Tetraphenylporphyrin dimers : an optical and magnetic resonance study

L. Benthem

Research output: Thesisinternal PhD, WU

Abstract

Tetraphenylporphyrin (TPP) molecules have been linked together to form dimers, using two positions of the phenyl groups at which the linking chain, which consisted mostly of 5 atoms, is attached (ortho or para position). The resulting dimers have different relative orientations of the porphyrin macrocycles w.r.t. each other and different centre-to-centre distances. Using standard procedures, metal ions have been inserted into the porphyrin ring; depending on the type of experiment different metal ions have been chosen. This thesis describes the effect of varying the geometry of these dimers on singlet-singlet energy transfer, excitonic- and spin-spin interaction and triplet state parameters.Using the optically detected magnetic resonance technique and the fluorescence fading method, the zero field splitting parameters (D and E) and the kinetic constants of the lowest excited triplet state have been determined for homo-dimers of the free base (H 2 TPP) 2 , of the zinc- compound (ZnTPP) 2 and for mixed dimers. These triplet state parameters can be compared with the results for both monomers. It is found that for the homodimers the results are the same as for the corresponding monomers. Also, no singlet-singlet energy transfer is observed, due to a very small overlap of the fluorescence and absorption spectra. For the mixed dimers we find the same triplet state parameters as for H 2 TPP, due to efficient singlet-singlet energy transfer (vide infra). From our results it can be concluded that the triplet states of both macrocycles in a TPP dimer can be considered as isolated from each other. This is not unexpected, since the triplet excitonic interaction is predicted to be small, due to the small S 0 -T 0 oscillator strength. Thus triplet-triplet energy transfer is unlikely to occur. Triplet excitonic interactions are expected to be observable by magnetic resonance techniques for much shorter distances between both porphyrin rings, than occur in our dimers.The average distance between the centres of both porphyrin macrocycles can be determined, using the spin-spin interaction between two paramagnetic ions, inserted into each of the porphyrin rings. This interaction is observable as a broadening in the solid state EPR powder spectra of Cu-Cu dimers. From these measurements the distances between ring centres in apolar solvents can be determined as: para-para (p,p) dimer 2.7, ortho-para (o,p) dimer 2.2 and ortho-ortho (o,o) dimer 2.1 nm. In polar solvents the relatively apolar porphyrin macrocycles are forced together, resulting in a shorter ring-ring distance. Steric hindrance of the phenyl groups can prevent this folding, however, for some of the dimers. The results obtained by this method are in good agreement with the distances derived from space filling models. It can also be concluded that an endoconformation (cofacial) for the o,o dimer is highly unlikely and that the preferential orientation is the exo- conformation, a situation very similar to the p,p dimer. These results are also confirmed by NMR measurements.We have performed a theoretical analysis of excitonic interaction on the Soret absorptionband. From this analysis, taking into account the presence of proton tautomers in free base porphyrin monomers and -dimers, the energy level shifts are determined as a function of the rotational angle around the linking chain. Comparing the theoretical and experimental data, the preferential orientation, determined from the shift and/or splitting of the absorption band, and the rotational angular distribution, determined by the bandwidth w.r.t. that of the monomer, can be found. For the p,p dimer the porphyrin macrocycles are approximately coplanar with a narrow orientational distribution; the o,o dimer has an exo-conformation and also coplanar macrocycles with a very narrow angular distribution. For the o,p dimer the result is an average perpendicular position of the porphyrin rings, but with a large rotational freedom. Because of steric effects of the bulky phenyl groups the spread in rotation does not extend over the full 2πrange.In covalently linked metal porphyrins, containing two different metal ions or consisting of a mixed metallo-free base porphyrin, singlet- singlet energy transfer occurs. We have analyzed S-S energy transfer between the zinc-containing and free base moieties in TPP dimers by emission spectroscopy and fluorescence kinetics. From fluorescence and fluorescence-excitation spectra the energy transfer efficiency, the rate of energy transfer, the lifetime of the donor fluorescence and the donor- acceptor distance can be calculated and compared with the results from fluorescence decay experiments. Both sets of values are in very good agreement. The donor-acceptor distance is also in agreement with the distance found by other methods. For the dimers the energy transfer rate constant is ≈2 magnitudes higher than that for intersystem crossing and the fluorescence decay, corresponding to an energy transfer efficiency of ≈85%. The energy transfer rate constant is dependent on <r -6 >, as well as on the relative orientation of the transition dipole moments. Due to the perpendicular position of both porphyrin macrocycles in the o,p dimer the energy transfer rate constant is lower than in the p,p dimer, although the distance between both rings is much shorter.From all measurements, it can be concluded that the properties of the singlet and triplet states of the o,o dimer, are very similar to those of the p,p dimer, apart from the distance effect. The o,p dimer has in some cases a different behaviour, due to the different position and orientation of the porphyrin macrocycles w.r.t. each other in comparison with the other two dimers.The appendices contain the mathematical formalism for the fluorescence fading method (appendix 1) and triplet decay in the presence of triplet- triplet energy tranfer (appendix 2) are presented.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Schaafsma, T.J., Promotor
Award date12 Oct 1984
Place of PublicationWageningen
Publisher
Publication statusPublished - 12 Oct 1984

Keywords

  • porphyrins
  • nuclear magnetic resonance

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