A theoretical study on the low-lying excited states of 2,2':5',2'-terthiophene and 2,2':5',2':5',2'-quaterthiophene.

The nature and properties of the low-lying singlet and triplet valence excited states of 2,2':5',2'-terthiophene (terthiophene) and 2,2':5',2':5',2'-quaterthiophene (tetrathiophene) are discussed on the basis of high-level ab initio computations. The spectroscopic features determined experimentally for short alpha-oligothiophenes are rationalised on theoretical grounds. Special attention is devoted to the nonradiative decay process through intersystem crossing (ISC) from the singlet to the triplet manifold, which is known to be relatively less efficient in tetrathiophene. Along the geometry relaxation of the S1 state of terthiophene, the S1 and T2 states become degenerate, which leads to a favourable situation for the occurrence of ISC. The parallel process is expected to be less favoured in tetrathiophene because of the less efficient spin-orbit coupling and the increase of the S1-T2 energy gap.     

Phosphorylation of 3',4',5,7-Tetramethyldihydroquercetin

3',4',5,7-Tetramethyldihydroquercetin was phosphorylated with phosphorous acid derivatives, and similar results were obtained with phosphorochloridites and phosphoramidites. The PIII derivatives obtained were subjected to oxidation. The structures of the products were determined by NMR spectroscopy.     

Synthesis of thiopyrano[4″,3″:4′,5′]pyrido[3′,2′:4,5]furo[3,2-d]pyrimidines

<正>Reactions of the 6-hydroxy-thiopyrano[3,4-c]pyridine-5-carbonitrile derivative 1 withα-halo-carbonyl compounds gave the ortho-substituted intermediates 2a-c which were converted into furo[2,3-b]thiopyrano[4,3-d]pyridines 3a-c by fusion of a furan moiety under basic conditions.Further cyclization of 3a-c led to a fusion of a pyrimidine ring,yielding the tetracyclic products 6,7 and 8.In addition,condensation of 6 with various aromatic aldehydes afforded the corresponding imines 9a,b.Mannich reaction of 7 gave products 10a,b.     

Hydrolytic reactions of guanosyl-(3',3')-uridine and guanosyl-(3',3')-(2',5'-di-O-methyluridine)

Hydrolytic reactions of guanosyl-(3',3')-uridine and guanosyl-(3',3')-(2',5'-di-O-methyluridine) have been followed by RP HPLC over a wide pH range at 363.2 K in order to elucidate the role of the 2'-hydroxyl group as a hydrogen-bond donor upon departure of the 3'-uridine moiety. Under neutral and basic conditions, guanosyl-(3',3')-uridine undergoes hydroxide ion-catalyzed cleavage (first order in [OH(-)]) of the P-O3' bonds, giving uridine and guanosine 2',3'-cyclic monophosphates, which are subsequently hydrolyzed to a mixture of 2'- and 3'-monophosphates. This bond rupture is 23 times as fast as the corresponding cleavage of the P-O3' bond of guanosyl-(3',3')-(2',5'-di-O-methyluridine) to yield 2',5'-O-dimethyluridine and guanosine 2',3'-cyclic phosphate. Under acidic conditions, where the reactivity differences are smaller, depurination and isomerization compete with the cleavage. The effect of Zn(2+) on the cleavage of the P-O3' bonds of guanosyl-(3',3')-uridine is modest: about 6-fold acceleration was observed at [Zn(2+)] = 5 mmol L(-)(1) and pH 5.6. With guanosyl-(3',3')-(2',5'-di-O-methyluridine) the rate-acceleration effect is greater: a 37-fold acceleration was observed. The mechanisms of the partial reactions, in particular the effects of the 2'-hydroxyl group on the departure of the 3'-linked nucleoside, are discussed.     

Conformational polymorphism and thermochemical analysis of 5,5' '-bis[(2,2,5,5-tetramethyl-1-aza-2,5-disila-1-cyclopentyl)ethyl]-2,2':5',2' ':5' ',2' '-quaterthiophene

The titled compound exists as two polymorphic solid phases (denoted form-I and form-II). Form-I obtained by as-synthesized material is a more stable phase. Form-II is a less stable phase. Spontaneous solid-solid transformation from form-II to form-I is observed in the temperature range between room temperature and the melting point of form-I (Tm = 156.5 degrees C), and its activation energy is estimated to be 96 kJ mol-1 by Arrhenius plot. The solid-solute-solid transformation (recrystallization from solution) from form-II to form-I is also observed. In contrast, form-II is obtained only by a solid-melt-solid transformation from form-I. Therefore, the system of two polymorphs is monotropic. The solid-state NMR measurement shows that form-I has the molecular conformation of complete S-syn-anti-syn in the oligothiophene backbone, whereas form-II has that of S-all-anti. With the solution NMR data, the polymorphism could not be observed. Therefore, the polymorphs originate from the different molecular packing involving the conformational change of the molecule. This unique property is attributed to the extra bulky terminal groups of the compounds. However, despite the extra bulky terminal groups, the mentioned polymorphism is not observed in the titled compound analogue which has S-all-anti conformation (like form-II).     

Crystal Structure of 1,2,4,1',2',4'-Hexamethylferrocene

Russian Journal of Coordination Chemistry - The crystal structure of 1,2,4,1′,2′,4′-hexamethylferrocene (Me6Fcsym) was determined by X-ray diffraction. In the Me6Fcsym molecule,...     

New approach to the synthesis of 2,2':5',2'-terthiophene-5,5'-and 2,2':5',2':5',2''-quaterthiophene-5,5''-dicarboxylic acids

Chemistry of Heterocyclic Compounds - The reaction of bromosuccinimide with esters of 3-substituted 2,2'-bithiophene-5-carboxylic acids was used to obtain their 5'-bromo derivatives, which...     

1,1',3,3',6,6',8,8'-Octachloro-9,9'-bifluorenylidene and perchloro-9,9'-bifluorenylidene,two exceedingly twisted ethylenes

The syntheses of 1,1',3,3',6,6',8,8'-octachloro-9,9'-bifluorenylidene (1), its precursors, and the byproduct 3,3',5,5'-tetrachloro-4-(trichloromethyl)biphenyl (5) are described. Accurate structural X-ray data on 1 and on perchloro-9,9'-bifluorenylidene (2) are reported and discussed. Because of chlorine overcrowding, the dihedral angles between their two identical fluorenylidene moieties are abnormally large, the central-ethylene twist angles being 55 and 66 degrees, respectively. Significant out-of-plane carbon-chlorine bond bending is likewise exhibited. Their ESR spectra and magnetic measurements prove that they are singlet species. The exceptionally large bathochromic displacements of their UV-vis absorption spectrum with respect to that of their parent hydrocarbon are mainly attributed to bond bending and molecular warping.