Sterically Induced Conformational Relaxation and Structure of <i>meso</i>-Diaryloctaalkyl Porphyrins in the Excited Triplet State:  Experimental and DFT Studies

The excited triplet state conformations of zinc and free base 5,15-diaryloctaalkylporhyrins are studied by experimental and computational (density functional theory, DFT) methods. From the observations of an unusual triplet state dynamics, i.e., fast nonradiative and biexponential decay, it has been suggested that these porphyrins exist in two distinguishable conformers in the lowest excited triplet state. X-ray crystallography and DFT (B3LYP/6-31G(d)) optimization of the ground state show that the porphyrins are planar prior to excitation. However, in the excited triplet state, the planar structure relaxes to an out-of-plane distorted saddle-shaped conformer. This distorted conformer and the lowest triplet potential energy surface are characterized by DFT calculations. It is suggested that the conformational relaxation explains the unusual triplet dynamics of this class of porphyrins.