One-pot synthesis of core-modified rubyrin, octaphyrin, and dodecaphyrin: characterization and nonlinear optical properties

Abstract

Modified 26π rubyrin, 36π octaphyrin, and 54π dodecaphyrin systems have been synthesized in moderately good yields through acid-catalyzed condensations of terthiophene diols and tripyrranes. The product distributions are decided both by the acid catalyst concentration and by the nature of the meso substituents. For example, a new isomer of [26]hexaphyrin(1.1.1.1.0.0) (rubyrin) was obtained with 0.3 equiv. of p-toluenesulfonic acid, when the meso substituent was mesityl in at least one of the precursors. A change of the mesityl substituent for a p-methoxy substituent in terthiophene diol resulted in the formation of a [3+3+3+3] condensation product [54]dodecaphyrin(1.1.1.1.0.0.1.1.1.1.0.0) in addition to the expected rubyrin. Furthermore, an increase in the acid concentration to 0.6 equiv. resulted in the formation of a new [36]octaphyrin(1.1.1.1.1.1.0.0), in addition to the rubyrin and dodecaphyrin. A single-crystal X-ray analysis of octaphyrin represents the first example of a planar conformation of an octaphyrin with six meso links. In rubyrin 19, one thiophene ring, opposite to the terthiophene subunit, is inverted, while in octaphyrin 30 one pyrrole ring and two thiophene rings are inverted. The various conformational possibilities tested for the unsubstituted dodecaphyrin 28, at semiempirical level, suggest that the most stable conformation is a figure-eight. The final geometry optimization of figure-eight dodecaphyrin was done at the B3LYP/6-31G^∗ level of DFT. Octaphyrins and dodecaphyrins bind trifluoroacetate anion effectively in their diprotonated forms, the binding constants (K) being 638 M¹ for dodecaphyrin 28, and 415 M¹ for octaphyrin 30. Electrochemical data reveal HOMO destabilization with increasing p electron conjugation, consistently with the large red shifts of the absorption bands. Preliminary studies on the use of these expanded porphyrins as third-order NLO materials were followed by measurements of their two-photon absorption (TPA) cross-sections [σ⁽²⁾]. The σ⁽²⁾ values increase upon going from the 26π rubyrins to the 54π dodecaphyrins, confirming our earlier observation that increases in π-conjugated electrons increase the TPA values.