Poly(aryleneethynylene) Type Polymers Composed of p-Phenylene and 2,5-Thienylene Units. Analysis of Polymerization Conditions and Terminal Group in Relation to the Mechanism of the Polymerization and Chemical and Optical Properties of the Polymer

Studies of photoinduced electron transfer and energy migration in a conjugated polymer system for fluorescence "turn-on" chemosensor applications

A series of poly[p-(phenyleneethynylene)-alt-(thienyleneethynylene)] (PPETE) polymers with variable percent loadings of the N,N,N'-trimethylethylenediamino group on the polymer backbone were synthesized and fully characterized. Photophysical studies show that changes in the loading of the amino group receptor on the backbone do not affect the polymer electronic structure in either the ground or excited states. The fluorescence quantum yields were found to be directly related to the loading of the amino groups and can be modeled by a Stern-Volmer type relationship. Photophysical studies related the total quenching efficiency to the inherent rate of photoinduced electron transfer (PET), the lifetime of the exciton, the rate of excitation energy migration along the polymer backbone, and the total loading of the receptor on the polymer. The role of the loading dependence on the application of these polymers as fluorescence "turn-on" sensors for toxic metal cations in dilute solution was also studied. Results showed that the fluorescence enhancement upon binding various cations was maintained even when the amino receptor loading along the polymer backbone was reduced.

Intermolecular interactions in pi-stacked conjugated molecules. Synthesis, structure, and spectral characterization of alkyl bithiazole oligomers

Syntheses are reported of new 4,4'-dialkyl-2,2'-bithiazole oligomers that have alkenoxy side chains that are capable of easy conversion to oligomers with functionalized side chains, e.g., terminally substituted hydroxy chains. The crystal structures of two representative oligomers (4,4',4' ',4' "-tetra-(2-propenoxymethyl)-2,2',5',5' ',2' ',2' "-quaterthiazole (3P2) and 4,4',4' ',4' "-tetra-(3-hydroxypropyloxymethyl)-2,2',5',5' ',2' ',2' "-quaterthiazole (3H2)) were determined; 3P2 crystallizes in a pi-stacked motif with two molecules per unit cell, whereas 3H2 forms pi-stacks that are linked with hydrogen bonds to form infinite two-dimensional sheets with one molecule per unit cell. A comparison of the UV-vis spectra of the compounds in solution and in the solid state provides unequivocal evidence for the presence of a Davydov splitting, W(D) approximately 0.2 eV, in solid 3P2. The spectra are interpreted in the framework of molecular exciton theory to extract a value of the intermolecular transfer integral, J approximately 0.2 eV, for a total exciton bandwidth of ca. 0.8 eV. Monte Carlo calculations were used to determine the density of states of the exciton band and the absorption and emission line shapes of the 0 <-- 0 transition. It is suggested that the "three-humped" absorption profile typical of partially crystalline, regioregular polymers is the "optical signature" of pi-stacking.

Synthesis and characterization of regiorandom and regioregular poly(3-octylfuran)

Poly(3-octylfuran) has been synthesized with three regioregularities: P3OF-95, P3OF-75, and P3OF-50, where the number signifies the percentage HT content. The 95% HT material is highly crystalline with a structure similar to that of HT-poly(3-octylthiophene), P3OT. The lamellar spacing is 22.1 A and the pi-stacking distance is 3.81 A. UV-vis spectroscopy reveals that P3OF-95 is aggregated in CHCl(3) solution, and solid films of P3OF-95, but not P3OF-75 or -50, show Davydov and exciton band splitting due to the interactions of the pi-systems in the stacked morphology. An estimate of the Davydov splitting is 0.15 eV (1200 cm(-1)). P3OF is reversibly oxidized at 0.32 V vs ferrocene/ferrocenium, but increasing the potential to 1.15 V leads to irreversible oxidation. Films of P3OF may be p-doped with iodine vapor. Doped P3OF-95 and -75 films have electrical conductivities of 10(-2) and 10(-7) S/cm, respectively. The UV-vis-NIR spectra of the iodine-doped films are interpreted in terms of molecular-like transitions involving the LUMO, HOMO, HOMO-1, and transitions across a Peierls distortion-induced gap in the intermolecular conduction band that is formed by the overlap of the pi-systems of the stacked partially oxidized chains. The conduction band gap estimated for P3OF-95 is 0.34 eV, and that for P3OF-75 is 0.9 eV. The P3OF samples are thermally stable in N(2) atmosphere to between 275 degrees C (P3OF-50) and 380 degrees C (P3OF-95), but suffer thermal oxidation above 150 degrees C or light-induced oxidation at room temperature.