CHARGE-TRANSFER INTERACTIONS IN MONOLAYERS AT THE AIR-WATER-INTERFACE

We have investigated the charge-transfer (CT) interactions between an amphiphilic cyanine dye N,N'-dioctadecyloxacyanine perchlorate (S9) as donor and octadecyltetracyanoquinodimethane (C18TCNQ) as acceptor components in mixed monolayers at the air water interface. UV-vis surface reflection and fluorescence along with the conventional surface pressure- and surface potential-area isotherms and spectral measurements were used for the investigation of the CT complexation process. The reflection spectrum of the mixed monolayer of S9 and C18TCNQ, molar ratio 1:1, shows a new broad band with a maximum at 490 nm, which we have ascribed to the CT transition between S9 and C18TCNQ. The CT band shows bathochromic shifts with increasing surface pressure. Progressive dilution of the 1:1:0 monolayer (S9/C18TCNQ/DMPC) with matrix molecule DMPC leads to a decrease in the CT band intenity until at 1:1:5 molar ratio of the components no CT band is seen. Upon direct excitation of the CT band at 480 nm or via excitation of the donor at 365 nm, a broad band fluorescence emission with a maximum at 560 nm is observed. Evidence for Forster type energy transfer between S9 and the CT complex was found. The excitation spectrum of the CT fluorescence band resembles closely the reflection spectrum. The fluorescence quantum yield was found to increase with decreasing temperature. The normalized (at constant surface density) fluorescence intensity-area isotherms show that the CT band intensity increases continuously with increasing surface pressure, whereas the donor fluorescence intensity decreases sharply above 25 mN/m. The results are discussed in terms of the theoretical model of CT interactions developed by Mulliken.