Titrimetric and Equilibrium Studies Using Indicators Related to Nile Blue A.

Predicting solvatochromic shifts and colours of a solvated organic dye: The example of nile red

“…The absorption maximum undergoes a spectral shift that is roughly correlated with increasing solvent polarity [25]. Furthermore, in some non-polar solvents like cyclohexane, the single absorption feature is found to exhibit two separate identifiable peaks, while in solvents with stronger polarity only a single peak is found.…”

“…It was noted by Davis et al that while nile red exhibits a spectral shift whose magnitude correlates with solvent polarity, there are also a number of clear outliers from this general trend [25]. To demonstrate this we take a selection of 20 solvents for which the peak absorption wavelengths of nile red are published [32], ranging from non-polar to strongly polar, and As can be seen from Fig.…”

“…Furthermore, in some non-polar solvents like cyclohexane, the single absorption feature is found to exhibit two separate identifiable peaks, while in solvents with stronger polarity only a single peak is found. There are a number of further subtle solvent-dependent changes to the spectra that go beyond a simple shift of the peak position, with nonpolar solvents cyclohexane and 2,2,4-trimethylpentane showing an asymmetric absorption peak profile with a tail in the blue wavelengths, while more polar solvents like acetone produce a more broadened peak profile [25] (see also section 8 in the supplementary material). The significant changes to the absorption spectrum in the visible region cause solutions stained with nile red to differ strongly in colour depending on the solvent used.…”

“…In this work, we consider nile red, a standard fluorescent dye soluble in organic solvents and alcohol, whose absorption and fluorescence properties are highly sensitive to its environment [25][26][27]. The dye consists of a 2-diethylamino group connected to an aromatic ring system via a single C-N bond (see Fig.…”

“…Studies with both TDDFT [28] and higher order quantum chemistry methods [29] have shown that rotation around the C-N bond is a key feature influencing the dye's absorption and fluorescence spectra. Nile red is very stable in alkaline aqueous environments [25] compared to other dyes like phenol blue [30] and its fluorescent properties have proven to be useful in a number of biological applications, for example as a stain for the detection of intracellular lipid droplets [31]. For this reason, the absorption and emission properties of nile red are well characterised experimentally in a large number of different solvent environments [25,32].…”