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

Abstract

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.