A combined computational and experimental study of dichromic crystals

Myburgh, Dirkie (2018-03)

Thesis (MSc)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: A combination of theoretical and experimental techniques were used in three studies related to dichromic crystals. The first study involves a porous crystal of a Schiff base-containing trianglimine compound undergoing a yellow-to-red colour conversion above a humidity of 55%. Schiff bases are well known for undergoing enolimine-ketoenamine transformations as iono-, solvato-, thermo- or photochromic processes, altering the visible colour properties of the system. In this study a similar transformation occurs owing to the combination of a sorptiochromic and a hydrochromic process, which are both considered as subcategories of solvatochromism. The enolimine-ketoenamine transformation is caused by H-bond formation between water and the oxygen atom of the enol group. This weakens the OH bond allowing a proton transfer from the oxygen to the nitrogen atom. In the presence of H2O the ketoenamine state is thermodynamically preferred, whereas in the gas phase the enolimine is preferred. Secondly, a phase study was performed on a diarylethene compound (1,2-bis(5-pyridyl-2-propyl-3-thienyl)perfluorocyclopentene), DA-2-[o]. Diarylethenes are well known for undergoing a ring-closure reaction under UV light and have been shown to exhibit photochromic behaviour in the crystalline state, as is the case for the three polymorphic phases identified for DA-2-[o], DA-2-[o]-α, DA-2-[o]-β and DA-2-[o]-γ. The first two are obtained by recrystallisation from acetone and hexane, while the third can be obtained by heating of either DA-2-[o]-β above 70 °C or DA-2-[o]-α above 100 °C. The DA-2-[o]-α phase is converted to the DA-2-[o]-β phase by heating above 100 °C and subsequent cooling to room temperature. The opposite conversion can be achieved by leaving DA-2-[o]-β in a cupboard under ambient conditions for a period of six months. This result combined with lattice-energy calculations suggests that the DA-2-[o]-α phase is the thermodynamically preferred phase. The final study provides a fundamental explanation for the photoisomerisation reaction of diarylethenes. This was done by developing a method whereby all the major parameter differences between the ring-opened and ring-closed forms are scanned followed by the calculation of the excited state potential energy surface map. This method was then used to illustrate how various structural modifications will affect the ring-closure or ring-opening reaction. The photochromism of the open form was ascribed to the formation of an electron hole after electron excitation from the HOMO to the LUMO. This results in an attractive force between the two adjoining carbon atoms and a downward energy gradient, over the conical intersection, towards the closed form. For the ring-opening it was observed that a twisting action is required before the ring-opening can take place making this an asynchronous process.

AFRIKAANSE OPSOMMING: 'n Kombinasie van teoretiese en eksperimentele tegnieke is gebruik in drie studies wat handel oor dichromatiese kristalle. Die eerste studie behels 'n poreuse Schiff-basis bevattende trianglimien verbinding wat 'n geel-tot-rooi kleur verandering ondergaan bo 'n humiditeit van 55%. Schiff basisse is bekend daarvoor dat hulle enolimien-ketoenamien transformasies ondergaan via iono-, solvato-, termo- of fotochromiese prosesse wat gepaard gaan met 'n kleur verandering. In hierdie studie vind 'n soorgelyke transformasie weens 'n kombinasie van 'n sorptiochromiese en 'n hidrochromiese proses plaas. Beide word as sub-kategorieë van solvatochromatisme beskou. Die enol-keto transformasie word aangedryf deur die vorming van waterstofbindings tussen H2O en die enol groep. Die OH binding word verswak waarna die waterstof atoom na die stikstof verskuif. In die teenwoordigheid van water is die ketoenamien vorm termodinamies meer gunstig terwyl die enolimien in die gas fase verkies word. Tweedens is 'n fase studie uitgevoer op 'n diarieleteen verbinding (1,2-bis(5-piridiel-2-propiel-3-tiëniel)perfluorosiklopenteen), DA-2-[o]. Diarieleteen verbindings ondergaan 'n ring-vorming onder UV lig. Soms kan hierdie proses plaasvind selfs in die kristal vorm wat die geval is vir die drie polimorfiese fases wat geïdentifiseer is vir DA-2-[o], DA-2-[o]-α, DA-2-[o]-β en DA-2-[o]-γ. Die eerste twee kan verkry word deur DA-2-[o] vanaf asetoon en heksaan respektiewelik te kristalliseer terwyl die derde verkry kan word deur verhitting van of DA-2-[o]-β bo 70 °C of DA-2-[o]-α bo 100 °C. Verder kan die DA-2-[o]-α fase omgeskakel word na die DA-2-[o]-β fase deur verhitting bo 100 °C en daaropvolgende verkoeling tot kamer temperatuur. Die teenoorgestelde omskakeling kan bereik word deur DA-2-[o]-β in 'n kas vir ses maande te laat staan. Hierdie resultaat, saam met rooster-energie berekeninge, vir beide sisteme, dien as bewys dat die DA-2-[o]-α fase termodinamies verkies word. In die laaste studie word die fotoisomerisasie reaksie van diarieleteen verbindings verduidelik deur hierdie fotochemiese reaksie te bestudeer vanaf 'n nuwe benadering. Hierdie metode is dan gebruik om te illustreer watter effek verskeie strukturele modifikasies op die ring-vorming- of ring-openingsreaksie sal hê deur na kombinasies van potensiële-energie kurwes van die grond en opgewekte toestande te kyk. Die fotochromatisme van die oop vorm kan aan die vorming van 'n elektron leemte na die opwekking van 'n elektron vanaf die HOMO na die LUMO toegeskryf word. Dit veroorsaak 'n aantrekkingskrag tussen die twee koolstof atome, waartussen die binding vorm, en ‘n afwaartse energie gradient in die rigting van die geslote-ring toestand. Vir die ring-openings-reaksie word 'n draai-aksie vereis voordat die ringopening kan plaasvind wat hierdie reaksie dus 'n twee-stap proses maak.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103913
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