Theoretical study of dynamic electron-spin-polarization via the doublet-quartet quantum-mixed state and time-resolved ESR spectra of the quartet high-spin state

Abstract

The mechanism of the unique dynamic electron polarization of the quartet (S = 3/2) high-spin state via a doublet-quartet quantum-mixed state and detail theoretical calculations of the population transfer are reported. By the photo-induced electron transfer, the quantum-mixed charge-separate state is generated in acceptor-donor-radical triad (A-D-R). This mechanism explains well the unique dynamic electron polarization of the quartet state of A-D-R. The generation of the selectively populated quantum-mixed state and its transfer to the strongly coupled pure quartet and doublet states have been treated both by a perturbation approach and by exact numerical calculations. The analytical solutions show that generation of the quantum-mixed states with the selective populations after de-coherence and/or accompanying the (complete) dephasing during the charge-recombination are essential for the unique dynamic electron polarization. Thus, the elimination of the quantum coherence (loss of the quantum information) is the key process for the population transfer from the quantum-mixed state to the quartet state. The generation of high-field polarization on the strongly coupled quartet state by the charge-recombination process can be explained by a polarization transfer from the quantum-mixed charge-separate state. Typical time-resolved ESR patterns of the quantum-mixed state and of the strongly coupled quartet state are simulated based on the generation mechanism of the dynamic electron polarization. The dependence of the spectral pattern of the quartet high-spin state has been clarified for the fine-structure tensor and the exchange interaction of the quantum-mixed state. The spectral pattern of the quartet state is not sensitive towards the fine-structure tensor of the quantum-mixed state, because this tensor contributes only as a perturbation in the population transfer to the spin-sublevels of the quartet state. Based on the stochastic Liouville equation, it is also discussed why the selective population in the quantum-mixed state is generated for the "finite field" spin-sublevels. The numerical calculations of the elimination of the quantum coherence (de-coherence and/or dephasing) are demonstrated. A new possibility of the enhanced intersystem crossing pathway in solution is also proposed.

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

“…Thus, the direct pathway illustrated in Figure 1b leads to ac ommonD EP pattern. However,f or chromophore-radical linked spin systemsc onnecting with functional groups,a ni ndirect pathway via an intermediate state (S IM )i s possible, as illustrated in Figure 1c.I tw as first speculated [19][20] and later confirmed theoretically [21][22] that changes in the spin wavefunction from quantum-mixed (QM) states (f 1 Àf 6 )t ot he pure Da nd Qs tates upon either ac harger ecombination or polarization transfer (Figure 9) can lead to an unusual DEP, when the S IM state is aQ Ms tate between the quartet (Q IM )a nd doublet (D IM )s tates.H ere, it should be noted that the spin parts (spin functions) in the wavefunctions of the D IM and Q IM states (j Q IM,Ms > and j D IM,Ms >)a nd those of the Da nd Qs tates ((j Q Ms > and j D Ms >)c an be expressed by the same formula. Therefore, when the wavefunction mixing between the chromophorea nd radical moiety is negligible (the limiting case in the weak exchange coupling), the spin functions of the Da nd Qs tates (j Q Ms > and j D Ms >)c an be relatedt ot he weakly coupled QM states ((j aT Ms' > and j bT Ms' >)a ccordingt oE quation (1):…”

“…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.…”

“…[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).…”

“…However, these properties can be used to effectively generate high‐spin photoexcited states as well as improve the stability of photochemically unstable aromatic compounds such as pentacene . The appending of π‐radical moieties to a chromophore/functional moiety can be used to control or modify the excited‐state dynamics . Furthermore, luminescent organic π‐radicals have also been developed .…”

Combining Molecular Spintronics with Electron Paramagnetic Resonance: The Path Towards Single-Molecule Pulsed Spin Spectroscopy

“…The theoretical understanding of all these effects is both crucial and challenging. First developments of a toolbox for the investigation of spin dynamics and EISC are under development [227][228][229][230]. This becomes particularly important for the observation of unusual effects, e.g.…”