Analysis of charge transfer transitions in stacked π-electron donor-acceptor complexes

Charge-transfer (CT) interactions have recently attracted enhanced interest in creating ordered ferroelectric networks of electron donors (D) and acceptors (A) and in providing the basis of functional photonic devices. In the present work, the CT character of three distinct donor-acceptor (D-A) complex interactions (pyromellitic diimide (PDI)/1,5-diaminonaphthalene (DAN), para-chloranil (pClA)/tetramethyl-para-phenylenediamine (TMPD) and tetracyanobenzene (TCNB)/1,2-di(4-pyridyl)ethylene (Bpe)) has been investigated in their ground and excited states using high-level quantum chemical methods (second-order algebraic diagrammatic construction (ADC(2)) and time-dependent density functional theory (TD-DFT) using a long-range corrected functional (ωB97xD)). The calculations show that the lowest electronic excitation has pronounced CT character in all the three dimers investigated. On the contrary, the ground states possess only smaller amounts of CT degree except for the pClA/TMPD complex, which has a strong amount of CT of 0.4 e. Optimization of the S1 state and calculation of the resulting vertical fluorescence transitions led to the interesting finding of a zero-energy gap for pClA/TMPD. The next smallest energy gap is computed for PDI/DAN (1 eV) followed by TCNB/Bpe (2.6 eV). The analysis of the electronic charge distribution of the D-A complex in the excited state shows a significant variation even though practically a full electron charge has been transferred from D to A.