Probing of local polarity in poly(methyl methacrylate) with the charge transfer transition in Nile red

Abstract

The permittivity of polymers and its spatial distribution play a crucial role in the behavior of thin films, such as those used, e.g., as sensor coatings. In an attempt to develop a conclusive approach to determine these quantities, the polarity of the model polymer poly(methyl methacrylate) (PMMA) in 600 nm thin films on a glass support was probed by the energy of the charge transfer transition in the oxazine dye Nile red (NR) at 25 °C. The absorption and fluorescence spectra of NR were observed to shift to the red with increasing solvent polarity, because of the intramolecular charge transfer character of the optical transition. New types of solvatochromic plots of emission frequency against absorption frequency and vice versa afforded the Onsager radius-free estimation of the ground and excited states dipole moment ratio. With this approach the values of these dipole moments of 11.97 D and 18.30–19.16 D, respectively, were obtained for NR. An effective local dielectric constant of 5.9–8.3 for PMMA thin films was calculated from the solvatochromic plot and the fluorescence maximum of NR observed in the PMMA films. The fluorescence band of NR in the rigid PMMA films shifted to the red by 130 cm−1 with increasing excitation wavelength from 470 to 540 nm, while in a series of liquids the position of the emission maximum of NR remained constant within same range of the excitation wavelength. It is concluded that the fluorescence spectrum of NR in PMMA undergoes inhomogeneous broadening due to different surroundings of NR molecules in the ground state and slow sub-glass transition (T g) relaxations in PMMA.

Reappraisal of Empirical Solvent Polarity Scales for Organic Solvents

“…In future, complementary conceptual methods as the Kawski model [76] should be also used in comparison to the conception of this work. The analysis of solvatochromic effects of appropriate solvatochromic probe molecules such as Nile red is recommended [77–81] . It is suggested that this probe may be useful to better understand the different relationships f( n ) as a function of N for the different solvent classes, because the solvatochromic property of Nile red is strongly determined by the solvent class [79,81] …”

Probing the Local Polarity in Biocompatible Nanocarriers with Solvatofluorochromism of a 4-Carbazole-1,8-naphthalimide Dye

“…The first observation can be attributed to a much lower Δµ in the case of NR (established at ca. 6.30 D) [7], which leads to a less pronounced bathochromic shift in emission (ca. 3000 cm −1 against ca.…”

“…Steady-state emission spectroscopy also allows probing local permittivity in polymers and has been used to characterize spatial heterogeneity of ε in polymeric films through single molecule or fluorescence microscopy studies [3,5]. Nile Red (NR) has been widely used as a luminescent dye in these approaches, owing to its photostability and convenient optical properties including high photoluminescence quantum yield (PLQY) and absorption and emission spectra strongly sensitive to the local environment [7,8]. NR has also been particularly used in biological applications, for instance in the detection of proteins from the spectral position of its fluorescence, or to determine intracellular local polarity from the variation of its lifetime (τ F ) in fluorescence lifetime imaging spectroscopy (FLIM) [9,10].…”

“…The photoluminescent behavior dependent on the environment displayed by NR has been attributed to the formation of a twisted intramolecular charge transfer (TICT) state, arising from a conformation change of the electron-donating diethylamino moiety [11]. This process results in a large dipole moment change between the ground and excited state, which influences their interaction in solvent or surrounding molecules [7]. Despite numerous and intensive studies, both on practical applications of theoretical and NR photophysics, there is still debate on whether the TICT is controlled by medium polarity, rigidity, or dielectric constant [12][13][14].…”

“…Despite numerous and intensive studies, both on practical applications of theoretical and NR photophysics, there is still debate on whether the TICT is controlled by medium polarity, rigidity, or dielectric constant [12][13][14]. In addition, it has been demonstrated that the photophysics of NR is significantly affected by particular solvent parameters, such as high polarizability, or specific solvent-solute interactions, hindering attempts to establish models that accurately predict its behavior [7,15]. Under this framework, we propose a novel 1,8-naphthalimide dye as a polarity probe whose photophysical behavior can be adequately described using medium polarity parameters.…”