THE EFFECT OF POLYELECTROLYTES ON THE AGGREGATION OF CYANINE DYES IN LANGMUIR-BLODGETT FILMS AND IN...
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Transcript of THE EFFECT OF POLYELECTROLYTES ON THE AGGREGATION OF CYANINE DYES IN LANGMUIR-BLODGETT FILMS AND IN...
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
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.
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