π-Topology and Spin Alignment Utilizing the Excited Molecular Field: Observation of the Excited High-Spin Quartet (S = 3/2) and Quintet (S = 2) States on Purely Organic π-Conjugated Spin Systems

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Abstract
As a model system for the photoinduced/photoswitched spin alignment in a purely organic π-conjugated spin system, 9-[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1a), 9-[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1b), 9,10-bis[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2a), and 9,10-bis[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2b) were designed and synthesized. In these spin systems, 9-phenylanthracene and 9,10-diphenylanthracene were chosen as photo spin couplers and iminonitroxide was chosen as a dangling stable radical. Time-resolved electron spin resonance (TRESR) spectra of the first excited states with resolved fine-structure splittings were observed for 1a and 2a in an EPA or a 2-MTHF rigid glass matrix. Using the spectral simulation based on the eigenfield method, the observed TRESR spectra for 1a and 2a were unambiguously assigned as an excited quartet (S = 3/2) spin state (Q) and an excited quintet (S = 2) spin state (Qu), respectively. The g value and fine-structure splitting for the quartet state of 1a were determined to be g(Q) = 2.0043, D(Q) = 0.0235 cm-1, and E(Q) = 0.0 cm-1. The relative populations (polarization) of each MS sublevel in Q were determined to be P+1/2‘ = P-1/2‘ = 0.5 and P+3/2‘ = P-3/2‘ = 0.0 with an increasing order of energy in zero magnetic field. The spin Hamiltonian parameters for Qu are g = 2.0043, D = 0.0130 cm-1, and E = 0.0 cm-1, and the relative populations in Qu were determined to be P0‘ = 0.30, P-1‘ = P+1‘ = 0.35 and P-2‘ = P+2‘ = 0.0. These are the first observations of a photoexcited quartet and a quintet high-spin state in π-conjugated triplet−radical pair systems. In contrast high-spin excited states were not observed for 1b and 2b, the π-topological isomers of 1a and 2a, showing the role of π-topology in the spin alignment of the excited states. Since a weak antiferromagnetic exchange interaction was observed in the ground state of 2a, the clear detection of the excited quintet high-spin state shows that the effective exchange coupling between the two dangling radicals through the diphenylanthracene spin coupler has been changed from antiferromagnetic to ferromagnetic upon photoexcitation. Thus, a photoinduced spin alignment utilizing the excited triplet molecular field was realized for the first time in the purely organic π-conjugated spin system. Furthermore, the mechanism for the generation of dynamic electron spin polarization was investigated for the observed quartet and quintet states, and a plausible mechanism of the enhanced selective intersystem crossing was proposed. Ab initio molecular orbital calculations based on density functional theory were carried out to determine the electronic structures of the excited high-spin states and to understand the mechanism of the spin alignment utilizing the excited molecular field. The role of the spin delocalization and the spin polarization mechanisms were revealed on the photoexcited state.

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