Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation

Abstract

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices.

Figure 1

X-band EPR spectra recorded at T = 90 K: Panel (a) The spectrum of FG powder under dark conditions (lower spectrum, 0.6 mW of applied microwave power) compared to that obtained after 10 min of in situ continuous UV light irradiation (upper spectrum, at 325 nm, 200 mW). Panel (b) The spectra of FG powder freshly dispersed in (a) cyclohexane, (b) benzene, and (c) acetone, recorded after 10 min of in situ UV irradiation at 325 nm (orange) and in dark conditions (blue). Experimental conditions: (a) 9.077 GHz frequency, 1.6 mW microwave power; (b) 9.080 GHz frequency, 1.0 mW microwave power; (c) 9.088 GHz frequency, 0.6 mW microwave power. The solvents’ respective dielectric constants are shown next to the recorded EPR traces (a–c).

Figure 2

(a) In situ light-induced X-band EPR spectra (LEPR) of FG powder (C₁F_1.1) freshly dispersed in an oxygen free benzene solution. The sample was kept in a nitrogen saturated atmosphere. Experimental parameters: 9.081 GHz frequency, 1.0 mW microwave power, 100 kHz modulation frequency, 0.5 mT modulation width, and 30 s acquisition time for each sequential spectrum. Signal acquisition sequence: 2.5 min under dark conditions, followed by 7.5 min under light irradiation (at 325 nm), then 20 min under dark conditions. (c) The correspondent 2D LEPR plot of the spectra shown in panel (a). (b) LEPR spectra of FG powder (C₁F_1.1) freshly dispersed in oxygen free acetone solution. Experimental parameters: 9.075 GHz frequency, 1.0 mW microwave power, 100 kHz modulation frequency, 0.5 mT modulation width, and 30 s acquisition time for each sequential spectrum. Signal acquisition sequence: 2.5 min under dark conditions, followed by 7.5 min under light irradiation (at 325 nm), then 20 min under dark conditions. (d) The correspondent 2D LEPR plot of the spectra shown in panel (b).

Figure 3

(a) Absorption spectra of defected FG in cyclohexane (CHX, green line), benzene (BEN, blue line), and acetone (ACE, red line) solutions computed without (short-dashed lines) and with (solid lines) an explicit solvent molecule in the vicinity of a radical defect. Black vertical dashed line denotes the wavelength of the applied UV light. Green, blue, and red vertical dashed lines denote the positions of the charge transfer (CT, S₀ → S₁) state in CHX, BEN, and ACE, respectively, (b) HOMO and lowest unoccupied molecular orbital (i.e., SOMO) of the solvated system in acetone, benzene, and cyclohexane (from left to right) corroborating the CT character of transitions shown in (panel a). (c) Proposed mechanism of the initial phase of the photoactivation of the FG-solvent interface.

Figure 4

(a) X-band EPR spectra of FG powder freshly dispersed in oxygen-free acetone solvent and recorded at T = 90 K under dark conditions (trace a), FG/acetone recorded after 3 min under UV irradiation (@325 nm) (trace b), FG/acetone recorded after 7.5 min under UV irradiation followed by 10 min under dark conditions (trace c), FG/acetone after UV irradiation (7.5 min), thawed at room temperature and finally cooled back under dark conditions at T = 90 K (trace d), and FG/deuterated acetone (acetone-d₆) recorded at T = 90 K after 7.5 min under UV irradiation followed by 10 min under dark conditions (trace e, bottom spectrum). (b) FG radical/acetone system; the charge density difference due to an extra electron (w.r.t. the charge density of the neutral system).