Photophysical and Quantum Mechanical Investigation on Exciplex Formation Mechanism

Abstract

The exciplex is an emissive complex of partially charged electron donor (Dδ+) and acceptor (Aδ-) formed via photoinduced electron transfer (PET). In this work, I combine spectroscopic data and quantum mechanical computation data of exciplex to investigate salient factors for PET and exciplex emissivity. Pyrene (Py) and N,N-dimethylaniline (DMA) were chosen for a model system, and julolidine (Jul) and N,N,2,6-tetramethylaniline (NN26) were used to compare Cphenly-Namine torsional angle (ϕ) effect. First, I found that the efficiency of PET, the primary step of exciplex formation, depends on the conjugation of non-bonding electrons of N to phenyl group in D. PET does not occur when the conjugation is limited with large ϕ forced by steric hindrance in NN26, whereas PET occurred most efficiently when the conjugation is promoted with small ϕ forced by the six-member ring in Jul. Second, I found that exciplex emissivity depends on the contribution of Py character on HOMO of exciplex with TD-DFT calculation. According to the quantum mechanical calculation, the major character of LUMO of exciplex is Py for both Py-DMA and Py-Jul exciplex, whereas HOMO of Py-DMA exciplex possesses more Py character of than that of Py-Jul exciplex. Therefore, stronger emission is expected from Py-DMA exciplex than Py-Jul exciplex, which is in good accordance with experimental observation. The findings in this work are very critical to understand the exciplex formation mechanism, and expected to shed light on developing new exciplex systems, understanding solvent effect and magnetic field effect of the exciplex systems.