Effects of high pressure on the electronic spectra and crystal structure of molecular materials
Embargo end date31/07/2021
Sussardi, Alif Nur Patriya
The fluorescence of solid-state molecular materials is a field of growing research interest, stimulated by technological applications, such as organic light-emitting diodes and optical sensing. Investigation of the relationship between pressure-induced changes in the structure and electronic spectra of such materials offers opportunities for understanding the influence of intermolecular interactions and conformational changes on optical properties. However, there have been few studies that directly correlate the results of high-pressure X-ray crystallography and high-pressure optical spectroscopy. An apparatus for the in situ measurement of UV-visible absorption and fluorescence emission spectra of crystals in a diamond anvil cell (DAC) has been developed. The effects of pressure (up to several GPa) on the structure and spectra of metal-organic frameworks (MOFs), molecular rotors, conjugated aromatic molecules and thermally activated delayed-fluorescence (TADF) materials have been studied. A Luminescent MOF material, Hf-peb, was studied. Hf-peb MOF is a MOF with two-fold interpenetrated linker, 1,4-phenylene-bis(4-ethynylbenzoate) (peb2-). X-ray crystallography reported in this thesis showed that the linker exists in two conformational states at ambient pressure (and room temperature), one in which the central phenyl ring is coplanar with the two terminal phenyl rings, and the second is the newly reported twisted conformer, where the central phenyl ring is perpendicular to the terminal phenyls. The fractional population of the twisted conformer increased with increasing pressure, from 28% at ambient pressure to 100% at 2.1 GPa. Both the absorption spectrum and the emission spectrum shifted to longer wavelength with increasing pressure. It was also found that the observed emission spectra, across the pressure range, can be well-fitted by linear combinations of the 2.1 GPa spectrum, assigned to the twisted conformer, and the ambient pressure spectrum. The fractional population of the twisted conformer at each pressure estimated in this manner was in good agreement with the values determined from the X-ray diffraction data. The close correlation indicates that the contribution of each conformer to the observed emission spectrum is determined by its ground-state population, and hence the two conformers must have very similar fluorescence brightness. A combined high-pressure UV-vis absorption spectroscopy and computational study on Zr-abdc MOF, a MOF containing an azobenzene dicarboxylate (abdc2-) linker has been carried out. It is revealed the effect of pressure on the absorption spectra in penetrating (methanol) and non-penetrating (FC-70) pressure media. Penetration of methanol into the porous MOF framework resulted in a hypsochromic shift that can be attributed to solvent-induced stabilisation of the more polar the ground state. In the non-penetrating FC-70 medium, pressure-induced compression of the unit cell volume caused a decrease in length of the abdc2- linker. DFT calculations predicted a consequent bending of the linker structure with increasing pressure. TDDFT calculations then predicted a decrease in the energy of the transition to the nπ* state, with increased bending. The TDDFT-predicted trend was in good agreement with the experimentally observed spectral shift. The effects of pressure on the fluorescence properties of two related molecular rotors, sym-pentaphenylcyclopentadiene (Ph5C5H) and sym-heptaphenylcycloheptatriene (Ph7C7H), have also been studied. The redshift in UV-vis absorption and emission spectra with increasing pressure on Ph5C5H could be attributed to stronger interphenyl interactions, which are already present at ambient pressure. On the other hand, X-ray crystallography on Ph7C7H demonstrated the influence of specific interphenyl interactions, both intramolecular and intermolecular, on the optical spectra. For Ph7C7H at high pressures, interphenyl interactions that closely resemble effective displaced-stacked benzene dimers can be identified. The observed fluorescence spectra could be interpreted in terms of relaxed excimer emission from these dimer-like species which occur only at high pressure. These observations elucidate the interactions that lead to aggregation-induced emission in molecular rotors of this type. The 1,4-bis(4-carbomethoxyphenylethynyl)benzene (BCPEB) is an example of a linear π-conjugated system, and also a molecular rotor, with three phenyl rings connected by acetylene linkers. The latter chromophore, commonly known as bis(phenylethynyl)benzene (BPEB), is considered to be a model system for one-dimensional molecular wires that have numerous applications in optoelectronics. The photophysical properties of BPEB are known to be strongly influenced by torsional isomerism. The solution phase and low-temperature fluorescence measurements showed very similar photophysical properties of both BPEB and BCPEB. The structure of BCPEB was investigated as a function of pressure, by single-crystal X-ray diffraction in a DAC, using synchrotron radiation. The pressure-induced hypsochromic shift and spectral profile evolution with decreasing pressure in the UV-vis absorption and emission spectrum can be related to decrease in intermolecular stacking interaction and increase in torsional movement, due to less restricted molecular movement in the crystal. Pressure-dependent properties were also studied for the well-known TADF material of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) and its derivative, 4CzIPN-tBu8. Single-crystal X-ray diffraction of 4CzIPN, obtained up to 4.16 GPa, showed a decrease in the intermolecular inter-carbazole distance with increasing pressure. In addition to steady-state UV-vis and fluorescence spectroscopy, time-resolved measurements of delayed fluorescence were conducted as a function of pressure. Both 4CzIPN and 4CzIPN-tBu8 show different response to pressure on the steady-state electronic spectra and its emission kinetics, in which the 4CzIPN-tBu8 experienced change in singlet-triplet energy gap at pressures higher than ~0.8 GPa. The observed pressure-dependence of the delayed fluorescence lifetime can be interpreted in terms of the effect of intermolecular interaction between the carbazole groups on the TADF process in the two systems. In summary, this thesis reports the relation between the crystal structure and the electronic spectra of photo-active materials, using a custom made high-pressure optical spectroscopy measurement system to elucidate various photophysical processes under high-pressure conditions.