Nanoscale Organization and Solid-State Dynamics of Carbazole-[π]-Carbazole Rotors Edged with Aliphatic Chains

The aggregation behavior and its influence on the intramolecular motion of three highly conjugated solid-state molecular rotors 2-4 with alkyl groups were investigated by a combination of fluorescence spectroscopy, transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), and variable-temperature solid-state H-2 nuclear magnetic resonance (VT H-2 ssNMR). Their long conjugation made them highly emissive in tetrahydrofuran solution (Phi(f) = 0.41, 0.21, and 0.18, for rotors 2, 3 and 4, respectively), and the elongated aromatic cores favored pi-pi intermolecular interactions upon aggregation in hexane. The TEM micrographs and PXRD studies of the resulting bulk showed that shorter chains generated larger amorphous domains in 2, as compared to 3 and 4. The H-2 ssNMR experiments showed fast intramolecular rotation of their central 1,4-diethynylphenylenes with the lowest activation energy to rotation for compound 2, E-a = 5.34 kcal/mol. This relationship between amorphous/crystalline domains and intramolecular motion has not been reported before, and it could guide the design of new solid-state molecular rotors.