Solvent-dependent ultrafast deactivation processes with phenylpropyl indigo derivatives: a step forward in the understanding of indigo decay mechanisms

Abstract

Two indigo derivatives, N-phenylpropylindigo (NPhC₃Ind) and N,N′-diphenylpropylindigo (N,N′PhC₃Ind), were synthesized using green chemistry techniques and their decay mechanisms were elucidated from ultrafast spectroscopic techniques, as well as density functional theory (DFT) and time-dependent DFT (TDDFT) studies. For NPhC₃Ind, in methylcyclohexane (MCH) and n-dodecane, a very fast (<1 ps) excited state proton transfer (ESPT) process is observed. In contrast, a bi-exponential decay is found in 2-methyltetrahydrofuran (2MeTHF), with a rising component of 9 ps and a decay of 72 ps, reminiscent of the behavior observed in indigo (IND). DFT/TDDFT calculations rationalize that the excitation of an intermolecular dimer structure, formed between hydrogen bonds involving the CO and H–N of two NPhC₃Ind units, leads to the formation of dark (S₁) and bright (S₂) states. Due to the structural distortion of the dimer, the emission is localized in one of the monomer units. Consequently, the absorption is considered to originate from a dimer while the emission (locally excited state, LE) originates from a monomer unit. In the case of N,N′PhC₃Ind, the formation of two conformers (C_IN and C_OUT) in the excited state is observed in viscous solvents, collapsing into a single conformer in 2MeTHF.