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

Abstract

A novel spin‐polarization mechanism involving the intramolecular ion pair *A.−‐D.+‐R, which competes with a spin–orbit intersystem crossing, is proposed for the formation of the uniquely spin‐polarized quartet photoexcited state observed for 1, which consists of a bodipy acceptor (A), a phenylanthracene donor (D), and a stable verdazyl radical (R).

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

“…In the case of bodipy-bridge-chromophore molecules with an appended p-radicala si llustrated in Figure 7a (bodipy:f luorophore boron dipyrromethene), our group reported the first observation of photoexcited quartet high-spin states with an unusualD EP pattern (aee/aae pattern; a/e:a bsorption/emission of the microwave), as shown in Figure 8a.T his unusual DEP pattern couldb er ationalized as as elective population (polarization)w ithin high-field spin sublevels. [19][20] Later,s imilar DEP patterns were also observed for othera cceptor-bridgechromophores linked to p-radical systems [103] as well as ac hromophore linked to an s-bonded radical. [27] Such unusual DEP patterns polarized in the high-field spin sublevels( aee/aae) have been previously observed only in the case of systems in which ar adical pair was formed andu nderwent RP-ISC followed by charger ecombination to the local triplet state in either am utated photosynthetic reactionc enter [53] or its model systems.…”

“…Thus, the direct pathway illustrated in Figure b leads to a common DEP pattern. However, for chromophore–radical linked spin systems connecting with functional groups, an indirect pathway via an intermediate state (S IM ) is possible, as illustrated in Figure c. It was first speculated and later confirmed theoretically that changes in the spin wavefunction from quantum‐mixed (QM) states ( ϕ 1 − ϕ 6 ) to the pure D and Q states upon either a charge recombination or polarization transfer (Figure ) can lead to an unusual DEP, when the S IM state is a QM state between the quartet (Q IM ) and doublet (D IM ) states. Here, it should be noted that the spin parts (spin functions) in the wavefunctions of the D IM and Q IM states (|Q IM, M s > and |D IM, M s >) and those of the D and Q states ((|Q M s > and |D M s >) can be expressed by the same formula.…”

“…[27] Such unusual DEP patterns polarized in the high-field spin sublevels( aee/aae) have been previously observed only in the case of systems in which ar adical pair was formed andu nderwent RP-ISC followed by charger ecombination to the local triplet state in either am utated photosynthetic reactionc enter [53] or its model systems. [104] Figure 8a andbshows the typical time-resolved ESR spectra of ab odipy-bridge-chromophore linked to a pradicals ystem [19][20] and a2 ,2,6,6-tetramethylpiperidin-1-yl)oxyl oroxidanyl (TEMPO)-PDI s-bonded radicals ystem, [27] respectively.A sD EP corresponds to an on-equilibrium physicalq uantity, it depends on the specific pathway how an observed state is generated, that is, its excited-state dynamics. Photoexcited high-spins tates usually exhibit ac haracteristicD EP pattern owing to an on-Boltzmann population in the zero-field spin sublevels( aaa/eee pattern, see Figure 5), which is similart o that of the usual photoexcited triplet states generated by SO-ISC (aa/ee pattern).…”

“…[16][17][18] The appending of p-radical moieties to ac hromophore/functional moiety can be used to control or modify the excited-state dynamics. [19][20][21][22][23][24][25][26][27][28][29][30][31] Furthermore, luminescent organic p-radicalsh ave also been developed. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] One of the straightforwarda pplications of luminescence is organic light-emitting devices (OLEDs).…”

Intersystem Crossing in Naphthalenediimide–Oxoverdazyl Dyads: Synthesis and Study of the Photophysical Properties

“…For instance, a compact NO radical labeled perylene dyad was reported, for which the triplet state of the perylene unit was observed with fs TA spectroscopy, but the triplet state lifetime was not exactly determined . In some cases, only the high spin state (for instance, quartet or quintet state) but no doublet state (D 1 state) was observed . The observed lifetime of the triplet state of the NDI unit in the current dyads is much shorter than the intrinsic triplet state lifetime of the NDI chromophore (ca.…”