Structural, Magnetic, and Optical Studies of the Polymorphic 9′-Anthracenyl Dithiadiazolyl Radical

Abstract

The fluorescent 9′-anthracenyl-functionalized dithiadiazolyl radical (3) exhibits four structurally determined crystalline phases, all of which are monomeric in the solid-state. Polymorph 3α (monoclinic P21/c, Z' = 2) is isolated when the radical is condensed onto a cold substrate (enthalpically favored polymorph) whereas 3β (orthorhombic P212121, Z' = 3) is collected on a warm substrate (entropically favored polymorph). The α and β polymorphs exhibit chemically distinct structures with 3α exhibiting face-to-face π−π interactions between anthracenyl groups while 3β exhibits edge-to-face π−π interactions. 3α undergoes an irreversible conversion to 3β on warming to 120 o C (393 K). The β-phase undergoes a series of reversible solid-state transformations on cooling; below 300 K a phase transition occurs to form 3γ (monoclinic P21/c, Z' = 1) and on further cooling below 165 K a further transition is observed to 3δ (monoclinic P21/n, Z' = 2). Both 3β  3γ and 3γ  3δ transitions are reversible (single-crystal X-ray diffraction) and the 3γ  3δ process exhibits thermal hysteresis with a clear feature observed by heat capacity measurements. Heating 3β above 160 o C generates a fifth polymorph (3ε) which is distinct from 3α -3δ based on PXRD data. The magnetic behavior of both 3α and the 3β/3γ/3δ system reflect an S = ½ paramagnet with weak antiferromagnetic coupling. The reversible 3δ ↔ 3γ phase transition exhibits thermal hysteresis of 20 K. Below 50 K the value of χmT for 3δ approaches 0 emu•K•mol -1 consistent with formation of a gapped state with an S = 0 ground state configuration. In solution both paramagnetic 3 and diamagnetic [3][GaCl4] exhibit similar absorption and emission profiles reflecting similar absorption and emission mechanisms for paramagnetic and diamagnetic forms. Both emit in the deep-blue region of the visible spectrum (λem ~ 440 nm) upon excitation at 255 nm with quantum yields of 4% (3) and 30% ([3][GaCl4]) affording a switching ratio [ΦF(3 + )/ΦF(3)] of 7.5 in quantum efficiency with oxidation state. Solid-state films of both 3 and [3][GaCl4] exhibit emission bands at longer wavelength (490 nm) attributed to excimer emission.

A survey of thermal expansion coefficients for organic molecular crystals in the Cambridge Structural Database

“…{JOGVEJ03}, R 2 = 0.1881 (four structures): the family comes from a study of several polymorphs over a range of temperatures (Beldjoudi et al, 2019). The linear fit is disrupted by JOGVEJ, which is recorded in the CSD at 293 K. Tables in the supporting information of the publication attribute the unit cell of JOGVEJ to 173 K, suggesting that errors have arisen somewhere in the publication/archiving process.…”

Solid‐State Room‐Temperature Near‐Infrared Photoluminescence of a Stable Organic Radical

“…This feature has inspired intensive research into the unique properties of luminescent organic solids, including mechano-and vapor-responsive luminescence [6][7][8] and aggregation-induced emission. [9,10] Luminescent organic radicals have attracted considerable attention [11][12][13][14][15][16][17][18] because of their unusual photophysical properties such as long-wavelength emissions, absence of heavy-atom effects, [19,20] and high electroluminescence quantum efficiency. [21][22][23][24] These characteristics stem from the doublet spin state of isolated radical molecules with an unpaired electron.…”

“…[32,33] Luminescent radical solids thus have potential as a new class of luminescent materials with novel properties and functions. However, the photoluminescence of organic radicals in a pure solid state has rarely been studied [14,15,34] because in many cases the emission is significantly quenched, especially at room temperature. [20,25] Here we report the electronic structure and luminescence properties of a novel stable organic (2,4-dichloro-3-pyridyl)bis (2,4,6-trichlorophenyl)methyl radical (metaPyBTM; Figure 1), which is a structural isomer of the previously reported (3,5dichloro-4-pyridyl)bis (2,4,6-trichlorophenyl)methyl radical (PyBTM).…”

Two different mechanisms of stabilization of regular π-stacks of radicals in switchable dithiazolyl-based materials

“…55 Other organic radicals exhibiting magnetic bistability include additional 1,3,2-dithiazolyls 33,56 , bis--1,2,3--dithiazolyls 57,58 , nitroxides 59 , spirobiphenalenyls 60,61 , and dithiadiazolyls. 62,63 The hysteresis loop of 1 is centered around 323 K and spans 46 K (see Figure 1, right). The different magnetic response of its two polymorphs (LT features diamagnetism, while HT features a weak paramagnetism 45 ) results from changes in the antiferromagnetic interactions between adjacent radicals along the π--stacks.…”