Anharmonic vibrational effects in linear and two-dimensional electronic spectra.

For the description of vibrational effects in electronic spectra, harmonic vibrations are a convenient and widespread model. However, spectra of larger organic molecules in solution usually exhibit signs of vibrational anharmonicity, as revealed by deviation from the mirror image symmetry between linear absorption and emission spectra of the harmonic case. For perylene and terylene, two molecules with rigid Pi-electron systems and strong vibrational-electronic coupling, we employ a simple but effective theoretical model, which introduces cubic anharmonicity in the potentials of electronic surfaces. Vibrational anharmonicity is then readily quantified based on the experimentally measured peak ratio of the first vibronic progression peaks in linear absorption and emission. This method is straightforward but not applicable if emission from the initially excited state is short lived. For such a case, we employ two-dimensional electronic spectroscopy in the visible as a comprehensive time-resolved technique for the experimental determination of the vibrational anharmonicity of pinacyanol iodide, a solvated dye molecule exhibiting ultrafast excited state isomerization. We show that the ratio between certain cross peak amplitudes in two-dimensional electronic spectra is a direct measure of vibrational anharmonicity.

A Unified Picture of S* in Carotenoids.

In π-conjugated chain molecules such as carotenoids, coupling between electronic and vibrational degrees of freedom is of central importance. It governs both dynamic and static properties, such as the time scales of excited state relaxation as well as absorption spectra. In this work, we treat vibronic dynamics in carotenoids on four electronic states (|S0⟩, |S1⟩, |S2⟩, and |Sn⟩) in a physically rigorous framework. This model explains all features previously associated with the intensely debated S* state. Besides successfully fitting transient absorption data of a zeaxanthin homologue, this model also accounts for previous results from global target analysis and chain length-dependent studies. Additionally, we are able to incorporate findings from pump-deplete-probe experiments, which were incompatible to any pre-existing model. Thus, we present the first comprehensive and unified interpretation of S*-related features, explaining them by vibronic transitions on either S1, S0, or both, depending on the chain length of the investigated carotenoid.