The Characterization Of Charge Transfer Of Extended π-Stacked Donor-Acceptor Clusters And The Effect Of Polar Substitution On The Ring Rotation In Pillar[5]arenes

The organic charge-transfer（CT）complexes investigated in this work are composed of ap-electron donor（D）and aπ-acceptor（A）and have the promising features such as electrical conductivity,ferroelectricity,photovoltaic effects,and optical properties.They have been applied in several areas such as chemistry,materials science,biology and medicine.In terms of theoretical studies about stackedπ-donor-acceptor complexes,some studies of intermolecular charge transfer have greatly supported the understanding of molecular materials based on organic CT complexes.However,more attention was focused so far on the donor-acceptor（D-A）dimer complexes.Few is known about the extended clusters.Commonly,in the system of cocystals,it includes more than one donor and acceptor.Therefor it is significant to investigate more extended systems to show details of the CT states with increasing cluster size.In present work,to characterize the charge transfer in excited states with increasing size of the clusters,two complexes withπ-stacked donor and acceptor,the 1,5-naphthalene diol（NDI）donor and pyromellitic diimide（PDI）acceptor,5-amino-1-naphthol（AMN）donor and pyromellitic diimide（PDI）acceptor,were investigated.The electronic excitations were calculated by using the scaled opposite-spin（SOS）variant of ADC（2）,time-dependent density functional theory（TD-DFT）using a long-range corrected（LC）functional（ωB97xD）and the TD-LC approach within density functional based tight-binding（TD-LC-DFTB）.Face-to-face mixed stacks and edge-to-face crossed stacks up to hexamers were investigated.The calculations show that the ground state of the complexes does not possess significant CT character.On the other hand,the lowest excited state（S₁）shows in all clusters a strong charge transfer.In several cases,the second excited state and also higher excited singlet states possess significant CT character as well.The orbitals involved in the excitation are mostly well localized and located on adjacent donor/acceptor pairs.Comparing different stacking directions,the lowest excitation energy for crossed stack complexes have a larger excitation energy than that mixed stacks by ～0.4 e V.Strong red shifts in vertical fluorescence emission transitions have been computed which could even lead to intersection between ground and first excited states,resulting in to ultrafast radiationless decay and fluorescence quenching.Pillar[5]arenes are a type of novel macrocycles,containing di-substituted hydroquinone units linked by methylene bridges in para-positions.Since discovered in2008,they have attracted extensive attention in supramolecular chemistry as interesting candidates to be used in the preparation of host-guest complexes.Functionalization by means of rim substitution and sustaining an ordered substituent arrangement on both sides of the rim is important for the development of new pillararene-based materials.In order to achieve this,the rim inversion process of rotating the hydroquinone units through the pillar[5]arenes has to be controlled.In order to show the process of different polar groups at the hydroquinone units leading to the destruction of the columnar structure of pillar[5]arenes,the influence of polar groups（-CH₂F,-CH₂Cl,-CH₂OH,-CH₂SH,-CH₂NH₂）on the rotation mechanism on transition state barriers and different local minima was investigated in present work.The stabilization of the intermediate structures by non-covalent van der Waals and interactions and also by hydrogen bonds constitute a major factor affecting barrier heights.By rotating,we got the inversion barriers of different degrees of rotation.Density functional theory combined with the hybrid M06-2X functional was performed in all calculations of the study of pillar[5]arenes.In conclusion,the polar substituent–CH₂OH facing inside of pillar[5]arenes’cavity have the largest barrier of about 10.1kcal/mol of all substitutions investigated.For lowest ones（-CH₂SH and-CH₂NH₂）,all the inversion barrier values were negative for their entire rotation process.Besides,the stabilization of the intermediate structures by non-covalent van der Waals and interactions and also by hydrogen bonds constitute a major factor affecting barrier heights.In summary,we have gotten the important conclusions by expanding theoretical studies.It is of importance to further understand the CT character ofπ-stacked clusters and P5A with polar groups.We also believe our theoretical study will provide a reliable theoretical reference for other new research in the future.