Pavol Hrdlovič

Spectral Properties of Substituted Coumarins in Solution and Polymer Matrices

The absorption and fluorescence spectra of substituted coumarins (2-oxo-2H-chromenes) were investigated in solvents and in polymer matrices. The substitutions involved were: (1) by groups with varying electron donating ability such as CH3, OCH3 and N(CH3)2, mainly, but not exclusively, in positions 7 and (2), by either CHO or 4-PhNHCONHN=CH- in position 3. While the spectra of non-substituted coumarin-3-carbaldehyde has absorptions at approximately 305 and 350 nm, substitution at position 7 leads to remarkable changes in the shape of the absorption spectrum and shifts the absorption to a longer wavelength. Similarly, the replacement of the formyl group with a semicarbazide group substantially influences the shape of the absorption spectrum, and coumarins which have only N(CH3)2 in position 7 experience small changes. These changes are associated with the increasing intramolecular charge transfer (ICT) character and increasing conjugation length of the chromophoric system, respectively, in the studied molecules. The fluorescence is almost negligible for derivatives which have H in this position. With increasing electron donating ability, and the possibility of a positive mesomeric (+M) effect of the substituent in position 7 of the coumarin moiety, the fluorescence increases, and this increase is most intense when N(CH3)2 substitutes in this position, for both 3-substituted derivatives. Spectral measurements of the studied coumarins in polymer matrices revealed that the absorption and fluorescence maxima lay within the maxima for solvents, and that coumarins yield more intense fluorescence in polymer matrices than when they are in solution. The quantum yield of derivatives which have a dimethylamino group in position 7 in polymer matrices approaches 1, and the fluorescence lifetime is within the range of 0.5–4 ns. The high quantum yield of 7-dimethylamino derivatives qualifies them as laser dyes which have kF higher than knr in the given medium. This is caused by stiffening of the coumarin structure in polar polymer matrices, such as PMMA and PVC, due to higher micro-viscosity than in solution and intermolecular dipole-dipole interaction between chromophore (dopant) and matrix.

Spectral characteristics and photochemical stability of fluorescence probes based on 1,8-naphthaleneimide in solution and in polymer matrix

Abstract Fluorescence probes of type N -(1′- R -2′,2′,6′,6′-tetramethyl-4′-piperidinyl)-1,8-naphthaleneimide exhibit spectral properties which are influenced by environment and substitution ( R ) on sterically hindered nitrogen of piperidine moiety. Absorption spectra of parent amine and its N substituted derivatives show the longest wavelength band around 340 nm (log ɛ ∼ 4.0) which is well resolved in cyclohexane. In polar methanol it is only slightly blue shifted. Substitution on sterically hindered nitrogen does not influence the absorption spectra in near UV region. Fluorescence spectra of the probes have maximum around 385 nm in methanol and comparable intensity as anthracene. In nonpolar cyclohexane the fluorescence intensity is low (more than 100×). In nonpolar isotactic polypropylene matrix the red shifted excimer like emission in the range 440–470 nm was observed. The N -oxy and N -hydroxy derivatives under the same conditions do not yield the excimer emission. In other polymer matrices as polystyrene, poly(methyl methacrylate) and polyvinylchloride similar emission as in methanol was observed, the emission in latter matrix being the most intense. The fluorescence lifetime in polar solvent and matrices is less then 1 ns and deviates slightly from monoexponential. In nonpolar iPP, the fluorescence lives longer around 5 ns and its decay is more complex. Intermolecular quenching of the probes by 1-oxy-2,2,6,6-tetramethyl-4-hydroxypiperidine in methanol occurs at the rate which is above the diffusion controlled limit what might indicate the involvement of the polar medium in the process. Intramolecular quenching, expressed as the ratio I NH / I NO for parent and oxidized amine respectively, strongly depends on the medium as well. In polar methanol this ratio is about 30 while in polymer matrices 2–4 only. Photolysis of probes doped in polymer matrices with radiation above 300 nm revealed that the parent amine is about 3× more stable as 1,8-naphthoic anhydride or N -octadecyl-1,8-naphthaleneimide.

Stabilising effect of monomers, homopolymers and copolymers of vinyl derivatives of sterically hindered amines on the photo-oxidation of polypropylene

Abstract Vinyl derivatives of sterically hindered amines, N-(2,2,6,6-tetramethyl-4-piperidyl)methacrylamide, 2,2,6,6-tetramethyl-4-piperidyl acrylate and methacrylate and 1,2,2,6,6-pentamethyl-4-piperidyl acrylate and methacrylate were prepared. Monomers, homopolymers and copolymers with dodecyl methacrylate (DMA) and octadecyl acrylate (ODA) were tested as stabilisers of the photo-oxidation of polypropylene film. The copolymers exhibited higher efficiency and lower extractability. The photostabilising efficiency increased in the order: homopolymer

Influence of polarity of solvents on the spectral properties of bichromophoric coumarins.

Absorption and fluorescence spectra of bichromophoric coumarins were investigated in different solvents and in polymer matrices. These bichromophoric coumarins were composed of a coumarin dimethylamino-substituted at position 7 or unsubstituted coumarin and phthalimide or a 1,8-naphthylimide linked with an iminomethyl bridge to the position 3 or 8 of the coumarin ring. Absorption spectra of 7-dimethylamino derivatives in position 3 of coumarin were quite similar, exhibiting broad bands around 430-440 nm like the parent compound 7-dimethylaminocoumarin-3-carbaldehyde. For coumarin derivatives substituted in position 8, the absorption maximum was shifted to shorter wavelength as for derivatives without position 7 dimethylamino substitution. The most intense fluorescence was observed for 7-(N,N-dimethylamino)-3-[(N-phtalimidoyl)iminomethyl]coumarin in polar solvent, while intense fluorescence was observed for 7-(N,N-dimethylamino)-3-[N-(1,3-dioxobenz[de]isoquinolinyl)iminomethyl]-coumarin in non polar solvent (chloroform), comparable with the fluorescence of 7-amino-4-methylcoumarin. Spectral measurements of bichromophoric coumarins in polymer matrices revealed that the maxima lies in between those for chloroform and methanol yielding more intense fluorescence then in solutions. Completely different solvent effects were observed for 7-(N,N-dimethylamino)-3-[N-(1,3-dioxobenz[de]isoquinolinyl)imino-methyl]coumarin and 7-(N,N-dimethylamino)-3-[(N-phtalimidoyl)iminomethyl]coumarin. With addition of polar methanol the intensity of fluorescence decreases, yielding a Stern-Volmer-like constant of 0.54 dm3mol−1 for 7-(N,N-dimethylamino)-3-[N-(1,3-dioxo-benz[de]isoquinolinyl)iminomethyl]coumarin and an even higher one of 1.08 dm3mol−1 for 7-dimethylaminocoumarin-3-carbaldehyde compared to the rather low one of 0.024 dm3 mol−1 for 7-amino-4-methylcoumarin. Contrary to this, addition of methanol under identical conditions brings about an increase in fluorescence intensity of 7-(N,N-dimethylamino)-3-[(N-phtalimidoyl)iminomethyl]coumarin (about 60-fold). The reasons for these different solvent effects are discussed.

Preparation and photochemical properties of polymers containing 1-(4-benzylphenyl)-2-propen-1-one or 1-(4-benzoylphenyl)-2-propen-1-one structural units☆

Abstract The new monomers 1-(4-benzylphenyl)-2-propen-1-one (4) and 1-(4-benzoylphenyl)-2-propen-1-one (6) and their homopolymers P4 and P6 and copolymers with styrene P4/S and P6/S were prepared. The polymers exhibit an n—π* band in their UV spectra and vibrationally well-resolved low-temperature phosphorescence. Both properties are characteristic of photoreactive polyketones. Polymers containing structure 6 show greater near-UV absorption and lower triplet energy in films at 77 K than the photoreactive polyvinylketones. The absorption and emission spectra of the transients, their lifetimes and their quenching by triplet quenchers in benzene at 300 K were determined by laser flash photolysis. Lamp photolysis of the title polymers at 366 nm in benzene results in a decrease in the molecular mass due to the Norrish type II reaction. The quantum yields of main-chain scission are comparable with those of other reactive polyvinylketones (around 0.2). The Stern—Volmer constants based on photolysis and transient quenching indicate the involvement of the long-lived triplet state in the chain scission process.

Spectral Properties of Probes Containing Benzothioxanthene Chromophore Linked with Hindered Amine in Solution and in Polymer Matrices

Absorption and emission spectroscopy as well as laser flash photolysis was employed in order to characterize the spectral properties of novel probes based on benzothioxantheneimide chromophore covalently linked with different types of sterically hindered amines. These were chosen as 2-(2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINH), the equivalent stable nitroxyl radical, i.e. 2-(1-oxo-2,2,6,6-tetramethyl-4-piperidyl)thioxantheno[2,1,9dej]isoquinoline 1,3-dione (BTXINO) and the alkoxy derivative 2-(1-(1′-phenylethoxy)-2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINOR). Spectral properties, in solutions and in various polymer matrices such as polystyrene, polymethyl methacrylate, polyvinyl chloride and polypropylene, were compared with the compound 2-(1-dodecyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXID) taken in the present study as a reference compound. By means of the fluorescence decay and in the contrary to three other probes, BTXINO probe clearly showed a biexponential decay while the three other probes led to monoexponential decay. Two different singlet excited states with lifetimes of about 0.4 and 5 ns were proposed. They correspond to two dispositions of the nitroxyl radical chain above and along the fluorescent moiety of the molecule. Such behaviour depends on the surrounding media. Moreover, an efficient intramolecular quenching of the fluorescence emission was only observed with the short lived singlet excited state. The ratio BTXID/BTXINO was found equal to about 4 and 9 in solutions and polymer matrices respectively. Laser flash photolysis indicated that the novel probes as well as the model compound yielded transient absorption with maximum at 530 nm, corresponding to the triplet states. The intermolecular quenching of such species by molecular oxygen and by free N-oxyl, such as 1-oxy-2,2,6,6-teramethylpiperidine (TEMPO) and 1-oxy-2,2,6,6-teramethyl-4-hydroxypiperidine (TEMPOL), and the intramolecular quenching was not efficient.

Preparation, photochemical stability and photostabilizing efficiency of adducts of 1,8-naphthaleneimide and hindered amine stabilizers in polymer matrices

Adducts of 1,8-naphthaleneimide and hindered amine stabilizer (HAS) such as 1-R-substituted-2,2,6,6-tetramethyl-4-aminopiperidine were prepared where R is –H, –O., –OH, –COCH3 and –OCOCH3. The photochemical stability of adducts was determined at photolysis in isotactic polypropylene, polystyrene and polyvinylchloride. Their photostabilizing efficiency was determined at photooxidation of isotactic polypropylene. The photochemical stability of the adducts is low in all matrices. The N-oxy derivative was the most stable in all matrices. The photolysis of all derivatives is the slowest in polystyrene. Intramolecularly combined chromophore/HAS are less effective stabilizers than in 1:1 mixture of separated components. The 1,8-naphthaleneimide chromophore decreases the stabilization efficiency of hindered amine structural units as compared to its absence.

Spectral properties of coumarin derivatives substituted at position 3. Effect of polymer matrix

Abstract The spectral properties of coumarin (2H-1-benzopyran-2-one) derivatives substituted at position 3 with bulky substituents, such as phenyl, phenylthio and benzylthio, were investigated in solvents of different viscosity and in polymer matrices. In order to evaluate the heavy atom effect, some derivatives were substituted by bromine at positions 6 and 8. The absorption spectra exhibit a broad band with a maximum at 340 nm (log e ≈ 4) which is not influenced by the medium. Coumarin derivatives in solvents of low viscosity exhibit broad band fluorescence with a low quantum yield of 0.005 and a lifetime shorter than 0.1 ns. In solvents of higher viscosity and in polymer films, the quantum yields are 0.05–0.2 higher and the lifetime is increased by approximately 2 ns. On the basis of the spectral data, the rate constant of radiationless deactivation k nr is 10 11 s −1 in solution and 4 × 10 8 s −1 in the polymer matrix. The activation energy of the radiationless process in 3-phenylcoumarin is about 34 kJ mol −1 in glycerol. The activation energy of viscosity change of glycerol is 72 kJ mol −1 . This comparison indicates that the torsional rotation of the substituent at position 3 is partially influenced by the friction viscosity of the medium.

Spectral characteristics of multifunctional probes based on pyrene in solution and in polymer matrices

Abstract The spectral properties of multifunctional probes derived from 4-(1-pyrene)butyric acid and 2,2,6,6-tetramethyl-4-hydroxypiperidine in solution and in polymer matrices were investigated. In the absorption spectra, the well-resolved absorption of pyrene dominates and is little influenced by the medium. The absorption of the hydrochloride derived from the amine in non-polar matrices is rather diffuse. All the fluorescence probes with different functional groups at the sterically hindered centre yield well-resolved emission spectra, which are the same in solution and in polymer matrices at low concentration. The main features of the emission spectra are not influenced by the polarity of the medium. A red-shifted excimer emission is observed for the hydrochloride at concentrations in solution (10 −3 mol dm −3 ) and in solid matrices (0.1 mol kg −1 ) at which the parent probe yields the monomer emission only, indicating the formation of ground state aggregates. Steady state and lifetime measurements indicate that the emission of the N -oxyl-type probe is quenched by about 50% irrespective of the medium. This new radiationless channel in the N -oxyl-type probe is comparable with the rate of the radiation channel.

The photolysis of polymers with benzoyl and para methoxybenzoyl structural units

Abstract The emission spectra of polyacrylophenone, copolymers styrene/acrylophenone (28.6, 19.0, 10.2 wt/wt% poly-p-methoxyacrylophenone and copolymer styrene/p-methoxyacrylophenone (22.7 wt/wt%) were measured for films at liquid nitrogen temperature. The emission spectra indicated a weak interaction between benzoyl structural units and a strong interaction in poly-p-methoxyacrylophenone. Photolysis in solution was not affected by these interactions. Quantum yields of main chain scissions in chlorobenzene by 366 nm radiation were about 0.2 for polyacrylophenone and copolymers styrene/acrylophenone and about 0.1 and lower for poly-p-methoxyacrylophenone and copolymer styrene/p-methoxyacrylophenone. The Stern-Volmer constants for photolysis quenching for styrene/acrylophenone copolymers increased in solution and decreased in film. The presence of oxygen lowers the quantum yield and Kw constant of poly-p-methoxyacrylophenone and styrene/p-methoxyacrylophenone copolymer. GPC measurements show that the molecular weight distribution widens during photolysis of styrene/acrylophenone copolymer (28.6 wt,/Wt %) indicating inhomogeneity in the copolymer.