We report on S and Se substituted tetraphenylporphyrins, where S and/or Se replace the central NH groups. S2TPP shows fluorescence with λma, = 705 nm, lifetime 1.3 ns, and quantum yield ϕf = 0.01. No phosphorescence is observed at 77 K in a solvent with 20% ethyl iodide. The acid dication of S2TPP also fluoresces with λmax = 766 nm, lifetime 0.49 ns, and quantum yield &ϕf = 0.11. However, SSeTPP and Se2TPP show no emission nor do their acid dications. Iterative extended Hückel (IEH) calculations predict an allowed charge-transfer (CT) transition Se(σ*) → π* at much lower energy than the analogous S(σ*) → π*. The lack of fluorescence is attributed to this transition occurring at lower energy than the π → π*. Curve fitting of the lowest energy optical absorption bands in H2TPP, S2TPP, SSeTPP, and Se2TPP reveals that the Se-containing compounds have an extra band in the long-wavelength tail with λmax = 715 nm (Se2TPP) and 705 nm (SSeTPP) that we attribute to the charge-transfer transition. Analysis of the IEH bond orders shows that contracted outer-valence d orbitals must be included to explain the cross-ring bonding, which increases in the order SS < SSe < SeSe. The calculations raise the possibility that, while S2TPP+ is a π radical, Se2TPP+ may be either a π or a Se(σ*) radical.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry