Tetraphenylporphyrin Molecules Containing Heteroatoms Other Than Nitrogen. 7. Emission and Electronic Structure of Rings Containing Sulfur and Selenium

We report on S and Se substituted tetraphenylporphyrins, where S and/or Se replace the central NH groups. S₂TPP shows fluorescence with λ_ma, = 705 nm, lifetime 1.3 ns, and quantum yield ϕ_f = 0.01. No phosphorescence is observed at 77 K in a solvent with 20% ethyl iodide. The acid dication of S₂TPP also fluoresces with λ_max = 766 nm, lifetime 0.49 ns, and quantum yield &ϕ_f = 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 λ_max = 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 SS < SSe < SeSe. The calculations raise the possibility that, while S₂TPP⁺ is a π radical, Se₂TPP⁺ may be either a π or a Se(σ*) radical.