Richard E. Gill

Color tuning of luminescence via molecular engineering of the active polymer is important for the commercial application of π‐conjugated polymers in photonic devices. The synthesis of a series of regiospecific alkylated polythiophenes is described, in which the effective conjugation length could be varied by altering the length of the coplanar blocks between head‐to‐head dyads. It is demonstrated that the color‐tuned materials obtained by this novel approach are potentially suitable for applicationin displays and other devices.

The combination of wide‐ and narrow‐band‐gap polymers in a block copolymer can provide a means of increasing the luminescence efficiency of the materials. This occurs because the excitons that are formed are trapped within specific units in the polymer chains and therefore cannot migrate to quenching sites caused by the presence of dopants. Color‐tunability is also possible through adjusting the band gaps of the components. Devices based on silanylene'thiophene lock copolymers are reproted.

Synthetic routes to alternating copolymers consisting of oligosilylene blocks and oligothiophene blocks (T-x; x = 1, 2, 3, 4, or 6 rings) are presented. Solubility requirements for obtaining acceptable molecular weights and, eventually, for film formation are met by the introduction of butyl groups replacing methyls on the silicon atoms and by employing T-6 blocks carrying two octyl substituents. Additionally, substituted oligothiophenes are synthesized as an aid in the interpretation of NMR, absorption, and fluorescence spectra. Regarding the electronic configuration of the oligothiophene blocks, NMR spectra show clear differences between plain oligothiophenes, end-substituted oligothiophenes, and polymers, indicative of pi-sigma interactions with the oligosilylene blocks and possible through-conjugation to adjacent blocks in polymers. Red shifts in optical spectra show a parallel trend across the various compounds based on the same oligothiophene unit, related to the stabilization of photoexcited states on the oligothiophene by the oligosilylene substituents. These effects are strong in T-2-based compounds and reduced fdr longer T-n. The main feature of the spectra is the decrease of the transition energies with the size of the oligothiophene blocks in the polymers. Since this effect is also found in fluorescence, it enables one to adjust the luminescence wavelength by choosing the proper block length (''chemical tuning''). Fluorescence quantum efficiencies in solution are found to be remarkably high in polymers based on T-2 blocks. Spin-coated films of T-2-based (or T-3-based) polymers show evidence of T-4 (T-6) impurity blocks that act as an exciton trap.

Conjugated polymers, for example PPV and polythiophene, show great potential for applications in thin‐film microelectronic and optoelectronic devices. An understanding of structure–property relationships in these materials is therefore of great importance. Here, the successful preparation of two novel substituted five‐ring oligo‐PPVs and the first crystallographic characterization of a single‐crystal all‐ trans PPV oligomer are reported.

Two approaches toward control of the luminescence wavelength of polythiophenes have been explored: (i) block copolymers in which oligothiophene blocks alternate with oligosilanylene blocks and (ii) regioregular polythiophenes in which oligothiophene sequences are delimited by n-octyl substituents placed in a ''head-to-head'' fashion on adjacent rings. Both methods aim at restricting the pi-conjugation to the oligothiophene sequences. The block copolymer approach is very effective, whereas the (solution) luminescence spectra of the regioregular polymers are strongly red-shifted with respect to absorption and confined to a narrow range of wavelengths. This is due to the quinoid character of the excited singlet state, in which there is a strong electronic driving force toward coplanarity of adjacent thiophene rings, which offsets the steric hindrance of the octyl substituents and increases the size of the conjugating pi-system. This explanation is supported by calculations and by spectral data of substituted bithiophenes.