ESTRATEGIA VACUNAL EN PACIENTES CON ENFERMEDAD CELÍACA

The relationship between BLA and the molecular NLO effect has been identified previously where BLA can be used as a universal parameter determining the mo- lecular nonlinearity of a push-pull chromophore [18, 23, 19, 21, 134]. However, the effects of BLA on the photostability of chromophores has not been investigated so far.

Table 7.6 – Relationship between the magnitude of BLA values and corresponding photostability (B) of IND-7 & IND-7R (∼2µm thick, light intensity 35 mW/mm2_{), and}

QUN-7 & QUN-7R (∼5µm thick, light intensity 10 mW/mm2_{) for similar chromophore}

loading. |BLA| B |BLA| B IND-7 0.0144 27258 QUN-7 0.091 53290 IND-7R 0.0350 20171 QUN-7R 0.062 216300 (c) 0 0 BLA B (a.u.) (b) BLA (a) β (a.u ) QUN-7 QUN-7R IND-7 IND-7R |β| (a.u) B (a.u.) B max 0

Figure 7.28 – (a) Schematic representation of shifting the BLA values of IND-7 and QUN-7 when ring-locking. BLA approaching zero reduce the molecular hyperpolarisa- bility responsible for the NLO property of chromophore. (b) Schematic representation of change in photostability for IND-7 and QUN-7 with BLA from the data given in Table 7.6. (c) Schematic of the relation between first hyperpolarisability β0 and and

photostability B. Change in BLA may or may not be linear, it can be other forms depending on the change in bond strength when the nature of the conjugatedπ-bridge approches neutral polyene limit.

162 Chapter 7. Photostability of NLO Chromophores

Our results show that, for a push-pull system with a given donor and acceptor, there is a trade-off between the optical stability and NLO property of the chromo- phores. That is a larger BLA correspond to a large first hyperpolarisability [19]. However, at the same time our results show that larger the magnitude of BLA value lead to a reduced photostability. Also, we found the reduction in photostability with BLA is related only to the magnitude of the BLA value and not the sign. In the case of first hyperpolarisability also, the hyperpolarisability is related to the magnitude and not the sign of the bond order alternation. The findings are summarized in Table 7.6 and a schematic representation is shown in Fig. 7.28.

Combining the result from our photostability experiments, and the well esta- blished concept of the relation between BLA and first hyperpolarisability show a trade-off between both. That is for a given donor-acceptor pair, any structural alterations leading to increase the molecular NLO effect result in a reduced photo- stability. There is a trade-off between the NLO effect and optical stability of NLO chromophores. The experiments have been made only on four different chromophore with two set of donor and acceptor. A more detailed study is required to genera- lize this effect as a universal relationship as observed in the case of BLA and first hyperpolarisability.

7.9

Conclusions

Photodegradation quantum efficiency of the chromophores at different chromophore concentration as well as light intensities were performed and the mechanism of en- hanced photodegradation quantum efficiency at high optical intensity is explained using a model based on oxygen mediated photodegradation. Photodegradation mo- del based on multiple photodegradation quantum efficiencies has been used to ex- plain a more realistic photodegradation mechanism. The effect of chromophore concentration on photostability is explained using the inter-chromophore energy transfer mechanism. Methods to enhance optical stability via quenching of singlet oxygen and by quenching of triplet excited state of the chromophore were investi- gated using a singlet oxygen quencher β-carotene. Effects of structural alterations on photostability of chromophores are investigated. Addition of bulky group for enhanced EO response showed no reduction in optical stability demonstrating an over-all enhancement in efficiency due to the bulky substitution. A paradoxical result from ring-locking in two different chromophores were explained using BLA

7.9. Conclusions 163 parameter. A trade-off between the NLO property and photostability is observed in the chromophore of a given donor-acceptor pair.

Further studies need to be done on the relationship between BLA and photosta- bility. Currently, investigations were done only on a set of four chromophores. Also, two of the chromophores did not give good crystals that made the crystallography study difficult. To extend this study experiments need to be done on a number of chromophores made of same donor and acceptor but different BLA values. This will help to develop a general relationship between BLA and photostability that leads to develop a correlation between the optical stability and molecular first hyperpo- larisability.

Chapter 8

Material Characterisation at

THz Frequencies

This chapter is divided mainly into two sections. The first part deals with the evaluation of the efficiency of polymer electro-optic (EO) materials to use as active components of terahertz (THz) devices. The second part of this chapter discusses the characterisation of some organic materials in the THz frequency range using Raman and THz spectroscopy.

One of the key benefits of using organic EO polymer materials as active compo- nents of THz generation and detection system is their applicability in the wider THz region. This is possible because of the lack of a strong absorption or refractive index dispersion in amorphous polymer materials in the THz frequency range [33]. The properties of polymers can vary from one material to another. For example, PMMA polymers show absorption of THz radiation above 1.6 THz [10]. This demands the polymer host used in film fabrication as well as the polymer containing the chromo- phore to be tested in THz frequency range. This will help ensure sufficient coherence length for THz generation from optical radiation using the property of the EO poly- mers. The power of THz radiation generated using EO process is proportional to the square of the thickness of EO material as long as the phase matching conditions are satisfied. The efficiency of THz detection is also proportional to the thickness of the polymer film. However, it is important that the maximum thickness of the material should not exceed the coherence length. Another key requirement to qualify an EO material for THz applications is the absence of crystalline phonons that are active in the THz region. The efficiency of THz generation and detection using a polymer

166 Chapter 8. Material Characterisation at THz Frequencies

EO material made of PYR-3 chromophore and amorphous polycarbonate (APC) is discussed in this chapter. The following paragraph gives an introduction to the experiments performed to investigate the properties of some organic materials in the THz region using infrared (IR) and Raman spectroscopy.

Hydrogen bonded carboxylic acid dimers are being extensively studied to ob- tain a better understanding of systems with intermolecular hydrogen bonds, which are strong enough to determine the structure of the compound. Researchers have proposed novel techniques to acquire more information about the Fermi resonance in hydrogen bonded compounds by analysing both low and high frequency vibra- tional modes of the molecules [135]. A detailed understanding of the spectra at low wavenumbers is necessary to achieve further improvements in this field. In a molecule with a centre of symmetry it is expected that vibrations that are Raman active are IR inactive and vice-versa because of the different selection rules. Both IR and Raman active spectra are required to obtain a more detailed information about the vibrational modes of materials under study. Conventional Fourier transform IR (FTIR) spectrometers can cover most of the THz region. However, exploring the IR active vibrational modes in the lower THz frequency region is primarily hindered by poor access to the THz frequencies. Most of the vibrational modes related to intermolecular hydrogen bonding have clearly identifiable signatures in this region. Low wave-number Raman and the corresponding IR spectra in the THz region for benzoic acid and some of its derivatives as well as other materials such as Aspi- rin, 4-nitrotoluene (4-NT) and 2,4-dinitro toluene (2,4-DNT) are discussed. Effects of temperature on the low energy vibrational modes of the 2,4-DNT and salicylic acid are investigated using low energy Raman spectroscopy. The experimental data were interpreted using the literature values as well as results from density-functional theory (DFT) calculations using theGaussian 09 [136] package.

8.1

THz generation and detection using EO polymers

Difference frequency generation (DFG) or optical rectification (OR) using the electro- optic (EO) effect of materials is an efficient method to generate THz radiation [33]. Inorganic EO materials are usually not suitable for wideband THz generation and detection because of the presence of crystalline phonons leading to the absorption of THz radiation. Strong absorption and dispersion of refractive index are characte- ristic to the EO crystals in the wider THz region. This section discusses the results

8.1. THz generation and detection using EO polymers 167 from the calculations using the THz dielectric properties of polymer EO materials containing organic NLO polymers to investigate their efficiency for THz generation and detection. In particular, investigating their refractive index and absorption in the THz frequency range as well as estimating the coherence length for THz gene- ration and detection. This is important because THz power as well as the detector sensitivity are proportional to the thickness of the EO material if the thickness of the material is less than its coherence length [9, 137].