J-aggregation of a sulfonated amphiphilic porphyrin at the air–water interface as a function of pH

π–A isotherms, ellipsometric measurements, Brewster angle microscopy (BAM) and reflection spectroscopy have been utilized to characterize the films of an amphiphilic porphyrin ((OD)3TPPS3) at the air–water interface as a function of pH. This porphyrin forms stable mono-molecular layers at such interfaces, and exhibits different J-aggregation as a function of pH. The J-aggregation of (OD)3TPPS3 on neutral pH subphases is notable considering that the nitrogen atoms at the central macrocycle have a pKa ≈ 4.9. The type of aggregates at neutral pH is like those detected at pH < 4, because the central porphyrin ring is already protonated. However at basic pH the aggregation happens without protonation of the central ring but can be instead controlled by application of the surface pressure. At the air–water interface, (OD)3TPPS3 shows two bands, a red component and a blue component, which have characteristics of non-degenerate linear oscillators being perpendicularly polarized between each other. The spectral behavior observed on subphases at different pHs is qualitatively interpreted by means of exciton coupling theory, assuming that the degenerate transitions attributed to the Soret band are split. Additionally, highly oriented molecular films of these J-aggregates were deposited onto transparent quartz slides.

The aggregation of (OD)3TPPS3 in MONOLAYERS at neutral and acid pHs is triggered by ZWITTERION formation, whereas in the basic subphase, aggregation is reversibly controlled by applying surface pressure.Figure optionsDownload high-quality image (43 K)Download as PowerPoint slideResearch highlights
► The amphiphilic porphyrin ((OD)3TPPS3) forms stable mono-molecular layers at the air–water interface under different conditions of pH.
► J-aggregation is triggered by electrostatic interactions and may be reversibly controlled as a function of pH.
► The splitting of the Soret band is also affected by the subphase pH.