Landscape Evolution and Human Settlement in the Iroise Sea (Brittany, France) during the Neolithic and Bronze Age

International audienceThe Molène archipelago appears to be particularly rich in Neolithic and Bronze Age remains and an exceptional concentration of megaliths has been brought to light. Several settlements are confirmed by dry-stone structures or by shell middens. These data give precious indications on the occupation chronology of the area. Moreover they allow us, for the first time in Brittany, to reconstruct everyday life during the late Prehistoric period.A prerequisite to this reconstruction was a better understanding of the evolution of the environment during this period, which locally implies a better knowledge of paleogeographic changes related to Holocene sea-level rise as well as on floral and faunal resources. The results obtained through paleogeographic reconstructions show that the archipelago since 4500 BC was already disconnected from the mainland. The megalithic monuments must therefore have been erected and used by islanders present on the archipelago from the middle of the 5th to the 2nd millennium BC. The distribution of the megalithic tombs reveals landscape occupation strategies which respond to both cultural choices and natural constraints. Throughout the entire period, geographic isolation has continued to increase, although it did not imply strong cultural specificities. Nevertheless, the increasing remoteness of the islands has fostered the search for livelihoods based on the intense exploitation of coastal resources. Despite their focus on the sea, these people did not neglect what inland areas could offer as evidenced by the early agro-pastoral practices in the archipelago

Substituent Effects in Pentacenes: Gaining Control over HOMO−LUMO Gaps and Photooxidative Resistances

A combined experimental and computational study of a series of substituted pentacenes including halogenated, phenylated, silylethynylated and thiolated derivatives is presented. Experimental studies include the synthesis and characterization of six new and six known pentacene derivatives and a kinetic study of each derivative under identical photooxiclative conditions. Structures, HOMO-LUMO energies and associated gaps were calculated at the B3LYP/6-311+G**//PM3 level while optical and electrochemical HOMO-LUMO gaps were measured experimentally. The combined results provide for the first time a quantitative assessment of HOMO-LUMO gaps and photooxiclative resistances for a large series of pentacene derivatives as a function of substituents. The persistence of each pentacene derivative is impacted by a combination of steric resistance and electronic effects as well as the positional location of each substituent. Silylethynyl-substituted pentacenes like TIPS-pentacene possess small HOMO-LUMO gaps but are not the longest lived species under photooxiclative conditions, contrary to popular perception. A pentacene derivative with both chlorine substituents in the 2,3,9,10 positions and o-alkylphenyl substituents in the 6,13 positions is longer lived than TIPS-pentacene. Of all the derivatives studied, alkylthio- and arylthio-substituted pentacenes are most resistant to photooxidation, possess relatively small HOMO-LUMO gaps and are highly soluble in a variety of organic solvents. These results have broad implications for the field of organic molecular electronics where OFET, OLED, and other applications can benefit from highly persistent, solution processable pentacene derivatives