Photophysics of a butadiyne-linked porphyrin dimer:: Influence of conformational flexibility in the ground and first singlet excited state

The photophysics of a butadiyne-linked porphyrin dimer has been investigated by spectroscopy and quantum mechanical calculations. Primarily, the influence of conformation on the ground and first singlet excited states was studied, and two spectroscopically distinct limiting cases were identified. Experiments show that the twisted and planar conformers are separate spectroscopic species that can be selectively excited and have unique absorption and emission spectra. Calculated ground-state spectra compare well with experimental spectra of the two species. A spectrum of the planar conformer was obtained by the addition of a dipyridyl pyrrole ligand, which forms a 1:1 complex with the dimer and thus forces it to stay planar. The absorption spectrum of the twisted conformer could be deduced from the excitation spectrum of its emission. The interpretation of the ground-state spectrum of the free noncomplexed dimer is that it represents an average of a broad distribution of conformations. Calculations support this conclusion by indicating that the barrier for rotation is relatively small in the ground state (0.7 kcal/mol). Studies of the temperature dependence of the fluorescence spectrum of the dimer indicate a mother-daughter relationship between the twisted and planar conformations in the excited state, where the former has approximately 3.9 kcal/mol higher energy. Furthermore, time-correlated single-photon counting experiments also suggest that the twisted population adopts a planar configuration in the first singlet excited state with a rate constant of k(rot) = 8.8 x 10(9) s(-1) in 2-MTHF at room temperature. The temperature dependence of the fluorescence lifetimes indicated that an activation energy barrier of approximately 2 kcal/mol, in part related to solvent viscosity, is associated with this rate constant.