A Raman and SERS study on the interactions of aza[5]helicene and aza[6]helicene with a nanostructured gold surface

We studied the interaction of 5-aza[5]helicene and 5-aza[6]helicene with gold nanostructured substrates by SERS spectroscopy and DFT calculations. By comparing Raman and SERS experiments we observe significant changes for the normal modes involving nitrogen. This behavior, together with further evidence coming from SERS experiments carried out on the quaternary salts of the same azahelicenes, clearly highlight the direct interaction of nitrogen with the gold surface. DFT calculations on azahelicene models interacting with Au20 gold clusters reveal the charge transfer between azahelicene and gold. The information obtained from this study is useful in the field of sensor applications, where knowledge of the molecular mechanisms of interaction is fundamental for an appropriate design of the device.