Cătălin Didraga

Optical spectra and localization of excitons in inhomogeneous helical cylindrical aggregates

We study the linear optical properties of helical cylindrical molecular aggregates accounting for the effects of static diagonal disorder. Absorption, linear dichroism, and circular dichroism spectra are presented, calculated using brute force numerical simulations and a modified version of the coherent potential approximation that accounts for finite size effects by using the appropriate open boundary conditions. Excellent agreement between both approaches is found. It is also shown that the inclusion of disorder results in a better agreement between calculated and measured spectra for the chlorosomes of green bacteria as compared to our previous report, where we restricted ourselves to homogeneous cylinders [Didraga, Klugkist, and Knoester, J. Phys. Chem. B 106, 11474 (2002)]. For the excitons that govern the optical response, we also investigate the disorder-induced localization properties. By analyzing an autocorrelation function of the exciton wave function, we find a strongly anisotropic localization behavior, closely following the properties of chiral wave functions which previously have been found for homogenoeus helical cylinders [Didraga and Knoester, J. Chem. Phys. 121, 946 (2004)]. It is shown that the circular dichroism spectrum may still show a strong dependence on the cylinder length, even when the exciton wave function is localized in a region small compared to the cylinders size.

Exchange narrowing in circular and cylindrical molecular aggregates: degenerate versus nondegenerate states

Abstract We study the optical absorption line shape of circular and cylindrical molecular aggregates with static Gaussian diagonal and off-diagonal disorder. We focus on the weak-disorder situation and analyze the effect of exchange narrowing of the disorder due to exciton delocalization. In particular, we show that the absorption lines originating from pairs of states that are degenerate in the ordered aggregate (e.g., the k=±1 pair in rings) is always broader than the one originating from a nondegenerate state. If we only account for diagonal disorder, this difference amounts to a factor of 2 . Moreover, in contrast to the nondegenerate case, the degenerate states give rise to a nonGaussian absorption profile that may even reflect its origin in two states through the emergence of a double-peak structure. The latter occurs if the off-diagonal disorder dominates and if the circumference of the ring or the cylinder is small enough (

Absorption and dichroism spectra of cylindrical J aggregates and chlorosomes of green bacteria

We study the absorption and linear and circular dichroism spectra of molecular aggregates having the shape of a cylinder. Examples are the chlorosomes of green bacteria and recently synthesized cyanine J aggregates, called amphipipes. We use a Frenkel exciton model and exploit the cylinder symmetry to separate the exciton states into bands with different transverse wavenumbers. Only three of these bands are optically active. The delicate interference between the contributions of these three bands to the CD spectrum, make this spectrum very sensitive to system parameters, such as the cylinder length. This offers a natural explanation for the observed strong variation of the CD spectra of chlorosomes with sample preparation technique.

Chiral exciton wave functions in cylindrical J aggregates

We study the exciton wave functions and the optical properties of cylindrical molecular aggregates. The cylindrical symmetry allows for a decomposition of the exciton Hamiltonian into a set of effective one-dimensional Hamiltonians, characterized by a transverse wave number k2. These effective Hamiltonians have interactions that are complex if the cylinder exhibits chirality. We propose analytical ansatze for the eigenfunctions of these one-dimensional problems that account for a finite cylinder length, and present a general study of their validity. A profound difference is found between the Hamiltonian for the transverse wave number k2=0 and those with k2 not equal 0. The complex nature of the latter leads to chiral wave functions, which we characterize in detail. We apply our general formalism to the chlorosomes of green bacteria and compare the wave functions as well as linear optical spectra (absorption and dichroism) obtained through our ansätze with those obtained by numerical diagonalization as well as those obtained by imposing periodic boundary conditions in the cylinders axis direction. It is found that our ansätze, in particular, capture the finite-length effect in the circular dichroism spectrum much better than the solution with periodic boundary conditions. Our ansätze also show that in finite-length cylinders seven superradiant states dominate the linear optical response.