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Electronic structure and energy descriptors for molecular crystals from quantum crystallography and X-ray charge density analysis

Periodic Reporting for period 1 - XQCR (Electronic structure and energy descriptors for molecular crystals from quantum crystallography and X-ray charge density analysis)

Reporting period: 2018-04-01 to 2020-03-31

The broad objectives of the project were to develop and optimize new methods in high-resolution X-ray crystallography and quantum crystallography to explore electronic properties and energy descriptors for molecular materials. The project has achieved the major objectives - the development and application of methods based on charge density analysis and quantum crystallography techniques using high-resolution X-ray data and their applications in the field of crystal engineering of molecular crystals.
The specific scientific objectives can be essentially summarized into the following:
i) Developing and optimizing quantum crystallographic methods for accurate intermolecular interaction energies and lattice energies in molecular crystals
ii) Developing protocols combining high-resolution X-ray charge density analysis and quantum crystallography techniques such as Hirschfield Atom Refinement
iii) Applying quantum crystallographic methods to explore the electronic band structure in functional molecular materials

The results from the project will help to understand the stability and mechanical properties of molecular materials such as pharmaceutical drugs, and the electronic properties functional molecular materials such as molecular semiconductors and photovoltaic crystals. Hence the fundamental insights from the project can have societal benefits.
The work carried out during the MSCA period was aimed at the objectives mentioned above, and covered by different Work packages.
The research work involved crystallization and screening of single crystals of the following compounds:
Polymorphs of the drug hydrochlorothiazide – by slow evaporation method
Polymorphs of the drug paracetamol and acetazolamide – by slow evaporation method
Isostructural compounds diphenyl disulfides (dpdS), diphenyl diselenides (dpdSe) and diphenyl ditelluride (dpdTe) – by slow evaporation method
Donor-acceptor complexes formed by benzoquinone derivatives and pyrene by slow evaporation method
Ammonia-borane:18-crown-6 complex by slow evaporation method
Organic/ metal-organic semiconductor/ photovolatic materials phthalocyanine and various
Metal phthalocyanines (Cu, Co, Zn) - by three-zone sublimation condensation method
Organic light emitting diode (OLED) components such as Alq3 (tris(8-hydroxyquinolinato)aluminium) and its various metal analogues such as CrQ3, InQ3, GaQ3. The compounds were synthesized by complexation reaction and the resultant products were crystallized by a three-zone sublimation condensation method. (related to the objectives in WP3)


Further the work involved experimental diffraction data collection for the following systems:
1. Routine single crystal data collection on all the above-mentioned systems using laboratory X-ray diffractometer (Supernova) at Aarhus university for their crystal quality screening as well as for structure determination.

2. Ultra-high resolution X-ray diffraction data collection on Cu-glycine complex at Spring-8 synchrotron facility, Japan
3. Single crystal neutron diffraction data collection on diphenyl disulfides (dpdS), diphenyl diselenides (dpdSe) and diphenyl ditelluride (dpdTe) at 100 K and 295 K at ANSTO, Sydney, Australia
4. Ultra-high resolution X-ray diffraction data collection on the donor-acceptor complex formed by tetrachlorobenzoquinone and pyrene at Spring-8 synchrotron facility, Japan
5. UV-Visible Diffuse reflectance spectroscopic data collection and analysis energy bandgaps for the following crystalline systems:
(a) diphenyl disulfides, diphenyl diselenides, and diphenyl ditelluride
(b) Metal phthalocyanines (Cu, Co, Zn analogues)
(c) Tris(8-hydroxyquinolinato)aluminium and its various metal analogues such as CrQ3, InQ3, GaQ3.
(d) Donor-acceptor complexes formed by benzoquinone derivatives and pyrene
(e) The solid solution crystals formed by compounds in (a), (b) and (c).

6. Energy dispersive X-ray spectroscopic data collection on the solid solution crystals formed by diphenyl disulfide (dpdS), diphenyl diselenide (dpdSe).
7. Raman spectroscopic data collection on:
(a) diphenyl disulfide (dpdS), diphenyl diselenide (dpdSe) and their solid solution crystals
(b) Ammonia-borane:18-crown-6 complex
Progress beyond the state of the art was made by means of the work packages that involve quantum crystallographic and computational studies that explore intermolecular interactions, chemical bonding and band gaps in molecular crystals utilising the experimental data. It also involves optimising new methods for lattice energies and accurate electron density modelling of molecular crystals.
a) Studies aimed at obtaining accurate interaction energies and lattice energies were carried out on the following systems: (i) a series of isostructural compounds diphenyl disulfide (dpdS), diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe) – in order to predict their relative stabilities/enthalpy of sublimation. The lattice energies from our method utilising experimental crystal data were found to be superior to the conventional quantum periodic methods and hence this outcome provides a novel and accurate method to predict and compare the thermodynamic stabilities of molecular crystals. The method has been further employed to a series of pharmaceutical drug polymorphs as well.
b) The study of electronic band gaps in molecular crystalline systems have resulted in important outcomes related to band gap tuning in materials. The studies led to the discovery of a new approach in molecular crystal engineering for property tuning i.e solid solution/alloy formation as a strategy for band gap tuning. This strategy has been also extended to a series of molecular functional materials such as organic light emitting diode and photovoltaic materials which has led to two ongoing research projects that combines both the fundamental and applied aspects of the optoelectronic properties in these materials.
c) Towards the development of methods that combine quantum crystallographic methods and X-ray multipole charge density modelling we studied the following systems: (i) Ammonia-borane:18-crown-6 complex, (ii) the donor-acceptor complex formed by tetrachlorobenzoquinone and pyrene. The combination of the quantum crystallographic Hirshfeld atom refinement and multipole modelling using ultra-high resolution X-ray diffraction data from Spring-8 synchrotron facility resulted in highly accurate electron density models in these systems.
d) Multipole charge density modelling using ultra-high resolution synchrotron X-ray diffraction data collection on Cu-glycine complex crystal was performed to obtain insights into the chemical bonding and intermolecular interactions in this crystal in connection with the unusual mechanical property of elastic bending exhibited by this system. This problem was addressed by a combination of quantum chemical studies of the electron density distribution, interaction energies, their topologies, and elastic tensors of this system. Further, to obtain more insights into the structure property relations we also employed high pressure crystallographic studies using diamond anvil cell.
Although these results fall under fundamental research, the insights derived from these studies can have implications in the solid-state formulation of pharmaceutical drugs and the design of functional molecular materials.
the synergy between quantum crystallography and crystal engineering of molecular materials