Synthesis and characterization of SERS-active heteroaromatic molecules

Azahelicenes are extensively conjugated, intrinsically chiral molecules with interesting optical and optoelectronic properties. They show interesting SERS activity as well as fluorescence. While synthesizing 5-aza[6]helicene (2) we observed concurrent formation of byproduct (3), phenanthreno[2,3-k]phenanthridine in non-negligible amount. Therefore, we decided to characterize it respect to its spectroscopic and emission properties. We recorded UV-Vis absorption and concentration-dependent fluorescence spectra of (3) excited at 314 nm. We observed an excellent linear dependence of the emission maximum (429 nm) vs. concentration in diluted MeOH solutions (5x10-5 - 3x10-4 mM). We studied its IR and Raman spectra and, once assessed its Raman activity, we adsorbed (3) on nanostructured gold substrates produced by PLD with a plasmon resonance close to 785 nm, with the purpose of investigating its SERS response. We also carried out density functional theory calculations, aiming at the interpretation of its observed SERS features. We concluded that this molecule, just like azahelicenes, shows promising features for applications in the manufacturing of SERS-active surfaces.