Electric-field Quantum Sensing Exploiting a Photogenerated Charge-transfer Triplet State in a Molecular Semiconductor

TL;DR

The paper addresses the challenge of electric-field sensing with molecular spins by demonstrating coherent electric-field sensing in a photogenerated charge-transfer triplet of the organic molecule ACRSA. Using a modified Hahn echo sequence that interleaves controlled $E$-field pulses, the authors quantify a spin-electric coupling with a maximum strength of $\kappa\approx0.59$ Hz/(V/m) and a peak sensitivity of $\delta f/E\approx0.51$ Hz/(V/m), despite negligible atomic SOC. The results show that heavy atoms are not required for SEC in organic systems, aided by a sizable molecular dipole moment and a strongly anisotropic $D$, with orientation-dependent measurements validating the linear $D(E)$ model $D(E)=D(0)+\kappa E\cos(\theta)$. The work establishes organic CT triplets as versatile, directionally sensitive quantum sensors of electric fields and outlines pathways to improve sensitivity—through molecular alignment, higher spin density, and longer coherence times—toward practical devices and potential room-temperature operation. These findings open routes for nanoscale electric-field sensing in organic molecular systems and near-surface environments where high spatial resolution is essential.

Abstract

Molecular spin systems are promising platforms for quantum sensing due to their chemically tunable Hamiltonians, enabling tailored coherence properties and interactions with external fields. However, electric field sensing remains challenging owing to typically weak spin-electric coupling (SEC) and limited directional sensitivity. Addressing these issues using heavy atoms exhibiting strong atomic spin-orbit couplings (SOC) often compromises spin coherence times. Here, we demonstrate coherent electric field sensing using a photogenerated charge-transfer (CT) spin triplet state in the organic molecule ACRSA (10-phenyl-10H,10' H-spiro\[acridine-9,9'-anthracen]-10'-one). By embedding electric field pulses within a Hahn echo sequence, we coherently manipulate the spin triplet and extract both the magnitude and directional dependence of its SEC. The measured SEC strength is approximately 0.51 Hz/(V/m), comparable to values reported in systems with strong atomic SOC, illustrating that heavy atoms are not a prerequisite for electric-field sensitivity of spin states. Our findings position organic CT triplets as chemically versatile and directionally sensitive quantum sensors of E-fields that function without atomic-SOC-mediated mechanisms.

Figures (9)

• Figure 1: (a) The schematic molecular structure of ACRSA. (b) The spin density of the charge-transfer triplet state induced by photoexcitation with $\mathrm{\lambda}$ = 355 nm, computed with the ORCA software (B3LYP/EPR-II basis). The blue (red) molecular orbitals correspond to the lower (higher) singly occupied molecular orbitals, coinciding with the hole (electron) density. The black arrow is the predicted orientation of the longitudinal zero-field splitting tensor/electric dipole moment, and it is defined as the $z$-axis for the experiment. (c) The (X-band) experimental and simulated field sweep of ACRSA doped into a PMMA matrix ($\sim 90$$\mu$M) at 20 K. The three starred points correspond to the fields where we conducted the spin-electric coupling measurements. (d) Schematic showing the optical pathway that leads to the formation of the triplet charge-transfer state to be electrically modulated, including the initial photoexcitation with $\lambda = 355$ nm, intersystem crossings (ISC), and internal conversion (IC). Inset: The simplified Zeeman energy diagrams when the longitudinal zero-field splitting ($D$) is parallel (top) and perpendicular (bottom) to the principal magnetic field, together with the allowed EPR transitions. The light-induced triplet state is spin polarised with $\sim 95\%$ population of $|m_s = 0\rangle$ (indicated by the thick lines). (e) The electronic phase-memory time ($T_{2\mathrm{e}}$) as a function of temperature. $T_{2\mathrm{e}}$ exceeds 2.5 $\mu$s at 20 K, and remains above 1.0 $\mu$s at 77 K.
• Figure 2: (a) The modified Hahn echo sequence for SEC measurements. A laser pulse at 355 nm generates the $\mathrm{^3CT}$ state with a spin-polarised initial population. After a fixed delay, a Hahn-echo sequence measures the spin coherence of the $\mathrm{^3CT}$ state. An $E$-field pulse is inserted immediately after the $\pi/2$ microwave pulse and the echo signal is recorded as a function of the duration and/or amplitude of the $E$-field pulse. (b) The echo intensity as a function of the $E$-field pulse duration, t$_E$. The data were recorded at 20 K with $\tau = 2$$\mu$s and $B_0$ at the "Int." field-position (as indicated in Fig. \ref{['fig: Intro']}(c)) with an $E$-field of 1.5 $\times 10^6$ V/m. The absence of an electric field response in the quadrature channel arises from the combination of a linear spin–electric coupling and the random orientation of spins within the ensemble.
• Figure 3: (a) (Simulated) Normalized angular distribution of resonant molecules as a function of the angle $\theta$ between the external magnetic field $\mathbf{B_0}$ and the molecular dipole/orientation axis p/$D$, as illustrated in the inset of panel (b). The three distributions correspond to the magnetic-field values used in the SEC study (see Fig. \ref{['fig: Intro']}(c)). The observed peaks in the XY and Int. distributions arise from the angular dependence of the resonant field, as detailed in Fig. S3 in the Supporting Information. (b) (Simulated) Effective electric field, calculated as the projection of the electric field along the p/$D$ axis ($E \cdot \cos(\theta)$, with $E =$ 1.5 $\mathrm{MV/m}$) weighted by the $\mathbf{B_0}$-dependent molecular population shown in (a). This represents the strength of the interaction between the electric field and the molecular ensemble, which takes into account the angular distribution variations at different $B_0$.
• Figure 4: (a-b) Integrated spin-echo intensity as a function of the electric-field pulse duration varied from 0 $\mu$s to $\tau$ (= 2 $\mu$s), for the three field positions considered in the study (see insets). The two configurations correspond to the electric field being parallel (left) and perpendicular (right) to $\mathrm{\mathbf{B_0}}$. In the parallel configuration, the most pronounced electric modulation occurs at Z, while in the perpendicular configuration, it occurs at XY. These positions correspond to the largest alignment between the electric dipole moment/$\mathrm{D}$ and the applied $E$-field, supporting the model in which $D$ is the primary contribution to the electric modulation. (c-d) Simulations of the electric-field modulation for (a-b), respectively. The model assumes that $\mathrm{D}$ is the only spin Hamiltonian term modulated by the electric field. This modulation is described by $D(E) = D(0) + \kappa E \cdot \cos(\theta)$, where $\mathrm{\theta}$ is the angle between $\mathrm{D}$ and the applied $E$-field. The coupling strength, $\kappa = 0.59 \,\mathrm{Hz/(V/m)}$, quantifies the interaction between the $E$-field and the magnetic anisotropy.
• Figure S1: EPR spectrum of a 5% ACRSA-doped PMMA film measured at 20 K under 355 nm photoexcitation, shown as a function of the time after the laser pulse (known as delay after flash). The data reveal a long-lived triplet state, with the spectral features in Fig. 1(c) persisting for more than 15 $\mu$s, i.e., significantly longer than the electron spin-phase memory time at the same temperature ($\sim 2.5~\mu$s; see main text). Blue and red areas indicate the emissive and absorptive components of the spectrum, respectively.