Modeling solvatochromism of Nile red in water

Abstract

Spurred 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(r X−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 λ max 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 λ max 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(r X−O ) rdf yields λ max of 596 nm which is in good agreement with the experimentally reported value 593 nm. The change in λ max due to inclusion of the fourth solvation shell is MURUGAN, RINKEVICIUS, AND ÅGREN 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.

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

“…In line with previous investigations carried out with ONETEP [11] and studies on nile red in water [7] we consider a large shell of solvent molecules explicitly in the TDDFT calculation. Here, we choose R shell = 11Å for all systems (see Section III C), corresponding to roughly 1050 to 1150 atoms per snapshot (see Tab.…”

“…Rather, it becomes necessary to fully include the first few solvation shells [7,11,33] in order to shift the energy levels associated with the solvent molecule to the right place and thus converging We point out that an inclusion of explicit solvent environment tends to lower the oscillator strengths of excitations compared with implicit solvent models. The reason for this can be seen in the different way the two approaches account for the dynamic polarisation effects in the solvent environment.…”

“…QM/MM approaches have been successfully applied to to the modelling of chromophores in complex environments, such as pigments in pigment-protein complexes [3][4][5]. However, recent work has shown that a range of properties of interest of systems embedded in complex environments, including the absorption properties of small solvated dyes, require the explicit inclusion of large parts of the embedding environment into the QM region in order to obtain consistently converged results [6][7][8][9][10][11]. In a recent work we demonstrated that to converge the solvatochromic shift of alizarin in water, system sizes of O(1000) atoms have to be treated fully quantum mechanically [11] in order to capture both short range polarisation and long range electrostatic effects on the excitation energies.…”

“…Nile red has been the focus of a considerable number of theoretical studies [7,28,29,[33][34][35][36], aimed at explaining its absorption and fluorescence properties in solution. For the purpose of this work, we investigate how well both the solvatochromic shift and the colour of the dye in solution can be predicted using state-of-the-art large-scale TDDFT calculations for the solute and large parts of the solvent environment.…”

Fiber-optic pulse width modulation sensor for low concentration VOC gas

“…As a result, it can be used to determine the polarities of biological objects and polymeric and organic-inorganic systems. This includes proteins and ormosils for synthesizing optical sensor materials for the detection of various solvent vapors [29,31,32].…”