Interfacial behaviors and aggregate structure of atropisomers of “picket-fence” porphyrin at the air/water interface

We have synthesized four atropisomers of the pivalamido phenyl substituted porphyrins, α,α,α,β-PivPP (abbreviation of meso-α,α,α,β-tetra(o-pivalamidophenyl)porphyrin), α,β,α,β-PivPP, α,α,β,β-PivPP and α,α,α,α-PivPP. The latter one is usually called the “picket-fence” substituted porphyrin. The interfacial behaviors of the four compounds have been studied by using Langmuir–Blodgett (LB) technique, Ultraviolet–visible (UV–vis) and Atomic Force Microscopy (AFM). The obtained π–A isotherms show that the Langmuir monolayers of PivPPs are formed at the air/water interface except that of α,α,β,β-PivPP which exhibits a platform at the surface pressure in the range of 11–14 mN/m indicating a monolayer transition. The UV–vis spectra disclose that the red-shift happens in the porphyrin Soret bands of the four atropisomer LB films deposited on quartz slides compared with the corresponding absorption bands in the diluted chloroform solution, which results from the strong π–π interaction between porphyrin rings. Except for α,α,β,β-PivPP, AFM images also suggest that the LB films formed by atropisomers of the substituted porphyrin are homogeneous monolayer. Generally, film-forming molecules pack more tightly during compressing the mononlayers, which is consistent with the UV–vis spectra results that the absorption intensities of LB films enhance and the Soret band varies with the increase of surface pressure. On the other hand, AFM morphologies of α,α,β,β-PivPP at higher surface pressure reveal that bilayers are formed, which coincides with the presence of a platform in the π–A isotherm.

Graphical abstractThe graph shows the arrangement of four atropisomers of α,α,α,α-PivPP, (a) α,α,β,β-PivPP, (b) α,β,α,β-PivPP, (c) α,α,β,β-PivPP, and (d) α,α,α,α-PivPP; green ball means the trimethyl groups with the steric hindrance, while yellow ball without the steric hindrance.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ Four atropisomers of α,α,α,α-PivPP are synthesized and separated by a special method. ▶ The Langmuir monolayers of PivPPs are formed at the air/water interface except α,α,β,β-PivPP, which demonstrate the bilayer forms. ▶ α,α,β,β-PivPP monolayer is the most instable among the four atropisomers due to that four pivalamido groups of α,α,β,β-PivPP have less steric hindrance.