Excited-state Dynamics and Spin-exchange Coupling of Anthracene–Verdazyl Radical in Frozen Glass Matrix Investigated by Transient Absorption Spectroscopy

Abstract

Excited-state dynamics of an anthraceneverdazyl radical diluted in a frozen glass matrix (2-MTHF) has been clarified by transient absorption spectroscopy. The magnitude of the intramolecular exchange coupling was determined from the temperature dependence of the decay time of the transient absorption signal of the high-spin quartet state, which was consistent with the theoretical prediction. since a large negative magneto-resistance of organic radical system was demonstrated by Sugawara et al. 3 Intra-or intermolecular spin alignment and exchange interaction through π-conjugation or π-orbital packing in purely organic spin systems play key roles in their research fields. However, most of these studies are limited to ground-state systems. 4 The studies of the exchange interaction between the triplet excited state and radical spin(s) will give very important knowledge on the forthcoming excited-state organic spintronics and photocontrol of the magnetic properties. 5 This knowledge leads to a new strategy for the photoinduced/switching magnetic spin systems, the photocontrol of the molecule-based magnetism and future organic spintronics devices using photoexcitation. Only for very weak exchange coupling, ESR spectroscopy gives direct information on the magnitude of the exchange coupling. Actually, the magnitude of the exchange coupling between the photoexcited triplet state and radical spin in the weak exchangecoupled spin systems were determined using time-resolved ESR (TRESR) spectroscopy.6 When the target system is emissive, the magnitude of the stronger spin-exchange coupling in the photoexcited-tripletdoublet coupled spin systems can be also determined by the temperature dependence of the emission intensity as demonstrated for a fluorescent metal-complex radical hybrid; 7 the lifetime of the spin polarization in this hybrid was also temperature-dependent, as obtained for the emission lifetime. In our previous papers, 8 we reported the first observation of a quartet (S = 3/2) photoexcited state and a quintet (S = 2) photoexcited state in organic π-radicals that were generated by a robust spin alignment through π-conjugation between stable iminonitroxide radicals (S = 1/2) and the triplet (S = 1) excited state of a phenyl-or diphenylanthracene derivative, which are ideal model systems in order to study the relationship between the π-topology and photoinduced spin alignment. We demonstrated that the spin multiplicities of the photoexcited state observed by the TRESR were dependent on the π-topology.8b,9 We have also investigated π-conjugated spin systems constructed from other polycyclic aromatic hydrocarbons (pyrene or pentacene) and covalently-linked stable radicals. 10,11 The magnitude of the intramolecular exchange coupling of their π-radical spin systems was, however, estimated only from theoretical calculations, 12 because the magnitude was much larger than that accessible by ESR spectroscopy and highspin photoexcited state was almost non-emissive (very weak phosphorescent).

Theoretical investigation of multi-spin excited states of anthracene radical-linked π-conjugated spin systems by computational chemistry

“…3a. The J DQ of pOV0a calculated from eqn (2) was close to the experimental result 29 and the previous theoretical calculation. 30,31 The dihedral angles (f) between the anthracene moiety and the adjacent phenyl group were 73.11 and 58.61 in the ground and excited states of pIN0a, respectively.…”

Low-Energy and Long-Lived Emission from Polypyridyl Ruthenium(II) Complexes Having A Stable-Radical Substituent

“…There are two possible excited-state pathways in such triplet–doublet hybrid systems; direct deactivation from the quartet excited state ( 4 [ 3 MLCT Ru – 2 OV]* in Scheme ) and partially spin-allowed process via the doublet excited state ( 2 [ 3 MLCT Ru – 2 OV]* in Scheme ). While the competition of these pathways gives the decay profile analyzable by a sum of two exponential decay functions, in the present study, there was no evidence for a contribution of the partially spin-allowed process in the excited-state decay of 1a 2+ or 2a 2+ . The result can be explained as follows.…”

“…The spin-exchange coupling between the 3 MLCT excited state in the [Ru(bpy) 3 ] 2+ moiety ( 3 MLCT Ru ), which is given by the photoexcitation followed by ISC, and doublet spin in the OV moiety ( 2 OV) generates the doublet and quartet MLCT-type excited states ( 2,4 [ 3 MLCT Ru − 2 OV]*). 26,28 In these excited states, the spin−orbit coupling originating from the unpaired electron in the OV moiety and the existence of partially spin-allowed pathways ( 2 [ 3 MLCT Ru − 2 OV]* → 2 [ 1 GS Ru − 2 OV]) are expected to have influence on both radiative and nonradiative processes of the complexes.…”

“…Scheme illustrates expected excited-state processes and spin-exchange coupling of the complexes. The spin-exchange coupling between the 3 MLCT excited state in the [Ru­(bpy) 3 ] 2+ moiety ( 3 MLCT Ru ), which is given by the photoexcitation followed by ISC, and doublet spin in the OV moiety ( 2 OV) generates the doublet and quartet MLCT-type excited states ( 2,4 [ 3 MLCT Ru – 2 OV]*). , In these excited states, the spin–orbit coupling originating from the unpaired electron in the OV moiety and the existence of partially spin-allowed pathways ( 2 [ 3 MLCT Ru – 2 OV]* → 2 [ 1 GS Ru – 2 OV]) are expected to have influence on both radiative and nonradiative processes of the complexes. Although the emissive metal complexes having the stable-radical substituents in a ligand(s) have been rarely reported, most of them show ligand-based emission, , and the number of the reports on the complex showing MLCT-type emission is still limited .…”

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…[100] The magnitude of the intramolecular ferromagnetic exchange interaction (J = 123 cm À1 [calcd] [100] and 117 AE 20 cm À1 [estimated from exp.] [101] )w as much larger than that of the dipolar interaction. Therefore, the possibility of dipolar mixingt aking place could be ruled out.…”