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

The study addresses how and why distinct solid–solid phase transition mechanisms—cooperative (martensitic-like) versus nucleation-and-growth—arise in a single n-type organic molecular crystal and how these mechanisms impact electronic and mechanical functionality. Cooperative transitions, characterized by diffusionless, concerted molecular displacements, are prized for ultrafast kinetics, low barriers, and reversibility, but are rare and poorly understood in molecular crystals. Prior reports in p-type semiconductors (e.g., TIPS-pentacene) link cooperativity to side-group dynamics (rotations, tilting, order–disorder). In contrast, n-type analogs lag in understanding due to challenges stabilizing low-lying LUMO levels and the presence of quinoidal backbones that can adopt biradical ground states. The molecule 2DQTT-o-B combines a quinoidal core (often enabling temperature-tunable biradical character) with long alkyl chains (for processability and environmental stability). The central objective is to unravel the molecular origins of two thermally induced polymorphic transitions in 2DQTT-o-B single crystals—one cooperative and one nucleation-and-growth—and to determine the roles of alkyl side-chain conformations and biradical formation in driving these mechanisms, with an eye toward leveraging them for dynamic electronic devices (actuators, switches).

Cooperative transitions in molecular systems are well documented in inorganic martensites but are seldom observed in organic crystals. In organic semiconductors, cooperative behavior has been reported in p-type systems such as TIPS-pentacene, where bulky side-group rotation and molecular tilting/order–disorder underpin ultrafast transitions, reversible shape change, and super-/ferroelasticity. These phenomena enable rapid switching of electronic properties and thermally activated actuation. For n-type systems, development lags because stabilizing deep LUMOs is difficult. Quinoidal oligothiophenes, stabilized by electron-withdrawing groups (e.g., cyano), yield planar backbones and can exhibit temperature-sensitive singlet biradical character. This biradical form impacts intermolecular interactions (including spin–spin interactions and potential pancake bonding), self-doping effects, and electronic transport. Despite the established importance of quinoidal cores and alkyl chains in designing n-type semiconductors, their influence on structural transition mechanisms in single crystals has been largely unexplored. This work fills that gap by demonstrating, within one n-type crystal (2DQTT-o-B), both a cooperative I–II transition linked to alkyl side-chain reorientation and a nucleation-and-growth II–III transition driven by biradical formation and side-chain disorder.

Materials and crystal growth: 2DQTT-o-B synthesized following prior methods. Single crystals grown via (i) dropcasting 10–15 mg mL−1 solutions in 1:1 para-dichlorobenzene:decane at 100 °C onto PTS-treated SiO2/Si (small crystals, 50–300 µm), and (ii) slow evaporation of 1 mg mL−1 solutions in 1:1 dichloromethane:ethyl acetate under N2 over weeks (large crystals, 1–3 mm). Polymorph I verified by SCXRD (C2/c, 1-D π-stacks along crystal long axis).

In situ polarized optical microscopy (POM): Crystals heated/cooled at 5 °C min−1 on a Linkam stage (Nikon H550S, Infinity 1 camera). Temperature calibrated using a thermocouple to correct stage readings. Image analysis via Python: crystal masked and average RGB intensity extracted per frame to track transition kinetics and avalanche behavior upon cooling.

Thermal shape-change assessment: Length change quantified from POM before/after transitions; hysteresis assessed via heating/cooling cycles. Observation of thermosalient motion for unpinned large crystals.

Grazing-incidence X-ray diffraction (GIXD): Performed at APS 8-ID-E (10.91 keV) using a Pilatus 1M detector. Films prepared by solution coating and annealed to polymorph I. In situ thermal annealing in He with controlled ramps (10 °C min−1) and continuous exposures (5 °C min−1) for videos. Data processed with GIXSGUI; incident angle 0.14°. Unit cells and peak evolution analyzed; structural interpolation used to simulate I→II.

Quantum-chemical simulations: Periodic semi-empirical GFN2-xTB (DFTB+ v21.2) used for adiabatic relaxations along the I→II path by linear lattice interpolation with fractional coordinates relaxed at each step; predicted structures validated against experimental powder patterns and π-stacking directions. DFT/TD-DFT (Gaussian16): ωB97X-D/def2-TZVPP for ground states; TD-DFT for UV-Vis (10 lowest singlet/triplet excitations). Broken-symmetry B3LYP/6-31G(d,p) used to assess biradical character, S–T gap, exchange interactions, and spin density for dimers approximating condensed phase. For aromatic-form modeling, cyano groups were substituted or BLA constrained to emulate quinoidal/aromatic states.

Raman spectroscopy: HORIBA LabRAM HR 3D, 532 nm laser (max 50 mW) with OD=0.2, 50× LWD objective; 300 g mm−1 grating; exposure 20–60 s. Variable-temperature with Linkam THMS600 (10 °C min−1), equilibrated and refocused (≈5 min) at each temperature. Spectra collected 900–1800 cm−1 (intramolecular) and <500 cm−1 (phonons). Peak deconvolution (OriginPro) with Gaussian/Lorentzian line-shapes depending on core vs alkyl modes; assignments assisted by DFT and literature.

EPR spectroscopy: Large crystals (polymorph I) measured in an EMXplus EPR with variable temperature. Temperature stepped by 25 °C through II–III; doubly integrated intensity plotted vs temperature to quantify spin concentration.

Electronic/device tests: Two-probe conductivity measured on pre-bent single-crystal devices attached with PEDOT:PSS electrodes (Keysight B1500A). Actuator devices fabricated by attaching crystals without pre-bending; thermal cycling across I–II used to reversibly detach/attach one contact for on/off switching.

• Two reversible thermal phase transitions in single crystals of 2DQTT-o-B: I→II at ≈164 °C and II→III at ≈223 °C (calibrated), each with distinct mechanisms.
• I→II transition is cooperative (martensitic-like): ultrafast propagation often within one video frame (<0.1 s), implying ≥2000 µm s−1 front speed; sharp phase front; avalanche behavior upon cooling (intensity plateaus) due to pinning at defects; phase fronts oriented along (010) and (025) planes linked to π-stacking, indicating domino-like transmission along 1-D π-stacks.
• Macroscopic shape change at I→II: crystal length decreases by 3.6 ± 1.3% (mean), consistent with unit-cell b-axis contraction by 2.9% from GIXD fitting. Large crystals exhibit thermosalient motion; pinned/small crystals can crack due to strain at the phase front.
• II→III transition is nucleation-and-growth: multiple nucleation sites (often at edges/defects); diffuse front; minutes-long kinetics; large hysteresis (~100 °C in single crystals; DSC on powders shows 75 °C); frequent kinetic trapping of III at RT; reverse III→I sometimes requires extended cooling. Crystal shows slight expansion without thermosalient effect; single crystallinity breaks into multi-domain microstructure.
• Structural signatures: I→II retains monoclinic symmetry with pretransition q2 shifts and a significant β-angle increase; II→III shows reconstructive change with disappearance of polymorph II peaks, emergence of polymorph III peaks without pre-shift, and likely higher symmetry (hexagonal-like) lattice while retaining strong π-stacking features.
• Molecular drivers of I→II: In situ Raman shows δ(CH2) at 1409 cm−1 undergoes a sudden intensity drop and redshift by ≈7 cm−1 at transition, indicating substantial alkyl chain reorientation and reduced interdigitation; phonon intensity modulation matches cooperative transitions. GIXD/xTB indicate increased tilt of the conjugated core (≈−12° relative to (100): from ≈66° in I to ≈54° in II), opening space for side chains and enabling cooperative tilt propagation.
• Molecular drivers of II→III: Raman reveals weakening of quinoidal ν(C=C)ECC,Q at 1387 cm−1 and appearance of new peaks at 1442 and 1497 cm−1 assigned to aromatic ν(C=C)ECCA and ν(C=C)asym, evidencing a quinoidal-to-aromatic (biradical) shift. The aromatic/quinoidal intensity ratio rises steeply at II→III; concurrently, the alkyl trans/gauche intensity ratio drops from ≈8 to ≈1, indicating pronounced side-chain disorder and mobility.
• EPR directly confirms biradical formation: ≈40× increase in spin concentration upon reaching II→III. Broken-symmetry DFT indicates notable biradical character (y≈0.35), singlet–triplet energy gap ≈−8.5 kcal mol−1, exchange interaction ≈−4.2 kcal mol−1, and closed-shell singlet favored over open-shell singlet by ≈2.7 kcal mol−1 at 0 K—consistent with biradicals emerging at elevated temperatures.
• Proposed mechanisms: I→II—cooperative, domino-like tilting of cores along π-stacks driven by alkyl side-chain reorientation/interdigitation changes; II→III—nucleation-and-growth driven by abrupt biradical increase that reconfigures core–core interactions (e.g., dimer-like motifs), facilitated by side-chain disorder.
• Device implications: Conductivity decreases ≈6× at I→II (recoverable with hysteresis); modest conductivity increase at II→III, likely from self-doping by biradicals. A thermally actuated single-crystal switch exploiting the I→II shape change achieves on/off ≈500; conductivity in polymorph I recovers ≥94% over cycles.

The coexistence of cooperative and nucleation–growth transitions in a single n-type molecular crystal enables a direct mechanistic comparison and reveals distinct molecular origins. The cooperative I→II transition is initiated and propagated by alkyl side-chain reorientation that modifies interdigitation and relieves steric constraints, thereby tilting conjugated cores in a concerted, diffusionless manner along π-stacks. This martensitic-like process preserves single crystallinity, proceeds ultrafast, and couples to a macroscopic shape change that can be harnessed for thermosalient actuation. In contrast, the II→III transition is reconstructive and proceeds via nucleation and growth. It is driven by a temperature-induced increase in biradical population that changes core–core interactions (spin–spin coupling, potential dimerization), while side-chain disorder provides the mobility to accommodate large packing rearrangements; the result is slow kinetics, large hysteresis, loss of single crystallinity, and domain formation that mitigates strain. These findings clarify that alkyl chains are not merely solubilizing groups but active components that can trigger cooperative transitions, while quinoidal-biradical equilibria in the core can drive reconstructive transitions. The delineation of these mechanisms suggests design rules: engineering side-chain flexibility and interdigitation to access cooperative behavior; tuning core electronic structure and biradical propensity to control reconstructive transitions. Functionally, the work demonstrates temperature-programmable modulation of conductivity and reliable mechanical switching in single-crystal devices, pointing to dynamic electronics that integrate fast mechanical actuation with electronic control.

This study uncovers two distinct, thermally activated polymorphic transition mechanisms in a single high-performance n-type semiconductor crystal (2DQTT-o-B). The I→II transition is a cooperative, ultrafast, shape-changing process driven by alkyl side-chain reorientation that tilts conjugated cores; the II→III transition is a nucleation-and-growth, reconstructive transformation driven by biradical formation in the core and facilitated by side-chain disorder. Direct structural, spectroscopic, and computational evidence (in situ POM, GIXD, Raman, EPR, DFT/xTB) support these assignments. By leveraging the cooperative shape change, a thermally actuated single-crystal switch with on/off ≈500 is demonstrated, along with reversible conductivity modulation across polymorphs. Future directions include resolving the full crystal structure and packing motif of polymorph III, quantitatively controlling biradical populations and their lifetimes, engineering side-chain architectures to tune cooperative thresholds and hysteresis, and optimizing device geometries to improve cycling stability and harness out-of-plane mechanics for multifunctional dynamic electronics.

• The full crystal structure of polymorph III could not be determined/simulated due to significant thermal disorder of alkyl chains and possible core structural changes accompanying biradical formation, limiting definitive assignment of packing motifs.
• Large hysteresis and kinetic trapping of polymorph III introduce variability in reverse transitions and complicate precise thermodynamic characterization.
• Raman-derived aromatic/quinoidal ratios carry substantial fitting uncertainties at high temperature due to spectral broadening and disorder.
• Exact alkyl chain positions and interdigitation changes during I→II are inferred from spectroscopy and modeling; direct crystallographic localization is limited by disorder.
• Device cycling stability is constrained by out-of-plane bending and contact reliability; further engineering is needed for long-term endurance.