ESIPT in a binary mixture of non-polar and protic polar solvents: Role of solvation dynamics

Excited-state intramolecular proton transfer (ESIPT) was studied for two different fluorophores M2N3HF and 2P3HBQ in binary solvent mixtures of n-heptane and methanol. While both fluorophores share a degree of structural resemblance, they exhibit sharply contrasting ESIPT dynamics in the solvent mixtures. For 2P3HBQ, ESIPT is ultra-fast in n-heptane, followed by monotonic retardation in the solvent mixtures as the proportion of methanol increases, due to the predominance of 2P3HBQMIDLINE HORIZONTAL ELLIPSISMethanol intermolecular H-bonding over intra-molecular H-bonding within the 2P3HBQ molecule, at high methanol concentrations. For M2N3HF on the other hand, ESIPT is known to occur within sub-picosecond time-scales in alkanes and within few tens of picoseconds in methanol. However, addition of minor quantities of methanol into an alkane solvent like n-heptane causes the ESIPT to be dramatically arrested: even at 3 % mole fraction of methanol, it requires several 100 ps to complete, thus becoming -1000-fold slower than that in pure n-heptane, and -5-fold slower than that in pure methanol. Our results demonstrate that the ESIPT time-constants of M2N3HF in the solvent mixtures follow the same trend as solvation time-constants, suggesting that solvent relaxation plays a major role in the re-distribution of intra-and intermolecular H-bonds in the excited state M2N3HF. Unlike 2P3HBQ, the ESIPT of M2N3HF is thus coupled to the solvation dynamics of the solvent medium.

Automated summary

Excited-state intramolecular proton transfer (ESIPT) was studied for two different fluorophores M2N3HF and 2P3HBQ in binary solvent mixtures of n-heptane and methanol. The ESIPT dynamics of these two fluorophores in the solvent mixtures were found to be contrasting, with 2P3HBQ exhibiting retardation and M2N3HF showing dramatic arrest in the presence of methanol.