Pyrazinoquinoxaline derivatives for flexible electronic devices: effect of the mechanical properties of the crystals on device durability

Understanding the interplay between the molecular structure and material properties of emerging p-type organic semiconductors marks a significant stride in the advancement of molecular electronics. Among the array of promising materials, mechanically flexible single crystals of π-conjugated molecules stand out due to their potential for cutting-edge applications in organic electronics. Notably, derivatives of pyrazinoquinoxaline (PQ) are recognized as versatile building blocks for constructing π-conjugated systems, showcasing good semiconductor performance in organic field-effect transistors (OFETs). In this study, we present an exploration into the p-type charge transport and mechanical characteristics of two newly synthesized PQ derivatives: 5,10-diphenyl-2,3,7,8-tetra(thiophen-2-yl)pyrazino[2,3-g]quinoxaline (DPTTQ) and 2,3,5,7,8,10-hexa(thiophen-2-yl)pyrazino[2,3-g]quinoxaline (HTPQ). HTPQ crystals exhibit flexural behaviour under applied stress, effortlessly returning to their initial configuration upon relaxation. Conversely, two polymorphic forms of DPTTQ crystals display brittle fracture when subjected to a similar stress. Specifically, DPTTQ molecules adopt a β-sheet packing, while HTPQ presents a γ-packing with a corrugated arrangement. Field-effect charge transport measurements reveal p-type charge transport in both DPTTQ and HTPQ, with HTPQ showcasing hole mobility up to 0.01 cm2 V-1 s-1, while DPTTQ exhibits mobility that is at least one order of magnitude lower. This variance in the field effect mobility can be directly correlated to the difference in crystal packing, highlighting a clear structure-property correlation. Moreover, taking advantage of the flexural nature of the HTPQ crystals, we fabricated durable electronic devices, which retain their conductivity for over 60 cycles of strain, indicating the efficacy of our chemical design in demonstrating high-performance flexible devices. These findings underscore the promise of semiconducting organics with γ-packing for achieving both better mobility and elasticity for integration into organic electronic devices.

Polaron Delocalization Dependence of the Conductivity and the Seebeck Coefficient in Doped Conjugated Polymers

Conjugated polymers are promising materials for thermoelectrics as they offer good performances at near ambient temperatures. The current focus on polymer thermoelectric research mainly targets a higher power factor (PF; a product of the conductivity and square of the Seebeck coefficient) through improving the charge mobility. This is usually accomplished via structural modification in conjugated polymers using different processing techniques and doping. As a result, the structure-charge transport relationship in conjugated polymers is generally well-established. In contrast, the relationship between the structure and the Seebeck coefficient is poorly understood due to its complex nature. A theoretical framework by David Emin (Phys. Rev. B, 1999, 59, 6205-6210) suggests that the Seebeck coefficient can be enhanced via carrier-induced vibrational softening, whose magnitude is governed by the size of the polaron. In this work, we seek to unravel this relationship in conjugated polymers using a series of highly identical pro-quinoid polymers. These polymers are ideal to test Emin's framework experimentally as the quinoid character and polaron delocalization in these polymers can be well controlled even by small atomic differences (<10 at. % per repeating unit). By increasing the polaron delocalization, that is, the polaron size, we demonstrate that both the conductivity and the Seebeck coefficient (and hence PF) can be increased simultaneously, and the latter is due to the increase in the polaron's vibrational entropy. By using literature data, we also show that this phenomenon can be observed in two closely related diketopyrrolopyrrole-conjugated polymers as well as in p-doped P3HT and PANI systems with an increasing molecular order.

Narrow-energy gap conjugated polymers based on benzobisthiadiazole and thiadiazoloquinoxaline: DFT and TDDFT study

Herein, the HOMO-LUMO energy gaps (Eg) and UV spectra of benzo[1,2-c:4,5-c']bis([1,2,5]thiadiazole (BBT) and [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ)-based donor-acceptor-donor type-conjugated polymers were computed by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) at B3LYP/6-31G(d,p) level. The donor groups consist of thiophene (TH), 3,4-ethylenedioxythiophene (EDOT), and 3,4-propylene dioxythiophene (ProDOT) units and the bisthiadiazole and thiadiazoloquinoxaline were chosen as electron acceptor groups. To examine the effects of the alkyl side chain on the molecular structure and Eg of the polymer, methyl groups were added at the 3,4-, 2,3-, and 3,3-positions of TH, EDOT, and ProDOT donor groups, respectively. Our calculated HOMO-LUMO energy gaps are in the range of 0.05 to 1.37 eV. The calculation results show that the energy gaps are in line with the available experimental values. The novel BBT and TQ derivatives with improved optical and electronic properties may find use in electronic applications. Graphical abstract.

Thiadizoloquinoxaline-Based N-Heteroacenes as Active Elements for High-Density Data-Storage Device

A novel thiadiazoloquinoxaline (TQ)-based donor-acceptor (D-A)-type N-heteroacene (Py-1-TQ) has been demonstrated for promising applications in organic multilevel resistive memory devices. Compared with its counterparts (Py-0-TQ and Py-2-TQ), which show flash-type binary memory behaviors, Py-1-TQ exhibits excellent nonvolatile write-once-read-many-times-type ternary memory effects with high ON2/ON1/OFF current ratios (10^5.8:10^3.4:1), which can be attributed to the different electron-withdrawing abilities between the pyrazine unit and TQ species that can induce stepwise D-A charge-transfer processes. These results suggest that TQ-based N-heteroacenes can be potentially useful in ultrahigh-density data-storage devices through the rational D-A tuning.

Highly Efficient Solid-State Near-infrared Organic Light-Emitting Diodes incorporating A-D-A Dyes based on α,β-unsubstituted "BODIPY" Moieties

We take advantage of a recent breakthrough in the synthesis of α,β-unfunctionalised 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) moieties, which we symmetrically conjugate with oligothienyls in an unexpectedly stable form, and produce a “metal-free” A-D-A (acceptor-donor-acceptor) oligomer emitting in the near-infrared (NIR) thanks to delocalisation of the BODIPY low-lying lowest unoccupied molecular orbital (LUMO) over the oligothienyl moieties, as confirmed by density functional theory (DFT). We are able to retain a PL efficiency of 20% in the solid state (vs. 30% in dilute solutions) by incorporating such a dye in a wider gap polyfluorene matrix and demonstrate organic light-emitting diodes (OLEDs) emitting at 720 nm. We achieve external quantum efficiencies (EQEs) up to 1.1%, the highest value achieved so far by a “metal-free” NIR-OLED not intentionally benefitting from triplet-triplet annihilation. Our work demonstrates for the first time the promise of A-D-A type dyes for NIR OLEDs applications thereby paving the way for further optimisation.

Tetraalkoxyphenanthrene-Fused Thiadiazoloquinoxalines: Synthesis, Electronic, Optical, and Electrochemical Properties, and Self-Assembly

π-Extended thiadiazoloquinoxaline (TQ) derivatives 1a,b-3a,b, in which a tetraalkoxyphenanthrene moiety is annulated with the TQ core and benzene rings are incorporated via the ethynylene spacer, were synthesized. They display absorption bands reaching into 750 nm and possess the electron-affinity comparable to [60]fullerene. The CF₃- and OMe-substituents on the benzene rings have moderate effects on modulation of the HOMO and LUMO levels. Tetraalkoxyphenanthrene-fused TQs 1a,b-3a,b aggregate in the solid state and assemble in solution through π-π stacking interactions. The self-assembly of 1a,b-3a,b into 1D superstructures was confirmed, and the difference in the alkoxy groups and the solvents for self-assembly proved to change their morphology. Comparison of the properties of 1a and those of reference compounds 4 and 5 clarified the effects of both the fusion of the phenanthrene moiety and the introduction of ethynylene spacers on the properties.

Tuning the optoelectronic properties of dual-acceptor based low-bandgap ambipolar polymers by changing the thiophene-bridge length

Dual-acceptor polymers based on DPP and TQ are prepared with very low bandgaps (0.6–0.9 eV) and ambipolar charge carrier transports.

Rational design of D–A1–D–A2 conjugated polymers with superior spectral coverage

Calculations and experiments elucidate factors governing how D–A₁–D–A₂ polymers offer fundamentally improved spectral coverage via allowed transitions to both acceptor LUMOs.