Jan K. Herrema

The genetically well-known strain Streptomyces coelicolor A3(2) produces the pH indicator (red/blue) antibiotic actinorhodin, but not all the "blue pigment" produced by this strain is actinorhodin. When the organism was subjected to various nutrient limitations (ammonium, nitrate, phosphate, or trace elements), and also during growth cessation caused by a relatively low medium pH, blue pigment production was initiated but the pigment and its location varied. At pH 4.5 to 5.5, significant formation of actinorhodin occurred and was located exclusively intracellularly. At pH 6.0 to 7.5 a different blue pigment was produced intracellularly as well as extracellularly. It was purified and identified as gamma-actinorhodin (the lactone form of actinorhodin). Analysis of act mutants of S. coelicolor A3(2) confirmed that both pigments are derived from the act biosynthetic pathway. Mutants with lesions in actII-ORF2, actII-ORF3, or actVA-ORF1, previously implicated or suggested to be involved in actinorhodin export, were impaired in production of gamma-actinorhodin, suggesting that synthesis of gamma-actinorhodin from actinorhodin is coupled to its export from the cell. However, effects on the level of actinorhodin production were also found in some mutants.

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.

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.