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The exchange narrowing of the J band of certain dye monomers upon aggregation in solution has been known since the 1930s. Here, we analyze the theoretical explanations put forward to account for these narrow absorption bands. Although the theories range from models of identical monomers interacting with vibrations to the opposite of rigid monomers with statistically distributed electronic site energies, all approaches exhibit exchange narrowing. However, we show that the origins of the narrowing are different. A unified theory incorporating the two approaches is presented in which features of both narrowing mechanisms are evident. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Stimulated by experiments by Zewail and co-workers [Chem. Phys. Lett. 172, 109 (1990)], we perform quantum calculations on the long time behavior of a wave packet which moves on coupled electronic surfaces of the NaI molecule. Beat structures in the simulated femtosecond pump/probe signal are shown to depend critically on the parameters used in the calculation. The adiabatic motion is compared to the dynamics including the nonadiabatic coupling and the consequences for the signal are explored. The results are compared to a semiclassical analysis of Chapman and Child [J. Phys. Chem. 95, 578 (1991)]. [ABSTRACT FROM AUTHOR]

We consider three distinct methods of calculating the vibronic levels and absorption spectra of molecular dimers coupled by dipole-dipole interactions. The first method is direct diagonalization of the vibronic Hamiltonian in a basis of monomer eigenstates. The second method is to use creation and annihilation operators leading in harmonic approximation to the Jaynes–Cummings Hamiltonian. The adiabatic approximation to this problem provides insight into spectral behavior in the weak and strong coupling limits. The third method, which serves as a check on the accuracy of the previous methods, is a numerically exact solution of the time-dependent Schrödinger equation. Using these methods, dimer spectra are calculated for three separate dye molecules and show good agreement with measured spectra. [ABSTRACT FROM AUTHOR]