We have measured the thermal conductivity of seven germanium crystals with different isotopic compositions in the temperature range between 2 K and 300 K. These samples, including one made of highly enriched 70 Ge͑99.99%͒, show intrinsic behavior at room temperature with the exception of a p-type sample with ͉N d-N a ͉Х2ϫ10 16 cm Ϫ3. The ''undoped'' samples exhibit a T 3 dependence at low temperatures, basically determined by boundary scattering. The maximum value of ͑which falls in the range between 13 K and 23 K͒ is found to be a monotonically decreasing function of the isotopic mass variance parameter g. The maximum m measured for the most highly enriched 70 Ge͑99.99%͒ sample is 10.5 kW/mK, one order of magnitude higher than for natural germanium. The experimental data have been fitted with the full Callaway theory, modified by treating transverse and longitudinal modes separately, using three free adjustable parameters for each set of modes to represent anharmonic effects plus the calculated contributions from isotopic and boundary scattering. For the isotopically purest 70 Ge͑99.99%͒ sample, dislocation scattering, or a similar mechanism, must be added in order to fit the data. We have also checked the effect of various surface treatments on the thermal conductivity in the low temperature region. The highest values of are found after polish etching with a SYTON suspension. ͓S0163-1829͑97͒00539-0͔
Hyperfluorescence (HF), a relatively new phenomenon utilizeing excitones transfer between two luminophores, requires careful pairwise tuning of molecular energy levels and is proposed to be the crucial step towards the...
Astrong conjugation present in fused systems plays acrucial role in tuning of the properties that would be showing adependence on the efficiency of p-electrons coupling.The pcloud available in the final structure can be drastically influenced by as ide-or al inear fusion of unsaturated and conjugated hydrocarbons.T he linear welding of naphthalene/ anthracene or quinoxaline/benzo[g]quinoxaline with triphyrin(2.1.1) gives structures where the competition between local and global delocalization is distinguished. The aromatic character observed in skeletons strongly depends on the oxidation state of the macrocyclic flanking and is either extended over the whole system or kept as ac omposition of local currents (diatropic and paratropic) of incorporated units. The hybrid systems showt he properties derived from the pconjugations that interlace one another but also showasignificant independence of (aza)acene subunits reflected in the observed spectroscopic properties.
Understanding of the aromatic properties and magnetically inducedc urrent densities of highlyc onjugated chromophores is importantw hen designing molecules with strongly delocalized electronic structure.L inear extension of the triphyrin(2.1.1) skeleton with an annelated benzo [b]heterocyclef ragment modifies the aromatic character by extending the electron delocalization pathway. Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring and an aromatic benzo[b]heterocycles ubunit. Current-density calculations provide detailedi nformation about the observed pathways and their strengths.Extending the p-electron system in conjugated molecules is an important area of research, because the conjugation pathway exhibits notable influence on the opticala nd magnetic properties. [1] Linear extensions of the acene p cloud by annelation of molecular rings lead to different aromatic behavior of five-and six-membered rings. [3,4] As idef usion of af ive-membered ring to acene cores results in major differences in the electron delocalization with the bridging heteroatom separating the aromatic and antiaromaticmoieties. [5] Triphyrins(2.1.1) (Scheme 1) are triangularly shaped, contracted porphyrinoids whose structural constraints significantly affect their electronic properties. [2] The C2 bridge (1,S cheme1) makes it possible to merge an acene to the triphyrin macrocycle potentially resulting in molecular structures with novel optical andm agnetic properties as well as an unusual aromatic character. [2d,e] We have synthesized novel molecular structures with benzo[b]furan and benzo[b]thiophene annelated to triphyrin(2.1.1) (2,S cheme 1). The annelated arene ring may influence the global aromatic properties by incorporating the external ben-zoic (C6) ring into the delocalization pathway.T he synthesized macrocycles can also functiona sb oron(III)l igands forminga ntiaromatic complexes as shown in this work.Triphyrins 2a and 2b wereo btained by acid-catalyzed condensation of reagents 3a and 3b,r espectively.T hey were efficiently synthesized via Suzuki-Miyaura coupling from easily obtainable or commercially availables tarting materials (2,3-dibromobenzo[b]heterocyclea nd pyrrole-boronic acid), [6] followed by at hermolytic deprotection. [2d,e] 2a and 2b were isolated with ay ield of 10 %a nd 12 %, respectively.T he small coordination core of the macrocycle leads to av ery efficient entrapment of ap rotonm aking it possible to isolate the cationic form. As the counterion of the condensationp roduct was unknown,w ereplaced it with ac hloride anion by using ap reviously reported approach. [2d,e] Characterization and further studies were performed on the chloride derivatives. The X-ray analysis performed for 2b has confirmed the postulated pattern ( Figure S44).2a (2b)h as aU V/Vis spectrum that is typical for aromatic porphyrinoids with aS oret-like band at 415 nm (430 nm) and a set of Qb ands that reach 600 nm (620 nm). The bands are redshiftedb y4 0-50nmc ompared to triphyrins(2.1.1) such as 1b [2b] and 1c, [2f]...
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