Search

A series of luminescent complexes based on {Ir(phpy)2} (phpy = cyclometallating anion of 2-phenylpyridine) or {Ir(F2phpy)2} [F2phpy = cyclometallating anion of 2-(2',4'-difluorophenyl)pyridine] units, with an additional 3-(2-pyridyl)-pyrazole (pypz) ligand, have been prepared; fluorination of the phenylpyridine ligands results in a blue-shift of the usual ³MLCT/³LC luminescence of the Ir unit from 477 to 455 nm. These complexes have pendant from the coordinated pyrazolyl ring an additional chelating 3-(2-pyridyl)-pyrazole unit, separated via a flexible chain containing a naphthalene-1,4-diyl or naphthalene-1,5-diyl spacer. Crystal structures show that the flexibility of the pendant chain allows the naphthyl group to lie close to the Ir core and participate in a p-stacking interaction with a coordinated phpy or F2phpy ligand. Luminescence spectra show that, whereas the {Ir(phpy)2(pypz)} complexes show typical Ir-based emission--albeit with lengthened lifetimes because of interaction with the stacked naphthyl group--the {Ir(F2phpy)2(pypz)} complexes are nearly quenched. This is because the higher energy of the Ir-based ³MLCT/³LC excited state can now be quenched by the adjacent naphthyl group to form a long-lived naphthyl-centered triplet (3nap) state which is detectable by transient absorption. Coordination of an {Eu(hfac)3} unit (hfac = 1,1,1,5,5,5-hexafluoro-pentane-2,4-dionate) to the pendant pypz binding site affords Ir-naphthyl-Eu triads. For the triads containing a {Ir(phpy)2} core, the unavailability of the 3nap state (not populated by the Ir-based excited state which is too low in energy) means that direct Ir?Eu energy-transfer occurs in the same way as in other flexible Ir/Eu complexes. However for the triads based on the{Ir(F2phpy)2} core, the initial Ir→³nap energy-transfer step is followed by a second, slower, ³nap→Eu energy-transfer step: transient absorption measurements clearly show the 3nap state being sensitized by the Ir center (synchronous Ir-based decay and ³nap rise-time) and then transferring its energy to the Eu center (synchronous ³nap decay and Eu-based emission rise time). Thus the ³nap state, which is energetically intermediate in the {Ir(F2phpy)2}-naphthyl-Eu systems, can act as a "stepping stone" for two-step d→f energy-transfer. [ABSTRACT FROM AUTHOR]

Abstract: Employees are significantly more likely to quit small rather than large firms to found new ventures. I examine empirically three alternative theories that might explain this finding: a transmission theory; a blocked mobility theory; and self-selection of workers. A representative sample of British panel data containing information on workplace, job and personal characteristics from 1991 to 2003 are used to explore the relationship between firm size and transitions into self-employment. The transmission and blocked mobility theories do not receive consistent empirical backing whereas the selection theory does. The implications of these findings for researchers and policy-makers are discussed. [Copyright &y& Elsevier]

Abstract: I explore the economic implications of learning by doing by borrower entrepreneurs who are initially imperfectly informed about their innate abilities. It is shown how entrepreneurial learning worsens the adverse effects of moral hazard, causing the possible disappearance of otherwise viable credit markets and thereby decreasing social welfare. The reason is that learning reduces subjective uncertainty about risky future outcomes, thus encouraging excessive risk-taking behaviour. [Copyright &y& Elsevier]

The paper proposes a human capital-based model of choice between paid employment and entrepreneurship that can give rise to multiple equilibria in occupational structure. The model is consistent with evidence of pronounced, persistent, and hitherto unexplained regional variations in entrepreneurship. Whether multiple equilibria emerge is shown to depend on the occupation-specific productivity of human capital and the costs of human and financial capital. The paper goes on to discuss the implications for governments seeking to promote entrepreneurship as a tool of regional development policy. [ABSTRACT FROM AUTHOR]

Verbal and mathematical reasoning are key cognitive skills which individuals use throughout their lives to create economic value. We argue that individuals undertaking entrepreneurial tasks also draw on these skills, and we study how best these skills should be combined in entrepreneurial teams. To that purpose we conduct a randomized field experiment using data from the BizWorld entrepreneurship education program. Four different types of teams are created which differ in terms of their cognitive skill composition. Our results show that balanced skills are beneficial for a team's venture performance only if it comes from within-person skill balance, and that combining team members with different skills in mixed teams does not compensate for a lack of members who individually possess balanced cognitive skills. [ABSTRACT FROM AUTHOR]

Novel molecular Re and Mn tricarbonyl complexes bearing a bipyridyl ligand functionalised with sterically hindering substituents in the 6,6′-position, [M(HPEAB)(CO)3(X)] (M/X = Re/Cl, Mn/Br; HPEAB = 6,6′-{N-(4-hexylphenyl)-N(ethyl)-amido}-2,2′-bipyridine) have been synthesised, fully characterised including by single crystal X-ray crystallography, and their propensity to act as catalysts for the electrochemical and photochemical reduction of CO2 has been established. Controlled potential electrolysis showed that the catalysts are effective for electrochemical CO2-reduction, yielding CO as the product (in MeCN for the Re-complex, in 95 : 5 (v/v) MeCN : H2O mixture for the Mn-complex). The recyclability of the catalysts was demonstrated through replenishment of CO2 within solution. The novel catalysts had similar reduction potentials to previously reported complexes of similar structure, and results of the foot-of-the-wave analysis showed comparable maximum turnover rates, too. The tentative mechanisms for activation of the pre-catalysts were proposed on the basis of IR-spectroelectrochemical data aided by DFT calculations. It is shown that the typical dimerisation of the Mn-catalyst was prevented by incorporation of sterically hindering groups, whilst the Re-catalyst undergoes the usual mechanism following chloride ion loss. No photochemical CO2 reduction was observed for the rhenium complex in the presence of a sacrificial donor (triethylamine), which was attributed to the short triplet excited state lifetime (3.6 ns), insufficient for diffusion-controlled electron transfer. Importantly, [Mn(HPEAB)(CO)3Br] can act as a CO2 reduction catalyst when photosensitised by a zinc porphyrin under red light irradiation (λ > 600 nm) in MeCN : H2O (95 : 5); there has been only one reported example of photoactivating Mn-catalysts with porphyrins in this manner. Thus, this work demonstrates the wide utility of sterically protected Re- and Mn-diimine carbonyl catalysts, where the rate and yield of CO-production can be adjusted based on the metal centre and catalytic conditions, with the advantage of suppressing unwanted side-reactions through steric protection of the vacant coordination site. [ABSTRACT FROM AUTHOR]

A series of blue-luminescent Ir(III) complexes with a pendant binding site for lanthanide(III) ions has been synthesized and used to prepare Ir(III)/Ln(III) dyads (Ln = Eu, Tb, Gd). Photophysical studies were used to establish mechanisms of Ir→Ln (Ln = Tb, Eu) energy-transfer. In the Ir/Gd dyads, where direct Ir→Gd energy-transfer is not possible, significant quenching of Ir-based luminescence nonetheless occurred; this can be ascribed to photoinduced electron-transfer from the photo-excited Ir unit (*Ir, 3MLCT/3LC excited state) to the pendant pyrazolyl-pyridine site which becomes a good electron-acceptor when coordinated to an electropositive Gd(III) centre. This electron transfer quenches the Ir-based luminescence, leading to formation of a charge-separated {Ir4+}•—(pyrazolyl-pyridine)•− state, which is short-lived possibly due to fast back electron-transfer (<20 ns). In the Ir/Tb and Ir/Eu dyads this electron-transfer pathway is again operative and leads to sensitisation of Eu-based and Tb-based emission using the energy liberated from the back electron-transfer process. In addition direct Dexter-type Ir→Ln (Ln = Tb, Eu) energytransfer occurs on a similar timescale, meaning that there are two parallel mechanisms by which excitation energy can be transferred from *Ir to the Eu/Tb centre. Time-resolved luminescence measurements on the sensitised Eu-based emission showed both fast and slow rise-time components, associated with the PET-based and Dexter-based energy-transfer mechanisms respectively. In the Ir/Tb dyads, the Ir→Tb energy-transfer is only just thermodynamically favourable, leading to rapid Tb→Ir thermally-activated back energy-transfer and non-radiative deactivation to an extent that depends on the precise energy gap between the *Ir and Tb-based 5D4 states. Thus, the sensitised Tb(III)-based emission is weak and unusually short-lived due to back energy transfer, but nonetheless represents rare examples of Tb(III) sensitisation by a energy donor that could be excited using visible light as opposed to the usually required UV excitation. [ABSTRACT FROM AUTHOR]

Synthetic methods toward ruthenium(II) complexes incorporating the benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine-11,16-quinone ligand, qdppn, are reported. In several cases, it was found that complexes containing coordinated benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine, dppn, could be chemically or photochemically oxidized to their qdppn analogues. Since this method was not possible in all the cases, a new, higher yielding, convenient synthesis of qdppn was developed. The crystal structure of the complex [Ru(phen)2(qppn)](PF6)2 (phen = 1,10-phenanthroline) which was synthesized from free qdppn reveals that a combination of π-π stacking between coordinated phen and qdppn units, as well as anion-ligand hydrogen bonding, define large hexagonal channels which are occupied by anions and solvent molecules. Electrochemical and photophysical studies reveal that the new qdppn-based complexes are not luminescent and, in contrast to their dppn analogues, they are also poor singlet oxygen sensitizers. Time-resolved studies and density functional theory (DFT) calculations indicate that optical properties of the new complexes are due to a short-lived charge separated state involving the quinone moiety of qdppn. The DNA binding properties of the new complexes have also been investigated. It was found that they are intercalators, displaying binding affinities which are comparable to their dppn analogues. [ABSTRACT FROM AUTHOR]