Plasmonically-Enhanced Vibrational Excitations of the Stable Blatter Radical

Stable radicals are intriguing species with a plethora of proposed applications in fields such as energy storage, molecular electronics and quantum communication^(<i>1</i>). However, surprisingly little is known of their optical properties and vibrational energy levels^{(<i>2</i>, <i>3</i>)}. Furthermore, it is not clear how these are affected by the radical oxidation state, which is key for understanding electrical transport through these species in molecular electronics devices. Here, we explore the properties of 1,2,4-benzotriazin-4-yl, a doubly-thiolated variant of the stable Blatter radical^(<i>1</i>), revealed through surface-enhanced Raman scattering (SERS). We investigate the vibrational modes of this molecule, its photoluminescence, and its optical response to reduction\oxidation and interactions with adjacent metallic surfaces.<br/>To investigate these molecules, we bind them to gold (Au) films as a self-assembled monolayer (SAM) and combine this in Nanoparticle-on-Mirror^{(<i>4</i>, <i>5</i>)} (NPoM) plasmonic nano-cavities. NPoM nano-cavities are an efficient platform for light-matter interactions owing to extreme light confinement^(<i>4</i>), enhancing processes such as Raman scatter by ~10⁹. This allows us to probe molecular processes at the few-molecule level by measuring their time-varying SERS and extract new understanding of their behaviour.<br/>We find the Blatter radical to have a photoluminescent (PL) excited state with LUMO\SOMO bandgap of ~1.9eV and vibronic side-band at ~1.7eV (shifted 1365cm^-1) which is the main vibration of the conjugated core as calculated by DFT. SERS signals from this vibration exhibit time-jitter on timescales &lt;1sec and we show evidence that this is likely from coupling between the molecular vibration and the LUMO\SOMO orbital of the unpaired electron.<br/>Our findings establish a previously- unknown coupling between the molecule SOMO\LUMO orbital, the radical spin, and molecular vibration and pave the way towards realizing efficient information-storage materials and molecular quantum devices.<br/>1. J. Hurtado-Gallego <i>et al.</i>, <i>Nano Lett</i>. <b>22</b>, 948–953 (2022).<br/>2. G. Karecla <i>et al.</i>, <i>New Journal of Chemistry</i>. <b>41</b>, 8604–8613 (2017).<br/>3. J. Z. Low <i>et al.</i>, <i>Nano Lett</i>. <b>19</b>, 2543–2548 (2019).<br/>4. J. J. Baumberg, J. Aizpurua, M. H. Mikkelsen, D. R. Smith, <i>Nature Materials 2019 18:7</i>. <b>18</b>, 668–678 (2019).<br/>5. F. Benz <i>et al.</i>, <i>Journal of Physical Chemistry Letters</i>. <b>7</b>, 2264–2269 (2016).