Developing Novel Building Blocks For Organic Materials:Bowl-shaped Conjugated Molecules And Cyclopentadienone Dimers

The research work of this dissertation mainly focused on developing the functional molecules with unique structural features,and understanding the relationship between novel molecular structures and properties(i.e.optical,charge-transport and photoswitching properties).Our accomplishments are summarized as follows.In the first part,we developed a series of bowl-shaped skeletons based on corannulene,and tried to understand the relationship between the structures and semiconducting properties of bowl-shaped molecules from molecular orbital energies and molecular arrangements.Firstly,we presented a novel bowl-shaped acceptor unit based on corannulene and firstly incorporated it into donor-acceptor copolymers.The donor units were used to modulate the energy levels and thus charge-transport properties of the copolymers.The ambipolar and electron transport were achieved based on different donor units.Secondly,we introduced electron-rich groups into the conjugated systems of corannulene to tune the energy level and molecular packing.The resulting ?-extended corannulene exhibits a one-dimensional layered packing motif.The single crystal of the corannulene deri vative shows the hole mobility of 0.06 cm2 V-1 s-1,which is the highest hole mobility among those of corannulene derivatives up to now.Thirdly,we used open-shell electronic structures to alter the energy level of corannulene.The open-shell corannulene derivative shows an optical gap of 0.81 eV and can be potentially used as an ambipolar transport material.Additionally,the biradical exhibits a high stability with a half-life of 37 days in solution,outperforming those of most biradicals.Finally,we explored the effects of stereoisomerism on molecular packing and thus charge-transport properties.Through theoretical calculations,we found the stereoisomerism significantly influences the charge-transport mobility and even charge-transport polarities.In the second part,we reported the synthesis of intramolccularly ?-stacked dimeric compounds using the one-pot method serendipitously found in the first part.Without changing the conjugated skeleton,simply altering the substituents(hydrogens or methyl groups)on the aromatic core results in completely different intramolecular staking modes(i.e.endo or exo configurations).Density functional theory(DFT)calculations were performed to rationalize the mechanism of dimerization processes,revealing that the steric hindrance together with ?-? interactions led to the two distinct stereoselectivities.The dimeric molecule with hydrogens on the aromatic skeleton(endo-dimer)exhibits larger intramolecular ?-overlap,displaying excimer-like characteristics,whereas the dimer with methyl groups on the core(exo-dimer)has less?-overlap,mainly sharing the features of the monomeric model compound.Such phenomena were further rationalized by time-dependent DFT(TD-DFT)calculations and revealed the importance of control over the intramolecular stacking modes.To our knowledge,this is the first example to modulate the intramolecular stacking modes of rigidly linked ?-stacked molecules by the substituent effects.These dimers provide an excellent platform for investigating intramolecular electronic couplings.Further extension of the ?-scaffolds is undergoing to explore the potential applications of this system,such as in single-molecule electronics and singlet fissions.In the third part,we disclosed how intramolecular stacking modes of ?-stacked dimers dictate photoswitching properties.The dimer possessing cofacially stacked chromophores(endo-dimer)displays a quasi-reversible photoswitching process based on intramolecular photochemical[2+2]cycloaddition reaction and its reverse reaction.In contrast,the dimer bearing crosswisely stacked chromophores(exo-dimer)exhibits a highly reversible photoswitching process.The photoisomerization mechanism includes a homolytic cleavage of a carbon-carbon single bond,followed by the radical rearrangement and the formation of a new carbon-carbon single bond.The photoisomerization of chiral exo-dimer,undergoing through diradical intermediates,shows excellent fatigue resistance under ambient conditions and large changes in geometries and dipole moments during photoisomerization.This work not only proved that intramolecular stacking modes of ?-stacked dimers can significantly influence their photoswitching properties but also provided a novel kind of chiral photoswitches bearing excellent photoswitching properties.In summary,this dissertation focused on the development of novel molecular materials.We developed a series of bowl-shaped conjugated molecules and systematically investigated their semiconducting properties.We provided the preliminary understanding of the relationship between the structures and properties of such bowl-shaped skeletons.We also developed a novel kind of chiral photoswitches,which is a significant complementation of well-known photoswitches.