Correlation Between Molecular Conformation, Packing, and Dynamics in Oligofluorenes: A Theoretical/Experimental Study

Abstract

Fluorene-based systems have shown great potential as components in organic electronics and optoelectronics (organic photovoltaics, OPVs, organic light emitting diodes, OLEDs, and organic transistors, OTFTs). These systems have drawn attention primarily because they exhibit strong blue emission associated with relatively good thermal stability. It is well-known that the electronic properties of polymers are directly related to the molecular conformations and chain packing of polymers. Here, we used three oligofluorenes (trimer, pentamer, and heptamer) as model systems to theoretically investigate the conformational properties of fluorene molecules, starting with the identification of preferred conformations. The hybrid exchange-correlation functional, OPBE, and ZINDO/S-CI showed that each oligomer exhibits a tendency to adopt a specific chain arrangement, which could be distinguished by comparing their UV/vis electronic absorption and 13C NMR spectra. This feature was used to identify the preferred conformation of the oligomer chains in chloroform-cast films by comparing experimental and theoretical UV/vis and 13C NMR spectra. Moreover, the oligomer chain packing and dynamics in the films were studied by DSC and several solid-state NMR techniques, which indicated that the phase behavior of the films may be influenced by the tendency that each oligomeric chain has to adopt a given conformation.

Photophysical, amplified spontaneous emission and charge transport properties of oligofluorene derivatives in thin films

“…3c). In contrast, a sharp small-angle reflection appears in the X-ray diffractogram of the pentafluorene film after annealing at 100 1C, which is likely related to crystallization, in agreement with previous results on bulk pentafluorene 31 and with the DSC trace. The completion of the cold crystallization process during annealing at 100 1C was finally confirmed with transmission diffraction experiments in temperature (Fig.…”

“…A previous study has shown by a combination of DSC and solid-state NMR measurements that the oligofluorenes tend to adopt distinct molecular conformations according to the number of monomeric units. 31 A plausible explanation for our photophysical results is that the oligofluorenes of different lengths show various degrees of twisting between fluorene units in the solid-state and this could substantially affect the actual conjugation length of the molecules. Overall, our results suggest that the actual conjugation length is structurally dependent and, as a consequence, the oligomer chain length should be distinguished from the effective conjugation length in the oligofluorene films.…”

Internal plasticization of chitosan with oligo(dl-lactic acid) branches

“…Indeed, the differences between the CPMAS and SPEMAS spectra increase for CG01 to CG04 samples, confirming that the molecular dynamics increases in the OLA moieties upon substitution. As already mentioned, this is mostly associated with the presence of motions in the intermediate regime time scale, which reduces the crosspolarization efficiency and interferes with the heteronuclear decoupling during the signal acquisition [30]. Therefore, this feature shows that for samples CG04, CG05 and CG06, the chitosan chains also become mobile at room temperature, so further increase in the OLA substitution do not significantly affect the overall dynamic behavior of the samples.…”

Synthesis of a PPV‐fluorene derivative: Applications in luminescent devices

“…According to the Arrhenius law, the activation energy for the glass transition was calculated by fitting the frequency f versus 1/ T of the E″ (Figure ): f is the frequency (Hz), f 0 is the pre‐exponential factor (Hz), E a is the activation energy (J mol −1 ), T is the absolute temperature where the maximum of tan δ occurs for each relaxation process, and R is 8.32 J K −1 mol −1 . The calculated activation energy was approximately 595 kJ mol −1 .…”