Synthesis of Polymers with Isolated Thiophene-Based Chromophores

Synthesis and characterization of the ground and excited states of tripodal-like oligothienyl-imidazoles

Six new thiophene oligomers, here designated as tripodal-like oligothienyl-imidazoles, were synthesized and have been investigated in ethanol solution at room and low temperature. The oligomers bear a common core where two or more thiophenes are linked to one or more imidazole units that further links through its alpha-position to a different number of incremental thiophene units. The study involves a comprehensive spectral and photophysical investigation where the properties of the singlet and triplet states have been investigated regarding absorption, fluorescence and phosphorescence, transient triplet-triplet absorption together with all relevant quantum yields (fluorescence, phi(F), internal conversion, phi(IC), intersystem crossing, phi(T,) and singlet oxygen, phi(Delta)) and lifetimes. In addition, density functional theory quantum chemical calculations were performed to gain a detailed understanding of the molecular geometry and optical properties of the investigated oligomers. From the overall data, the radiative (k(F)) and radiationless (k(NR), k(IC), and k(ISC)) rate constants have been determined and it is shown that, in contrast with the parent oligothiophenes, the radiative competes with the radiationless deactivation channels. The results show that, by comparison with the oligothiophene counterparts, there is an augment of the relative contributions of the internal conversion and fluorescence processes relative to the S(1)~~-->T(1) intersystem crossing. Phosphosphorescence emission was found for the simplest member of the investigated compounds with a low quantum yield (phi(Ph) = 0.009) and a lifetime of 8 micros. The data also show that the introduction of a 4,5-dithienyl-imidazole moiety in a bi- or terthiophene oligomer results in, respectively, a 20-fold and a 3-fold increase of the fluorescence quantum yield relative to their oligothiophene counterparts. The synergy of the structural and photophysical properties, combined with the exceptional thermal stabilities, opens new perspectives related to the copolymerization of oligothiophenes with thienyl-imidazole units with potential application as organic light emitting devices.

Synthesis and characterization of novel optically active polyarylene vinylenes with controlled effective conjugation length

New chiral and soluble binaphthyl derivatives (12 and 13) endowed with carboxaldehyde or cyanomethyl functional groups have been prepared as suitable building blocks for the synthesis by Knoevenagel condensation of a series of optically active block copolymers (1-7) with controlled effective conjugation length. A variety of functionalized co-monomers (14-19) have been prepared by different synthetic procedures to be used in further polymerization reactions with binaphthyl derivatives 12 and 13. Depending upon the nature of the co-monomers, it is possible to tune the wavelength of the new polymers, which is very close to that of the respective repeating units. Fluorescence measurements on polymers 1-3 reveal a strong blue-green emission with Stokes shifts of 74-107 nm. Theoretical calculations at the semiempirical AM-1 level have been carried out on model compounds, and the calculated torsion angles are in agreement with the electronic spectra data. Finally, the redox properties of the polymers prepared (1-7) were determined by cyclic voltammetry, and an amphoteric behavior with oxidation potentials ranging from 1.1 to 1.6 V and reduction potentials close to -1.5 V was found.