Characterization of Charge Transfer in Excited States of Extended Clusters of π-Stacked Donor and Acceptor Complexes in Lock-Arm Supramolecular Ordering.

Abstract

Lock-arm supramolecular ordering cocrystals formed by π-stacked materials constitute an interesting class of materials, which exhibits ferroelectric behavior at room temperature. To characterize the charge transfer in excited states, two complexes with π-stacked donors and acceptors, the 1,5-naphthalene diol (NDI) donor and pyromellitic diimide with diethylene glycol arms (PDIA) acceptor, 5-amino-1-naphthol (AMN) donor and PDIA acceptor, were investigated. The electronic excitations were calculated using the scaled opposite-spin 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. The orbitals involved in the excitation are mostly well localized and located on adjacent donor/acceptor pairs. Comparing different stacking directions, the vertical excitation energies for the NDI-PDIA crossed stacks are larger than those for the mixed stacks by 0.2-0.4 eV. In the case of the AMN-PDIA system, the energy differences are smaller (∼0.1 eV) with mostly the same energetic ordering as for the NDI-PDIA case. 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 ultrafast radiationless decay and fluorescence quenching.