Unraveling two distinct polymorph transition mechanisms in one n-type single crystal for dynamic electronics

Cooperativity, a phenomenon where concerted molecular displacements circumvent energetic and entropic barriers, is prevalent in biological systems but rare in molecular crystals. This study focuses on understanding the origins of cooperative transitions and their potential for applications in organic electronics. While cooperative transitions, characterized by ultrafast kinetics and low transition barriers, have been observed in some p-type organic semiconductors, their presence and underlying mechanisms in n-type counterparts remain largely unexplored. N-type semiconductors, crucial for complete logic circuit design, require relatively low-lying LUMO levels, which are challenging to stabilize. 2DQTT-o-B, a high-performing n-type organic semiconductor featuring a quinoidal structure with electron-withdrawing cyano groups and solution-processable alkyl chains, provides an ideal system to investigate these transitions. The quinoidal core stabilizes an exotic biradical ground state that exists in equilibrium with the quinoidal form, potentially influencing electronic properties and structural transitions. This research investigates the coexistence of cooperative and nucleation and growth mechanisms in 2DQTT-o-B single crystals, aiming to elucidate the molecular origins of these distinct transition behaviors and their impact on electronic properties.

The literature extensively documents cooperativity in various systems, including spin flipping in magnetic materials and protein folding. In solid-state phase transitions, cooperative mechanisms involve diffusionless molecular displacement, resulting in ultrafast kinetics and low energy barriers compared to nucleation and growth mechanisms. Cooperative behavior has been reported in some organic semiconductors, such as TIPS-P, where the rotation of bulky side chains plays a key role. However, studies on n-type semiconductors have lagged due to the challenges in stabilizing low-lying LUMO levels. The design of stable n-type molecules often involves forming quinoidal structures with electron-withdrawing groups and attaching long alkyl chains for solution processability and environmental stability. Biradical formation in these quinoidal cores has been shown to affect spin-spin interactions and self-doping effects, which can influence charge carrier densities and electronic properties. Despite the importance of both quinoidal cores and alkyl chains, their impact on structural transitions remains less understood.

Single crystals of 2DQTT-o-B were grown via dropcasting or slow evaporation. Their polymorph transitions were investigated using polarized optical microscopy (POM), grazing incidence X-ray diffraction (GIXD), Raman spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. POM, with high-speed imaging, revealed the kinetics of the two reversible phase transitions (I-II at 164 °C and II-III at 223 °C). Image analysis quantified the transition speeds and revealed avalanche behavior during cooling for the I-II transition. GIXD, performed in situ during heating, determined unit cell changes and provided insights into molecular orientations. Raman spectroscopy tracked changes in intramolecular and intermolecular vibrations, providing information on alkyl chain conformation and core structure changes. EPR spectroscopy directly measured the concentration of unpaired spins, confirming the formation of biradical species. Density functional theory (DFT) calculations were performed to simulate the electronic structure and vibrational spectra, aiding in peak assignments. Adiabatic relaxations via semi-empirical quantum chemistry simulations further investigated the structural changes during the transitions. Finally, two-point-probe conductivity measurements on single crystal devices assessed the electronic consequences of the phase transitions. Thermal actuation devices were fabricated to demonstrate the application of cooperative shape changes in organic electronics.

The study revealed two distinct polymorph transition mechanisms in 2DQTT-o-B single crystals. The I-II transition, occurring within milliseconds, exhibited cooperative behavior with a well-defined phase front propagating through the crystal, often initiated at crystal tips or defects. This transition was accompanied by a 3.6 ± 1.3% decrease in crystal length and thermosalient behavior in larger crystals. Analysis showed that this transition is driven by a change in alkyl chain conformation, resulting in a 12° decrease in conjugated core tilt and reduced alkyl chain interdigitation. The II-III transition, significantly slower (minutes), displayed nucleation and growth behavior with diffuse spreading of the new phase. This transition was characterized by a slight crystal expansion but no thermosalient effect. Raman spectroscopy revealed the appearance of new peaks associated with an aromatic form, indicating a quinoidal-to-aromatic structural change accompanied by increased alkyl chain disorder. EPR measurements confirmed a dramatic increase in spin concentration across the II-III transition, confirming the formation of biradical species. DFT calculations provided support for the proposed structural changes and biradical formation. The study showed that the I-II transition results in a 6-fold decrease in conductivity, while the II-III transition leads to a moderate increase, potentially due to self-doping effects from biradicals. Finally, a single-crystal actuator device was successfully demonstrated utilizing the shape change associated with the cooperative I-II transition, showcasing its potential for practical applications in dynamic electronics.

This study successfully unravels the distinct molecular mechanisms driving two polymorph transitions within a single n-type organic semiconductor crystal. The I-II transition highlights the crucial role of alkyl side chain flexibility in driving cooperative behavior, while the II-III transition underscores the surprising impact of biradical formation on inducing a nucleation and growth transition. The contrasting kinetics and driving forces of these transitions are clearly demonstrated. The successful demonstration of a single-crystal actuator device based on the cooperative shape change opens exciting new avenues for designing functional materials and devices. These findings offer invaluable insights into designing organic semiconductors with tunable properties through controlled polymorphic transitions and provide a pathway to creating new functionalities.

This research demonstrates the coexistence of cooperative and nucleation and growth mechanisms within a single n-type organic semiconductor crystal, 2DQTT-o-B. The study elucidates the molecular origins of these distinct transitions, attributing the cooperative I-II transition to alkyl side chain reorientation and the nucleation and growth II-III transition to biradical formation and alkyl chain disorder. The findings highlight the importance of alkyl chain engineering in controlling polymorphic behavior and the potential of cooperative transitions for creating novel organic electronic devices. Future research could explore the impact of other side chain modifications on cooperative transitions, investigate the full crystal structure of polymorph III, and expand the applications of this phenomenon in other functional materials.

The exact positions of the alkyl chains in the crystal structure were difficult to determine due to disorder, potentially limiting the precision of the proposed models. The lack of a full crystal structure for polymorph III restricts the detailed understanding of its packing motif. While the single-crystal actuator device demonstrates the potential of the cooperative shape change, the limited cyclability due to crystal bending warrants further investigation for improved device design and stability.