Chemical control of single-molecule fluorescence

The study of the influence of the environment on the fluorescence properties of molecules has been carried out in various fields of research, including scanning tunneling microscopy (STM). In the present manuscript, the focus is shifted from these extrinsic environmental effects to effects related to internal changes in the molecule. Thus, the results presented here demonstrate how minute changes in the chemical structure of a molecule can affect its photonic response. For this, two different schemes are considered: The first approach consists in modifying 'in-situ' a single free base phthalocyanine (H₂Pc) molecule. Here, the core of the molecule is modified by successively removing individual protons with the STM tip. The electrons remaining in the molecule after this procedure are localized in sigma-like molecular orbitals and generate an electric field that affects the STM-induced fluorescence properties of the molecule, a phenomenon known as the Stark effect. This 'internal' Stark effect generates a shift in the fluorescence line that follows a parabolic dependence with the number of remaining charges in the molecule. The vibronic emission lines also experience energy shifts which are well reproduced by DFT calculations, and which highlight a vibrational Stark effect. The second experimental approach is conducted on a series of Zinc-phthalocyanine molecules (ZnPc). Here, ZnPc derivatives containing an increasing number of isoindole units were simultaneously evaporated on our substrates. The study of the STM-induced luminescence of these different species shows very particular and systematic fluorescence changes, such as the lifting of degeneracy of excited states of the molecule and the appearance/disappearance of specific vibronic states. (author)