Modulation of the Photophysical Properties of Pyrene by the Microstructures of Five Poly(alkyl methacrylate)s Over a Broad Temperature Range

Abstract

Pyrene fluorescence spectra have been recorded in five poly(alkyl methacrylate)s (where alkyl is ethyl butyl, isobutyl, cyclohexyl and hexadecyl) over a 20-400 K temperature range. The changes in the position and the full width at half maximum (FWHM) of the 0-0 emission band (peak I) have been correlated with the structural characteristics of the alkyl groups in the different relaxation regimes of the polymers to assess the degree of coupling of the excited singlet states with the polymer cybotactic regions. Data treatment of the peak I positions using an electron-phonon model indicates that longitudinal optical modes are involved, and that the magnitude of coupling depends on the polymer structure and follows the same trend as the glass transition temperatures. The same spectral parameters have been correlated also with "hole" free volumes from positron annihilation spectroscopy over temperature ranges which span the glass or melting transitions of the polymers. Reasons why free volume and FWHM measurements follow the same trends, and other aspects of the systems, are discussed.

Effects of temperature and CO2 pressure on the emission of N,N′-dialkylated perylene diimides in poly(alkyl methacrylate) films. Are guest-host alkyl group interactions important?

“…64 During the long excited singlet-state lifetimes of the pyrene molecules (∼200 ns), the distance traversed in polyethylene is estimated to be <1 Å. However, as noted above, the results presented here and in previous studies with PAMAs [12][13][14] indicate that the fluorescence properties of aromatic guest molecules depend much more on chain segment relaxation rates and micro-diffusion than on hole free-volume. Thus, the nature of the side chains in the PAMAs is a very important parameter in understanding the behaviors of the different perylenes.…”

“…Accordingly, the PAMAs can be placed into the same three categories that were based upon results with pyrene reporter molecules. 13,14 These considerations and concepts are illustrated in scheme 1 as cartoon representations of the predicted, preferred solubilization sites of the different perylenes in the 5 PAMAs.…”

“…Also, the preferred locations, conformations, and orientations of guest molecules inside the polymer matrixes depend on the magnitudes of intimate probe−polymer and polymer−polymer interactions. 8,9 Others 10-12 and we [13][14][15] have observed interesting properties of different PAMAs based on the comportment of photochemical and photophysical probes in ensemble average measurements; a cartoon representation of the most probable site locations within the PAMAs is presented in figure 3 of ref 14. These studies differ from single molecule excitations in which stochastic measurements can be made multiple times to generate ensemble averages.…”

“…An interesting example of this technique has been reported in the glassy phase of poly(methyl methacrylate) using several perylene derivatives as the probe. 16,17 In our previous photophysical and photochemical investigations with 5 different PAMAs, it was found that they can be placed in three different categories, depending upon the nature of the dynamic and structural characteristics of the alkyl side groups: [13][14][15] (i) in PAMAs with short, linear alkyl chains, such as poly(ethyl methacrylate) (PEMA) and poly(butyl methacrylate) (PBMA), the sizes and shapes of the alkyl groups allow guest molecules to approach closely the ester functionalities and the main chains; (ii) In PAMAs with more rigid and bulkier side-chains, such as poly(isobutyl methacrylate) (PIBMA) and poly(cyclohexyl methacrylate) (PCHMA), the nature of the alkyl groups forces guest molecules to reside farther from the ester functionalities and main chains of the polymer; (iii) in PAMAs with long n-alkyl side chains, such as poly(hexadecyl methacrylate) (PHDMA), guest molecules are excluded from the volume segments of the microcrystallites, but can enter those regions after chain melting.…”

“…In that regard, we have shown that the size and length of side-chain alkyl groups in PAMAs play an important role in determining the rates of inter-and intramolecular photophysical processes of pyrenyl guest molecules. 13,14 It was concluded that the dynamics of the pyrenyl-guest molecules in these media are controlled primarily by side chain relaxation rates of the polymer chains. However, based on both photophysical and photochemical studies, the most important factor in polyethylene films was found to be 'hole' free volume.…”