The observation of persistent oscillatory signals in multidimensional spectra of protein-pigment complexes has spurred a debate on the role of coherence-assisted electronic energy transfer as a key operating principle in photosynthesis. Vibronic coupling has recently been proposed as an explanation for the long lifetime of the observed spectral beatings. However, photosynthetic systems are inherently complicated, and tractable studies on simple molecular compounds are needed to fully understand the underlying physics. In this work, we present measurements and calculations on a solvated molecular homodimer with clearly resolvable oscillations in the corresponding two-dimensional spectra. Through analysis of the various contributions to the nonlinear response, we succeed in isolating the signal due to inter-exciton coherence. We find that although calculations predict a prolongation of this coherence due to vibronic coupling, the combination of dynamic disorder and vibrational relaxation leads to a coherence decay on a timescale comparable to the electronic dephasing time.

The structural aspects for the complexation of xanthone and fluorenone to R-as well as -cyclodextrins (CDxs) were explored by combining absorption, fluorescence, and induced circular dichroism studies with theoretical calculations of the optimized structures for the ketone-CDx complexes and their predicted ICD spectra. Detailed information on the similarities and differences for the complexes of these ketones with CDxs was obtained. For both CDxs, the ketones are bound to the rim of the cavity, and deeper penetration is observed for the complexes with -CDx. In addition, a 1:2 complex involving R-CDx was only observed in the case of fluorenone. Although the two ketones have similar structures, their molecular recognition properties and the resulting structures of the CDx complexes show distinct differences.

Stereoselective binding of the 1-naphthyl-1-ethanol (1-NpOH) and 2-naphthyl-1-ethanol (2-NpOH) enantiomers with β-cyclodextrin (β-CD) was investigated by photophysical and theoretical studies. The latter are based on structural calculations of the complexes and their respective induced circular dichroism spectra. For the enantiomers of 1-NpOH, where the complexes with β-CD have a 1:1 (guest:CD) stoichiometry, a mixture of structures with different geometries was observed for each enantiomer. No stereoselectivity was apparent for these complexes. (R)- and (S)-2-NpOH form complexes with β-CD that have both 1:1 and 2:2 stoichiometries. Structural calculations indicate that the naphthyl moiety of both enantiomers is preferentially included in the CD cavity for the 1:1 complex. In the case of the complexes with 2:2 stoichiometry, a mixture of complexes is present that either have the naphthyl moieties at a distance or in close proximity. The former geometry explains the observation of long-lived excited triplet states for 2-NpOH, whereas the latter geometry is responsible for the excimer emission. The stereoselectivity observed for the monomer to excimer intensity ratios when (R)- and (S)-2-NpOH are complexed to β-CD is due to the different contributions of 2:2 complexes with different geometries.

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