Spin Polarization from Circularly Polarized Light Induced Charge Transfer

Abstract

We show how a spin polarization can be generated through the photo-induced electron transfer of an achiral donor-acceptor complex following chiral light excitation. In particular, we illustrate the basic energetic and symmetry requirements for chirality induced spin selectivity where the chirality emerges from the electronic degrees of freedom following excitation with circularly polarized light. We study this effect in a simple model of a metalloporphyrin complex with an axial acceptor ligand using quantum mechanical rate theories and numerical simulations. We find that the spin polarization emerges due to the selective excitation of a ring current within the porphryin, breaking the degeneracy of the two degenerate spin states. The resultant spin polarization increases with the spin orbit coupling between the metal in the porphyrin and the axial ligand, and is transient, with a lifetime dependent on the rate of dephasing from the Jahn-Teller distortion mode. This proposed effect should be observable in spin-resolved photoemission spectroscopy.

Figures (3)

• Figure 1: Electron transfer in an achiral donor-acceptor complex driven by circularly polarized light. Left- and right-circularly polarized light excite the donor singlet states $\ket{^1\mathrm{D}_{\lambda}}\equiv$$^1$[D$^*_{\lambda}$-A] with $\lambda=\pm$ from the ground state $\ket{\Psi_0}\equiv$$^1$[D-A]. Singlet and triplet charge transfer states $\ket{^1\mathrm{CT}_{\lambda}}\equiv$$^1$[D$^{\bullet+}_{\lambda}$-A$^{\bullet-}$] and $\ket{^3\mathrm{CT}_{\lambda}}\equiv$$^3$[D$^{\bullet+}_{\lambda}$-A$^{\bullet-}$] are nearly degenerate and detuned from $\ket{^1\mathrm{D}_{\lambda}}$ by $\Delta$, while $v_0$ and $v_{\mathrm{SOC}_{\lambda}}$ mediate spin-conserving and spin-orbit coupled electron transfer.
• Figure 2: Unitary dynamics. (a) The populations in the singlet donor $p_{\ket{^1\mathrm{D}_+}}$ (black), singlet charge transfer $p_{\ket{^1\mathrm{CT}_+}}$ (yellow) and the triplet charge transfer $p_{\ket{^3\mathrm{CT}_+}}$ (red) states and (b) the spin polarization $\langle\hat{P}_{z}(t)\rangle$ following excitation into $\ket{^1\mathrm{D}_{+}}$. (c) Mean spin polarization $\bar{P}_{z}$ for different detunings $\Delta$ and its dependence on initially excited state. Black star indicates condition in (a) and (b).
• Figure 3: Relaxational dynamics. (a) Population difference $\langle \Delta \hat{N}(t)\rangle=\langle\hat{N}_+(t)\rangle-\langle\hat{N}_-(t)\rangle$ (solid light orange) and its exponential fit $\exp(-\Gamma_{N}t)$ (dashed orange) and (b) the spin polarization $\langle\hat{P}_{z}(t)\rangle$ (solid light blue) and the exponential fit to its envelope $\exp(-\Gamma_{P}t)$ (dashed blue) for $\Delta=-30\,\mathrm{meV}$ and $f_{\mathrm{D}}=f_{\mathrm{CT}}=0.4$. (c) The decay rates $\Gamma_{P}$ (blue circles) and $\Gamma_{N}$ (orange squares) as a function of dimensionless coupling strengths $f_{\mathrm{D}}$ and $f_{\mathrm{CT}}$. The solid line is the expected rate from our analytic expression in Eq. \ref{['Eq:popdif']}.