Selected aspects of photochemistry in polymer media

Abstract

-A nurober of selected aspects of the reciprocal interactions of polymers with excited solutes or polymer-hound chromophores are given. The role of free volume, glass-transition temperature (Tg), microscopic viscosity, and polarity or environments to which excited molecules are exposed is emphasized in order to illustrate possible manners in which the photophysical, photochemical, and subsequent chemistries can be influenced.Many of these factors can affect several monomolecular and bimolecular processes encountered in the sensitization of photocrosslinkable polymers: the photophysics of the sensitizer, energy transfer to the reactive sites on the polymer, and the formation of crosslinks. In fact, the triplet yields of aroylnaphthothiazole derivatives, a widely used class of sensitizers, are considerably higher in the more viscous polymeric matrices. These ~-in the polymer approach, however, a value of only ca. 0.7. As a resuitca search for more efficient triplet sensitizers led to a new class of compounds: 5-and 7-substituted 3-ketocoumarins.As a model for study of the bimolecular processes in polymers, we chose exciplex and excimer probes. The following conclusions were drawn: 1. Several exciplex and excimer emissions in polymeric media are considerably shifted to shorter wavelengths as compared with the maxima measured in fluid media, indicating that interactions are impaired in polymeric matrices. 2.Emissions from the polymer matrix above the glass-transition temperature are similar in wavelength and temperature dependence thereof to those observed in fluid solutions. 3.

Photochemical and Photophysical Studies of and in Bulk Polymers

“…The excimer band of a guest molecule, methyl 4 -(1 -pyrenyl)butyrate ( P -1 , Chart 11.1 ) in PS fi lms, is blue shifted by 40 nm compared with that in fl uid solutions. 56 The dearth of emission from excimers of P -1 in PS has been attributed to the diffi culty of molecular pairs to attain the necessary complex geometry. With increasing temperature, this excimer band becomes red shifted and reaches the wavelength maximum at 348 K ( T g = 327 K) found in fl uid media.…”

“…With increasing temperature, this excimer band becomes red shifted and reaches the wavelength maximum at 348 K ( T g = 327 K) found in fl uid media. 56 These spectral changes have been attributed to increased mobility of polymer chains at higher temperatures.…”

“…As expected on the basis of viscosity considerations, the rates of translational diffusion of small molecules in glassy polymers are slower than in rubbery polymers and the latter are lower than in low -viscosity liquids. 56 For example, the mobility of polymer chains and side groups of polydimethylsiloxane ( PDMS ) has been related to quenching rate constant ( k q ) of pyrene fl uorescence by phthalic anhydride:…”

“…66 Rates of rotational or translational diffusion and conformational changes of chromophores can also yield information about their average environments in polymers. 56,67 Exciplex formation is attenuated when the constituent molecules have diffi culties to adopt their preferred orientations or cannot diffuse rapidly in a polymer. Thus, amount of intermolecular exciplex formation between anthracene or 4 -(1 -pyrenyl) butyrate and dimethyl toluidine is lower in PS fi lms compared with that in fl uid media, but increases as temperature approaches T g or T m from lower values (and chain mobility increases).…”

“…Thus, amount of intermolecular exciplex formation between anthracene or 4 -(1 -pyrenyl) butyrate and dimethyl toluidine is lower in PS fi lms compared with that in fl uid media, but increases as temperature approaches T g or T m from lower values (and chain mobility increases). 56 Absence of intramolecular exciplex formation in these polymer matrices has been attributed to slower folding of alkyl groups linking the exciplex constituents (e.g., rate of folding process < 10 6 s − 1 , T < 348 K for N , N -dimethyl -4 -[3 -(pyren -1 -yl)propyl]aniline) (Chart 11.2 , Py3Anil ). 56 Also, from their diffusion coeffi cients, ∼ 10 − 8 cm 2 s − 1 in PE31 at 385 K, anthracene guests are calculated to be able to move a distance of about one molecular diameter (4 -5 Å ) during their excited -singlet state lifetime ( ∼ 5 ns).…”