Langmuir and Langmuir-Blodgett films of perylene tetracarboxylic derivatives with varying alkyl chain length: Film packing and surface-enhanced fluorescence studies

The synthesis, film-packing properties in Langmuir and Langmuir-Blodgett (LB) films, and surface-enhanced fluorescence (SEF) of a new family of perylene tetracarboxylic derivatives (PTCD) are reported. The molecular organization on water subphase is extracted from surface pressure-area (pi -A) isotherms recorded under varying compression speeds and subphase temperatures. It was found that the pi -A isotherms are temperature dependent and the area per molecule decreases with increasing temperature of the subphase. The effect is more pronounced for molecules with longer lateral chains. The isotherm for these molecules can reach high pressures, indicating strong packing on the water surface. The presence of hysteresis with and without collapsing of the film indicates the formation of aggregates on the water surface. Further, the shift to smaller areas per molecule for successive compressions shows that the aggregates are stable. The pi -A isotherms were found to be independent of the compression speed. The area values for Langmuir films show that PTCD derivative molecules have similar arrangements on the water surface. From the extrapolated area per molecule values, it is concluded that the perylene moiety in these molecules is preferentially organized with the molecular axis perpendicular to the surface. Bis(n-pentylimido)perylene (5C-PTCD), a molecule with a shorter alkyl chain, presents a lower degree of molecular packing and vertical alignment. The Langmuir monolayers were transferred onto glass substrates with a transfer ratio of 1. The UV-visible spectra using polarized light show that the packing of PTCD molecules is maintained in Langmuir-Blodgett (LB) films. The distance dependence of SEF provides indirect evidence that the molecular alignment observed in Langmuir films is also kept in LB films. It was found that the SEF enhancement factor for a bis(n-pentadecylimido)perylene (IE;-PTCD) monolayer on silver islands is larger than that of 5C-PTCD molecules, a finding that further supports the head-on organization in LB films.