Investigating the Halochromic Properties of Azo Dyes in an Aqueous Environment by Using a Combined Experimental and Theoretical Approach

Abstract

The halochromism in solution of a prototypical example of an azo dye, ethyl orange, was investigated by using a combined theoretical and experimental approach. Experimental UV/Vis and Raman spectroscopy pointed towards a structural change of the azo dye with changing pH value (in the range pH 5-3). The pH-sensitive behavior was modeled through a series of ab initio computations on the neutral and various singly and doubly protonated structures. For this purpose, contemporary DFT functionals (B3LYP, CAM-B3LYP, and M06) were used in combination with implicit modeling of the water solvent environment. Static calculations were successful in assigning the most-probable protonation site. However, to fully understand the origin of the main absorption peaks, a molecular dynamics simulation study in a water molecular environment was used in combination with time-dependent DFT (TD-DFT) calculations to deduce average UV/Vis spectra that take into account the flexibility of the dye and the explicit interactions with the surrounding water molecules. This procedure allowed us to achieve a remarkable agreement between the theoretical and experimental UV/Vis spectrum and enabled us to fully unravel the pH-sensitive behavior of ethyl orange in aqueous environment.

Substituent effects on absorption spectra of pH indicators: An experimental and computational study of sulfonphthaleine dyes

“…We recently elucidated the pH-sensitive behaviour of the azo dye Ethyl Orange. [6] Accurate UV/Vis spectra of the dye in a water environment were simulated using molecular dynamic simulations combined with TD-DFT; which were in good agreement with experiment. The focus of the present paper is to use a similar combined theoretical and experimental approach on a set of sulfonphthaleine dyes.…”

“…The previous rationale also applies for another compound, for example 2, but do note this is at a different pH (more specifically pH 8.3 for 2). The ratio of both equilibrium constants can then be written as (5) and simplified to (6).…”

“…[3] The most exploited chromic types are thermochromism and photochromism, [4,5] but halochromism (pH sensitivity) is also increasingly studied and put into practice. [6][7][8][9][10][11][12] Our main interest lies in the possible application of halochromic dyes in pH-responsive polymers, or more precisely intelligent textiles. This is part of a larger research field, namely functionalized polymers.…”

Vibrational fingerprint of the absorption properties of UiO-type MOF materials

“…3.2). This methodology and similar level of theory (employing BLYP instead of PBE) have proven to yield accurate results for these types of systems [22,31]. The PBE functional is used in this work in correspondence with Refs.…”

“…This study followed the procedure as proposed by the group of Barone et al, who demonstrated that averaging over different configurations-generated during an MD simulationleads to reliable dynamic estimations of transition properties in good agreement with experimental data [26][27][28][29]. Such methodology was previously successfully applied for a large variety of systems, ranging from small gas phase molecules over solvated dyes to dyes occluded in a porous zeolitic environment [29][30][31][32]. In the majority of the studies, the electronic transitions are calculated using TD-DFT in its linear response formulation, since it offers a good accuracy/computational cost ratio [33][34][35].…”