Modeling solvatochromism of Nile red in water

Abstract

AbstractSpurred by the pioneering modeling studies of Sylvio Canuto on absorption spectra and solvatochromic shifts of organic molecules in polar and nonpolar solvents, we report in this work a computational study for the common optical dye probe Nile red (NR) to elucidate the origin of its absorption shift between gas phase and aqueous solution. The Car‐Parrinello molecular dynamics (CPMD) technique is used for gas phase NR, whereas for NR in water solvent, a hybrid quantum mechanics‐molecular dynamics (CPMD‐QM/MM) approach has been utilized. For the configurations obtained from CPMD and CPMD‐QM/MM, the absorption spectrum has been calculated using the INDO/CIS method as implemented in the ZINDO program. Different solvation shells for NR in water have been defined based on solute‐all‐atoms and solvent center of mass radial distribution function (g(rX–O)rdf). The electronic excitation energies for these solvation shells were calculated in a systematic way to evaluate their individual contributions. In addition, calculations of absorption spectra were performed for NR (excluding solvent molecules) obtained from CPMD‐QM/MM calculations to isolate the contribution to the solvatochromic shift just due to solvent‐induced geometrical change. Interestingly, this geometrical change in NR itself contributes as much as 50 nm to the solvatochromic shift. The calculated λmax for gas phase is around 488 nm and is comparable to the values reported for NR in nonpolar solvents, whereas the inclusion of solvent molecules in the hydration shell yields a λmax of 565 nm which contributes to almost 77 nm of the solvatochromic shift. The inclusion of solvent molecules up to the fourth solvation shell in the g(rX–O) rdf yields λmax of 596 nm which is in good agreement with the experimentally reported value 593 nm. The change in λmax due to inclusion of the fourth solvation shell is only 1 nm, indicating that the spectrum has converged with respect to the solvent effect. Overall, this study suggests that the combined use of CPMD‐QM/MM and ZINDO can be successfully used to model and to interpret solvatochromic and thermochromic behavior of NR and other organic dye molecules. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010