Theory of the absorption and circular dichroism spectra of helical molecular aggregates

Abstract

A theory of the electronic circular dichroism (CD) and optical rotatory dispersion (ORD) of infinite aggregates exhibiting cylindrical symmetry is presented in which, to the authors' knowledge, for the first time vibrational structure is included explicitly. It is shown that, with the coherent exciton scattering approximation in the Green function approach, the detailed vibrational structure of the aggregate absorption. CD and ORD bands can be calculated from a knowledge of the electronic coupling and the monomer absorption line shape alone. Detailed model calculations for a single helix are made and the results are used to expose the origin of different spectral features. A good reproduction of experimental J-aggregate spectra is obtained, using the same electronic interaction to fit both absorption and CD spectral line shapes. The theory allows some prediction of aggregate geometry to be made, but it is shown that an unambiguous geometrical assignment can only be made where experimental spectra for light of different propagation directions with respect to the cylinder axis are available.

Electromagnetic Study of the Chlorosome Antenna Complex of Chlorobium tepidum

“…2. A similar approach, the CES method, has also been used for excitonic systems 14 and leads essentially to the same equations that are used in the present work. Now, in the quantum case, the simplest approach to obtain the absorption spectrum consists in applying Fermi's Golden rule (qtm) abs (ω) = 4πω 3c k |μ| 2 πδ (E k − ω) , (9) here, E k is the k − th eigenvalue of the Hamiltonian given in Eq.…”

The influence of geometry on the vibronic spectra of quantum aggregates

“…Although trivial in this case, the above formula (13) will be useful when vibrations are included. The shape of the absorption spectrum is given by −ImA(E) where the spectral function A(E) is [28,33,34],…”

“…When this is done, the Green operators g(E) and G(E) become dependent on these degrees of freedom. Then the spectral function (15) must be generalised to [28,34,35] …”

“…Even though one is dealing with large organic molecules, in statistical models they are treated as structureless entities characterised only by a given electronic transition energy. As in our previous work on spectra and energy transfer [27][28][29], we consider first the purely electronic model of identical coupled monomers and then the realistic case of broad structured spectra arising from interaction with both intra-molecular vibrational modes and with external vibrational modes of the environment. We use the "coherent exciton scattering" (CES) approximation generalised to be applicable to aggregates of arbitrary size and geometry.…”

Non-Markovian Quantum State Diffusion for temperature-dependent linear spectra of light harvesting aggregates

“…The influence of vibrations on molecular aggregates has been studied using various methods (see for example [3,4,6,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]). …”