Reversible Absorption and Emission Responses of Nile Blue and Azure A Derivatives in Extreme Acidic and Basic Conditions

Abstract

Oxazinium derivatives have recently played an important role in bioanalysis attributing to the distinguished properties, thus a detailed study of the structure-property relationship is especially significant. Herein, pH-sensitive optical properties of Nile Blue (1a), N-monoalkyl-Nile Blue (1b) and Azure A (1c) have been carried out in extreme acid and base conditions. Dyes 1a and 1c showed colorimetric changes by the protonation of nitrogen atom in strong acidic condition (pH < 2.0), and dyes 1a - c exhibited colorimetric changes by equilibrium between amino and imide groups in very strong basic case (pH > 7.6). Besides, their fluorescent properties were closed to ON - OFF and OFF - ON emissions at 640-820 nm under strong acidic and basic conditions. Moreover, the absorption and emission properties were reversible, and there were no remarkable optical intensity changes of dyes 1a - c under subacidic and neutral solutions (pH = 3.0-7.0). The (TD) DFT calculations were used to optimize the most stable structures of their corresponding protonated and deprotonated forms, and their absorption and emission properties were also explained. Their fluorescent properties nearly ON-OFF and OFF - ON in strong acidic and basic conditions at near-infrared region will give the possible application in pH detection for extreme conditions. Graphical abstract ᅟ.

TD‐DFT absorption spectrum of Azure A in aqueous solution: Vibronic transitions and electronic properties

“…(LUMO+1), λ vert2 = 410 nm). A similar calculated result (B3LYP/TZVP/CPCM theory level) was obtained for AA + in aqueous solution in Wang et al [36]: S 0 ! S 1 (f 1 = 0.4420, (HOMO-1) !…”

“…This indicates the importance of considering the vibronic coupling when calculating the absorption spectra. Although direct comparison of λ vert and λ exp is widespread in the literature (see References [36,42,43] in the case of AA + ), it is more correct to compare λ vibron and λ exp [50]. Table S1 and Figure 3 show that all functionals with IEFPCM solvent model give an underestimated λ vibron values in comparison with the experimental value λ max = 632 nm = 1.96 eV ("cyanine failure").…”

“…LUMO and HOMO!LUMO, λ vert2 = 425 nm). At the same time, the authors of References [36,43] did not correlate the S 0 ! S 2 transition with the experimental spectrum.…”

“…Therefore, to calculate the vibronic absorption spectrum of AA + in an aqueous solution, we used all the hybrid functionals supported by Gaussian16. Note what in the three above-mentioned (see Section 1) theoretical papers [36,42,43], the most popular hybrid functional B3LYP [59] was used. Singlet transitions HOMO!LUMO in the visible region of the spectrum were analyzed.…”

The polarity sensitivity factor of some fluorescent probe molecules used for studying supramolecular systems and other heterogeneous environments

“…However, each has a unique shape and fluorescence properties, which may potentially make them more useful in specific applications. Nile Blue A 6[41][42][43] is an oxazine dye, closely related to the more commonly used Nile Red. However, whereas Nile Red is insoluble in water (thus not allowing for a determination of its PSF), Nile Blue A is water soluble.…”