Size Enhancement of Transition Dipoles to One- and Two-Exciton Bands in a Photosynthetic Antenna

Abstract

Measurement of nonlinear absorption and differential optical density spectrum as well as fluorescence behavior of the light harvesting antenna LH2 from purple bacterium Rhodobacter sphaeroides indicates exciton delocalization over 16 6 4 bacteriochlorophyll-a molecules, corresponding to probably the full physical length of the circular aggregate responsible for the 850 nm absorption band. The giant dipole moments for the consecutive transitions ground state ! one-exciton band ! two-exciton band are 25.5 6 2.5 D and 21.5 6 3.5 D, respectively. [S0031-9007(96)

Exciton (De)Localization in the LH2 Antenna of Rhodobacter sphaeroides As Revealed by Relative Difference Absorption Measurements of the LH2 Antenna and the B820 Subunit

“…We conclude that the calculated dipole strength of the lowest level, i.e., 2.41, is in good agreement with the experimental value of 2.8 revealed by the superradiance data. 9 But the dipole strength of 16 suggested by Leupold and co-workers 22 is difficult to interpret, because the upper limit for a circular aggregate of 18 BChls (with the in-plane orientation of transition dipoles) is equal to 9. For the disordered aggregate for our parameter set this value is 6-7.…”

“…9 Nonlinear absorption measurements have revealed the giant absorption cross-section in the B850 band of the same complex. 22,23 The corresponding dipole strength was estimated as 16 ( 4 (in the units of the monomeric dipole strength). 22 According to our model the B850 absorption band is determined by 3-5 of the lowest exciton levels (the k ) 0, (1, and (2 levels).…”

“…22,23 The corresponding dipole strength was estimated as 16 ( 4 (in the units of the monomeric dipole strength). 22 According to our model the B850 absorption band is determined by 3-5 of the lowest exciton levels (the k ) 0, (1, and (2 levels). The dipole strengths of these levels are 2.41, 6.40, 6.52, 0.95, and 0.68 (in increasing order of energy).…”

“…5 These reports have stimulated new studies of the excitonic structure of these light-harvesting complexes [6][7][8][9][10][11][12][13][14][15][16][17] including experimental studies and theoretical simulations of the nonlinear optical response of the antenna. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] However, the dynamics of electronic excitations in the antenna is still the subject of intensive debates. It is clear that the BChl arrangement in the form of a tightly-packed high-symmetry aggregate tends to delocalize electronic wavefunctions, but the intrinsic static and dynamic disorder of a pigment-protein complex may give rise to more localized states.…”

Local and Nonlocal Contributions to the Linear Spectroscopy of Light-Harvesting Antenna Systems

“…Estimates of the size of the exciton based on various experiments vary between a dimer and at least half the LH2 ring. [17][18][19][20][21][22][23][24][25] The timedependent properties of the antenna systems are interpreted in terms of a small hopping exciton: excitation of the system rapidly leads to a localized quasi particle, which by a random walk hops to neighboring sites [26][27][28][29][30] and eventually to the special pair of the RC. The localization is thought to be the result of energy disorder and/or electron-phonon coupling.…”