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

Two indigo derivatives, -phenylpropylindigo (NPhC3Ind) and ,'-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 -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 CO and H-N of two NPhC3Ind units, leads to the formation of dark (S) and bright (S) 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 ( and ) in the excited state is observed in viscous solvents, collapsing into a single conformer in 2MeTHF.