Optical spectra of molecular aggregates and crystals: testing approximation schemes

Abstract

The issue of the non-conservation of the oscillator strength in molecular aggregates is solved and several approximation schemes are validated.

Aggregates of polar dyes: beyond the exciton model

“…In this respect, the proposed two-step approach to aggregates of polar dyes may offer a good starting point towards the development of few-particle approaches, that are successfully exploited for aggregates of non-polar dyes. 7,17,22,25…”

“…The term in the fourth line is proportional to the squared transition dipole moment of the mf dye (see eqn (10)) and contains both the exciton hopping term, as well as the twoexciton terms that are usually neglected in the exciton model. 25 In the last line the first term is an exciton-exciton interaction term (proportional to the squared mesomeric moment): it conserves the exciton mumber and it may enter the exciton model, but is of course relevant only to aggregates of polar dyes. 31 The very last term is again an ultraexcitonic term mixing states whose exciton number changes by one unit.…”

“…In the cyanine limit then the electronic part of the Hamiltonian in eqn (6) reduces to the Hamiltonian for aggregates of non-polar dyes. 25…”

“…15,16 Deviations from the exciton model are mainly recognized in the intensity of optical transitions as well as of CD spectra. 25 However, in aggregates of non-polar dyes with a large quadrupolar character, large deviations from the exciton model are observed: [26][27][28][29][30] in these systems, the dense excitation spectrum at the molecular level, and the large molecular polarizability, make the approximations of the exciton model critical, with impressive effects on linear and nonlinear optical spectra.…”

Supramolecular chirality: a caveat in assigning the handedness of chiral aggregates

“…As detailed in the ESI, this fundamental inconsistency is related to the approximation used in the exciton model and is solved by accounting for the intermolecular interactions that mix gAgB, and eAeB states. 13 To validate our results, we compare CD spectra calculated in the exciton approximation with TD-DFT results. We select a linear phenyleneethynylene dye (PE, Fig.…”