Molecular origins of polymorphic phase transitions for dynamic organic electronics

Davies, Daniel

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https://hdl.handle.net/2142/115401 Copy

Description

Title

Molecular origins of polymorphic phase transitions for dynamic organic electronics

Author(s)

Davies, Daniel

Issue Date

2022-04-20

Director of Research (if dissertation) or Advisor (if thesis)

Diao, Ying

Doctoral Committee Chair(s)

Diao, Ying

Committee Member(s)

• Evans, Christopher
• Peters, Baron G.
• Yang, Hong

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Polymorphism
• Organic Semiconductors
• Phase Transitions
• biradical

Abstract

The correlation between polymorph discovery and the time and effort spent researching a compound was famously proposed by McCrone in 1965. Nearly 60 years later, and despite the importance of polymorphic structure to a variety of properties, polymorphs are still mostly found through tried-and-true serendipity. However, control over the polymorphic structure is critical for fully realizing the performance of organic semiconductors requiring a deep understanding of the molecular origins regulating these polymorphic structures. This is particularly the case in quinoidal n-type semiconductors which have significantly lagged behind p-type counterparts. One way to control polymorphism is through reversible phase transitions which may occur through different pathways. Typically transitions occur by nucleation and growth, which is widely observed among solid-to-solid transitions. These transitions occur via diffusive, molecule by molecule mechanisms which completely reconstructs the crystal and often disrupt the material structural integrity. In a few, but growing, number of crystals, cooperative structural transitions are found. These transitions, in contrast, involve simultaneous, concerted displacement of molecules in a crystalline material. Cooperative transitions have acquired much attention in the research community for their low transition barrier, ultrafast kinetics, and structural reversibility. Moreover, due to the displacive nature of the mechanism, many times shape changes and even thermosalient effects may be observed. Harnessing these behaviors offer a route to dynamic electronics not possible previously. For our portion of time and effort devoted to the understanding of these polymorphic transitions, we uncover the prolific polymorphism in a high performing n-type organic semiconductor, 2 dimensional quinoidal terthiophene (2DQTT-o-B), obtaining at least 6 different polymorphs. These polymorphs exhibited vastly different structural characteristics as analyzed by single crystal X-ray diffraction (SCXRD) and grazing incidence X-ray diffraction (GIXD). Via the selection of these different polymorphic structures, we show tuning of the electron charge carrier mobility by 5 orders of magnitude. Moreover, we explore a thermally induced self doping effect in these devices. Along with the rich polymorphic space, we observed the presence of both a cooperative and nucleation and growth transition via two reversible thermally triggered transitions in single crystals under in situ polarized optical microscopy (POM). The presence of both mechanisms offered an unprecedented chance to compare the origins of these mechanisms on an equal footing to isolate structural and molecular design effects on the transitions. Through Raman spectroscopy and GIXD, We show the cooperative behavior is driven by a reorientation of the alkyl chains, resulting in a change in the tilt of the molecules, much like dominos falling. On the other hand, the nucleation and growth behavior was driven by biradical formation and facilitated by melting alkyl chains. Moreover, the cooperative behavior is used to design novel single crystal thermally actuated organic semiconductor devices. By utilizing the cooperative shape change, the crystal can break and reform the contact with PEDOT:PSS, providing a thermally switchable device. The molecular mechanisms suggested that the alkyl chains play a key, yet distinct role in both reversible phase transitions. We investigated the effect of alkyl chain length on both transition mechanisms by successively shortening the branched alkyl chain length and observing the transition behavior under in situ POM. We showed the at the shortest side chains, the cooperative behavior was suppressed, which was confirmed through Raman spectroscopy. Moreover, we showed the nucleation and growth behavior was present in all three systems, but the transition temperature increased with shorter side chains, consistent with a higher alkyl chain order to disorder transition. Moreover, the biradical formation showed a coupling with the phase transition, providing a new route to tuning the electronic structure behavior. This provides a generalizable design rule for controlling the cooperative behavior and fine tuning nucleation and growth pathways. Finally, we explore the kinetic trapping of polymorph III at room temperature, resulting in significant breaking of the hexagonal symmetry, despite no first order transition, as well as changes to the electronic properties. Despite the property changes, this kinetically trapped state still retains insulating like behavior, orders of magnitude below the conductance from the stable form. We demonstrate a method using laser induced heating as a way to control the conversion of this kinetically trapped polymorph back to the stable form, allowing for tuning of the conductance state in the device. Using this setup, we fabricated a memristor device based on the electronic switching properties from this transition.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Daniel Davies

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