No abstract

The photophysical properties and their temperature dependence are reported for the sterically encumbered nonplanar zinc and free base 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrins (ZnDPP and H2DPP), and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrins (ZnOETPP and H2OETPP), and the zinc complex of 5,10,15,20-tetra-tert-butylporphyrin (ZnT(t-Bu)P). Compared to planar 5,10,15,20-tetraphenylporphyrins (ZnTPP and H2TPP), the above compounds exhibit reduced lifetimes of the lowest excited singlet state, reduced fluorescence yields, and large shifts between their absorption and emission maxima at room temperature. ZnT(t-Bu)P, which is known to adopt a ruffled conformation, displays dramatically altered photophysical properties including a 7 ps 1(π,π*) lifetime compared to one of ∼2 ns for ZnTPP at 296 K. Equally noteworthy is the return of the ZnT(t-Bu)P singlet lifetime to a “normal” value of 2.5 ns at 78 K. An analogous temperature dependence has been observed previously for the free base analog H2T(t-Bu)P. The other porphyrins investigated, with different modes of nonplanarity, display smaller temperature variations but also tend toward more normal properties at low temperatures. A more extreme case of perturbation to the tetrapyrrole electronic structure is found in zinc 2,3,5,5‘,7,8,12,18-octamethyl-13,17-bis(3-methoxy-3-oxopropyl)isoporphyrin perchlorate, a porphyrin tautomer with an interrupted π system. This zinc isoporphyrin also exhibits a short excited state lifetime of 130 ps at 296 K, which again increases to 0.8 ns at 78 K. The results for the various nonplanar porphyrins and for the isoporphyrin in several solvents indicate that the principal cause of the altered excited state lifetimes is the ability of the molecules to traverse multiple conformational surfaces in the excited state. These surfaces appear to be separated by only small energy barriers that vary with the types of conformational distortions and their concomitant perturbations of the electronic structures of the chromophores.

With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E(1/2) for the second oxidation varies considerably with the type of supporting electrolyte. E(1/2) values for the first reduction to give a porphyrin pi-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a pi-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (Delta/Ox(2) - Ox(1)/) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of Delta/Ox(2) - Ox(1)/ is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO(4)(-) or PF(6)(-)) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating Delta/Ox(2) - Ox(1)/ and thus the binding affinity of the anion to the porphyrin dication. One is the type of pi-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.