pi-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 pi-conjugated spin systems

As a model system for the photoinduced/photoswitched spin alignment in a pu rely organic pi -conjugated spin system, 9-[4-(4,4,5,5-tetramethyl-1-yloxyi midazolin-2-yl)phenyl]anthracene (1a), 9-[3-(4,4,5,5-tetramethyl-1-yloxyimi dazolin-2-yl)phenyl]anthracene (1b), 9,10-bis[4-(4,3,5,5-tetramethyl-1-ylox yimidazolin-2-yl)phenpl]anthracene (2a), and 9,1 0-bis[3-(4,4,5,5-tetrameth yl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2b) were designed and synthesi zed. In these spin systems, 9-phenylanthracene and 9,10-diphenylanthracene were chosen as photo spin couplers and iminonitroxide was chosen as a dangl ing stable radical. Time-resolved electron spin resonance (TRESR) spectra o f the first excited states with resolved finestructure splittings were obse rved for 1a and 2a in an EPA or a 2-MTHF rigid glass matrix, Using the spec tral simulation based on the eigenfield method, the observed TRESR spectra for 1a and 2a were unambiguously assigned as an excited quartet (S = 3/2) s pin 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 M-s sublevel in Q were de termined to be P+1/2' = P-1/2' = 0.5 and P+3/2' = P-3/2' = 0.0 With an incr easing order of energy in zero magnetic field. The spin Hamiltonian paramet ers for Qu are g = 2.0043, D = 0.0130 cm(-1), and E = 0.0 cm(-1), and the r elative populations in Qu were determined to be P-0' = 0.30, P-1' = P+1' = 0.35 and P-2' = cm P+2' = 0.0 These are the first observations of a photoex cited quarter and a quintet high-spin state in pi -conjugated triplet-radic al pair systems. In contrast high-spin excited states were not observed fdr Ib and 2b, the pi -topological isomers of 1a and 2a, showing the role of p i -topology in the spin alignment of the excited states. Since a weak antif erromagnetic exchange interaction was observed in the ground state of 2a, t he clear detection of the excited quintet high-spin state shows that the ef fective exchange coupling between the two dangling radicals through the dip henylanthracene spin coupler has been changed from antiferromagnetic to fer romagnetic upon photoexcitation. Thus, a photoinduced spin alignment utiliz ing the excited triplet molecular field was realized for the first time in the purely organic pi -conjugated spin system. Furthermore, the mechanism f or the generation of dynamic electron spin polarization was investigated fo r the observed quarter 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 molec ular field. The role of the spin delocalization and the spin polarization m echanisms were revealed on the photoexcited state.