Nicholas Y. Tan

Direct evidence to support the restriction of intramolecular rotation hypothesis for the mechanism of aggregation-induced emission: temperature resolved terahertz spectra of tetraphenylethene

In contrast to the traditional fluorescent dyes that exhibit a decrease in fluorescence upon aggregation, Aggregation-Induced Emission (AIE) molecules are a family of fluorophors which exhibit increased fluorescence upon aggregation. Consequently, AIE molecules represent an interesting new material with potential applications in fluorescent chemo/biosensors, light emitting devices and medical diagnostics. Numerous mechanisms have been proposed to explain this phenomenon, including E–Z isomerization, and restriction of intramolecular rotations (RIR). However, there has not been any direct experimental evidence to support either one of these hypotheses. Here we use terahertz time-domain-spectroscopy (THz-TDS) and solid-state computational simulations of an AIE molecule to link the increase in intensity of intramolecular rotation and rocking modes to the measured fluorescence and reveal direct evidence supporting the RIR hypothesis. This is the first time that terahertz spectroscopy has been used to directly probe such molecular motions in AIE materials and in doing so we have found conclusive evidence to fully explain the AIE mechanism.

High-birefringence nematic liquid crystal for broadband THz applications

ABSTRACT Liquid crystals (LCs) have been studied extensively in the visible range for their dielectric tunability, and the characterisation in the terahertz (THz) range has gained increasing interest due to the need for active THz modulation and switching devices. In this paper, we use THz time-domain spectroscopy to measure the frequency-dependent birefringence and the absorption coefficient of a number of commercial and non-commercial nematic LCs, including E7, BL037, MDA-98-1602, LCMS-107, GT3-23001 and 1825, over a range of bias voltages at room temperature. Furthermore, several basic components of LC mixture are analysed to establish their contributions to birefringence and theoretical model is used to fit the absorption spectra. The large tunability and low loss measured for a range of samples show that the LCs are useful tunable dielectrics for compact, efficient and broadband THz devices. GRAPHICAL ABSTRACT

Probing phase transitions in simvastatin with terahertz time-domain spectroscopy.

Simvastatin is known to exist in at least three polymorphic forms. The nature of polymorphism in simvastatin is ambiguous, as the crystal structures of the polymorphs do not show any significant change in crystal packing or molecular conformation. We utilize terahertz time-domain spectroscopy to characterize each of the polymorphs and probe the phase transitions in the range of 0.2-3.0 THz and for temperatures ranging from 90 to 390 K. In form III, vibrational modes are observed at 1.0, 1.25, and 1.7 THz. For form I, we find that the spectrum is dominated by a baseline corresponding to libration-vibration motions coupled to the dielectric relaxations, which is characteristic of a disordered hydrogen bonding material but with additional broad vibrational modes at 0.8 and 1.4 THz. In addition, the baseline shifts with temperature similar to that observed in disordered materials. This background absorption exhibits pronounced changes around the phase transition temperatures at 232 and 272 K. The results are in agreement with molecular dynamics simulations, which indicate that changes in the rotational freedom of the ester tail in the molecule govern the polymorphism in simvastatin.

Terahertz study on porosity and mass fraction of active pharmaceutical ingredient of pharmaceutical tablets

In this study, terahertz time-domain spectroscopic (THz-TDS) technique has been used to ascertain the change in the optical properties, as a function of changing porosity and mass fraction of active pharmaceutical ingredient (API), of training sets of pharmaceutical tablets. Four training sets of pharmaceutical tablets were compressed with microcrystalline cellulose (MCC) excipient and indomethacin API by varying either the porosity, height, and API mass fraction or all three tablet parameters. It was observed, as far as we know, for the first time, that the THz time-domain and frequency-domain effective refractive index, as well as, the frequency-domain effective absorption coefficient both show linear correlations with the porosity and API mass fraction for training sets of real pharmaceutical tablets. We suggest that, the observed linear correlations can be useful in basic research and quality inspection of pharmaceutical tablets. Additionally, we propose a novel optical strain parameter, based on THz measurement, which yields information on the conventional strain parameter of a tablet as well as on the change of fill fraction of solid material during compression of porous pharmaceutical tablets. We suggest that the THz measurement and proposed method of data analysis, in addition to providing an efficient tool for basic research of porous media, can serve as one of the novel quality by design (QbD) implementation techniques to predict critical quality attributes (CQA) such as porosity, API mass fraction and strain of flat-faced pharmaceutical tablets before production.

Noninvasive porosity measurement of biconvex tablets using terahertz pulses

Biconvex pharmaceutical microcrystalline cellulose (MCC) compacts were investigated by the detection of terahertz (THz) pulse delay in the transmission measurement mode. The dimensions of the tablets were kept as constants but the porosity was a priori known variable. It is shown that the porosity of the biconvex compact has a linear correlation with the THz pulse delay. By constructing a calibration line between these two parameters (i.e. porosity and THz pulse delay), it is possible to non-invasively detect porosity of biconvex tablets. We suggest that this preliminary study could be the starting point of in-depth future studies on the screening of porosity and related properties of real biconvex pharmaceutical tablets using terahertz sensing techniques.