Tautomerization in Porphycenes

Abstract

Porphyrins and metalloporphyrins are objects of intense studies in both basic and applied sciences. Owing to their crucial role in biological processes, such as photosynthesis, oxygen transport and activation, porphyrins have been labeled "Pigments of Life" [1]. The accuracy of this characterization is additionally confirmed by the use of porphyrins in medicine as phototherapeutic agents [2][3][4][5][6]. The potential of porphyrins as building blocks is enormous and includes, for example, artificial light-harvesting and photosynthetic systems [7-9], molecular memories [10,11], photovoltaic devices [12], and many other advanced materials [13][14][15][16][17][18]. The prospect of applications stimulated a rapid development of synthetic procedures that resulted in obtaining new classes of compounds, such as expanded, contracted, or inverted porphyrins [19,20]. Of particular interest in this area are constitutional isomers of porphyrin (also dubbed "reshuffled" porphyrins) [20], tetrapyrrole macrocycles which differ from the parent molecule in the way of linking the pyrrole rings by methine groups (Fig. 8.1). The research in this field started with the synthesis of porphycene in 1986 [21]. Since then, three more "nitrogen-in" isomers have been obtained: corrphycene [22], hemiporphycene [23,24], and isoporphycene [25,26]. Closely related to this class is an "inverted" ("N-confused") porphyrin [27,28].All these molecules have been shown to be planar and aromatic. They also exhibit electronic spectra characteristic for porphyrins, with the lower intensity Q transitions in the red part of the visible region followed by stronger Soret bands in the near-UV (Fig. 8.2). However, the relative intensities of the Q and Soret transitions differ significantly among the series. The lowest intensity of Q bands is observed in porphyrin and corrphycene. Hemiporphycene reveals stronger Q bands, and porphycene even stronger ones. This behavior can be rationalized upon inspecting the energies of the frontier p orbitals. The analysis of HOMO and LUMO splitting based on the perimeter model [29] has led to correct predictions not only for relative absorption intensities, but also for signs and patterns in magnetic circular dichroism. These predictions were experimentally confirmed for porphycene [30], corrphycene [31], and hemiporphycene [32].Hydrogen-Transfer Reactions. Edited by

Metal Complexes of Porphycene, Corrphycene, and Hemiporphycene: Stability and Coordination Chemistry

“…The magnitudes of the orbital splittings can be estimated qualitatively from the inspection of the shape of the orbitals of the parent, unperturbed perimeter, C 20 H 20 2 . [22] Appropriate B3LYP/6-31G(d,p) calculations yield DHO-MO values of 0.16, 0.06, and 0.03 eV for porphyrin, porphycene, and corrphycene, respectively. For the two possible trans-tautomers of hemiporphycene, values of 0.04 and 0.11 eV are obtained.…”

“…Synthesis: 2, 3,7,8,12,13,17, (Aldrich 97 %) was used as received to make the corresponding zinc(ii) (9), magnesium(ii) (13), nickel(ii) (17), and cobalt(ii) (21) complexes in accord with previously reported procedures. [1,26,74] 2, 3,6,7,12,13,16, and its zinc(ii) (10), nickel(ii) (18), and cobalt(ii) (22) complexes were synthesized as reported in the literature. [4,30,75] 2,3,6,7,11,12,17,18-Octaethylcorrphycene (7) [6] and 2,3,7,8,11,12,17,18-octaethylhemiporphycene (8) [8] were synthesized by using published procedures.…”

2 + 2 Can Make Nearly a Thousand! Comparison of Di- and Tetra-Meso-Alkyl-Substituted Porphycenes

“…The data accumulated so far strongly suggest a link between the photophysical and tautomeric properties; the latter have been extensively investigated for porphycenes in various environments, ranging from liquid and solid condensed phases, − supersonic jets, − and helium nanodroplets, to the regime of single molecules probed by fluorescence, − Raman, and scanning probe microscopies. − …”

Spectroscopy and Tautomerization Studies of Porphycenes

“…Finally, tautomerism of porphycenes is discussed. Even though some of these subjects have been partially covered previously, − the goal is to include the relevant literature on electronic spectroscopy and tautomerism published since Vogel et al’s first work on porphycene synthesis.…”