Indigo and isoindigo-based small molecules have emerged as promising electron-deficient cores for ambipolar organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). Their strong electron-accepting character, structural versatility, and relatively good air stability make them competitive alternatives to other high-performance n-type and ambipolar semiconductors. This article examines their molecular design, synthetic routes, electronic properties, and performance in OFETs and OPVs, while comparing them to other electron-deficient cores.
**Molecular Structure and Electronic Properties**
Indigo and isoindigo are structurally similar yet exhibit distinct electronic characteristics. Indigo consists of two fused indole units connected by a central double bond, while isoindigo features two oxindole units linked via a central double bond. The lactam rings in both systems contribute to their electron-deficient nature, with isoindigo typically displaying stronger electron affinity due to its more rigid and planar structure.
The lowest unoccupied molecular orbital (LUMO) levels of indigo and isoindigo small molecules typically range between -3.5 eV to -4.2 eV, facilitating electron transport. Their highest occupied molecular orbital (HOMO) levels lie around -5.5 eV to -6.0 eV, which contributes to reasonable air stability by minimizing oxidative degradation. The extended π-conjugation and intramolecular hydrogen bonding in these cores enhance molecular packing, leading to improved charge carrier mobility.
**Synthesis and Functionalization**
The synthesis of indigo and isoindigo small molecules often begins with commercially available precursors. Indigo derivatives are typically synthesized via condensation reactions of isatin or its substituted analogs, while isoindigo derivatives are prepared through the condensation of brominated oxindole intermediates. Functionalization at the nitrogen atoms or the aromatic rings allows tuning of solubility, energy levels, and solid-state packing.
Common modifications include alkylation of the lactam nitrogens to improve solubility and the introduction of electron-withdrawing groups (e.g., fluorine, cyano) to further lower LUMO levels. For example, N-alkylated isoindigo derivatives exhibit enhanced solution processability without significantly compromising electron affinity. Compared to other electron-deficient cores like diketopyrrolopyrrole (DPP) or naphthalene diimide (NDI), indigo and isoindigo offer simpler synthetic routes and lower material costs.
**Ambipolar OFET Performance**
Indigo and isoindigo small molecules demonstrate balanced ambipolar transport in OFETs, with electron and hole mobilities often exceeding 0.1 cm²/Vs. Their performance stems from efficient intermolecular π-π stacking and low-lying LUMO levels, which facilitate electron injection from common electrodes like gold or silver.
Isoindigo-based small molecules generally outperform indigo derivatives due to their higher planarity and stronger electron-withdrawing ability. For instance, alkylated isoindigo-thiophene hybrids have achieved electron mobilities up to 1.0 cm²/Vs and hole mobilities around 0.5 cm²/Vs in optimized devices. In contrast, indigo derivatives typically exhibit slightly lower mobilities but better air stability due to their deeper HOMO levels.
Compared to other electron-deficient cores:
- DPP-based small molecules often show higher mobilities but poorer air stability.
- NDI derivatives exhibit superior electron transport but limited hole transport.
- Benzothiadiazole (BT) cores require additional structural modifications to achieve ambipolarity.
**OPV Applications**
In OPVs, indigo and isoindigo small molecules serve as non-fullerene acceptors or as donors in bulk heterojunction devices. Their narrow bandgaps (1.4 eV to 1.8 eV) enable broad light absorption, while their energy level alignment with common donor materials facilitates efficient charge separation.
Isoindigo-based acceptors paired with polymer donors like PBDB-T have achieved power conversion efficiencies (PCEs) exceeding 8%, comparable to some DPP and NDI-based systems. Their fill factors often surpass 70%, indicating effective charge extraction. Indigo derivatives, while less efficient in OPVs, offer better stability under prolonged illumination and ambient conditions.
**Air Stability Comparison**
Air stability is a critical factor for practical applications. Indigo and isoindigo small molecules exhibit moderate to good stability due to their deep HOMO levels, which resist oxidation. Unencapsulated OFETs based on these materials often retain over 80% of their initial performance after two weeks in air, outperforming many DPP and BT-based systems.
In contrast:
- DPP derivatives degrade rapidly without encapsulation due to their higher-lying HOMO levels.
- NDI systems show better electron transport stability but are prone to morphological degradation.
- Rylene diimides (e.g., PDI) exhibit excellent electron affinity but poor solubility and film-forming properties.
**Conclusion**
Indigo and isoindigo small molecules offer a compelling balance of performance, stability, and synthetic accessibility for ambipolar OFETs and OPVs. While isoindigo derivatives excel in charge transport and OPV efficiency, indigo-based systems provide superior air stability. Their competitive performance relative to more complex electron-deficient cores positions them as viable candidates for next-generation organic electronics. Future research may focus on further optimizing side-chain engineering and solid-state packing to enhance device performance without compromising stability.