Excited-state properties of TCTA:PO-T2T exciplex in solvents investigated by density function theory and transient absorption spectroscopy

Abstract

The solvent-dependent excited-state properties of TCTA:PO-T2T mixture are investigated through a combination of quantum chemical calculations with ultrafast transient absorption spectroscopy. Photoluminescence spectroscopy reveals distinct exciplex emission peaked at 580 ± 5 nm in low-polarity solvents (ε = 2.38 for toluene and ε = 17.3 for 2Me-THF), while this emission band completely disappears in high-ε solvents like DMF (ε = 36.7) and DMSO (ε = 46.7). This solvent-dependent behavior aligns with the calculated exciplex binding energies, which decrease from −20.6 kcal/mol (toluene) to −13.5 kcal/mol (DMSO), confirming the thermodynamic instability of exciplexes in high-ε media. Electronic structure calculations identify solvent-dependent polarization effects that perturb exciplex excited-state energies. These energies are modestly higher in 2Me-THF than in toluene due to the presence of electron-rich oxygen, resulting in a blue shifted emission. Femtosecond transient absorption spectroscopy delivers direct evidence of exciplex formation through the emergence of excited-state absorption between 575 and 700 nm, which evolves on the timescale of hundreds of picoseconds to nanoseconds. This study offers valuable insights into exciplex behavior in solution.