Micro- vs Macrosolvation in Reichardt’s Dyes

Abstract

Solvation is a complex phenomenon involving electrostatic and van der Waals forces as well as chemically more specific effects such as hydrogen bonding. To disentangle global solvent effects (macrosolvation) from local solvent effects (microsolvation), we studied the UV−vis and IR spectra of a solvatochromic pyridinium-N-phenolate dye (a derivative of Reichardt's dye) in rare gas matrices, in mixtures of argon and water, and in water ice. The π−π* transition of the betaine dye in the visible region and its C−O stretching vibration in the IR region are highly sensitive to solvent effects. By annealing argon matrices of the betaine dye doped with low concentrations of water, we were able to synthesize 1:1 water−dye complexes. Formation of hydrogen-bonded complexes leads to small shifts of the π−π* transition only, as long as the global polarity of the matrix environment does not change. In contrast, changes of the global polarity result in large spectral band shifts. Hydrogen-bonded complexes of the betaine dye are more sensitive to global polarity changes than the dye itself, explaining why E T values determined with Reichardt's dyes are very different for protic and nonprotic solvents, even if the relative permittivities of these solvents are similar.

The Negative Solvatochromism of Reichardt‘s Dye B30 – A Complementary Study

“…In HBD solvents, the susceptibility (slope) as function of polarity is significantly smaller compared to non‐HBD solvents. According to the modern view, the formation of hydrogen bonds at the phenolate‐betaine dye alters the inherent dipole moment of the dye, which is reflected in an additional influence of the global polarity of the solvent on and as recently discovered [26] …”

“…According to the modern view, the formation of hydrogen bonds at the phenolate-betaine dye alters the inherent dipole moment of the dye, which is reflected in an additional influence of the global polarity of the solvent on e ñmax ð Þ and ñmax as recently discovered. [26] Thus, hydrogen bonding in combination with high global polarity has a stronger effect on lowering the inherent dipole moment of the dye. Therefore, changes in the polarity of solvents that cannot form H-bridges have a stronger effect on the changes in e ñmax ð Þ, because H-bridges have a damping, levelling effect.…”

“…The spectral footprint of the electronic transition of B30 as function of solvent property has been subject of numerous experimental and theoretical studies in literature. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Due to the electronic ground state being more polar than the first exciting state, strong dipole/dipole interactions of the dye with the solvent in the ground state should determine the UV/Vis absorption property and lead to significant negative solvatochromism. [11][12][13][14][15] This is, nowadays, the classic teaching.…”

“…[1,2] Nevertheless, there are still doubts about the correct physical interpretation of the E T (30) parameter. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Since dipolar influences of the solvent in terms of the static dielectric constant (ɛ r ) play an essential role with respect to the dielectric function via the Clausius-Mossotti relation f(ɛ r ) = (ɛ r -1)/(2ɛ r + 2), [5,18] the different effects of hydrogen bond donating (HBD) compared to non-HBD solvents were already recognized in the first work on this topic. [7] For both solvent families, separate logarithm-like curves of E T (30) against the dielectric constant f(ɛ r ) are found.…”

Reaction of electrons trapped in cryogenic matrices with benzophenone

“…The unpolar argon matrix and the highly polar amorphous water ice represent opposite ends of the solvent polarity scale. [ 67 ] Although sodium atoms can be isolated in solid argon and visible light excitation is necessary for their ionization, they spontaneously dissociate into Na + and hydrated electrons in the water matrix. The dissociation of sodium atoms in water is driven by the high electron affinity of water ice and by the stabilization of Na + in the polar matrix.…”