22π smaragdyrin molecular conjugates with aromatic phenylacetylenes and ferrocenes: syntheses, electrochemical, and photonic properties

Abstract

Syntheses, spectroscopic, electrochemical, and third-order nonlinear optical susceptibilities of a series of 22π smaragdyrins and their corresponding Rh(I) derivatives bearing phenylacetylene substituents and ferrocene-containing substituents are reported. The synthetic strategy involved a [3 + 2] acid-catalyzed oxidative coupling reaction of the appropriate dipyrromethane and oxatripyrrane. The desired meso substituents, such as phenylacetylenylphenyl and the ferrocenes, were incorporated to the dipyrromethane unit prior to the oxidative coupling reaction. The optical absorption, emission characteristics, and the quantum yield of the smaragdyrin conjugates depends on the nature of the substituent, nature of linker group, and the spacer length. Theoretical studies at the DFT level suggest high delocalization of electrons confined to only four of the five available heterocyclic rings for the free bases. However, upon Rh(I) metalation, the π-electron delocalization is extended to all the heterocyclic rings. The two-photon absorption cross section (TPA) values σ⁽²⁾ measured through the open aperture Z-scan method, increases linearly with enhanced π-electron delocalization for the smaragdyrins containing phenylacetylene substituents. The meta branching of substituents decreases σ⁽²⁾ values. Introduction of Rh(I) to the smaragdyrin cavity enhances the σ⁽²⁾ values by about 3-10 orders of magnitude, attributed to the increased aromatic character upon Rh(I) insertion. The calculated molecular electrostatic potential (MESP) and harmonic oscillator model of aromaticity (HOMA) for the free bases and the Rh(I) derivatives justifies such a conclusion. A linear correlation observed for the second oxidation potential of Rh(I) derivatives and corresponding free bases also support the increased aromaticity upon Rh(I) insertion. The electrochemical data for ferrocene-containing smaragdyrins reveal easier ring oxidation by about 50-130 mV and harder ferrocene oxidation by 40-180 mV suggesting electron-donating nature of the ferrocene upon linking with the smaragdyrin π system. The TPA cross section value of 88782 GM observed for 5g represents one of the highest values known for a metalloexpanded porphyrin derivative.