The Design of Radical Stacks: Nitronyl‐Nitroxide‐Substituted Heteropentacenes

The first alkyl chain‐anchored heteropentacene, dithieno[2,3‐d;2′,3′‐d′]benzo‐[1,2‐b;3,4‐b′]dithiophene (DTmBDT), mono‐ or disubstituted with a nitronyl nitroxide group has been prepared through a cross‐coupling synthetic procedure of the corresponding dibromo‐derivative (Br(2)‐DTmBDT) with a nitronyl nitroxide‐2‐ide gold(I) complex. The synthesized nitroxides possess high kinetic stability, which allowed us to investigate their structure and thermal, optical, electrochemical, and magnetic properties. Single‐crystal X‐ray diffraction of both mono‐ and diradicals revealed that the nitronyl nitroxide group lies almost in the same plane as the nearest side thiophene ring. Such arrangement favors formation of edge‐to‐edge dimers, which then form close π‐stacks surrounded by interdigitating alkyl chains. Before melting, these nitronyl nitroxide radical substituted molecules undergo at least two different phase transitions (PTs): for the monoradical, PTs are reversible, accompanied by hysteresis, and occur near 13 and 83 °C; the diradical upon heating shows a reversible PT with hysteresis in the temperature range 2–11 °C and an irreversible PT near 135 °C. PTs of this type are absent in Br(2)‐DTmBDT. Therefore, the step‐by‐step substitution of bromine atoms by nitronyl nitroxide groups changes the structural organization of DTmBDT and induces the emergence of PTs. This knowledge may facilitate crystal engineering of π‐stacked paramagnets and related molecular spin devices.