Scheme 1 The electron exchange integral, electronic configurations and singlet–triplet energy gap of a diradical. However, such phenomena have not been realized experimentally. Furthermore there is the possibility of reversibility by external stimuli. However, control of Δ E ST of a crystalline diradical by external stimuli (temperature, pressure, light, etc.) is highly desirable especially in the design of functional materials because no geometric modifications would be needed. 1–10 The use of different substituents and incorporation of heteroatoms in the structures have proven effective ways to alter the energy gap. Estimation, determination and the control of the singlet–triplet energy gap have thus become the central themes in the study of diradicals. The singlet–triplet energy gap (Δ E ST = E S − E T = 2 J) provides direct information on the extent of the electron exchange interaction and is essential for understanding the diradical character. A diradical either has a singlet ground state (S, S = 0, spin multiplicity = 1) or has a triplet ground state (T, S = 1, spin multiplicity = 3), which is indicated by the negative or positive electron exchange interaction ( J) between two unpaired electrons ( Scheme 1). Because of their interesting optical, electronic and magnetic properties, diradicals also play important roles in materials chemistry expected to have promising applications as functional materials in quantum information processing systems, electronic devices, lithium ion batteries and organic spintronics. They are important in the understanding the nature of chemical bonds in theoretical chemistry, and have fundamental significance in the fields in mechanistic chemistry, synthetic chemistry and biological chemistry. Introduction Diradicals, 1–15 molecules with two unpaired electrons, are important reactive intermediates. The work presents an unprecedented instance of a thermally controllable singlet–triplet gap for a crystalline diradical and provides a novel diradical material relevant to the design of functional materials. The behavior is induced by the conformation change of the terphenyl bridge, the key structural unit of the species. The dication diradical of 4,4′′-di(bisphenylamino)- p-terphenyl exhibits two singlet states with different exchange coupling constants at different temperatures as determined by SQUID and EPR measurements. We now report that the energy gap of a diradical in the solid state can also be controlled by an external stimulus. Such knowledge is essential for understanding their diradical character, which is controllable internally by modification of the electronic and steric properties of the substituents. Their defining feature is the energy difference between their singlet and triplet states, which provides direct information on the extent of their electron exchange interactions. Diradicals, molecules with two unpaired electrons, are reactive intermediates that play an important role in many fields.

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