| Abstract
| - The photophysics of a butadiyne-linked porphyrin dimer has been investigated by spectroscopy and quantummechanical calculations. Primarily, the influence of conformation on the ground and first singlet excitedstates was studied, and two spectroscopically distinct limiting cases were identified. Experiments show thatthe twisted and planar conformers are separate spectroscopic species that can be selectively excited and haveunique absorption and emission spectra. Calculated ground-state spectra compare well with experimentalspectra of the two species. A spectrum of the planar conformer was obtained by the addition of a dipyridylpyrrole ligand, which forms a 1:1 complex with the dimer and thus forces it to stay planar. The absorptionspectrum of the twisted conformer could be deduced from the excitation spectrum of its emission. Theinterpretation of the ground-state spectrum of the free noncomplexed dimer is that it represents an average ofa broad distribution of conformations. Calculations support this conclusion by indicating that the barrier forrotation is relatively small in the ground state (0.7 kcal/mol). Studies of the temperature dependence of thefluorescence spectrum of the dimer indicate a mother−daughter relationship between the twisted and planarconformations 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 planarconfiguration in the first singlet excited state with a rate constant of krot = 8.8 × 109 s-1 in 2-MTHF at roomtemperature. The temperature dependence of the fluorescence lifetimes indicated that an activation energybarrier of approximately 2 kcal/mol, in part related to solvent viscosity, is associated with this rate constant.
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