Substituent and Solvent Polarity on the Spectroscopic Properties in Azo Derivatives of 2-Hydroxynaphthalene and Their Difluoroboranes Complexes

Abstract

Novel fluorescent dyes such as difluoroborane complexes of 1-phenylazonaphthalen-2-ol derivatives were successfully synthesized and characterized with a focus on the influence of a substituent and a solvent on the basic photophysical properties. 1H, 11B, 13C, 15N, and 19F nuclear magnetic resonance (NMR) spectra of substituted 1-phenylazonaphthalen-2-ol difluoroboranes and their parent azo dyes were recorded and discussed. The absorption and emission properties of synthesized compounds were investigated in solvents of varying polarity. They were found to be fluorescent despite the presence of the azo group. The azo group rotation was blocked by complexing with -BF2 to get a red shift in absorption. Solvent-dependent spectral properties of compounds were investigated using Lipper-Mataga and Bakhshiev plot. The calculated DFT energies and Frontier Molecular Orbitals calculations of the studied compounds were proved to be consistent with the experimental observations.

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

“…It is hard to find reasonable correlations of ɛ with any empirical or physically determined polarity parameter of the solvent. [1,[34][35][36][37][38][39] For example, ɛ clearly increases from 16 000 Lmol À 1 cm À 1 to 20 000 Lmol À 1 cm À 1 for the positive solvatochromic dye 4-nitroaniline with increasing solvent polarity going from 1,4-dioxane, propylene carbonate to DMSO. [37] However, in water a lower ɛ about 14 000 Lmol 1 cm À 1 is again found.…”

“…It is unsatisfactory that intensity changes in the UV/Vis‐spectrum of a dye depending on the nature of the solvent have so far not been accepted as a criterion for the evaluation of solvent influences [1a] . One reason for the nonacceptance is that the molar absorption coefficient ϵ of normal dyes, for example like crystal violet, [34] substituted 5‐methylisoindolo [2,1‐a] quinolines [35] or azobenzens, [36] is practically not influenced by the type of solvent. It is hard to find reasonable correlations of ϵ with any empirical or physically determined polarity parameter of the solvent [1,34–39] .…”

“…One reason for the nonacceptance is that the molar absorption coefficient ϵ of normal dyes, for example like crystal violet, [34] substituted 5‐methylisoindolo [2,1‐a] quinolines [35] or azobenzens, [36] is practically not influenced by the type of solvent. It is hard to find reasonable correlations of ϵ with any empirical or physically determined polarity parameter of the solvent [1,34–39] . For example, ϵ clearly increases from 16 000 Lmol −1 cm −1 to 20 000 Lmol −1 cm −1 for the positive solvatochromic dye 4‐nitroaniline with increasing solvent polarity going from 1,4‐dioxane, propylene carbonate to DMSO [37] .…”