Direct observation of exciton-exciton interactions.

Natural light harvesting as well as optoelectronic and photovoltaic devices depend on efficient transport of energy following photoexcitation. Using common spectroscopic methods, however, it is challenging to discriminate one-exciton dynamics from multi-exciton interactions that arise when more than one excitation is present in the system. Here we introduce a coherent two-dimensional spectroscopic method that provides a signal only in case that the presence of one exciton influences the behavior of another one.

Unraveling the structure and exciton coupling for multichromophoric merocyanine dye molecules.

The relative orientation of chromophores is a key factor in determining the relationship between the structure and the functionality in molecular multichromophore ensembles. In the case of structurally flexible molecular systems in solution, the task to clarify the relevant effects of accessible chromophore orientations with spectroscopic observations is very demanding. In this study, we address this issue by investigating a series of differently connected multichromophoric systems composed of highly dipolar merocyanine dyes that are systematically varied in their substitution pattern and the number of chromophores attached to a bridging benzene ring.

The role of the dipolar neighborhood on the relaxation dynamics of multichromophoric merocyanines.

The interactions between different chromophores within a molecular system are crucial to comprehend relaxation dynamics, regardless if one deals with small molecules or larger systems like polymers or aggregates. We investigate a series of merocyanine molecules that contain one, two or three highly dipolar (μg = 13.1 D) dyes in close vicinity to study the influence and origin of interactions, e.

Relaxation dynamics and exciton energy transfer in the low-temperature phase of MEH-PPV.

Understanding the effects of aggregation on exciton relaxation and energy transfer is relevant to control photoinduced function in organic electronics and photovoltaics. Here, we explore the photoinduced dynamics in the low-temperature aggregated phase of a conjugated polymer by transient absorption and coherent electronic two-dimensional (2D) spectroscopy. Coherent 2D spectroscopy allows observing couplings among photoexcited states and discriminating band shifts from homogeneous broadening, additionally accessing the ultrafast dynamics at various excitation energies simultaneously with high spectral resolution.

Energy Transfer Between Squaraine Polymer Sections: From Helix to Zigzag and All the Way Back.

We provide a joint experimental and theoretical study of squaraine polymers in solution. The absorption spectra show evidence that two different conformations are present in the polymer: a helix and a zigzag structure. This unique situation allows investigating ultrafast energy-transfer processes between different structural segments within a single polymer chain in solution.

Ultrafast Energy Transfer between Disordered and Highly Planarized Chains of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV).

Upon cooling a solution of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), a phase transition occurs, leading to the formation of aggregates. We have studied the dynamics of singlet excitons in MEH-PPV solution below the critical temperature of the phase transition using steady-state photoluminescence measurements and pump-probe fs-spectroscopy at different temperatures. Spectral analysis indicates the coexistence of disordered chromophores with highly planarized chromophores.

Monitoring ultrafast intramolecular proton transfer processes in an unsymmetric β-diketone.

We report the experimental determination of the intramolecular enol-enol tautomerization rate of an unsymmetric β-diketone, benzoylacetone, with femtosecond transient absorption in the ultraviolet. Initially, there is an equilibrium of two possible enolic structures in solution, which is disturbed upon UV excitation by exciting a disproportionate fraction of one enolic form. Comparison to symmetric β-diketones, acetylacetone and dibenzoylmethane, suggests that ground-state proton transfer gives rise to additional dynamics in benzoylacetone due to the dissimilarity of the two enolic forms.

Ultrafast UV-induced photoisomerization of intramolecularly H-bonded symmetric β-diketones.

In photoinduced molecular reaction dynamics, the effects of electronic charge redistribution can lead to multiple pathways that are determined by the nature of the initial structures involved and the environment the molecule of interest is studied in. The β-diketones are a common example of this complexity. They show keto-enol tautomerism that is almost totally shifted toward the enolic form.