Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States

Abstract

Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (2S0/D0) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronyl nitroxide, NN) covalently attached to a donor–acceptor chromophore via a m-phenylene bridge, (bpy)­M­(CAT-m-Ph-NN) (1) (bpy = 4,4′-di-tert-butyl-2,2′-bipyridine, M = PdII (1-Pd) or PtII (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy•–, CAT•+ = semiquinone (SQ), and NN•), charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a 2T1 (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective, and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. It is postulated that zero-field-splitting-induced transitions between the chromophoric 2T1 and 4T1 states (1-Pd and 1-Pt) and spin–orbit-induced transitions between 2T1 and NN-based quartet states (1-Pt) account for the differences in polarization.

Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization

“…Recently, we demonstrated that despite the short 2 T 1 excited state lifetime and absence of intermolecular collisions, strong ground state ESP can be generated in (bpy)Pt(CAT- m -Ph-NN) ( mPh-Pt , NN = nitronylnitroxide radical). 16 The intensity of the photoinduced ESP in mPh-Pt was also shown to be dramatically altered by changing the metal ion from Pt to Pd in what are otherwise identical complexes. 16 In the present work, we show that the nature of the ground state ESP is also related to the magnitude of the excited state chromophore-radical antiferromagnetic exchange ( J SQ-NN (= 2 J CR ) Fig.…”

“… 16 The intensity of the photoinduced ESP in mPh-Pt was also shown to be dramatically altered by changing the metal ion from Pt to Pd in what are otherwise identical complexes. 16 In the present work, we show that the nature of the ground state ESP is also related to the magnitude of the excited state chromophore-radical antiferromagnetic exchange ( J SQ-NN (= 2 J CR ) Fig. 2A ), which can be controlled by tailored bond torsions that reduce π-system mediated superexchange coupling.…”

“… 11,15–17 Interestingly, localized aryl-NN π→π* transitions are also observed in this spectral region, 39,40 but they are obscured in mPh-Pt and 6-Me-mPh-Pt by the more intense LL′CT band. 16 Despite the remarkably similar electronic absorption spectra observed for (bpy)Pt(CAT- bridge -NN) complexes, 17 the nature of the magnetic exchange coupling between spins in the excited states of (bpy)Pt(CAT- bridge -NN) systems can dramatically affect excited state processes when compared to the parent (bpy)Pt(CAT) chromophore that lacks an appended radical. 11 This was previously exemplified by the observation of both magnetic circular dichroism (MCD) activity, 17 and a dramatic modulation of excited state lifetimes in (bpy)Pt(CAT- bridge -NN) molecules.…”

“…We note that no ESP is generated in (DMSO) 2 Pt(CAT- m -Ph-NN), which lacks an LL′CT excited state, indicating that the LL′CT excited state is necessary to observe ground state ESP in these systems. 16 …”

“… 11–13,16,17 When these molecules are elaborated with a persistent radical attached to the CAT donor, the LL′CT excited states can be described as having three localized spin qubits that possess different pairwise exchange interactions. 16,17 A particular benefit of this tripartite entanglement 9,18–20 is that when the spins in the open-shell excited singlet- (S) and triplet (T) states of the chromophore exchange couple with the radical, two doublet states ( 2 S 1 and 2 T 1 ) and a quartet state ( 4 T 1 ) result, and magnetic exchange interactions admix the 2 S 1 and 2 T 1 states. 17 This excited state exchange mixing effectively mediates an enhanced intersystem crossing 21–24 that permits access to the localized high-spin triplet configuration of the chromophore.…”