Tunable Soret‐Band Splitting of an Amphiphilic Porphyrin by Surface Pressure

Abstract

Porphyrin aggregates, that is, Jand H-aggregates, have attracted much attention because of their peculiar structural and photophysical properties, which make them potential candidates for applications in nonlinear optics, nanometer-sized photoconductors, light-harvesting systems, and catalysis. Porphyrin aggregation can be detected through spectrum changes of the degenerated Soret band. For freebase porphyrins with D2h symmetry, the degeneracy is theoretically lost. However, the absorption is not detectably split and a sole increased bandwidth is apparent with respect to porphyrins with D4h symmetry. [2] Such splitting can be detected for densely packed porphyrins as a function of the molecular organization. Thus, the Soret band of freebase porphyrins can be described in terms of two mutually perpendicular oscillators (namely, Bx, By). Soret-band splitting of porphyrins has been described before. However, controlling this phenomenon is difficult because of the high tendency of porphyrins to autoaggregate. A tunable Soret-band splitting is described in this communication for the first time in the same experiment. A novel amphiphilic porphyrin, namely, (OD)3TPPS3 (see Figure 1)—with a selected balance between three meso-phenyl groups possessing hydrophilic sulfonato moieties and one meso-phenyl group bearing three long hydrophobic alkoxy chains—is synthesized. The used patterns are known to produce ordered array structures in porphyrins and fullerenes. Figure 1 shows the p–A isotherm of (OD)3TPPS3 as well as the ellipsotherm (dD–A isotherms) on the basic subphase (pH 11, NaOH). The monolayer stability is analyzed by means of area–time diagrams and compression–expansion cycles (see the Supporting Information). The analysis reveals the formation of a stable monomolecular layer of (OD)3TPPS3 at the air–water interface. Moreover, from the ellipsometric study (see the Supporting Information), we calculate a thickness of 3.17 nm (A= 0.9 nm), which corresponds to a monolayer of upright porphyrins with perpendicular alkyl chains at the air–water interface for such a full-packing phase. Reflection spectroscopy of monolayers can selectively detect molecules at the air–water interface because of the reflectionenhanced effect that originates from absorption. Figure 2 shows reflection spectra (DR) of monolayers of (OD)3TPPS3 on Figure 1. Molecular structure of 5,10,15-tris(4-sulfonatophenyl)-20-(3,4,5-trin-octadecyloxyphenyl)porphyrin (OD)3TPPS3. p–A isotherm (c) and ellipsotherm, dD–A (g) of a monolayer of (OD)3TPPS3 at the air–water interface with basic subphase (pH 11, NaOH).