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Charge-transfer character of excitons in
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Image of FIG. 1.
FIG. 1.

(a) Absorption spectra of copolymers in dilute solution (100 mg/l in chloroform). The inset above the graph shows chemical structures of F8 and BT units and repeat unit of copolymer. (b) Absorption spectra of copolymers in the optimized geometry determined by quantum-chemical calculations. All absorption spectra have been normalized to the peak of the higher energy absorption band at .

Image of FIG. 2.
FIG. 2.

Schematic electronic structure of a copolymer with 33.3% BT units showing the existence of two distinct bands for the unoccupied orbitals. The shapes of the HOMO (bottom), LUMO (middle), and LUMO (top) levels are also shown. The surface represents an isovalue plot of the electronic density (for ) visualized in GAUSVIEW. The color (black/white) represents the sign of the wave function. The color and size of the spheres represent the sign and amplitude of the LCAO (linear combination of atomic orbitals) coefficients, respectively.

Image of FIG. 3.
FIG. 3.

(a) HOMO and LUMO energy levels of F8 and copolymers measured by cyclic voltammetry. (b) HOMO and LUMO energy levels calculated in the optimized geometry. Values for F8 were obtained from literature (Ref. 13) for oligomers with five fluorene units.

Image of FIG. 4.
FIG. 4.

(a) Normalized PL spectra and (b) PL efficiencies of F8 and copolymers (6.0%–49.7% BT) in dilute solution (15 mg/l in chloroform) after excitation at 325 nm. (c) Normalized intensity of emission from F8-BT species measured at 550 nm after excitation at 470 nm for copolymers (8.9%–49.7% BT) in dilute solution (10 mg/l in chloroform and o-xylene) measured by TCSPC. All curves can be fitted with a single exponential, confirming that only one emissive species contributes to the emission at 550 nm. No dependence of the lifetime on the detection wavelength was found over the range of 480–680 nm. Excitation of the F8 species at 407 nm also did not change the lifetime of the emission from the F8-BT species since energy transfer from F8 to F8-BT occurs very quickly. The instrument response function is shown in gray.

Image of FIG. 5.
FIG. 5.

Normalized PL spectra of copolymers (8.0% and 49.7% BT) in dilute solutions (10 mg/l in chloroform and o-xylene) after excitation at 407 nm.


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Table I.

BT content, molecular weight, and polydispersity of copolymers used in the experimental part of this study. Copolymer compositions were determined by elemental analysis using the nitrogen content as standard.

Generic image for table
Table II.

BT content and structure of the model oligomers generated for the theoretical simulations.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Charge-transfer character of excitons in poly[2,7-(9,9-di-n-octylfluorene)(1−x)-co-4,7-(2,1,3-benzothiadiazole)(x)]