title: Bridging the gap between porphyrins and porphycenes: substituent-position-sensitive tautomerism and photophysics in meso-diphenyloctaethylporphyrins.
abstract: 2,3,7,8,12,13,17,18-Octaethyl-5,15-diphenylporphyrin (1) is characterized by an inner cavity with a rectangular shape and small NH⋅⋅⋅N distances. It resembles porphycene, which is a constitutional isomer of porphyrin known for its strong intramolecular hydrogen bonds and rapid tautomerization. Such distortion of the porphyrin cavity leads to tautomeric properties of 1 that are intermediate between those of porphyrin and porphycene. In particular, a tautomerization in the lowest excited singlet state of 1 has been discovered, occurring with a rate three orders of magnitude lower than that in porphycene, but three to four orders of magnitude higher than that in porphyrin. An isomer of 1, 2,3,7,8,12,13,17,18-octaethyl-5,10-diphenylporphyrin (2), exhibits a different kind of geometry distortion. This molecule is nonplanar, but the inner cavity shape and dimensions are similar to those of the parent porphyrin. The same hydrogen-bonding strength as that in porphyrin is observed for 2. In contrast, the nonplanarity of 2 significantly influences the photophysics, leading to a decrease in fluorescence quantum yield and lifetime. Absorption, magnetic circular dichroism, and fluorescence spectra are similar for 1 and 2 and resemble those of parent porphyrin. This is a consequence of comparable energy splittings of the frontier orbitals, ΔHOMO≈ΔLUMO. The results demonstrate that judicious selection of substituents and their position enables a controlled modification of geometry, hydrogen-bonding strength, tautomerization rate, and photophysical and spectral parameters of porphyrinoids.
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01-224 Warsaw, Poland.
authors.forename: Sylwester
authors.initials: S
authors.lastname: Gawinkowski
authors.forename: Grażyna
authors.initials: G
authors.lastname: Orzanowska
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Izdebska
authors.forename: Mathias O
authors.initials: MO
authors.lastname: Senge
authors.forename: Jacek
authors.initials: J
authors.lastname: Waluk
mesh_terms: D006860:Hydrogen Bonding; D055668:Photochemical Processes; D011166:Porphyrins / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: C568726:1, 2,3,7,8,12,13,17,18-octaethyl-5,10-diphenylporphyrin; C568725:2,3,7,8,12,13,17,18-Octaethyl-5,15-diphenylporphyrin; D011166:Porphyrins; C051407:porphycene