Cytosine–guanine base pairing in a hydrogen-bonded complex of stable open-shell molecules with S = 1 spins

Abstract

A bio-inspired motif of hydrogen-bonded nucleobases is introduced to stable organic biradical crystals. We have synthesized a hydrogen-bonded complex based on both a cytosine-substituted nitronyl nitroxide biradical (1) and a closed-shell alkyl-substituted guanine (2). From single-crystal X-ray structure analyses, the cytosine and guanine moieties are found to form a Watson–Crick pair with threefold hydrogen bonds. Magnetic susceptibility measurements reveal that the cytosine-substituted biradical 1 has a triplet (S = 1) ground state with a singlet–triplet energy gap of 2J/k_B = 27.0 K. This complex is the first example of Watson–Crick type base pair possessing stable high-spin organic radical moieties, which is fully characterized by single-crystal X-ray structure analysis. A face-to-face overlapping is found between the planar nucleobases, while side-by-side hydrogen bonds are found between the Watson–Crick pairs. Thus, the introduction of the radical substituent into the nucleobase results in little disturbance to the molecular arrangement usually found in nucleobase molecules: the ground-state spin multiplicity of biradical 1 is maintained.