Controlling Polymorphic Transitions in n-Type Organic Semiconductor Single Crystals by Alkyl Chain Engineering

Abstract

Control of polymorphic behavior is crucial for designing functional organic semiconductor devices as even a slight structural difference may translate to dramatically different electronic properties. One route to controlling structure is through stimulus-induced polymorph transitions, which allows for switching those electronic properties. However, despite advances in predicting crystal structures, the molecular design characteristics governing the polymorphic transition mechanism remains unknown. Here, we systematically investigate a series of n-type organic semiconductor molecules based on 2-dimensional quinoidal terthiophene with varying alkyl side chain lengths to modulate two distinct polymorph transitions, one cooperative martensitic transition and the other non-cooperative nucleation and growth transition. In the three molecular systems, we observe that shortening the alkyl chain past a critical length suppresses the cooperative polymorph transition by limiting the alkyl chain conformation change. On the other hand, the nucleation and growth transition temperature increases as the side chain length decreases, possibly driven by the increase in the melting point of the alkyl chains. We also found that tuning the alkyl chain length modulates the associated quinoidal to aromatic biradical switching that drives the nucleation and growth transition, suggesting a synergy between the crystal structure and electronic structure. Ultimately depending on the exact mechanism of the polymorph transition, adjusting the alkyl chain length may lead to tuning of the polymorph transition temperature or suppression of the transition altogether. This offers a potential molecular design rule to target a particular transition mechanism based on the desired behavior for the system.

Direct Laser Writing Crystal Polymorphs of Organic Semiconductors for Phase Change Electronics

“…Previous work showed that two-dimensional quinoidal terthiophene (2DQTT- o -B) (Figure a) exhibits a rich set of polymorph phase transitions with drastically different electronic properties. ,, Powders and films typically form a metastable polymorph I′ initially (depending on solvent and drying process), which are annealed to the stable polymorph I (Figure b). Additionally, the hysteresis for the II–III transition is substantial at 78 °C, suggesting a large energy barrier for the polymorph transition.…”

“…Due to the close proximity of the II–III polymorph transition temperature with the melting point for 2DQTT- o -B, accessing the I–III–IV transition with laser pulses becomes difficult. However, the polymorph transition behavior in 2DQTT is highly tunable via changing the length of the alkyl side chains . Shortening the alkyl chains increases the transition temperature to obtain polymorph III as well as increases the melting point.…”

“…Recently, two-dimensional quinoidal terthiophene (2DQTT- o -B) has shown promise for dynamic phase change electronics, exhibiting a diverse set of polymorphic transitions. ,− Our previous work detailed two thermally accessible phase transitions undergoing cooperative (I–II) and nucleation and growth (II–III) mechanisms. The polymorph II–III transition exhibited a large hysteresis between II-to-III and III-to-II transition temperatures, allowing for the potential of kinetic trapping by rapidly cooling polymorph III.…”

“…However, the polymorph transition behavior in 2DQTT is highly tunable via changing the length of the alkyl side chains. 21 Shortening the alkyl chains increases the transition temperature to obtain polymorph III as well as increases the melting point. The increased stability of polymorph III may provide a route for using the laser to induce the I−III−IV transition.…”

Solvent-Induced Polymorphism in a Schiff Base Compound and Their Distinguishing Effects on Semiconducting Behaviors for Utilization in Photosensitive Schottky Devices

“…Our focus is on achieving high electrical conductivity through either the continuous coordination bonds formed between metal centers and ligands or via noncovalent interactions such as the π–π stacking interaction between organic moieties possessing π-conjugation. − Notably, organic semiconductors offer numerous advantages over conventional inorganic counterparts, including lightweightness, flexibility, and the ease of depositing thin films over large surfaces. − Traditionally, the structure–function co-relationships of organic semiconductors have been investigated through chemical modifications of organic molecules. , However, polymorphism offers an ideal platform for such investigations by excluding the influence of chemical modification. Consequently, substantial progress has been made in understanding the impact of polymorphism on semiconducting properties using crystalline materials made of conjugated small molecules. − It has also been observed that slight changes in the crystal packing significantly affect charge transport in electronic devices. N -salicylideneanilines, derived from Schiff base condensation reactions between salicylaldehyde and aniline derivatives, deserve special mention in this context.…”