Cage Effect in Poly(alkyl methacrylate) Thin Films Studied by Nile Red Single Molecule Fluorescence Spectroscopy

Abstract

Interactions in a probe–polymer cage were monitored by spectral fluctuations in the emission spectroscopy of single molecules of Nile Red in poly(alkyl methacrylate) thin films (25–200 nm) in the 278–323 K temperature range. Three types of emission spectra were identified. The highest emission energy spectra show small amplitude fluctuations and a very low probability of changing their spectral emission features. On the other hand, the other two types of emission profiles exchange more frequently. The fluctuations are analyzed by the complementary cumulative distribution function of spectral emission energy difference between successive spectra in a time trace. The fluctuations show three components: two of them with zero mean average and distinctly different standard deviations and a third component with much lower frequency and an amplitude absolute value of 0.13–0.15 eV. This amplitude is the same in all conditions, pointing to a common feature of the probe–polymer cage as responsible for their presence: complex cage rearrangements, involving the carbonyl side chain of the polymer as the common actor are postulated to be the cause of these spectral fluctuations in the time range of seconds.