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 84720

ByHUSSEIN SAMHA

• Cyclic conjugationCyclic conjugation• Absorb in visible Absorb in visible

regionregion• Used as sensitizers Used as sensitizers

in many applicationsin many applications• High quantum yieldsHigh quantum yields• High extinction High extinction

coefficientscoefficients

1-ethyl-1’-octadecyl-2,2’-cyanine

NN

C2H5 C18H37

+

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

rK

nrK

AK

C* C + fluorescence

C

where Kr and Knr 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 R2VCl2 was used as the electron acceptor

Findings

• A slope of 1.9 x 1011 cm2.mol-1 is obtained. • Considering 10-10 s as an upper limit of the

excited state lifetime• A lower limit of 3.14 x 10-3 cm2.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 R2V2+

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 monolayerby R2V2+ 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