Intramolecular charge transfer and molecular flexibility: Key parameters to be considered in the design of highly fluorescent p-phenylene vinylene derivatives

The optoelectronic properties of a series of p-phenylene vinylene derivatives are studied herein to scrutinize their relationship with the molecular flexibility of the peripheral groups induced by the hybridization of the presented nitrogen atoms. All the molecules share a model fluorophore in the core, 1,3,5-tristyrylbenzene, as a structural leitmotif, while TPA and carbazole groups are located in the periphery. The studied compounds showed high luminescence, highlighting the TPA derivative (compound 1) with a fluorescence quantum yield of 91% in dichloromethane solution. In addition, a linear relationship between the fluorescence lifetime and the solvent polarity was found for this compound. The solvent dependence was associated with an intramolecular charge transfer (ICT) upon photoexcitation which leads to a large change of the dipole moment and geometrical modifications mainly involving a single branch of the molecule. The photophysical properties of the carbazole derivatives (compounds 2 and 3) are strongly influenced by the substitution position of the peripheral carbazole groups (C-N or C-C linkage, respectively). Interestingly, a linear relationship between the non-radiative rate constant and the solvent viscosity was found for compound 3. This behavior was associated to the higher rigidity of this compound because the carbazole groups are embedded in the branches of the molecule and not in a terminal position as in compound 2. Finally, the three studied compounds were used as electroluminescent material for non-doped OLEDs (organic light emitting diodes) in a proof-of-concept. This experiment also showed the importance of the molecular packing for having a good device performance.