Relaxations in Poly(vinyl alcohol) and in Poly(vinyl acetate) Detected by Fluorescence Emission of 4-Aminophthalimide and Prodan

Hydrogen bond strengthening induces fluorescence quenching of PRODAN derivative by turning on twisted intramolecular charge transfer

Researchers have proposed different effective mechanisms of hydrogen bonding (HB) on the fluorescence of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and its derivatives. Herein, excited state transition and dynamics analysis confirm that the fluorescence of PD (a derivative of PRODAN with ethyl replaced by 3-hydroxy-2,2-dimethylpropan) emits from the planar intramolecular charge transfer (PICT) state rather than twist ICT (TICT) state, because the fluorescence emission and surface hopping from the TICT state to the twist ground (T-S₀) state is energy forbidden. Nevertheless, the strengthening of intramolecular-HB (intra-HB) and intermolecular-HB (inter-HB) of PD-(methanol)₂ smooth the pathway of surface hopping from TICT to T-S₀ state and the external conversion going to planar ground state by decreasing the energy difference of the two states. This smoothing changes the fluorescence state of PD-(methanol)₂ to the TICT state in which fluorescence emission does not occur but surface hopping, leading to the partial fluorescence quenching of PD in methanol solvent. This conclusion is different from previous related reports. Moreover, the inter-HB strengthening of PD-methanol in PICT state induces the cleavage of intra-HB and a fluorescence red-shift of 54nm compared to PD. This red-shift increases to 66nm for PD-(methanol)₂ for the strengthening of the one intra-HB and two inter-HBs. The dipole moments of PD-methanol and PD-(methanol)₂ respectively increase about 10.3D and 8.1D in PICT state compared to PD. The synergistic effect of intra-HB and inter-HB induces partial quenching of PD in methanol solvent by turning on the TICT state and fluorescence red-shift. This work gives a reasonable description on the fluorescence red-shift and partial quenching of PD in methanol solvent, which will bring insight into the study of spectroscopic properties of molecules owning better spectral characteristics.

Optical and Thermal Stability of Oligofluorene/Rubber Luminescent Blend

This paper proposes to obtain homogeneous and stable blends of oligo(9,9-dioctylfluorene)-co-phenylene (OF), a conjugated oligomer with strong tendency of formation of excimers in the solid state, and nitrile rubber (NBR). This rubber protection reduces the formation of polymer excimers in the films. The fluorene oligomer was synthesized via Suzuki reaction and incorporated in the nitrile rubber. The films were formed by spin coating and casting techniques on the proportions of 1, 5, 10, 20 and 50 % (w/w) of OF in the nitrile rubber (NBR). The structural, optical and thermal properties of the films were evaluated with infrared, UV-Vis, fluorescence and thermogravimetry, respectively. The nitrile rubber proved to be essential for the preparation of homogeneous and stable films, since it was not possible to obtain films with only fluorene using the above-mentioned techniques. Furthermore, luminescent properties of OF are unchanged and the excimers formation in the solid state decrease suggesting the efficiency of nitrile rubber as the matrix for making films.

Isosteric and fluorescent DNA base pair formed by 4-amino-phthalimide and 2,4-diaminopyrimidine: melting, structure, and THz polar solvation dynamics

An artificial base pair in duplex DNA, formed by 2,4-diaminopyrimidine and fluorescent 4-aminophthalimide C-nucleosides, is characterized spectroscopically, for use in femtosecond solvation dynamics.

Modulation of the 4-aminophthalimide spectral properties by hydrogen bonds in water

TD-DFT and DFT calculations have been performed to examine the relationship between the spectral shifts of 4-aminophthalimide (4AP) and the formation of hydrogen bonds in water solution. The computations of the S0 state are at the IEFPCM-B3LYP/6-311++G(d, p) level and the S1 state at the TD-IEFPCM-B3LYP/6-311++G(d, p) level. The eleven structures of the hydrogen-bonded 4AP clusters formed with different number water molecules in both S0 and S1 states were optimized. The absorption, fluorescence and infrared spectra were calculated. The results of the hydrogen bond energy and length reveals that the hydrogen bonds formed by the nitrogen atom of the amine group with water molecule (A type) are significantly weakened from states S0 to S1. In contrast, the hydrogen bonds formed by the oxygen atoms of the two carbonyl groups (B type) with water molecules and those formed by the two hydrogen atoms of the amine group (C type) with water molecules are remarkably strengthened. Comparing with the 4AP monomer spectra, the weakening for the hydrogen bond of A type could be responsible for the blueshifts of the electronic absorption spectra and the stretching vibrational spectra of the two NH groups in 4AP from states S0 to S1. The significant redshifts of the electronic spectra and the S0-S1 downshifts of the stretching vibrational modes of the two NH groups and the two carbonyl groups in 4AP could be attributed to the strengthening of hydrogen bonds for B and C types.

A multi-property fluorescent probe for the investigation of polymer dynamics near the glass transition

In addition to the commonly observed single molecule fluorescence intensity fluctuations due to molecular reorientation dynamics, a perylene bisimide-calixarene compound (1) shows additional on-off fluctuations due to its ability to undergo intramolecular excited state electron transfer (PET). This quenching process is turned on rather sharply when a film of poly(vinylacetate) containing 1 is heated above its glass transition temperature (T g), which indicates that the electron transfer process depends on the availability of sufficient free volume. Spatial heterogeneities cause different individual molecules to reach the electron transfer regime at different temperatures, but these heterogeneities also fluctuate in time: in the matrix above T g molecules that are mostly nonfluorescent due to PET can become fluorescent again on timescales of seconds to minutes.

The two different mechanisms for intensity fluctuation, rotation and PET, thus far only observed in compound 1, make it a unique probe for the dynamics of supercooled liquids.

Fluorescent polymer coatings with tuneable sensitive range for remote temperature sensing

Polymer films of poly(vinyl alcohol) containing the fluorescent dyes 4-aminophthalimide (AP) or 6-propionyl-2-dimethylamino-naphthalene (Prodan) are used as temperature-sensitive fluorescent coatings for remote temperature sensing. Temperature can be obtained by a two-wavelength ratiometric-based emission intensity measurement. The coatings are sensitive in a 100K temperature range that can be tuned by polymer-solute interactions. The usable range is 200-300 K for AP and 280-380 K for Prodan.

Dynamic polar solvation is reported by fluorescing 4-aminophthalimide faithfully despite H-bonding

Solvation dynamics of 4-aminophthalimide (4AP) in methanol is measured by broadband upconversion of the fluorescence band. The peak emission frequency nu(t) is determined from 100 fs onward with 85 fs time resolution. Polar solvation based on simple continuum theory, including solute polarizability, describes the temporal shape of nu(t) quantitatively. Extrapolation nu(t-->0) points to an initial emission frequency which agrees with the result from stationary spectroscopy in a nonpolar solvent. The extent (4300 cm(-1)) of the dynamic Stokes shift is largely due (50%) to H-bonding, however. The observations imply that H-bonds with 4AP adiabatically follow the dielectric relaxation of the methanol network. The stimulated emission band is also used to measure solvation dynamics. The evolving band is monitored by transient absorption spectroscopy of supercontinuum probe pulses. But the excited-state absorption spectrum, its relative amplitude, and its evolution are needed to extract nu(t) from such measurements. These key data are obtained by comparison with the upconversion results. Thus calibrated photometrically, 4AP transient absorption can be used to monitor solvation dynamics in any solvent. The excited-state absorption spectrum is assigned with the help of time-dependent density-functional calculations. Fluorescence excitation and double-resonance spectroscopy of isolated 4AP, cooled in a supersonic jet, is used to determine optically active modes. An intramolecular reorganization energy is inferred which is consistent with the value in 2-methylbutane (2025 cm(-1)). The crystal structure is also provided.

Role of surfactants in the sorption of the whitening agent Tinopal CBS onto viscose fibers: a fluorescence spectroscopy study

In the present work, we studied the role of an anionic surfactant, sodium dodecyl sulfate, and a cationic surfactant, dodecyltrimethylammonium chloride, in the sorption of 4,4'-distyrylbiphenyl sodium sulfonate (Tinopal CBS) onto modified cellulose fibers. Fluorescence spectroscopy was used to quantify the amount of sorbed Tinopal CBS on the fiber surface. Differences in the spectral properties and the efficiency of sorption of the whitener/surfactant/fiber system are explained in terms of electrostatic interactions. Our results also show that the sorption efficiency is greater for solutions containing cationic surfactants only below the critical micelle concentration, while anionic surfactants show a smooth influence on the sorption process.