Kock-Yee Law

Squaraine chemistry: effects of solvent and temperature on the fluorescence emission of squaraines

The squaraines — a class of donor-acceptor-donor molecules — were shown to exhibit intense multiple fluorescence in the visible region. The three emission bands, designated α, β and γ in the order of decreasing energy, were found to be emissions from the excited state of squaraine, the excited state of the solute-solvent complex and a relaxed, twisted excited state respectively. In this work, the results of an investigation of the effects of the solvent and temeperature on the absorption and fluorescence emission of bis(4-dibutylaminophenyl)squaraine (1) are reported. The data confirm that squaraine forms solute-solvent complexes in solvents. Evidence is provided that the solute-solvent interactions is short range, and that polar solvents usually exert a bathochromic effect on the absorption maximum wavelength λmax. Along with the bathochromic shift, a gradual change in the composition of the emission band, from being dominated by the α emission in ether to being dominated by the β emission in polar solvent, is observed. Both λmax and Keq (the equilibrium constant for complexation) are shown to correlate well with the solvent parameter π*. The results indicate that the solute-solvent complexation process is responsible for the bathromic shift in λmax and the composition change in the emission spectra. This is responsible for the bathochromic shift in λmax and the composition change in the emission spectra. This conclusion is supported by variable and low temperature spectral data. While the lifetime of excited 1 is shown to be independent of the solvent (2.3±0.1 ns), the lifetimes of the excited solute—solvent complex and the relaxed, twisted excited state are solvent sensitive, varying from 0.6 to 3.5 ns respectively. From the fluorescence lifetime and the radiative decay rate, the rate of the radiationless decay, which involves rotation of the Cue5f8C bond between the phenyl ring and the four-membered ring of squaraine, can be calculated. The rotation rate is shown to increase rapidly as the twisting of the squaraine chromophore increases. Finally, this work also shows that the intensity of the γ emission not only by lowering the temperature but also by a rigid matrix at room temperature. The results support the postulation that a twisting motion is required for the generation of the relaxed excited state.

Squaraine Chemistry: Synthesis, Characterization and Xerographic Properties of Bis(4-methylbenzylaminophenyl)-squaraine and Its Derivatives

Abstract Bis(4-methylbenzylaminophenyl)squaraine and a number of its derivatives (N-benzylsquaraines ) have been synthesized by condensation of squaric acid with the corresponding N-benzylaniline derivatives. Two azeotropic solvent systems, namely 1-heptanol and 1-butanol/toluene, were used. Consistently higher yields, but less pure samples, were obtained in 1-heptanol. The higher yield is attributable to the favorable arylation reaction of the n-heptyl squarate intermediate rather than its esterification reaction in 1-heptanol in the squaraine synthesis. Although the spectroscopic properties of N-benzylsquaraines are found to be very similar to those of N-alkyl-substituted squaraines, the physical and the electrical properties between these two groups of squaraines are quite different. N-Benzyl squaraines exhibit high melting points, low solubility in organic solvents and moderate-to-high xerographic sensitivity in bilayer photoreceptor devices. Our results show that N-benzyl-substitution of squaraine is probably the only viable approach to modify the solubility of squaraine without significantly reducing the xerographic sensitivity. The xerographic properties of N-benzylsquaraines are found to be sensitive to the synthetic history of the sample (reacting solvent, recrystallization) and these results are discussed in terms of an impurity effect.

Fluorescence probe for microenvironments: On the fluorescence properties of p-N,N-dialkylaminobenzylidenemalononitrile in polymer matrices

The effect of an alkyl group (R) in a series of poly(alkylmethacrylate) and poly(alkylacrylate) polymers on the fluorescence properties of p-N,N-dialkylaminobenzylidenemalononitrile derivative, 1, has been studied. The fluorescence emission maximum shifts to the blue and the fluorescence quantum yield decreases as the chain length of R increases. These results are interpreted in terms of a model which assumes that the location of 1 is dependent on R and the change in quantum yield reflects the difference in free volume or polymer chain flexibility in various locations in these polymers.

Squaraine chemistry. Effect of N-pyrrolidino substitution on the synthesis and solid state properties of squaraines

Abstract Several symmetrical and unsymmetrical squaraines bearing N-pyrrolidino groups have been synthesized by condensation of N-pyrrolidinoaniline derivatives with squaric acid or a 1-aryl-2-hydroxycyclobutene-3,4-dione derivative. As compared to squaraines synthesized from N,N-dimethylanilines, a consistently higher synthetic yield was obtained for the N-pyrrolidino-squaraines synthesized in the work. The yield improvement is attributable to the high nucteophilicity of N-pyrrolidinoanilines, owing to the rigidized N-pyrrolidino ring structure. N-Pyrrolidino substitution is shown to have very little effect on the physical and spectroscopic properties of squaraine. Examinations of the solid state properties of N-pyrrolidino substituted squaraines, by solid state absorption spectroscopy and by X-ray powder diffraction reveal that the N-pyrrolidino group is the only N-alkyl substituent identified thus far that exerts no effect on the aggregation of squaraine molecules in the microcrystalline state. This implies that N-pyrrolidino squaraines should be photoconductive and indeed photoconductivities have been observed in xerographic devices incorporating them.

Azo pigments and their intermediates. On the anomalous optical absorption of bisazo pigments from 4,4′-diamino-1,1′-dianthrimide and 2-hydroxy-3-naphthanilides

Abstract A number of bisazo pigments ( 2a–2o ), which were designed to extend the optical absorption of azo pigments to longer wavelengths (greater than 650 nm), enabling these pigments to be useful in photoreceptors in light-emitting diode (LED) (680 nm) and diode laser (780 nm) printers, were synthesized by coupling a red-absorbing amine ( 1 , 4,4′-diamino-1,1′-dianthrimide, λ max ≈ 680 nm) and 2-hydroxy-3-naphthanilide couplers using the classic azotization-coupling procedure. All bisazo pigments synthesized are dark blue solids with melting points higher than 300 °C. They absorb at λ max ≈ 610- nm in the visible region in the solid state. Their wavelengths of absorption, which are insensitive to electronic and steric effects exerted by the substituents in the coupler moiety, are unexpectedly at shorter wavelengths than that of 1 (λ max ≈ 680 nm). In order to facilitate our understanding of the hypsochromic shift of absorption from 1 to 2a–2o , the nature of the electronic transition of the long-wavelength absorption band of 1 was studied systematically by absorption spectroscopy. Results from structural effect and solvent effect experiments indicate that the long-wavelength absorption in 1 is a charge-transfer band resulting from an intramolecular charge-transfer interaction between the two aminoanthraquinonyl rings. In the bisazo pigment structure, owing to the steric repulsion between the two coupler moieties, such an intramolecular interaction is prohibited. This changes the conformation of the bisazo pigment and is shown to be responsible for the hypsochromic shift of the optical absorption from 1 to bisazo pigments 2a–2o .

Fluorescence lifetime and electron transfer in cyclobutenediylic dyes

Abstract The fluorescence spectra of “cyclobutenediylic dyes”, DMA and DEAF, reported recently by Rehak and Israel were found to be different from those observed in our laboratory. The detailed procedures for which multiple fluorescence emission bands were recorded in the solutions of these compounds are reported and the probable cause of the discrepancy is discussed.

Azo pigments and their intermediates: Effect of substitution on the diazotization and coupling reactions of o-hydroxyanilines

Abstract The effects of substituents on the diazotization and coupling reactions of o-hydroxyanilines have been studied. When the substituent is a strong electronwithdrawing group, e.g. NO 2 , the resulting substituted 2-hydroxy-lbenzenediazonium chloride is highly acidic, even in aqueous hydrochloric acid solution. It deprotonates and forms the corresponding substituted 1,2-benzoquinone-1-diazide. For less electron-withdrawing or even electronreleasing substituents, the so-generated diazonium chlorides are soluble in the aqueous hydrochloric acid solution. These diazotized products are isolated as hexafluorophosphate salts. The coupling reactions between various diazotized products of o-hydroxyanilines and 2-hydroxy-3-naphthanilide are shown to proceed through the 1,2-benzoquinone-1-diazide intermediate, regardless of their original structure. Synthetic results show that the substituted 1,2-benzoquinone-1-diazide intermediate is only reactive when it is activated by a substituent that is more electron-withdrawing than the chloro group.

Fluorescence probe for microenvironments: effect of solvent vapour on the properties of vapour-swollen polymers

Abstract The effect of solvent vapour on the properties of vapour-swollen vinyl chloride-vinyl acetate ( 83 17 ) copolymer has been studied by a fluorescence probe, a p-N,N-dialkylaminobenzylidenemalononitrile derivative (1). Results show that the fluorescence quantum yield ( Ф f ) of 1 in vinyl chloride-vinyl acetate ( 83 17 ) matrix decreases by a factor of ≈ 10, an indication of the increase in free volume or in polymer chain mobility, upon vapour swelling. The variations of Ф f observed in various swollen matrices, which correlate only with the density of the swelling solvent, indicate that there is a profound vapour effect on the properties of swollen polymer. A density effect on the mobility of polymer chains in swollen polymer is proposed.

Squaraine chemistry. Synthesis, structural characterization and xerographic properties of mixtures of halosquaraines

Abstract A number of mixtures of halosquaraines have been synthesized by co-reacting mixtures of N,N-dimethylaniline and N,N-dimethylhaloaniline with dibutyl squarate in water-saturated 1-butanol. Yield decreases as the concentration of N,N-dimethylhaloaniline increases and this decrease is attributable to the low reactivity of N,N-dimethylhaloaniline. This low reactivity is also revealed in the lower than expected halogen incorporation in all the mixtures studied. The chemical composition of each mixture was studied by high-resolution proton NMR spectroscopy and mass spectrometry. The merit of these two techniques in the identification and quantification of individual squaraines in the mixtures is presented and discussed. The xerographic properties of each mixture were studied by xerographic photodischarge technique in bilayer photoreceptor devices; improvement in photosensitivity is observed for squaraine mixtures synthesized from N,N-dimethylaniline (DMA) and 3-fluoro-N,N-dimethylaniline (3-FDMA). At a DMA/3-FDMA ratio of 75/25, the fluorosquaraine composition synthesized exhibits photosensitivity similar to the pure fluorosquaraine synthesized from 3-FDMA. The yield of such a composition is 30%, an improvement of 11% from the pure fluorosquaraine synthesized from 3-FDMA.

Spectroscopic studies of spin-coated dye-in-polymer films. On the relationship between compatibility and aggregation of small molecules in polymer matrices

Abstract Dye-in-polymer (DIP) films of various concentrations of I (a p - N , N -dialkyl-aminobenzylidenemalononitrile derivative) in four polymers of different dye-polymer compatibilities have been prepared by spin-coating technique using a wide range of coating speeds (600–8000 rpm). The aggregation of dye molecules in polymer matrix was studied by electronic spectroscopy. The dye-polymer compatibility of various DIP systems was examined by d.s.c. Our results indicate that there exists a staturated concentration of I in each polymer and this saturated concentration decreases as the dye-polymer compatibility decreases, e.g. it decreases from ∼20% in poly(vinyl acetate) and in poly(isobutyl methacrylate) to ∼10% in styrene/isobutyl methacrylate (8:2) copolymer to ∼5% in polystyrene. At dye concentrations lower than this saturated concentration, the degree of dye aggregation is not sensitive to varying spin-coating speeds. At dye concentrations higher than this saturated concentration, the degree of dye aggregation depends on the degree of dye-polymer incompatibility and on the spin-coating speed. The significance of the present work in solvent coating technology of small molecule/polymer systems, in general, will be discussed. Finally, the T g s of various DIP systems were found not to correlate with the dye concentration (by weight). This is attributable to the strong dye-dye interaction of I inside the polymer matrix.