THE EFFECT OF POLYELECTROLYTES ON THE AGGREGATION OF CYANINE DYES IN LANGMUIR-BLODGETT FILMS AND IN AQUEOUS SOLUTIONS; SOME KINETIC ASPECTS OF J-AGGREGATES DEPARTMENT OF CHEMISTRY SOUTHERN UTAH UNIVERSITY Cedar City, UT By HUSSEIN SAMHA

Cyclic conjugationCyclic conjugation Absorb in visible regionAbsorb in visible region Used as sensitizers in many applicationsUsed as sensitizers in many applications High quantum yieldsHigh quantum yields High extinction coefficientsHigh extinction coefficients

1-ethyl-1’-octadecyl-2,2’-cyanine 1,1’-diethyl-2,2’-cyanine

Pressure-Area (  –A) isotherm of the dye on the surface of PVS aqueous solution (100 mg/liter)

Normalized absorption of the dye on the surface of water as a function of the surface pressure

Aggregate Formation Normalized absorption and emission of the dye in LB films

Dye Electron Acceptor

C* C + fluorescence C where K r and K nr are the radiative and nonradiative deactivation rate constants of C*. The dependence of cyanine dye fluorescence on electron acceptor concentration can be derived from above equations. Plot of (I  /I)-1 of the J-aggregates as a function of the concentration of the quencher in the adjacent layer gives a straight line. C* C + + A - C + A

Stern-Volmer kinetics: fluorescence quenching in two component, photosensitizer-electron acceptor system. Cyanine dye was used as the photosensitizer and dialkyl viologen derivative R 2 VCl 2 was used as the electron acceptor

Findings A slope of 1.9 x cm 2.mol -1 is obtained. Considering s as an upper limit of the excited state lifetime A lower limit of 3.14 x cm 2.molecule -1.s -1 for the electron-transfer rate constant is calculated.

Fluorescence Quenching in LB Films The fluorescence of the cyanine dye J- aggregates in a system containing an LB monolayer of the dye covered by an R 2 V 2+ containing fatty acid LB monolayer in head to head contact, is quenched by > 90% with a maximum concentration of the viologen derivative in the adjacent layer was 10 mol percent.

Fluorescence quenching of the J-aggregates in LB monolayer by R 2 V 2+ in the adjacent layer, concentration was 10 mol percent

Notes The pressure-area isotherm of LB film of the dye on the surface of PVS aqueous subphase (100 mg/liter) is identical with that performed on the surface of pure water. However, the area per molecule obtained on the surface of PVS subphase is significantly larger (about 55A) than that found when water is used in the subphase (about 45A). The use of PVS in the subphase enables the deposition of multilayer LB films of the dye by the vertical deposition technique. Only monomers (no aggregates) are detected in the films by the absorbance measurements performed on films on the surface of the subphase and on solid (glass) substrates. The absorbance of the monomers in the presence of PVS is 15 nm red shifted compared to the absorbance of the monomer in chloroform solution Ion-pair interaction between negatively charged sulfate head groups of PVS and the cationic dye molecules restricts the aggregation and cause the red shift in the absorbance.

Aggregation in Aqueous Solution Absorbance of Dye (1.76 x 10-3 mM) in aqueous solution

Absorbance of the dye as a function of PVS added

Adsorption isotherm of dye on PVS

Notes J-aggregates of the dye were formed upon addition of PVS to the monomers of the dye in solution. The appearance of only one isosbestic point in the UV- vis spectra suggests that the dye monomers are quantitatively converted to J-aggregates A 1:2 mole ratio of dye/PVS was calculated at the maximum capacity of the polymer.

Concluding Remarks J-aggregates formed in LB films on water Ion-pair with PVS from the subphase prevents the aggregation J-aggregate in the bulk of the PVS aqueous solution Super quenching of the fluorescence of the dye in LB monolayer