23 Apr 05:50
by Shuya Yamada,
Takeshi Kaneda,
Philip Steib,
Kei Murakami,
Kenichiro Itami
Two become one: A palladium‐catalyzed dehydrogenative C2‐selective dimerization of pyridine is described. A variety of 2,2′‐bipyridyls can be prepared directly from unfunctionalized pyridine without additional pre‐halogenation or pre‐metalation steps. The reaction is applicable to a series of sterically hindered 3‐substituted pyridine derivatives and enables the concise synthesis of twisted 3,3′‐disubstituted‐2,2′‐bipyridyls, an underdeveloped class of ligands.
Abstract
2,2′‐Bipyridyls have been utilized as indispensable ligands in metal‐catalyzed reactions. The most streamlined approach for the synthesis of 2,2′‐bipyridyls is the dehydrogenative dimerization of unfunctionalized pyridine. Herein, we report on the palladium‐catalyzed dehydrogenative synthesis of 2,2′‐bipyridyl derivatives. The Pd catalysis effectively works with an AgI salt as the oxidant in the presence of pivalic acid. A variety of pyridines regioselectively react at the C2‐positions. This dimerization method is applicable for challenging substrates such as sterically hindered 3‐substituted pyridines, where the pyridines regioselectively react at the C2‐position. This reaction enables the concise synthesis of twisted 3,3′‐disubstituted‐2,2′‐bipyridyls as an underdeveloped class of ligands.
16 Jan 05:47
by Brendan Wright, Yukihiro Nakajima, Tracey M. Clarke, Kouichi Okuda, Heikki Paananen, Attila J. Mozer, Shogo Mori
A variety of charge extraction (CE) techniques have been developed to measure charge density and recombination coefficients in bulk heterojunction solar cells. Charge recombination during charge extraction as a major limitation of this method has not been systematically quantified. This study reports CE measurements using a newly designed fast switch, which enables the application of a reverse bias to the solar cells facilitating charge extraction. With applied reverse bias, more than 40% increase in the extracted charge is obtained in solar cells with thicker active layers or with fast recombination. The measured charge carrier lifetime increases by up to a factor of three at sufficiently high applied biases (up to 8 V), suggesting significant errors in CE measurements without applied bias. The increased extracted charges with increasing applied bias are attributed to a combination of three cases: (i) slightly faster charge extraction due to the larger electric field; (ii) increased charge extraction rate at high light intensities when the transients are space charge disturbed; (iii) increased charge separated lifetime during charge extraction attributed to the spatial separation of the electron and hole density due to the applied electric field.
Reverse bias during charge extraction is applied through a fast switch to bulk heterojunction solar cells. More than 40% increase in the extracted charge is seen and the recombination lifetime obtained from the relation between the extracted charge and switching delay time is increased up to three times, showing the importance of reverse bias to measure lifetime.
