Photodegradation of pyrene-labeled polystyrene adsorbed on silica surface in chloroform

Adsorption of polymers onto particles has been a subject of study because of its importance in the behavior of suspensions.1,2 To understand the adsorption/desorption process in detail, studies have been carried out with model systems. Some studies have employed polystyrene (PS) adsorbed onto metals, silica, glass, and mica.3-7 In previous work, we reported on the use of pyrene-labeled PS to study the dynamics of the rearrangement of adsorbed PS after exposing the PS adsorbed on a silica surface to desorption conditions, i.e., after replacing the bulk solution containing unabsorbed PS with pure solvent.8 Pyrene has been used widely in the literature as a probe of structure and dynamics in macromolecular systems.9-11 From the fluorescence emission properties of pyrene labels, such as the I3/I1 ratio, the excimerto-monomer ratio (Ie/Im), and fluorescence polarization value, one obtains information about the conformation, mobility, and dielectric environment of the pyrene probe within the macromolecule being studied. In this note, we report on the photodegradation of the pyrene labels on PS in chloroform (CHCl3) and adsorbed on silica surface contacting with CHCl3 caused by excitation light under conditions typical for photophysical studies of polymers. We feel this effect should be brought to the attention of the community because of the common use of pyrene as a probe.12,13 As published in a previous paper,8 after the adsorption of labeled PS onto a silica surface has proceeded for a predetermined time in dilute PS/CHCl3 solution, replacing the bulk polymer solution with pure solvent CHCl3 causes a progressive relative increase in excimer emission intensity with time, i.e., an increase in Ie/Im ratio over time. In this work, we observe that the increase in Ie/Im ratio was dependent on the total exposure time of the sample to the excitation light in the fluorescence spectrometer. The synthesis and characterization of the pyrenelabeled PS were reported previously.8 The polymer used has a molecular weight of 50K and label density of 15 pyrene labels per chain. Porous silica samples were obtained as gifts from Dupont and had average pore sizes of 57 or 122 A. Solvent CHCl3 was distilled before use. The preparation of adsorbed, pyrene-labeled PS layers on silica was described before.8 The bulk polymer concentration for adsorption was 0.1 g/L in CHCl3. After adsorption for a certain time, the bulk polymer solution was replaced with pure CHCl3 by rinsing the solid and transferring it quickly into a fluorescence cell with a 2 mm path length containing pure CHCl3. The fluorescence emission and excitation spectra were taken on a SPEX Fluorolog 1680 0.22 m double spectrometer with a 450 W xenon lamp using front face excitation. The excitation light beam was directed onto the polymer/silica sediment pack, not the supernatant. First, the effect of the excitation light on the pyrene labels of the polymer adsorbed on a porous silica was investigated. Typical excitation spectra obtained with successive scans are shown in Figure 1. The excitation spectrum monitored at 480 nm (maximum emission wavelength for pyrene excimer) has several broad unexpected bands in the range 350-450 nm when compared to other reports.13 It is well-known that the excitation spectra of the preassociated pyrene molecules have similar vibrational structure as the pyrene monomer excitation spectra but are 1-4 nm red-shifted.13 The observed bands do not correspond to a static excimer of pyrene, which is expected when pyrene molecules are preassociated in the ground state. It was speculated that these bands were caused by other species that emit in the same spectral region as the pyrene excimer. At this point, we decided to investigate whether photodegradation products formed during the spectral collection process while the sample was being irradiated in the sample chamber of the fluorescence spectrometer was the cause of the observation. On the basis of this hypothesis, it was found that the intensity of the excitation spectra of pyrene labels changed dramatically with cumulative exposure to the instrument excitation light under conditions typical for photophysical studies. The typical observation is that intensities in the monomer excitation spectra decrease with the number of scans taken, while that in the excitation spectra monitored at the excimer emission wavelength increase dramatically, especially in the region of 350-450 nm. These changes could be caused by rearrangement of polymer chains on the surface upon exposure to pure solvent, as suggested earlier, or by photodegradation in the instrument as suggested here. To determine which mechanism was operating, an adsorbed layer of labeled PS on silica was prepared at conditions similar to that described in ref 8 and exposed to solvent for 2 days in complete absence of light, and then its fluorescence excitation spectra were acquired. This sample showed changes in its fluorescence excitation spectra similar to those shown Figure 1 which were taken within minutes after sample preparation. Since, after 2 days under desorption conditions, chain rearrangements should have reached an equilibrium, photodegradation of labels is strongly suspected as the cause of spectrum changes rather than conformational changes for which the labels maintain their chemical integrity. Similar spectral and intensity changes were observed for all the silica samples used in ref 8 and for three other pyrene-labeled PS with different molecular weight and different labeling densities.14 For comparison, we tested pyrene-labeled PS in CHCl3 solution under the same conditions in the fluorescence instrument with the results shown in Figure 2. In the fluorescence emission spectra with excitation at 330 nm, multiple scans caused an intensity decrease * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Chemical Engineering and Applied Chemistry. 4151 Macromolecules 1999, 32, 4151-4153