Low-band-gap polymers with broad spectral absorption are highly sought after for application in organic photovoltaic cells and other optoelectronic devices. Thus, a conjugated copolymer based on two different indolenine squaraine dyes SQA and SQB was synthesized by Suzuki coupling, and its steady-state and time-resolved optical properties were investigated in detail. In CHCl3 the copolymer [SQA-SQB] n shows a strongly broadened and red-shifted absorption compared to that of its monomers, which was explained by exciton coupling of localized transition moments. The theoretical background of exciton coupling theory for copolymers was worked out in detail. In toluene, [SQA-SQB] n displays a spectral narrowing of the lowest excitation band which resembles the exchange narrowing effect found in cyanine J-aggregates. In this way [SQA-SQB] n behaves like a one-dimensional covalently bound hetero-J-aggregate. The photoinduced dynamics of the copolymer was investigated by transient absorption pump–probe spectroscopy with femtosecond resolution. Because of the unusually high exciton diffusion constant, singlet–singlet annihilation is the rate-limiting step for deactivation of the copolymer in solution at high laser fluencies. This is unlike the situation for many conjugated polymers in the solid state, where diffusion-limited annihilation is usually found. Thus, the [SQA-SQB] n copolymer is a unique model system which combines the excitonic features of J-aggregates with the chemical robustness of a polymer.

Vibronic absorption spectra of molecular aggregates consisting of up to N = 9 monomer units are calculated employing methods of time-dependent quantum mechanics. Taking one vibrational degree of freedom for each monomer into account and treating one-exciton excited electronic states leads to a problem with N vibrations and N electronically coupled states. The demanding quantum propagation is carried out within the multiconfiguration time-depended Hartree method (MCTDH). Spectral features of and population transfer in the aggregates are analyzed as a function of the aggregate size and the strength of the electronic coupling. With a model for oligomers of perylene bisimides, it is shown how measured temperature-dependent absorption spectra correlate with the aggregate size. Furthermore, the exciton localization and dynamics in these aggregates are investigated.

In the first part of this paper, the many-particle quantum hydrodynamics (MPQHD) equations for a system containing many particles of different sorts are derived exactly from the many-particle Schrödinger equation. It includes the derivation of the manyparticle continuity equations (MPCE), many-particle Ehrenfest equations of motion (MPEEM), and many-particle quantum Cauchy equations (MPQCE) for any of the different particle sorts and for the total particle ensemble. The new point in our analysis is that we consider a set of arbitrary particles of different sorts in the system. In MPQCEs, there appears a quantity called pressure tensor. In the second part of this paper, we analyze two versions of this tensor in depth -the Wyatt pressure tensor and the Kuzmenkov pressure tensor. There are different versions because there is a gauge freedom for the pressure tensor similar to that for potentials. We find that the interpretation of all quantities contributing to the Wyatt pressure tensor is understandable but for the Kuzmenkov tensor, it is difficult. Furthermore, the transformation from Cartesian coordinates to cylindrical coordinates for the Wyatt tensor can be done in a clear way, but for the Kuzmenkov tensor, it is rather cumbersome.

Population transfer between electronic molecular states can be effectively induced via the interaction with shaped laser pulses. Regarding a numerical example, it is demonstrated that perturbation theory, as is often applied in numerical simulations of field-matter interactions, might lead to divergences. The occurring error accumulating in the norm of the wavefunction can be decomposed into two contributions. The first one is a small numerical error, which is controllable by minimization of the time-propagation step, whereas the second one is related to the order of the perturbative expansion. These two contributions behave differently upon variations of the potential energy surface of the system and also the laser pulse parameters. An improved scheme is proposed in which the first part carrying the numerical error disappears.