The electronic structure of donor/acceptor polyenes (merocyanines) is studied by means of electroabsorption (Stark) spectroscopy. Molecules belonging to this class exhibit an extreme variability of their linear and nonlinear optical properties. Depending on the nature of the donor and acceptor end groups and the polarity of their solvent environment they can adopt ground state molecular structures between limits with mostly localized double bonds (polyene-like) or fully delocalized double bonds (cyanine-like). By comparing the results obtained for the change in dipole moment, Δμ, and change in polarizability, Δα, upon optical excitation of 12 donor/acceptor polyenes with theoretical predictions, each molecule's location in between these limits can be assigned. The results demonstrate that this position can be understood by considering only the relative electron donating and accepting strengths of the end groups. The solvent dependence of the electronic structure is also studied by measuring Stark spectra for two compounds in different frozen solvents. The results are consistent with a change of the molecule's ground state toward more dipolar structures in polar solvents. Taken together, the donor, acceptor, and solvent dependences of the electronic structure of donor/acceptor polyenes can be described by a simple model based on the gradual change of the molecule’s ground state between polyene-like and cyanine-like structures.

The rotational dynamics of phenylene and diamantane rotators in crystals of 1,9-bis(4-[3,3,3-triphenylpropynyl]phenyl)diamantane were analyzed independently within the same crystal structure. The dynamics of phenylene rotation were established by dynamic line shape analysis using 13C CPMAS NMR. The phenylene signals were selectively highlighted by deuteration of the aromatic trityls and the use of short contact times for cross polarization. The dynamics of the diamantane group were established by 1H spin-lattice relaxation under conditions where dipolar relaxation was shown to be the dominant mechanism between 250 and 425 K. A factor of 20 000 between the rates of rotation of the faster diamantane and the slower phenylene at 300 K supports expectations that higher symmetry rotors should have significantly faster dynamics.

No abstract