Tailoring spirobifluorenes for chiroptical applications

Resumo

Chiroptical responses arise from the diastereomeric interactions between chiral light and the enantiomers of a chiral system. Such fingerprint interactions form the basis of chiroptical techniques and they are routinely used for the structural characterization of chiral systems from different scales. The high accuracy of chiroptical responses and related techniques have also drawn the attention of the scientific community and a plethora of chiroptical systems have been developed through individual, collective, induced or multiple chirality approaches at different states of matter. However, the potential of everyday chiroptical applications still needs to be explored. In this context, the chemistry of allenes widely contributed to the construction of open, cyclic oligomers and cage-like structures with outstanding chiroptical properties in solution. The single-molecular handedness of allenes were also translated into metal surfaces, yet their applicability was hampered by photoisomerization and called for a more chiral building block. 9,9’-spirobifluorenes (SBFs) are considered to be photochemically and thermally stable building blocks and have many uses in the field of molecular electronics. Besides through-space orbital interactions in SBFs, the perpendicular arrangement of the two fluorene rings can give rise to axial chirality upon an appropriate substitution. The chemistry of 2-substituted SBFs is particularly well-established and they offer efficient π-conjugation. The versatility and robustness of chiral 2-substituted SBFs were confirmed by both theoretical and experimental studies. With the proof of concept in hand, this PhD thesis aims at the modulation of SBFs as a milestone toward chiroptical applications and three objectives are undertaken: 1) Understanding the impact of spiroconjugation in SBF derivatives. To draw conclusions on the relationship between spiroconjugation and both reactivity and chiroptical properties, chiral SBFs with different electronic character are developed. In addition to the experimental findings, the computational insights into their molecular orbitals, optical and chiroptical responses are provided. 2) Development of thioacetate-derivatized SBFs suitable for surface functionalization. In line with the strategy of self-assembled monolayers, thioacetate-derivatized SBFs CF-3, CF-4 and CF-5 are designed and synthesized. Their chiroptical properties are finally studied by means of ECD spectroscopy as well as TD-DFT calculations. 3) Preparation of SBF cyclooligomers and uncovering their structure-property relationship. Trimeric SBF cyclooligomers are designed and synthesized through a non-stereoselective approach in one-pot. Their optical and chiroptical properties are first investigated by ECD measurements and next supported by theoretical calculations. Finally, a comprehensive computational analysis is performed in order to visualize the helical morphologies of their molecular orbitals.