Far infrared reflectance and optical properties of organic superconductor (TMTSF)2ClO4 /

Polarized reflectance measurements of the quasi I-D charge-transfer salt (TMTSFh CI04 were carried out using a Martin-Puplett-type polarizing interferometer and a 3He refrigerator cryostat, at several temperatures between 0.45 K and 26 K, in the far infrared, in the 10 to 70 cm- 1 frequency range. Bis-tetramethyl-tetraselena-fulvalene perchlorate crystals, grown electrochemically and supplied by K. Behnia, of dimensions 2 to 4 by 0.4 by 0.2 mm, were assembled on a flat surface to form a mosaic of 1.5 by 3 mm. The needle shaped crystals were positioned parallel to each other along their long axis, which is the stacking direction of the planar TMTSF cations, exposing the ab plane face (parallel to which the sheets of CI04 anions are positioned). Reflectance measurements were performed with radiation polarized along the stacking direction in the sample. Measurements were carried out following either a fast (15-20 K per minute) or slow (0.1 K per minute) cooling of the sample. Slow cooling permits the anions to order near 24 K, and the sample is expected to be superconducting below 1.2 K, while fast cooling yields an insulating state at low temperatures. Upon the slow cooling the reflectance shows dependence with temperature and exhibits the 28 cm- 1 feature reported previously [1]. Thermoreflectance for both the 'slow' and 'fast' cooling of the sample calculated relative to the 26 K reflectance data indicates that the reflectance is temperature dependent, for the slow cooling case only. A low frequency edge in the absolute reflectance is assigned an electronic origin given its strong temperature dependence in the relaxed state. We attribute the peak in the absolute reflectance near 30 cm-1 to a phonon coupled to the electronic background. Both the low frequency edge and the 30 cm-1 feature are noted te shift towards higher frequcncy, upon cntering the superconducting state, by an amount of the order of the expected superconducting energy gap. Kramers-Kronig analysis was carried out to determine the optical conductivity for the slowly cooled sample from the measured reflectance. In order to do so the low frequency data was extrapolated to zero frequency using a Hagen-Ru bens behaviour, and the high frequency data was extended with the data of Cao et al. [2], and Kikuchi et al. [3]. The real part of the optical conductivity exhibits an asymmetric peak at 35 cm-1, and its background at lower frequencies seems to be losing spectral weight with lowering of the temperature, leading us to presume that a narrow peak is forming at even lower frequencies.

The chemistry of heterocyclic methine bases and of their acyl and thioacyl derivatives

The work described in this thesis has been dtvided into six sections . The first section involves the reaction of 3,5-diphenyl-2-methyl-l,3,4-oxadiazolium perchlorate with acetic and benzoic anhydrides. The second section deals with the preparation and reactions of 1,3,4-thia diazolium salts. Some monomeric 1,3,4-thiadiazoline methine bases have also been prepared by reacting 1,3,4-thia d iaz ol ium s al t s with concen trated ammonium hydroxide solution. Variable temperature p.m.r. of 2-(3-acetylacetonylidene)-3,5-diphenyl-A4 -1,3,4-thiadiazoline has also been described. The third section deals with prepar a tion and reactions of some compounds in benzoxazole series. The fourth section deals with the prep a ration and reactions of N-alkyl-2-methylbenzothi azolium salts with base , a nd with some a cetylating and thioacetylating agents. Treatment of 2,3-dimethylbenzothiazolium iodide and of 3-ethyl-2-methylbenzothia zolium iodide with base wa s found to give the corresponding dimeric methine b a ses and evidence supporting their structure is also given. Thiol acetic acid was found to exchange 0 for S in its reactions with 2-acetonylidene-3-methylbenzothiazoline and 2-acetophenonylidene-3-methylbenzothi a zoline. (ii) In th e fifth section, the r eactions of 2,3-dimethylbenzselenazolium iodide with a variety of ac e tylating and thioacetylating agents has been described. The treatment of 2,3-dimethylbenzselenazolium iodide with base was found to give rise to a dimeric methine base and evidence supporting its structure is also given. The reactions of this dimeric methine b a se with benzoic anhydride and phenylisothiocyanate have also been described. The sixth section deals with the preparation and reactions of l-alkyl-2-methylquinolinium salts. Treatment of 1,2-dimethylquinolinium iodide and l-ethyl-2-methylquinolinium iodide was found to give the corresponding monomeric methine bases and evidence supporting their structure is also given. The E-type geometry of the olefinic bond in 2-acetonylidene-l-methylquinoline has been established on the basis of an N.O.E. experiment.