteria for which, so far, n = 8, 9, and 16. Calculation of the localization/extendedness (chromophore occupation number) patterns and participation numbers of the exciton levels reveals that the effects of random disorder on the exciton levels that contribute to the critically important B850 or B875 absorption band can be understood in terms of a single BDP of e1 symmetry (“hidden correlation” effect). This is a consequence of the structures of the complexes, the complexes falling in the weak disorder regime and the strict symmetry selection rules that govern the couplings between zero-order exciton levels by BDPs. The above finding greatly simplifies computational studies on the effects of random disorder on the spectroscopic properties of the above bands. Our results show that interpretation of electric field (Stark) effects on the B850 and B875 absorption bands must include the effects of energy disorder. Similarities and differences between the effects of diagonal and off-diagonal energy disorder are discussed as is the relevance of our findings to the superradiant properties of the LH complexes.

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Section: Discussionmentioning

confidence: 99%

Symmetry-Based Analysis of the Effects of Random Energy Disorder on the Excitonic Level Structure of Cyclic Arrays: Application to Photosynthetic Antenna Complexes

and

Small

1998

J. Phys. Chem. B

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“…When excitonic couplings dominate the electronic dephasing from bath interactions, Förster theory is inapplicable. In this case interexciton level relaxation requires modulation of intermolecular interactions by librational or translational motions of the chromophores as discussed in the book Davydov (for reviews see refs and ). This mechanism has been invoked to explain the ∼200 fs excitation energy relaxation dynamics of the aforementioned FMO complex. , …”

Section: Introductionmentioning

confidence: 99%

Comparison of the LH2 Antenna Complexes of Rhodopseudomonas acidophila (Strain 10050) and Rhodobacter sphaeroides by High-Pressure Absorption, High-Pressure Hole Burning, and Temperature-Dependent Absorption Spectroscopies

et al. 1997

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The cyclic (C n ) light harvesting 2 (LH2 or B800−850) complexes of Rps. acidophila (strain 10 050) and Rb. sphaeroides, isolated under identical conditions, are compared using the title spectroscopies. Thermal broadening and shifting data for the B850 absorption band reveal a structural change near 150 K for both species in the glycerol:water solvent used. The linear regions of thermal broadening above and below this temperature are shown to be consistent with dephasing via phonon-assisted relaxation between the B850 ring's exciton levels, which contribute to the B850 absorption band. The theoretical model used predicts, for both species, that the nearest neighbor coupling(s) between bacteriochlorophyll a (BChl a) molecules of the B850 ring is (are) significantly stronger, ca. 35%, for the low-temperature structures. Moreover, the linear thermal broadening rates of Rb. sphaeroides are significantly lower than those of Rps. acidophila for both the low- and high-temperature regions. Analysis of the difference in rates with the above model indicates that the nearest neighbor BChl a−BChl a coupling(s) is ca. 20% weaker for Rb. sphaeroides at all temperatures. The observation that the thermal shift rate for the B850 band of Rb. sphaeroides is 2.2 times smaller than that of Rps. acidophila is consistent with this weaker coupling. Pressure shift data for the B800 band indicate that the compressibility (κ) for Rb. sphaeroides is significantly larger than for Rps. acidophila, suggesting that the weaker excitonic coupling between B850 molecules of Rb. sphaeroides stems, at least in part, from looser packing of its α,β-polypeptide pairs. A higher κ value for Rb. sphaeroides provides an explanation for the observation that the linear rates for pressure broadening and shifting of the B850 band for the two species are similar. Although the pressure- and temperature-dependent data for the B800 band of both species are consistent with weak excitonic coupling between nearest neighbor B800 molecules, the data for the B850 band (including pressure shifting of zero-phonon holes burned into the lowest exciton level of the B850 ring (B870)) require interpretation in terms of strong coupling. Although large, the pressure-shift rate for B870 holes burned on the high-energy side of the B870 band (−0.46 cm-1/MPa) is a factor of 1.3 lower than on the low-energy side. An interpretation for this variation in terms of energy disorder is given. Zero-phonon hole action spectra (4.2 K) for the B870 exciton level are presented that yield similar inhomogeneous widths for the B870 band of both species, ∼120 cm-1. For both species the apparent displacement of this band below the maximum of the B850 band is close to 200 cm-1. A theoretical discussion of the relationship between the apparent displacement and excitonic level structure in the absence of energy disorder is given in the accompanying paper.

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“…In the case of strong coupling, the zero-order states in the matrix element are delocalized (excitonic) and defined by a Hamiltonian that contains the static intermolecular potential energy. Thus, relaxation between the delocalized exciton levels is triggered by the dependence of the intermolecular potential energy on, for example, librational motions of the chlorophyll (Chl) pigments that modulate the excitonic interactions. − Such a mechanism for aromatic molecular crystals has been extensively studied, both experimentally and theoretically. − It appears to be important for the ultrafast (∼100 fs) downward interexciton-level relaxations observed for the B850 and B875 BChl a rings of the LH1 and LH2 complexes of purple bacteria …”

Section: Introductionmentioning

confidence: 99%

Chlorophyll a Franck−Condon Factors and Excitation Energy Transfer

1999

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The Franck−Condon factors for the S1(Q y ) ↔ S0 electronic transition of chlorophyll (Chl) molecules are important for understanding excitation energy transfer in photosynthetic complexes. Currently, there are two sets of Chl a Franck−Condon factors for over 40 modes, one determined by spectral hole burning and the other by fluorescence line narrowing. Those obtained by the latter spectroscopy are, on average, a factor of 30 times smaller than the hole burning values. Nonline-narrowed fluorescence results for the light-harvesting complex II of photosystem II at 4.2 K are presented that agree quite well with the hole burning but not the fluorescence line narrowing values.

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Energetics and Scattering of Mixed Exciton–Photon States in Organic Crystals

Cited by 5 publications

References 112 publications

Symmetry-Based Analysis of the Effects of Random Energy Disorder on the Excitonic Level Structure of Cyclic Arrays: Application to Photosynthetic Antenna Complexes

Symmetry-Based Analysis of the Effects of Random Energy Disorder on the Excitonic Level Structure of Cyclic Arrays: Application to Photosynthetic Antenna Complexes

Comparison of the LH2 Antenna Complexes of Rhodopseudomonas acidophila (Strain 10050) and Rhodobacter sphaeroides by High-Pressure Absorption, High-Pressure Hole Burning, and Temperature-Dependent Absorption Spectroscopies

Chlorophyll a Franck−Condon Factors and Excitation Energy Transfer

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