Thermosolvatochromism of Phenol Blue in Polar and Nonpolar Solvents

Abstract

The solvatochromic properties of phenol blue [N-(4-dimethylaminophenyl)-1,4-benzoquinoneimine] have been investigated in polar and nonpolar solvents using variable-temperature 1 H NMR and electronic absorption spectroscopy. In acetonitrile, chloroform, and cyclohexane, protons on the quinoneimine ring are inequivalent at all temperatures, while in methanol an exchange process increasingly broadens the signals from these protons as the temperature is raised. The temperature-dependent absorption spectrum of phenol blue in methanol, acetonitrile, chloroform, and cyclohexane is reported, and the observed increase in peak frequency and bandwidth with temperature is found to exceed the predictions of continuum theory. The large increase in dipole moment on excitation results in a strong inductive contribution to the solvatochromism, and leads to large thermochromic shifts even in nonpolar solvents. The possible contributions of solvent-dependent electronic structure, low-frequency intramolecular vibrations, and conformational flexibility to the thermosolvatochromic properties of phenol blue are discussed. Evidence is presented for solvent dependence of the dipole difference µ eµ g between the ground and excited electronic states.

Ultrafast Dynamics of a Solvatochromic Dye, Phenol Blue: Tautomerization and Coherent Wavepacket Oscillations

“…Phenol blue (PB), also known as N , N -dimethylindoaniline, N -(4-dimethylaminophenyl)-1,4-benzoquinoneimine, is a well-known nonfluorescent solvatochromic dye − which has been widely studied by spectroscopic methods, such as resonance Raman scattering, − time-resolved transient absorption (TRTA), , and NMR, , as well as theoretical simulations. − PB is composed of an electron-donating (dimethylaniline) moiety connected to an accepting (benzoquinone monoimine) one by a π-conjugated bridge. Upon photoexcitation, charge transfer (CT) occurs from the donor to the acceptor moiety, and the contribution from the zwitterionic resonance structure becomes stronger than that from the covalent quinoneimine one in the first excited-singlet (S 1 ) state (Chart , top).…”

“…It was suggested that because of the inhomogeneous distribution of the solvation environment, the narrow band of the excitation laser selects the individual “solvation state” at each wavelength. Meanwhile, the visible absorption band of PB is reported to exhibit not only solvatochromism but also thermochromism in MeOH solution . By increasing the temperature, the maximum absorbance decreases, and the absorption band slightly shifts to shorter wavelengths.…”

“…By increasing the temperature, the maximum absorbance decreases, and the absorption band slightly shifts to shorter wavelengths. Although the shift might as well be due to the temperature-dependent change of the solvent dielectric constant or refractive index, observation of the isosbestic point at 567 nm suggests that the system could be in thermal equilibrium between two distinct forms of PB . Existence of tautomers can also cause excitation wavelength dependence of the resonant Raman bands.…”

Ethanol-guided synthesis of (flower-on-leaf)-like aniline oligomers with excellent adsorption properties

“…In the FTIR spectrum ( Fig. 22 The signals at 7.42 and 7.31 ppm might correspond to the benzenoid protons of the 1,4-benzoquinoneimine ring, 23 while the peaks centered at 6.18 and 5.60 ppm should be related to the benzenoid protons of substituted 1,4-benzoquinone. The sharp band at 1444 cm À1 is attributed to the skeletal CQC stretching vibration of the substituted aromatic ring, and the signal observed at 1413 cm À1 could be assigned to symmetric stretching of the phenazine heterocyclic ring.…”

“…21 Concretely, the benzenoid aromatic protons could be identified by these signals centered at 7.47, 7.26 and 7.00 ppm, indicative of the ending benzene rings in these oligomers. 22 The signals at 7.42 and 7.31 ppm might correspond to the benzenoid protons of the 1,4-benzoquinoneimine ring, 23 while the peaks centered at 6.18 and 5.60 ppm should be related to the benzenoid protons of substituted 1,4-benzoquinone. 24 These results indicate the presence of 1,4-benzoquinoneimine and 1,4-benzoquinone segments in the aniline oligomers.…”