Atypical Energetic and Kinetic Course of Excited-State Intramolecular Proton Transfer (ESIPT) in Room-Temperature Protic Ionic Liquids

Abstract

The excited-state intramolecular proton-transfer (ESIPT) process in 1,8-dihydroxyanthraquinone (18DHAQ) dye has been investigated in protic ionic liquid (PIL) solvents using photochemical measurements. The results demonstrate noteworthy modulations in both steady-state and time-resolved emission characteristics of excited normal (N*) and tautomeric (T*) forms of the dye. That the emission of T* increases unexpectedly upon increasing solvent viscosity indicates that subsequent to the initial forward ESIPT, there is also a relatively slower back ESIPT process involved for the excited dye. It is inferred that the propensity of this back ESIPT process is determined by the dynamics of the diffusive solvent relaxation, a process that is known to be strongly viscosity-dependent in ionic liquids. Evidence of both forward and back ESIPT for the dye has been obtained from femtosecond fluorescence up-conversion measurements. While an unusually fast forward ESIPT is clearly observed in all of the PILs studied, the uncommon back ESIPT process is distinctly indicated in PIL solvents having lower viscosities, certainly due to reasonably fast diffusive solvent relaxation in these solvents that causes a temporal modulation in the energies of the normal and tautomeric forms within a reasonably short time and thereby brings down the energy of N* compared to that of T*, triggering the back ESIPT process. Observation of solvent-viscosity-dependent back ESIPT is an intriguing finding for the present study as to the best of our knowledge, such a behavior has so far not been reported in the literature for the ESIPT reaction.

Combined TDDFT and AIM Insights into Photoinduced Excited State Intramolecular Proton Transfer (ESIPT) Mechanism in Hydroxyl- and Amino-Anthraquinone Solution

“…Thus, a fast ESIPT process is expected for 1,5-DHAQ and 1,4-DHAQ with the double ESIPT process. Furthermore, the energy of 1,8-DHAQ became slightly higher after ESIPT (Figure S1 ), giving rise to the solvent-sensitive ESIPT process for 1,8-DHAQ 25 .…”

“…However, 1,4-DHAQ 37 , 38 and AAAQ 40 – 43 with the similar molecular structures don’t show the ESIPT progress. More interestingly, 1,8-DHAQ 25 , 37 – 39 , 44 – 47 and CAAQ 40 – 43 switch their properties in different solvents. Blank et al .…”

Introducing the Bipolar Solvent Media Using the Aqueous Mixtures of Amino Acid Anion-Based Ionic Liquids

“…To ascertain the above described proton transfer phenomenon in these mixtures, we have studied the UV–vis absorption spectrum of 18DHAQ in [N 2221 ]­PhAla. 18DHAQ dye possesses two hydrogen atoms in its native form (the structure of the dye is given in Figure c) and displays the UV–vis absorption maximum at around 430 nm in almost all of the molecular solvents and ionic liquids, irrespective of their polarity. − However, if the dye is dissolved into a basic medium, there is a possibility of proton transfer from the dye molecule to solvent. This proton transfer might significantly alter the absorption maximum of the dye.…”

Excited-State Intramolecular Proton Transfer Reaction and Ground-State Hole Dynamics of 4′-N,N-Dialkylamino-3-hydroxyflavone in Ionic Liquids Studied by Transient Absorption Spectroscopy

“…Ionic liquids (ILs) with long alkyl chains have an inhomogeneous structure because the polar and nonpolar parts are segregated. When the length of the alkyl chain exceeds a certain value, the polar and nonpolar parts form a heterogeneous microphase structure, as demonstrated by molecular dynamics simulations and scattering experiments. − This results in solvation heterogeneity about the solute molecules dissolved in the IL and affects the solvation, dynamics, and even chemical reactions significantly. − The heterogeneous solvation of solute molecules has been observed via monitoring the electronic states, vibrational frequencies, rotations, and diffusion of probe molecules by absorption, fluorescence, resonance Raman, extended X-ray absorption fine structure (EXAFS), and NMR spectroscopy. Depending on the hydrophilicity and polarity of the solute, the presence of preferential solvation, which can occur in mixed solvents, in a microphase-separated environment of anions and cations has been proposed.…”