Singlet–Singlet Exciton Annihilation in an Exciton-Coupled Squaraine-Squaraine Copolymer: A Model toward Hetero-J-Aggregates

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]_<i>n</i> 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]_<i>n</i> 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]_<i>n</i> 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]_<i>n</i> copolymer is a unique model system which combines the excitonic features of J-aggregates with the chemical robustness of a polymer

Ultrafast Exciton Self-Trapping upon Geometry Deformation in Perylene-Based Molecular Aggregates

Femtosecond time-resolved experiments demonstrate that the photoexcited state of perylene tetracarboxylic acid bisimide (PBI) aggregates in solution decays nonradiatively on a time-scale of 215 fs. High-level electronic structure calculations on dimers point toward the importance of an excited state intermolecular geometry distortion along a reaction coordinate that induces energy shifts and couplings between various electronic states. Time-dependent wave packet calculations incorporating a simple dissipation mechanism indicate that the fast energy quenching results from a doorway state with a charge-transfer character that is only transiently populated. The identified relaxation mechanism corresponds to a possible exciton trap in molecular materials

Identification of Ultrafast Relaxation Processes As a Major Reason for Inefficient Exciton Diffusion in Perylene-Based Organic Semiconductors

The exciton diffusion length (<i>L</i>_D) is a key parameter for the efficiency of organic optoelectronic devices. Its limitation to the nm length scale causes the need of complex bulk-heterojunction solar cells incorporating difficulties in long-term stability and reproducibility. A comprehensive model providing an atomistic understanding of processes that limit exciton trasport is therefore highly desirable and will be proposed here for perylene-based materials. Our model is based on simulations with a hybrid approach which combines high-level ab initio computations for the part of the system directly involved in the described processes with a force field to include environmental effects. The adequacy of the model is shown by detailed comparison with available experimental results. The model indicates that the short exciton diffusion lengths of α-perylene tetracarboxylicdianhydride (PTCDA) are due to ultrafast relaxation processes of the optical excitation via intermolecular motions leading to a state from which further exciton diffusion is hampered. As the efficiency of this mechanism depends strongly on molecular arrangement and environment, the model explains the strong dependence of <i>L</i>_D on the morphology of the materials, for example, the differences between α-PTCDA and diindenoperylene. Our findings indicate how relaxation processes can be diminished in perylene-based materials. This model can be generalized to other organic compounds