Normal & reversed spin mobility in a diradical by electron-vibration coupling

Abstract

π-conjugated radicals have great promise for use in organic spintronics, however, the mechanisms of spin relaxation and mobility related to radical structural flexibility remain unexplored. Here, we describe a dumbbell shape azobenzene diradical and correlate its solid-state flexibility with spin relaxation and mobility. We employ a combination of X-ray diffraction and Raman spectroscopy to determine the molecular changes with temperature. Heating leads to: i) a modulation of the spin distribution; and ii) a "normal" quinoidal → aromatic transformation at low temperatures driven by the intramolecular rotational vibrations of the azobenzene core and a "reversed" aromatic → quinoidal change at high temperatures activated by an azobenzene bicycle pedal motion amplified by anisotropic intermolecular interactions. Thermal excitation of these vibrational states modulates the diradical electronic and spin structures featuring vibronic coupling mechanisms that might be relevant for future design of high spin organic molecules with tunable magnetic properties for solid state spintronics.

Fig. 1. Phenoxyl and azobenzene building blocks of CAR and the bicycle pedal motion.

Phenoxyl (a) and azobenzene (b) building blocks of CAR with the bicycle pedal motion in azobenzene and in oligoenes. c Extreme quinoidal/aromatic canonical forms of CAR (note the differences between these extreme forms and the quinoidal-like, aromatic-like, and pseudoquinoidal forms discussed below that always correspond to hybrids with sizeable weights of the two extreme forms).

Fig. 2. Temperature-dependent single-crystal structures of CAR.

a Single-crystal X-ray structure with selected bond lengths of CAR at 130, 290, and 340 K with thermal ellipsoids at the 50% probability level. H atoms are not shown for clarity. b Selected bond lengths (Å) values of CARH and CAR.

Fig. 3. Solid-state magnetic and spectroscopic properties of CAR.

a Temperature-dependent plots of χT versus T and fitted χT–T curve (solid line) for CAR measured at 1.0 T from 2 to 400 K. The half-field transitions (Δm_s = ±2) are shown as insets. b ESR spectra of 1 mmol L⁻¹ CAR in a frozen glass (toluene) at 130 K. c UV-Vis-NIR electronic absorption spectrum in solid-state of CAR at room temperature; d room temperature solid-state Raman spectra of CAR taken with the 633 nm and 1064 nm excitation laser lines.

Fig. 4. Variable temperature vibrational Raman spectroscopy.

Solid-state variable temperature Raman spectra of CAR taken with the 785 nm (a) and (b) and with 633 nm excitation wavelength (c) and (d) in two different vibrational intervals where the four important (strongest) bands are denoted as (1) ν(CC)_ph; (2) ν(CC)_phO; (3) ν(CN)_azo1; and (4) ν(CN)_azo2. e Representation as a function of the temperature of the ratio of intensities of 1 (A1) and 2 (A2) as black circles and 3 (A3) and 4 (A4) bands as red squares as spectroscopic markers of the degree of quinoidal/aromatic character, which delineates three regions: small ratio means more quinoidal, large ratio more aromatic and pseudoquinoidal (PQ) is in between. f Topologies of the vibrational modes from the theoretical Raman spectra in Supplementary Figs. 12–14.

Fig. 5. Spin density distribution of CAR.

a Selected bond lengths and calculated spin population of CAR at different temperatures. b The integral spin density changes of phenoxyl and azobenzene moieties at temperatures of 130, 200, 250, 290, and 340 K.

Fig. 6. Sequential vs simultaneous action of torsion and bicycle pedal motions.

Simulations (not at scale) of the dihedral torsion motion of the azobenzene relative to the phenoxyls provoking the Q→A transformation (a), and of the combined torsional plus bicycle pedal motions of the central azobenzene moiety provoking the A→PQ transformation (b).

Fig. 7. Potential energy surface dependence of CAR with torsion and bicycle pedal motions.

a Torsional angle potential energy profiles of the CAR molecule inside the crystal structure at 290 K (A form) and of that at 340 K (PQ form). b Potential energy profiles of the CAR molecule inside the crystal structure at 290 K (A form) and of that at 340 K (PQ form) alongside the bicycle pedal motion distortion. Calculations were carried out with the QM/MM ONIOM model on the cluster of CAR molecules, B3LYP/6-31G* level with electronic embedding for the central molecule, Dreding force field, and charges determined with the Qeq approach for the MM part.