Solvatofluorochromism and twisted intramolecular charge‐transfer state of the nile red dye

Abstract

Profiles of the S1 potential energy surface of the Nile Red dye along the rotational coordinate of the amino group are computed using time‐dependent density functional theory (TDDFT) and XMCQDPT2/CASSCF. The calculated profiles exhibit two minima corresponding to a planar locally excited (LE) state and a twisted intramolecular charge transfer (TICT) state. The profiles calculated by time‐dependent density functional theory (TDDFT) depend on the weight of the hartree‐fock (HF) exchange in the functional: at 0% exchange, only the TICT minimum exists, whereas at 50% exchange and more there is only the LE minimum. The profiles obtained by TDDFT at 20–25% HF exchange are in qualitative agreement with that obtained by XMCQDPT2/CASSCF calculations. The energy of the charge transfer state is lowered due to the participation of doubly excited configurations and dynamic correlations, which is implicitly included in the density functionals. The solvent effects on the relative energies of the LE and TICT states and on the barrier height are studied using the polarizable continuum model. The effect of hydrogen bonds is studied for a complex of Nile Red with two water molecules. The solvatochromism of Nile Red fluorescence in aprotic polar solvents is explained by nonspecific solvation, which stabilizes the LE state and causes Nile Red fluorescence solvatochromism; in water and alcohols, it is explained by the formation of hydrogen bonds, which stabilize the TICT state and facilitate the LE‐to‐TICT transition. © 2012 Wiley Periodicals, Inc.

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

“…Any rotation around the C-N bond that moves the diethylamino groups out of the molecular plane causes the S 1 excitation to have an increasing amount of charge transfer character. In contrast to higher order quantum chemistry methods, semi-local functionals like PBE predict the excitation energy of the S 1 state to decrease for more twisted conformations, while range-separated hybrid functionals like CAM-B3LYP correctly predict an increase [28,29].…”

“…1) and its absorption spectrum in the visible region is dominated by the S 1 state corresponding to a HOMO-LUMO transition with strong oscillator strength. 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].…”

“…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.…”

Relation of molecular structure to Franck–Condon bands in the visible-light absorption spectra of symmetric cationic cyanine dyes

“…The second problem arises from the complex potential energy surface (PES) for those cyanines that undergo fast trans-cis photoisomerization from the ES, involving C-C bond rotations along the polymethine chain [19,20,[50][51][52]. The problem here is that the transition is no longer truly adiabatic, and the R ES most relevant for computing transition energies is not a relaxed geometry but a pseudo-stationary FC point on the PES [53,54,31].…”

“…Note that energies for dye I, and all calculations for dyes II-VII, were performed using the diffuse basis and B3LYP functional. A reasonable lambda diagnostic [60,51] of approximately 0.7 was obtained for all seven dyes.…”