Synthesis and Optical Features of Axially and Peripherally Substituted Subporphyrins. A Paradigmatic Example of Charge Transfer versus Exciplex States

We report here the synthesis of two novel subporphyrins (SubPs), in which the macrocycle has been functionalized at its meso (1) or axial (2) position with tetracyanobuta-1,3-diene (TCBD)–aniline. In-depth spectroscopic, spectrometric, and electrochemical analyses were carried out with both of them, whose molecular structures were determined by single-crystal X-ray diffraction studies. In the case of 2, its Ra and Sa enantiomers were separable by chiral HPLC and presented a fairly good configurational stability at room temperature, which enabled determining the activation parameters for the thermally induced racemization. Conversely, the enantiomers’ separation was unfeasible for 1 due to the conformational and/or configurational dynamics of the TCBD–aniline, a structural “flexibility” that could be drastically reduced at low temperatures. The physicochemical impact of placing the TCBD–aniline at either the axial or peripheral positions of SubPs is also rather significant. The HOMO–LUMO gap is reduced by as much as 0.35 eV in SubP–(TCBD–aniline)meso 1 (1.77 eV) and, in turn, enables an emissive charge-transfer (CT) state in virtually all environments. It is only in polar environments, where it links a local excitation with an indirect charge separation. In contrast, a much larger HOMO–LUMO gap of 2.12 eV in SubP–(TCBD–aniline)axial 2 disables an emissive CT state and enforces either an exciplex deactivation in apolar environments or a direct charge separation in polar environments.