Peter A. Martic

Ketocoumarins: A new class of triplet sensitizers

Abstract Several derivatives of 3-ketocoumarins were prepared and are shown to have many of the photophysical criteria required for efficient triplet sensitizers. These compounds include 3-aroylcoumarins ( 1 ) and 3,3′- carbonylbiscoumarins ( 2 ). The aryl groups in 1 are either phenyl and substituted phenyl derivatives or heterocyclic groups such as thienyl and benzofuryl. The substituents on the coumarin moiety in 1 and 2 , if any, are alkoxy or dialkylamino. These compounds, with absorption maxima between 330 and 450 nm, have extinction coefficients in the range of 10 4 to almost 10 5 , which is an important criterion for efficient sensitization of thin films of polymers as those used in photoresists and lithography. The singlet-triplet intersystem crossing (isc) efficiencies of several derivatives approach unity. In others, however, a radiationless decay process competes with the isc. The decay process is particularly dominant in the asymmetrically substituted derivatives of 2 , but seems to be considerably suppressed in polymeric matrices. The triplet energies of these compounds range from ca . 48 to 60 kcal/mol. Some of these ketocoumarins show phosphorescence spectra that suggest the presence of “frozen-in” rotamers.

Protein adsorption on pluronic copolymer-coated polystyrene particles

The effect of surface modification of polystyrene latex particles on subsequent protein adsorption was investigated by photon correlation spectroscopy, sedimentation, and fluorescence anisotropy decay techniques. The surface modifiers used are a series of Pluronic triblock copolymer surfactants (PEO)a(PPO)b(PEO)a (where PEO is polyethylene oxide and PPO is polypropylene oxide). The hydrodynamic thickness of the adsorbed Pluronic copolymers is proportional to the PEO chain length a as well as the molecular weight of the surfactant, with Pluronic F108 and Tetronic 908 having the thickest layers. Undetectable protein adsorption was observed on such surfactant-coated particles. This was demonstrated by monitoring the unchanged particle size upon addition of human albumin, fibrinogen, or whole plasma, in contrast with size increase for the uncoated particles. This result was also confirmed by direct adsorption isotherm measurement using sedimentation method. Furthermore, the unchanged rotational correlation time of the protein molecules upon addition of Pluronic F108-coated polystyrene particles is also consistent with the above finding of protein resistance of the coated particles. Rotational correlation time was determined from the fluorescence anisotropy decay of an extrinsic probe, 1-pyrenesulfonyl chloride, covalently attached to the protein molecules.

Protein adsorption and conformational change on small polymer particles

Abstract Adsorption of proteins and their conformational changes upon adsorption on artificial surfaces may play an important role in determining the subsequent biological process when a polymeric particle is brought in contact with biological fluids. Previously, we have shown that protein adsorptionon polystyrene particles (0.08 to 0.305 μm in diameter) can be reduced to a negligible amount if the particles were precoated with polymeric surfactants—Pluronic F108 or Tetronic 908. In the present study, we have extended our measurements from 25 to 37°C and also included the three major serum proteins—albumin, fibrinogen, and immunologlobulin—as well as whole plasma. Protein resistance was demonstrated by the unchanged particle size of the surfactant-coated polystyrene particles upon addition of proteins. This finding was further substantiated by the constancy of the rotational correlation times, φ, of the proteins in the presence of excess F108-coated polystyrene particles. The unchanged rotational correlation time of the protein observed is indicative of the unperturbed protein molecular rotation and is consistent with the absence of interaction between the protein molecule and the polymer particles. These φ parameters were determined by the technique of fluorescence anisotropy dcay using 1-pyrenesulfonyl chloride as the extrinsic probe for the three serum proteins. Previously, W. Norde et al. (J. Colloid Interface Sci. 66, 257, 295 (1978)) showed by circular dichroism that denaturation occurred for proteins which were desorbed from polymer particles. In the present work, we have demonstrated by fluorescence anisotropy decay that denaturation occurred not only for proteins desorbed from particles but also for those which remain adsorbed on the particle surface, in situ. In the case of protein adsorption on the uncoated polystyrene, the rotational correlation time of the protein, φ, increases with the increasing number of particles resulting from restricted molecular rotation of the protein upon adsorption. This φ parameter was reduced to zero, however, upon further addition of polymer, particularly those with large particle sizes. This is attributed to the unfolding of the adsorbed protein molecule, resulting in the exposure of the probe site to an aqueous environment. The zero φ value was also observed for proteins desorbed from plystyrene using Pluronic F108 as the desorbing agent. These results suggest that proteins become unfolded upon adsorption on polystyrene particles and also retain their denaturated state even after desorption.

Efficient organic electroluminescent device using a single bipolar carrier transport layer

An electroluminescent device was developed in which a single bipolar transport layer is used. High efficiencies can be obtained from this device by judiciously selecting a dopant that enhances the radiant recombination probability of carrier pairs and spaces this recombination zone away from the cathode.

The design of photoreactive polymer systems for imaging processes

Abstract The use of polymers in photoreactive imaging systems depends upon the interrelationship between the polymer physical properties and the photosensitive response. Modifications of the structure and physical properties by synthetic means permit control over the physical properties, such as solubility, melting point, glass transition temperature, and crystallinity. Specific polymer imaging systems depend upon these properties in order to function. Modifications of light absorbing chromophores and understanding the factors controlling sensitization permit adjustment of the wavelength response over the range from 250–650 nm. Triplet energy transfer from optical sensitizers to the photoreactive moieties is the most probable mechanism of sensitization. We have found that competing side reactions such as oxidation and photoreactions of the sensitizer can reduce the efficiency of sensitization.

Selected aspects of photochemistry in polymer media

Abstract A number of selected aspects of the reciprocal interactions of polymers with excited solutes or polymer-bound 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 0. in the polymer approach, however, a value of only ca. 0.7. As a result a 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. Improper orientation of the reactants in polymers is responsible for the shift observed in excimer emission and for a part or all of the shift in exciplex emissions. 4. Little if any difference is observed between exciplex emissions in polymers of low and moderate macroscopic polarities. This may be attributed to two causes: a) Due to improper orientation the dipole moment of the exciplex in the polymer is expected to be smaller and, therefore, less solvation energy can be gained. b) The segmental motion of the polymer required to properly solvate the complex is probably too slow at room temperature compared with the lifetime of the exciplex. 5. Bichromophoric molecules, which form exciplexes in fluid media, fail to reach an exciplex configuration when dissolved in polymers.

Inclusion complexes of pyrenylbutyrate with γ-cyclodextrin

Mixing 4-pyren-1-ylbutyrate ion with γ-cyclodextrin in aqueous solution leads to inclusion complexes that become evident from changed absorption spectra as well as protection of the included pyrene moieties from fluorescence quenchers. The inclusion proceeds in stages, yielding first a 1:1 complex, which then dimerizes to form a 2:2 complex. At 25 °C the respective equilibrium constants of complex formation are 1.3 × 103 and 5.2 × 104 l mol–1. The dimerization equilibrium has an unusual temperature dependence, with ΔH≠–16.7 kcal mol–1 and ΔS≠–35 cal K–1 mol–1.

Comparative photochemical behavior of some anthracenyl and naphthacenyl sulfonium salt derivatives

Abstract A series of anthracenyl and naphthacenyl mono- and bis-sulfonium salts was synthesized and their spectroscopic, redox and photoacid behavior investigated. When the thermodynamics of photoinduced electron transfer (PET) bond cleavage in the sulfonium salt derivatives is weakly exothermic, i.e. ΔG ET °=−1.8 kcal mol −1 , the rate of PET does not compete effectively with the rate of decay of the first excited singlet state by fluorescence emission. When ΔG ET ° is more exothermic than about 5 kcal mol −1 , the fluorescence emission is quenched and PET bond cleavage takes place. The quantum yield of acid formation ( Φ H+ ) in acetonitrile for the napthacenyl and anthracenyl sulfonium salt derivatives is in the range 0.003–0.28. In the anthracenyl sulfonium salts, the electron-withdrawing requirements of the substituent groups on the sulfonium moiety are greatly relaxed over the naphthacenyl derivatives due to the higher energy π ∗ level of the anthracene moiety.

Unique intramolecular exciplex luminescent behaviour of 9-(2-diethylaminomehylphenyl)anthracene in rigid media

9-(2-Diethylaminomethylphenyl)anthracene exhibits exciplex luminescent behaviour in solution, in a rigid polymer film, and in the solid state, owing to the restricted conformation of the phenylanthracene ring system as indicated from its X-ray crystal structure.

Hydrophobic Interactions of Aromatic Hydrocarbons Induced by Surfactants and Polyelectrolytes

The microenvironment of charged molecules can be significantly altered in the presence of ionic molecules such as surfactants, lipids, and polyelectrolytes. Such a change in the microenvironment of aromatic hydrocarbon chromophores can be conveniently studied spectroscopically.