Stable luminescent diphenylamine biphenylmethyl radicals with α-type D0 → D1 transition and antiferromagnetic properties

Abstract

Organic luminescent radicals, with their open-shell electronic structure, exhibit appealing optoelectronic properties and hold promise for a wide range of potential applications. Although a few series of luminescent radicals have been reported, most feature D₀ → D₁ transitions dominated by HOMOβ-SOMOβ (β-type transitions). In contrast, luminescent radicals that exhibit SOMOα-LUMOα properties (α-type transitions) are much rarer. Here, we designed and synthesized two stable novel diphenylamine biphenylmethyl (BTM) luminescent radicals, both characterized by α-type D₀ → D₁ transitions, and simultaneously maintained the D-A˙ structure for the first time. Density functional theory (DFT) calculations confirmed that fine-tuning the energy levels of frontier molecular orbitals can facilitate β-type to α-type D₀ → D₁ transition. Besides, in a 2(2Cl(m)PhN)-BTM crystal we for the first time observed strong antiferromagnetic interactions among luminescent radicals through SQUID measurements. This work offers design insights into luminescent radicals with α-type transition for future development.

Scheme 1. (a) Schematic representation of α and β transitions. (b) Structures of the stable luminescent diphenylamine biphenylmethyl radicals in this study.

Fig. 1. (a) EPR spectrum of 2PhN-BTM and 2(2Cl(m)PhN)-BTM in DCM solution (10⁻³ M) at room temperature. (b) Crystal structure of 2PhN-BTM and 2(2Cl(m)PhN)-BTM. (c) The crystal stacking structure of 2PhN-BTM and 2(2Cl(m)PhN)-BTM; black dashed lines: the distance between the central carbon atoms of adjacent radicals; red dashed lines: Cl⋯π halogen bonds between adjacent 2(2Cl(m)PhN)-BTM molecules.

Fig. 2. (a) Absorption and emission spectra of 2PhN-BTM and 2(2Cl(m)PhN)-BTM in cyclohexane solution (10⁻⁵ M) under excitation at 365 nm at room temperature. (b) Photoluminescence decay spectra of 2PhN-BTM and 2(2Cl(m)PhN)-BTM in cyclohexane solution (10⁻⁵ M) under excitation at 365 nm at room temperature. (c) Photoluminescence spectra of 2(2Cl(m)PhN)-BTM in cyclohexane solution at different temperatures under UV laser irradiation (365 nm) (the asterisk represents the harmonic peak of the light source).

Fig. 3. Donor/acceptor plane angles and frontier orbitals of Cz-BTM (a), 2PhN-BTM (b) and 2(2Cl(m)PhN)-BTM (c) according to DFT calculations at the ground state (UB3LYP, 6-31G(d,p)) (isovalue = 0.02).

Fig. 4. (a) Hole(blue)–electron(green) analysis of Cz-BTM, 2PhN-BTM and 2(2Cl(m)PhN)-BTM for D0 → D1 transition and distance between the hole center and electron center. (b) Photoluminescence spectra of Cz-BTM, 2PhN-BTM and 2(2Cl(m)PhN)-BTM in different solutions under 375 nm irradiation at room temperature.

Fig. 5. Molar magnetic susceptibility (χ_m) versus T for (a) 2PhN-BTM and (b) 2(2Cl(m)PhN)-BTM at a temperature range of 1.9 K to 300 K.