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

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.