Analysis of the vibronic fine structure in circularly polarized emission spectra from chiral molecular aggregates

Abstract

Using a Frenkel-exciton model, the degree of circular polarization of the luminescence (g lum ) from one-dimensional, helical aggregates of chromophoric molecules is investigated theoretically. The coupling between the electronic excitation and a local, intramolecular vibrational mode is taken into account. Analytical expressions for the fluorescence band shape and g lum are presented for the case of strong and weak electronic coupling between the chromophoric units. Results are compared to those from numerical calculations obtained using the three particle approximation. g lum for the 0-0 vibronic band is found to be independent of the relative strength of electronic coupling between chromophores and excitation-vibration coupling. It depends solely on the number of coherently coupled molecules. In contrast, for the higher vibronic transitions ͉g lum ͉ decreases with decreasing strength of the electronic coupling. In the limit of strong electronic coupling, ͉g lum ͉ is almost constant throughout the series of vibronic transitions but for weak coupling ͉g lum ͉ becomes vanishingly small for all vibronic transitions except for the 0-0 transition. The results are interpreted in terms of dynamic localization of the excitation during the zero point vibrational motion in the excited state of the aggregate. It is concluded that circular polarization measurements provide an independent way to determine the coherence size and bandwidth of the lowest exciton state for chiral aggregates.

Using circularly polarized luminescence to probe exciton coherence in disordered helical aggregates

“…5͑a͔͒ depend on a product of one-and twoparticle coefficients and, therefore, cannot be expressed solely in terms of the exciton coherence function. 11,33 For example, the 0-1 dissymmetry, using Eqs. ͑3͒, ͑8͒, and ͑12͒, is given by…”

“…In analyzing disorder-free helices in a prior work, we uncovered an unusual N-dependent sign change in g lum in the spectral region surrounding the 0-0 transition. 33 Here, N is the number of chromophores comprising the helical aggregate. In Ref.…”

“…In Ref. 33, we showed that the dissymmetry of the 0-0 transition ͑g lum 0-0 ͒ scales nonlinearly with N, changing its sign twice every complete helical turn. This surprising size dependence requires temperatures to be low enough to ensure that emission emanates from the lowest energy exciton.…”

“…In Ref. 33, we assumed periodic boundary conditions, in contrast to the present work, where more realistic open boundary conditions are employed throughout. As we will show, the change to open boundary conditions does not alter the unusual behavior of g lum 0-0 but does induce significant changes to the sideband dissymmetries ͑g lum 0-t , t ജ 1͒ when the exciton coupling is weak.…”

“…As we will show, the change to open boundary conditions does not alter the unusual behavior of g lum 0-0 but does induce significant changes to the sideband dissymmetries ͑g lum 0-t , t ജ 1͒ when the exciton coupling is weak. 33 This paper is organized as follows. After introducing the model details in the following section, we continue in Sec.…”