4 research outputs found
Synthesis of oligo- and polythiophenes in zeolite hosts
Oligomers and polymers of thiophene derivatives were synthesized in the channels of
zeolite Y and mordenite. Intrazeolite oxidation of monomers such as thiophene , 3-
methylthiophene , and bithiophene by Fe(lll) or Cu(ll) ions results in formation of insoluble
polymers that have spectroscopic properties similar to the corresponding bulk polymers. The
zeolites containing the polymers are nonconducting, but when extracted from the host, the
polymers show d.c. conductivities typical for t he bulk materials. Oligothiophene species with welldefined
electronic transitions could be produced in acidic zeolite Y
Polythiophenes and oligothiophenes in zeolite hosts
The polymerization of different thiophenes in the channels of molecular sieve zeolite hosts
is described. Thiophene, 3-methyIthiophene, 2,2'-bithiophene, and terthiophene were introduced
into dehydrated proton-, Cu(II)- or Fe(III)-containing zeolites (NaY and Na-mordenite) from
organic solvents or vapor-phase. In the large-pore hosts, green-black products are formed
from the monomers within several minutes. Spectroscopic characterization (IR, UV-NIR)
confirms the formation of oxidized polymer chains in the zeolite channels. UV-Near IR reflectance
spectra of the zeolite/polythiophene samples exhibit a broad absorption from 500 to about 2500 nm
as the bulk and not the resolved spectra of short oligomers, thus fairly long polymer chains are
formed in the zeolites. Conducting polymers can be recovered after dissolution of the zeolite host
in HF. 2, 2'-bithiophene and a-terthiophene in acidic H2Y and U^Y zeolites (2 and 6 protons per
super cage/ß-cage) yield yellow-green and purple products, respectively. UV-NIR reflectance data
indicate that the acidic zeolite hosts oxidize the thiophene oligomers to yield stable radical cations
and dications in their channel systems
Hipparcos open clusters and stellar evolution
By relying on recently improved Hipparcos parallaxes for the Hyades, Pleiades
and Ursa Major clusters we find that stellar models with updated physical
inputs nicely reproduce the location in the color magnitude diagram of main
sequence stars of different metallicities. Stars in the helium burning phase
are also discussed, showing that the luminosity of giants in the Hyades,
Praesepe and Ursa Major clusters appears to be in reasonable agreement with
theoretical predictions. A short discussion concerning the current evolutionary
scenarios closes the paper.Comment: 5 pages, 6 Postscript figures, accepted by MNRA
Stellar population models at high spectral resolution
We present new, high-to-intermediate spectral resolution stellar population
models, based on four popular libraries of empirical stellar spectra, namely
Pickles, ELODIE, STELIB and MILES. These new models are the same as our
previous models, but with higher resolution and based on empirical stellar
spectra, while keeping other ingredients the same including the stellar
energetics, the atmospheric parameters and the treatment of the
Thermally-Pulsating Asymptotic Giant Branch and the Horizontal Branch
morphology. We further compute very high resolution (R=20,000) models based on
the theoretical stellar library MARCS which extends to the near-infrared. We
therefore provide merged high resolution stellar population models, extending
from ~1000 AA to 25,000 AA. We compare how these libraries perform in stellar
population models and highlight spectral regions where discrepancies are found.
We confirm our previous findings that the flux around the V-band is lower (in a
normalised sense) in models based on empirical libraries than in those based on
the BaSeL-Kurucz library, which results in a bluer B-V colour. Most noticeably
the theoretical library MARCS gives results fully consistent with the empirical
libraries. This same effect is also found in other models using MILES, namely
Vazdekis et al. and Conroy & Gunn, even though the latter authors reach the
opposite conclusion. The bluer predicted B-V colour (by 0.05 magnitudes in our
models) is in better agreement with both the colours of Luminous Red Galaxies
and globular cluster data. We test the models on their ability to reproduce,
through full spectral fitting, the ages and metallicities of galactic globular
clusters as derived from CMD fitting and find overall good agreement.
{Abridged}Comment: 30 pages, 36 figures, Monthly Notices of the Royal Astronomical
Society in pres