Raman Spectroscopic Study on the Solvation of N,N-Dimethyl-p-nitroaniline in Room-Temperature Ionic Liquids

Abstract

Electronic absorption spectra and Raman spectra of N,N-dimethyl-p-nitroaniline (DMPNA) have been measured in various fluids from the gaseous-like conditions in supercritical fluids (SCFs) to highly polar room-temperature ionic liquids (RTILs). We found that the S0-S1 absorption band center of DMPNA in RTILs is mostly determined by the molar concentrations of ions. On the other hand, the bandwidth of the absorption spectrum does not follow the expectation from a simple dielectric continuum model. Especially in SCFs, the bandwidth of the absorption spectrum decreases with increasing solvent density, suggesting that the intramolecular reorganization energy is a decreasing function of the solvent density. The Raman shift of the NO2 stretching mode has been proven to be a good indicator of the solvent polarity; i.e., the vibrational frequency of the NO2 stretching mode changes from 1340 cm-1 in mostly nonpolar solvent such as ethane to 1300 cm-1 in water. The linear relationship between the absorption band center and the vibrational frequency of the NO2 mode, which was observed for conventional liquids in a previous paper (Fujisawa, T.; Terazima, M.; Kimura, Y. J. Chem. Phys. 2006, 124, 184503), holds almost well for all fluids including SCFs and RTILs. On the other hand, the vibrational bandwidth does not show a simple relationship with the absorption band center. The vibrational bandwidths in RTILs are generally larger in comparison with those in conventional liquids with similar polarity scales. Among the RTILs we investigated, the vibrational bandwidth loosely correlates with the molecular size of the anion. A similar dependence on the anion size is also observed for the bandwidth of the absorption spectrum. We have also investigated the excitation wavelength dependence of the Raman shift of the NO2 stretching mode in RTILs. The extent of the dependence on the excitation wavelength in all fluids is well correlated with the vibrational bandwidth.

Transient absorption studies of the photochromic behavior of 3H-naphtho[2,1-b]pyran linked to a p-nitroaniline group

“…The assumption that the S 1 state of 1 is essentially localized on the p-nitroaniline group is consistent with the observation of substantial analogies between the S 1 states of p-nitroaniline (PNA) 12 and N,N-dimethyl-p-nitroaniline (DPNA), 13 concerning their dynamics and spectral properties. Indeed, the S 0 -S 1 absorption band of 1 is quite comparable in shape and position to the strong S 0 -S 1 charge transfer band observed for PNA (l max 365 nm) 12 and DPNA (l max 385 nm) 13 in acetonitrile. The short-lived character of S 1 (about 1.5 ps) is also in agreement with the ultrafast S 1 decay kinetics reported for PNA (0.30 ps) and DPNA (0.63 ps) in acetonitrile.…”

Solute–solvent hydrogen-bonding in room temperature ionic liquids studied by Raman spectroscopy

“…For example, the vibrational spectrum is more sensitive to local solute-solvent interactions than the electronic spectrum, and many vibrational spectroscopy studies have been performed to investigate solute-solvent interactions in RTILs. [15][16][17][18] However, a direct comparison between the solvatochromic parameters and vibrational spectral shifts in RTILs has not been well demonstrated until now. Welton et al investigated the IR spectra of water dissolved in BMIm-cation based RTILs using ATR-IR spectroscopy and found that, relative to those in the vapor phase, both Fermi-splitting OH bands show a significant shift depending on the RTIL anion species, suggesting the existence of hydrogenbonding interactions between the water molecules and RTIL anions.…”

Anomalous ground-state proton transfer of 4′-N,N-diethylamino-3-hydroxyflavone in ionic liquids of imidazolium-based cations with tetrafluoroborate

“…4,5 We considered that this phenomenon is related to the local heterogeneity of the solvation in ILs, which has been studied using various experimental and theoretical methods. [6][7][8][9][10][11][12][13] In this study, we found a unique ground state reaction of DEAHF, i.e., formation of the tautomer in the ground state in specific ILs. We also found that water contamination strongly affects the equilibrium between the normal and tautomeric forms, and that there is a unique slow conversion process from the normal form to the tautomeric form in ILs.…”

“…This is in contrast to effects of water on other probe molecules. 9 on the Raman shift in [BMIm][BF 4 ] was not significant up to 13 700 ppm. We also tested the effect of BF 4 À in a conventional liquid solvent.…”