Excited State Exchange Control of Photoinduced Electron Spin Polarization in Electronic Ground States

Abstract

Ground-state electron spin polarization (ESP) is generated in radical elaborated (bpy)­Pt­(CAT-NN) and (bpy)­Pt­(CAT-p-Me2PhMe2-NN) (bpy = 5,5′-di-tert-butyl-2,2′-bipyridine, CAT = 3-tert-butylcatecholate, p-Ph = para-phenylene, NN = nitronylnitroxide). Photoexcitation produces an exchange-coupled, three-spin, charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to a 2T1 (T = chromophore excited spin triplet configuration) excited state. The SQ-bridge-NN bond torsions affect the magnitude of the excited state exchange interaction ( J SQ‑NN ), which determines the 2T1–4T1 energy gap. Ground state ESP is dependent on the magnitude of J SQ‑NN , and we postulate that this results from differences in 2T1 and 4T1 state mixing. Mechanisms that lead to the rapid transfer of the excited state ESP to the ground state are discussed. Although subnanosecond 2T1 state lifetimes are measured optically in solution, the ground state ESP decays very slowly at 20 K and is observable for more than a millisecond.

Excited State Magneto-Structural Correlations Related to Photoinduced Electron Spin Polarization

“…The energies and intensities of the higher energy charge transfer bands differ between the complexes and are a function of both the number and positions of the bridge methyl groups, reminiscent of the effects of bridge torsions on the spectra of the corresponding zinc complexes , shown in Figure B. A markedly weaker band is observed at 16500 (ε max = 783 M –1 cm –1 ) for (DMSO) 2 Pt­(CAT-NN) . This complex lacks the bpy acceptor that figures prominently in the CAT → bpy LL′CT transition.…”

“…The electronic absorption spectra of (bpy)­Pt­(CAT-NN) and (bpy)­Pt­(CAT- bridge -NN) LL′CT complexes and (DMSO) 2 Pt­(CAT-NN) are provided in Figure C. For all complexes, an LL′CT absorption band − ,, is observed between 17,000 and 18,000 cm –1 with ε max between 3000 and 4000 M –1 cm –1 . The energies and intensities of the higher energy charge transfer bands differ between the complexes and are a function of both the number and positions of the bridge methyl groups, reminiscent of the effects of bridge torsions on the spectra of the corresponding zinc complexes , shown in Figure B.…”

“…To date, the nature of the metal (Pd vs Pt) and the associated metal ion SOC, para vs meta connectivity effects on bridge-mediated exchange coupling, and the magnitude of J SQ‑NN have all been shown to affect ground state ESP in (bpy)­M­(CAT- bridge -NN) complexes. Here, we focus on (bpy)­Pt­(CAT-NN) and a series of (bpy)­Pt­(CAT-Ph-NN) LL′CT complexes with para bridge connectivity.…”

“…Control of excited state processes and how they affect ground state observables is at the forefront of modern magneto-structural studies, which are important for molecular quantum information science (QIS) . This derives from the fact that unpaired electron spins in molecules can function as qubits that are amenable to manipulation by magnetic field effects and microwave pulses. − Our prior studies have focused on understanding key relationships between molecular magnetic and electronic structure and seemingly disparate observables and processes. For example, these studies have correlated the rate of non-radiative excited state decay and magnetic circular dichroism intensity , with the magnitude of pairwise excited state magnetic exchange interactions, ground state NMR chemical shifts with excited state spin-orbit coupling (SOC) and excited state lifetimes, , and molecular conductance with organic bridge-mediated magnetic exchange interactions …”

“…More recently, we have turned our attention to understanding how to photogenerate and subsequently control ground-state electron spin polarization (ESP) in metal complex-radical dyads (Figure ) that possess low-energy ligand-to-ligand charge transfer (LL′CT) excited states. − Ultrafast generation of a charge-separated species by direct photoexcitation into the LL′CT states of these donor–acceptor chromophores generates a spin qubit pair that exists in a chromophoric S = 0 singlet quantum state. ,,, When a persistent radical is appended to the LL′CT chromophore, this allows for spin–spin interactions to occur with the promise of their control via the pairwise magnetic exchange interactions that connect these spins. − The LL′CT state is particularly interesting with respect to QIS sensing applications − since this state is very sensitive to the environment (e.g., the solvent dielectric), and this sensitivity can be exploited to engineer energy matches between the LL′CT chromophore and the localized radical states of radical-elaborated chromophores for spin polarization control.…”