π Topology and Spin Alignment in Unique Photoexcited Triplet and Quintet States Arising from Four Unpaired Electrons of an Organic Spin System

Abstract

Syntheses, electronic structures in the ground state, unique photoexcited states, and spin alignment are reported for novel biradical 1, which was designed as an ideal model compound to investigate photoinduced spin alignment in the excited state. Electron spin resonance (ESR), time-resolved ESR (TRESR), and laser-excitation pulsed ESR experiments were carried out. The magnetic properties were examined with a SQUID magnetometer. In the electronic ground state, two radical moieties interact very weakly (almost no interaction) with each other through the closed-shell diphenylanthracene spin coupler. On photoirradiation, a novel lowest photoexcited state with the intermediate spin (S = 1) arising from four unpaired electrons with low-lying quintet (S = 2) photoexcited state was detected. The unique triplet state has an interesting electronic structure, the D value of which is reduced by antiferromagnetic spin alignment between two radical spins through the excited triplet spin coupler. The general theoretical predictions of the spin alignment and the reduction of the fine-structure splitting of the triplet bis(radical) systems are presented. The fine-structure splitting of the unique photoexcited triplet state of 1, as well as the existence of the low-lying quintet state, is interpreted well on the basis of theoretical predictions. Details of the spin alignment in the photoexcited states are discussed.

First Evidence for a Uniquely Spin‐Polarized Quartet Photoexcited State of a π‐Conjugated Spin System Generated via the Ion‐Pair State

“…[5][6][7] In such cases, the SO-ISC mechanism generates selective population of the zero-field (ZF) wave functions of the quartet spin states, thus leading to an A/E (aaaeee) or E/A (eeeaaa) pattern. In contrast, for the two-spin system, it is well-known that the radical-pair (RP) mechanism [14] in solution leads to selective population of the high-field (HF) wave functions of the M s sublevels by singlet-triplet mixing S-T 0 (or S-T AE1 ).…”

“…The spectral pattern of the signals as a result of the photoexcited state is almost the same as that of the TRESR spectrum, in that all signals have similar phasememory times and spin-lattice relaxation times. [7] Under the condition w 1 ! w ZFS , the TN frequency w TN of the M s $(M s +1) transition is given by Equation (1, [13] where…”

“…[4] Studying spin alignment in photoexcited states will give key knowledge for the photocontrol of magnetic properties. We have reported photoexcited quartet (S = 3/2) and quintet (S = 2) high-spin states of p-conjugated organic compounds [5][6][7] constructed from aromatic hydrocarbons and pendant stable radicals. p-Conjugated spin systems lead to robust spin alignment compared with other triplet/radical-pair systems (s-bonded systems [8] and coordination complexes [9] ).…”

First Evidence for a Uniquely Spin‐Polarized Quartet Photoexcited State of a π‐Conjugated Spin System Generated via the Ion‐Pair State

“…[3] Recently, photoswitching between diamagnetic and paramagnetic phases was realized in a 1,3,5trithia-2,4,6-triazapentalenyl (TTTA) organic crystal. We have reported photoexcited quartet (S = 3/2) and quintet (S = 2) high-spin states of p-conjugated organic compounds [5][6][7] constructed from aromatic hydrocarbons and pendant stable radicals. We have reported photoexcited quartet (S = 3/2) and quintet (S = 2) high-spin states of p-conjugated organic compounds [5][6][7] constructed from aromatic hydrocarbons and pendant stable radicals.…”

“…[1] Manipulation of the magnetic properties of organic molecules in the spin ground state has been intensively studied by using electron-hole doping [2] and photochromic molecules. We have reported photoexcited quartet (S = 3/2) and quintet (S = 2) high-spin states of p-conjugated organic compounds [5][6][7] constructed from aromatic hydrocarbons and pendant stable radicals. [4] Studying spin alignment in photoexcited states will give key knowledge for the photocontrol of magnetic properties.…”