Molecular reorganization in Langmuir-Blodgett films of mesogenic Zn-porphyrin octaesters

The role of chain length in affecting the molecular organization in thin films of mesogenic Zn-porphyrin octaesters ZnC(n)OAP has been investigated by means of Langmuir isotherms and area-per-molecule relaxation measurements at the air-water interface, as well as by unpolarized and polarized optical absorption spectroscopy and polarized ATR-FTIR spectroscopy after transfer onto the solid substrate. The results clearly identify three different classes of behavior depending on the chain length. Intermediate chain length molecules (ZnC(n)OAP, where n = 6-12) feature clear coexistence regions in the isotherms as well as marked molecular relaxation due to self-organization effects in the coexistence region. Such behavior is absent in the short-chain derivative ZnC(3)OAP and strongly reduced in the longest side chain derivative ZnC(14)OAP, reflecting the tendency of intermediate chain length porphyrins to form columnar mesophases in the bulk. These data are confirmed by the time evolution of the optical absorption spectra for mono- and multilayers transferred at surface pressures corresponding to different zones of the compression isotherm. The role of the chain length is also evident in the optical absorption spectra of transferred layers as a modulation of the blue shift of the Soret band connected with the formation of molecular stacks. Molecular anisotropy has been investigated by polarized optical (Soret band) and infrared (C-H stretching bands) spectroscopy, finding tilting angles of 40-45 degrees for the macrocycles in transferred layers and evidence of quantum confinement effects.