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

Aydan Yadigarli,Sergey I Druzhinin,Holger Schönherr,Qimeng Song

doi:10.3762/bjoc.15.248

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 (Tg) relaxations in PMMA.

Highlights

The chain and segment mobility as well as the permittivity of polymers possess an enormous impact on the properties of polymers and polymer thin films
The maxima in the fluorescence spectra of Nile red (NR) in liquid solvents at 25 °C were observed to gradually shift to the red, from 17660 cm−1 in nonpolar n-hexane to 16090 cm−1 in polar acetonitrile, showing no indication of dual fluorescence (Figure 2a). This behavior for NR [35] is similar to donor–acceptor-substituted stilbenes [36,37], benzenes [38,39] and aminocoumarins [35,40,41]. This effect is caused by solvent relaxation around dipolar solutes that possess in the lowest excited state a substantially higher dipole moment than that in the ground state
New types of relative solvatochromic plots, in which the position of the emission maximum is plotted versus the position of absorption maximum or vice versa, allow one to estimate the ratio of the ground and excited state dipole moments

Summary

Introduction

The chain and segment mobility as well as the permittivity of polymers possess an enormous impact on the properties of polymers and polymer thin films. The same dye has been reported as probe for local permittivity in polymers, in particular, the spatial distribution of the dielectric constant measured for thin PMMA films was described using NR as a reported dye [15]. The spatial distribution of the dielectric constant for thin PMMA films was mapped by analyzing the position of the fluorescence of NR employed as a reporter dye [15]. For PMMA, the α-relaxation as slowest relaxation is observed at the glass transition (Tg = 95–110 °C) [34] It corresponds to long-range conformational changes of the polymer backbone. In PMMA, the dynamics of the ester group (β-relaxation) can be coupled with the α-relaxation [29]

Results and Discussion

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Conclusion