Débora T. Balogh

Two-photon absorption in azoaromatic compounds

Abstract We report on the two-photon absorption (2PA) cross-section for eight azoaromatic compounds in dimethyl sulfoxide solutions, using the Z-scan technique with femtosecond laser pulses at 775 nm. The 2PA cross-sections of these molecules are of the order of (1–6)×10 −50 cm 4 s photon −1 . A correlation was established between the molecular structure and the two-photon absorption process, which makes it possible to develop molecular design strategies to create structures with suitable two-photon absorption coefficients.

Optical storage in mixed Langmuir-Blodgett (LB) films of azopolymers and cadmium stearate

Abstract We present a comparative study on the optical storage characteristics of mixed Langmuir–Blodgett (LB) films from azobenzene polymers and cadmium stearate. The amplitude of induced birefringence depends on the azobenzene content in the polymer as well as on the rate of trans – cis – trans isomerization. Structural factors such as main chain rigidity seem to play an important role in the remaining stored information.

The influence of electrostatic and H-bonding interactions on the optical storage of layer-by-layer films of an azopolymer

Abstract The optical storage properties of layer-by-layer (LBL) films of a copolymer of methacrylic acid and 4-[[2-(methacryloyloxy)-ethyl]ethylamino]-2′-chloro-4′-nitroazobenzene (MA- co -DR13) alternated with the cationic poly(allylamine hydrochloride) are reported. The writing process is considerably slower than in cast films due to electrostatic and H-bonding interactions that hinder molecular orientation. The speed of the writing process can be increased if the LBL film is immersed into pure water prior to the storage experiments, probably because entrapped water helps preventing intermolecular interactions. This is a reversible effect and cannot be attributed to mere water absorption of the polymer film, since cast films of the same components do not exhibit the ‘water effect’. It is also demonstrated that the films display sensing capabilities, with their optical storage properties being significantly affected by gushes of hot and humid air.

Chromophore aggregation hampers photoisomerization in Langmuir–Blodgett films of stearoyl ester of Disperse Red-13 (DR13St)

Abstract Aggregation of azobenzene chromophores in Langmuir–Blodgett films of stearoyl ester of Disperse Red-13 (DR13St) and cadmium stearate (CdSt) hampers the trans–cis photoisomerization that normally leads to optical storage capabilities, since the required free volume around the chromophore is not available. Such aggregation has been inferred by UV–VIS. spectroscopy and X-ray diffraction. Optical storage properties could still be obtained if DR13St was mixed in a guest–host system with a polycarbonate matrix.

Langmuir and Langmuir–Blodgett films of a homopolymer of Disperse Red-13

We report on the formation of stable and uniformly transferable Langmuir monolayers of a methacrylic homopolymer of Disperse Red-13 (HPDR13). There was no need of covalently attaching long alkyl side chains or mixing the homopolymer with film forming materials. HPDR13 was derived from DR13-methacrylate by a homo polymerization reaction in the presence of 2,2′-azobisisobutyronitrile as initiator. Surface pressure isotherms were obtained under various subphase temperatures, pH conditions and at several compression speeds. The monolayer has also been characterized by surface potential isotherms. The results indicated that, in contrast to the poor stability of a methacrylate-DR13 monomer monolayer, stable monolayers of HPDR13 could be obtained at the air–water interface. A uniform layer by layer transfer has been demonstrated by the linear increase of maximum absorption with increasing film thickness. A red shift observed in the UV–Vis spectra of the Langmuir–Blodgett (LB) films revealed a possible J-type aggregation of molecules. LB films have also been characterized by Fourier Transform Infra Red (FTIR) spectroscopy.

Femtosecond laser induced synthesis of Au nanoparticles mediated by chitosan.

This paper reports the synthesis of Au nanoparticles by 30-fs pulses irradiation of a sample containing HAuCl4 and chitosan, a biopolymer used as reducing agent and stabilizer. We observed that it is a multi-photon induced process, with a threshold irradiance of 3.8 × 10(11) W/cm2 at 790 nm. By transmission electron microscopy we observed nanoparticles from 8 to 50 nm with distinct shapes. Infrared spectroscopy indicated that the reduction of gold and consequent production of nanoparticles is related to the fs-pulse induced oxidation of hydroxyl to carbonyl groups in chitosan.

Langmuir monolayers of lignins obtained with different isolation methods

Abstract The characteristics of Langmuir monolayers of lignins isolated via distinct methods, viz. acetone–oxygen–organosolv (termed Lignin-1), Kraft (Lignin-2), acetic acid–organosolv (Lignin-3) and acetone–organosolv (Lignin-4), are investigated using surface pressure and surface potential measurements. The overall behavior of these lignins is essentially the same, but some differences were observed which are attributed to the relative amount of carbonyl groups in the lignin skeleton. Lignin-1 and 3 displayed pressure–area isotherms considerably more expanded because they possess the largest amounts of carbonyls that are forced out of the water upon monolayer compression in the condensed phase. These carbonyls contribute negatively to the surface potential. For Lignin-1, where such groups appear in COOH moieties, the surface potential is much smaller than for the other lignins. Hysteresis and stability experiments showed that the lignin monolayers are metastable possibly owing to molecular reorganization at the air–water interface.

Optical storage in mixed Langmuir-Blodgett (LB) films of disperse Red 19

We report on the optical storage characteristics of mixed Langmuir-Blodgett (LB) films from an azobenzene polymer (disperse red 19 1,4 diphenylmethane polyurethane, DR19-MDI) and cadmium stearate. This mixed LB film showed a great remaining birefringence (80%) due to its high rigidity.

Aggregation in Langmuir and Langmuir -Blodgett films of azopolymers and its role for optically induced birefringence

Abstract Aggregation in azopolymers is reported to affect the Langmuir monolayer characteristics and the optically induced birefringence of Langmuir–Blodgett (LB) films from DR19 isophorone polyurethane (PIPDI) and DR19 4,4′ diphenylmethane polyurethane (PMDI). In mixed monolayers with cadmium stearate (CdSt), the folding of PMDI molecules appears to be substantially changed compared to the monolayer of the pure polymer, leading to a surface potential that is higher than observed for monolayers of pure PMDI and of pure CdSt. UV–Vis spectroscopy data of the deposited mixed LB films indicate H-type aggregation for PMDI/CdSt, in contrast to PIPDI/CdSt and other azopolymers investigated earlier. The H-type aggregation precludes photoisomerization, thus requiring a higher laser power for the maximum induced birefringence to be achieved in LB films of PMDI/CdSt.

Surface enhanced fluorescence and Raman imaging of Langmuir-Blodgett azopolymer films

The spectroscopic properties and surface-enhanced spectra of Langmuir-Blodgett (LB) films of methacrylic homopolymer (HPDR13) are presented. It is shown that LB film displays strong fluorescence attributed to the spatial restrictions imposed by its structure. The emission is observed in conjunction with photoisomerization, a process clearly demonstrated by the formation of surface-relief gratings in the LB film [C.R. Mendonça et al., Macromolecules 32 (1999) 1493]. Surface-enhanced Raman scattering (SERS), Surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF) were observed for LB films of HPDR13 deposited onto silver island films. SERS measurements were also carried out on a sample fabricated with one monolayer LB film deposited onto silver islands followed by one overlayer of silver (LB sandwiched between two layers of silver islands). The polymer interacts very weakly with the metal surface (physisorption), and the enhancement effect is determined by the local electric field enhancement. The strong SERS and SERRS signals were suitable for micro-Raman imaging. Line, area mapping and global images of the LB monolayer on silver island are reported. The transfer ratio in the fabrication of the LB suggests a homogeneous coating of the silver islands, thereby the chemical images show the variation of the SERS intensity due to surface enhancement.