Iratxe Arrechea-Marcos

Diindeno-fusion of an anthracene as a design strategy for stable organic biradicals

The consequence of unpaired electrons in organic molecules has fascinated and confounded chemists for over a century. The study of open-shell molecules has been rekindled in recent years as new synthetic methods, improved spectroscopic techniques and powerful computational tools have been brought to bear on this field. Nonetheless, it is the intrinsic instability of the biradical species that limits the practicality of this research. Here we report the synthesis and characterization of a molecule based on the diindeno[b,i]anthracene framework that exhibits pronounced open-shell character yet possesses remarkable stability. The synthetic route is rapid, efficient and possible on the gram scale. The molecular structure was confirmed through single-crystal X-ray diffraction. From variable-temperature Raman spectroscopy and magnetic susceptibility measurements a thermally accessible triplet excited state was found. Organic field-effect transistor device data show an ambipolar performance with balanced electron and hole mobilities. Our results demonstrate the rational design and synthesis of an air- and temperature-stable biradical compound.

Ladder-type Heteroarenes: Up to 15 Rings with Five Imide Groups

A series of novel imide-functionalized ladder-type heteroarenes with well-defined structure and controllable conjugation lengths were synthesized and characterized. The synthetic route shows remarkable efficacy for constructing the electron-deficient ladder backbones. π-Conjugation extension leads to narrowed band gaps with enhanced electron affinities. The ladder arenes are incorporated into organic thin-film transistors, and show encouraging electron mobilities of 0.013-0.045 cm2  V-1  s-1 . The heteroarenes reported here provide a remarkable platform for fundamental physicochemical studies and materials innovation in organic electronics.

(Semi)ladder-Type Bithiophene Imide-Based All-Acceptor Semiconductors: Synthesis, Structure–Property Correlations, and Unipolar n-Type Transistor Performance

Development of high-performance unipolar n-type organic semiconductors still remains as a great challenge. In this work, all-acceptor bithiophene imide-based ladder-type small molecules BTI n and semiladder-type homopolymers PBTI n ( n = 1-5) were synthesized, and their structure-property correlations were studied in depth. It was found that Pd-catalyzed Stille coupling is superior to Ni-mediated Yamamoto coupling to produce polymers with higher molecular weight and improved polymer quality, thus leading to greatly increased electron mobility (μe). Due to their all-acceptor backbone, these polymers all exhibit unipolar n-type transport in organic thin-film transistors, accompanied by low off-currents (10-10-10-9 A), large on/off current ratios (106), and small threshold voltages (∼15-25 V). The highest μe, up to 3.71 cm2 V-1 s-1, is attained from PBTI1 with the shortest monomer unit. As the monomer size is extended, the μe drops by 2 orders to 0.014 cm2 V-1 s-1 for PBTI5. This monotonic decrease of μe was also observed in their homologous BTI n small molecules. This trend of mobility decrease is in good agreement with the evolvement of disordered phases within the film, as revealed by Raman spectroscopy and X-ray diffraction measurements. The extension of the ladder-type building blocks appears to have a large impact on the motion freedom of the building blocks and the polymer chains during film formation, thus negatively affecting film morphology and charge carrier mobility. The result indicates that synthesizing building blocks with more extended ladder-type backbone does not necessarily lead to improved mobilities. This study marks a significant advance in the performance of all-acceptor-type polymers as unipolar electron transporting materials and provides useful guidelines for further development of (semi)ladder-type molecular and polymeric semiconductors for applications in organic electronics.

Benzotrithiophene versus Benzo/Naphthodithiophene Building Blocks: The Effect of Star‐Shaped versus Linear Conjugation on Their Electronic Structures

The synthesis, characterization, and optical properties of a novel star-shaped oligothiophene with a central rigid trithienobenzene (BTT) core and diketopyrrolopyrrole (DPP) units are reported and compared with homologous linear systems based on the benzodithiophene (BDT) and the naphthodithiophene (NDT) units end capped with DPPs. This comparison is aimed at elucidating the effect of the star-shaped configuration versus linear conformation on the optical and electrical properties. Electronic and vibrational spectroscopies, together with transient absorption spectroscopy, scanning electronic microscopy, and DFT calculations are used to understand not only the molecular properties of these semiconductors, but also to analyze the supramolecular aggregation in these derivatives. We conclude that although the subject star-shaped derivative is not optimal in terms of π-conjugation, its extended BTT unit significantly favors intermolecular π-stacking interactions, which is interesting for their applications in devices. Field-effect transistors and solar cells were fabricated with these new molecular semiconductors and the performance difference discussed.

Solution-processed N-trialkylated triindoles for organic field effect transistors

Three crystalline N-trialkyltriindoles in which the length of the alkyl chains attached to the nitrogen has been enlarged from a methyl to a butyl and to a hexyl group have been investigated in the search for semiconducting triindole easy to process from solution. We have found that the number of carbon atoms of the N-alkyl chains has a significant impact on how these molecules interact with each other in the bulk materials and it strongly influences the final morphology of the crystals, which grow as long crystalline wires, cubes or microbelts. Single crystal analysis allows us to recognize the contribution of several cooperative CH–π interactions to guide the self-assembly of these types of molecules. In addition, alkyl chain engineering allows triindole derivatives processable for solution, which render OFETs showing field effect mobilities of 0.03 cm2 V−1 s−1 and 6 × 10−3 cm2 V−1 s−1 when vapor deposited and drop casted, respectively. The results of this study represent a step forward towards the rational control of the supramolecular arrangement of this high performance semiconducting platform, this being a key fact for designing efficient solution-processed organic semiconductors.

Tuning of the Electronic Levels of Oligothiophene-Naphthalimide Assemblies by Chemical Modification.

Inversion of the connections of amidine linkers combined with controlled oligothiophene chain catenation in oligothiophene-naphthalimide assemblies provides an efficient method to tune the HOMO and LUMO values in this type of assemblies. This modification also suppresses the intramolecular charge transfer (ICT) band normally found in this type of derivatives, also delocalizing the frontier molecular orbitals over the whole conjugated skeleton. The resultant assemblies were used in the fabrication of field-effect transistors, which showed well-balanced ambipolar transport.

Stereoisomers of an azine-linked donor–acceptor conjugated polymer: the impact of molecular conformation on electrical performance

We herein report the synthesis of a pair of azine linked donor–acceptor type conjugated polymers by the use of palladium(II)-based direct arylation. The two stereoisomers of the azine molecule were synthesized, separated, characterized, and further incorporated with a thiophene–phenylene–thiophene-based fused lactam (TPTL) acceptor molecule to form the donor–acceptor skeletons. The effects of the azine linkage isomerism on the polymer skeleton were studied both theoretically and experimentally. Semiconducting properties of these polymers were also evaluated in thin film transistors.

D–A–D 2H-benzo[d][1,2,3]triazole derivatives as p-type semiconductors in organic field-effect transistors

A series of Donor–π–Acceptor–π–Donor compounds based on a 2H-benzo[d][1,2,3]triazole core branched with different alkynyl donor groups has been characterized and tested in organic field-effect transistors (OFETs). The electronic and molecular structures were elucidated through optical and vibrational spectroscopy in conjunction with DFT calculations. The results indicate that the planarity of the structure and the good intramolecular charge transfer from the electron-donating to the electron-withdrawing fragments play a critical role in the application of the compounds as semiconductors in OFET devices. The compounds were tested in a top-contact/bottom-gate thin film transistor architecture, and they behave as p-type semiconductors.