Luminescent properties: lifetimes

Luminescence properties (lifetimes and intrinsic quantum yields) of Ndnl f)-diketonates with dbm derivatives (ro(Nd) = 0.25 ms)... 

Arnaud, N., and Georges, J. (2003) Comprehensive study of the luminescent properties and lifetimes of Eu(3 + ) and Tb(3 + ) chelated with various ligands in aqueous solutions Influence of the synergic agent, the surfactant and the energy level of the ligand triplet. Spectrochim. Acta A Mol. Biomol. Spectrosc. 59(8), 1829-1840. 

Ley and Schanze have also examined the luminescence properties of the polymers Pq, Pio> P25> and P50 in solution at 298 K, and in a 2-methyltetrahydro-furan solvent glass at 77 K. These spectroscopic studies reveal that fluorescence from the 71,71" exciton state is observed at Amax=443 nm, 2.80 eV in the polymers P0-P50 at 298 and 77 K, but the intensity and lifetime of the fluorescence is quenched as the mole fraction of Re in the polymers is increased. This indicates that the metal chromophore quenches the 71,71" state. The quenching is inefficient even when the mole fraction is large, suggesting that interchain diffusion of the 71,71" exciton is slow compared to its lifetime [70]. Phosphorescence from the 71,71" state of the conjugated polymer backbone is observed at > max=b43 nm, 1.93 eV in P10-P50 at 77 K, and emission at Amax=690 nm, 1.8 eV is assigned to the d7i(Re) 7i oiy MLCT transition. 

The luminescent properties of [Os (N)(NH3)4]X3 (X = C1, CF3S03) and (Ph4As)2[Os -(N)(CN)5] have also been reported (Table 3). The complexes are emissive and have long excited state lifetimes both in the solid state and in fluid solutions at room temperature (Aen,= 550nm in MeCN). [Os(N)(NH3)4]Cl3 is a powerful one-electron oxidant in the excited state, with an excited state redox potential of 2.1 V vs. NHE. The emitting state has been suggested to be d y) dj f] in origin. 

The excited state lifetimes and luminescence properties of metal complexes are related to the relative positions of the potential energy wells shown in Figure 4.77. On the left we have a lowest excited state which resembles geometrically the ground state (the internuclear distances, r, are similar). The crossing between these states requires a high activation barrier E (in a classical picture) and the excited state lifetime is therefore relatively long. The Stoke s shift between the absorption band (a) and the emission band (e)... 

Abstract Mononuclear Ir(III)-polyimine complexes show outstanding luminescence properties, i.e., high intensities, lifetimes in the is time range, and emission wavelengths that can be tuned so as to cover a full range of visible colors, from blue to red. We discuss the approaches for the use of ligands that afford control on luminescence features. Emphasis is placed on subfamilies of cyclometalated complexes, whose recent enormous expansion is motivated by their potential for applications, including that as phosphorescent dopants in OLEDs fabrication. The interplay of the different excited states associated... 

Following the introduction to size-dependent nanophenomena presented in the previous sections, we now focus our attention on the luminescence properties of lanthanide ions at additional sites or distorted structure existing in nanophases. Phenomena of prolonged luminescence lifetime, anomalous thermalization, upconversion luminescence, dynamics of long-range interaction with two-level-systems (TLS), and quantum efficiency are to be discussed. 

By taking advantage of the keto-enol equilibrium, Yb(hf w/i T was prepared in methanol-rU and its luminescence properties were determined in several solvents, methanol-, thf-cfo, PO(OMe)3, and dmso- 6- Emission intensity is the largest in the latter solvent, by a factor 2.5 with respect to deuterated methanol and the Yb(2F5/2) lifetime reaches 66 ps, as compared to 10 ps in methanol- 4 (Kim and Park, 2003). 

Further enhancement of luminescence properties is possible through halogenation of the C-H group. With this improvement, the maximum theoretical quantum yield achievable (without water molecules and secondary ligand) is 14%, corresponding to a Yb(2Fs/2) lifetime of 140 ps (Tsvirko et al., 2001). 

Because of the higher sensitivity of Ndm ions towards deactivation through O-H oscillators, the complexes with this lanthanide have much lower quantum yields and lifetimes when compared to Ybm. The best photophysical properties are obtained with phthalexon S and since complexes with PS contain 4-5 water molecules, depending on the lanthanide ion, it is quite clear that exclusion of these water molecules from the first coordination sphere will lead to much enhanced luminescent properties. This is indeed demonstrated by bis(cyclen)-substituted PS, H736 (see fig. 36), which increases quantum yields to 0.23 and 1.45% in D2O for Ndm and Ybm, respectively (Korovin and Rusakova, 2002). 

Excitation of the Lnm ion by a d-transition metal ion is an alternative to chromophore-substituted ligands, and proof of principle has been demonstrated for several systems. The lack of quantitative data, however does not allow an evaluation of their real potential, except for their main advantage, which is the control of the luminescent properties of the 4f-metal ion by directional energy transfer. In this context, we note the emergence of self-assembly processes to build new edifices, particularly bi-metallic edifices, by the simultaneous recognition of two metal ions. This relatively unexplored area has already resulted in the design of edifices in which the rate of population, and therefore the apparent lifetime, of a 4f-excited state can be fine-tuned by energy transfer from a d-transition metal ion (Torelli et al., 2005). 

The high sensitivity and specificity of photoluminescence analysis should make it possible to individualize clue materials, e.g., hair and glass, by the characteristic luminescence properties of trace constituents or impurities. Of particular significance are the newer techniques of analyzing the luminescence decay curves. For example, even when the absorption and luminescence spectra of the impurities are similar, it is possible to determine their concentrations if their luminescence lifetimes differ. The usefulness of this technique is illustrated in Figs. 1 and 2, where it is shown that the fluorescence spectra of naphthalene (N) and 1,6-dimethyl napthalene (DMN) are too similar for fluorescence spectral analysis of their mixtures (Fig. l) yet their relative concentrations can be readily determined from the fluorescence decay curve (Fig. 2). As indicated by the dashed curve in Fig. 2, the observed decay is the sum of exponential decays from a shorter lived component, i.e., DMN (lifetime 50 nsec) and a longer lived component, i.e., N (lifetime 100 nsec). St. John and Winefordner (j) have discussed this technique in general and Hoerman and co-workers (8,9) have been... 

The electronic absorption spectra and luminescent properties of [Osvl(OEP)(N)X] (X = OMe, OC103) have also been studied (223). The emission spectra of [Osvl(OEP)(N)X] and [OsVI(OEP)(0)2] are similar and they have comparable double lifetimes. The phosphorescence has been assigned as derived from a triplet Tx(tt, tt ) level. 

An unusual feature is that the luminescence intensity and lifetime of 12b increases substantially in nonpolar solvents relative to the lifetime in polar solvents. Moreover, as the emission lifetime increases, the decay kinetics become distinctly nonexponential. The explanation for the unusual solvent-dependent luminescence properties of 12b is that in a low dielectric solvent the MLCT and charge separated states (14 and 15, respectively, in Scheme 8) are similar in energy and equilibrium is established between the two states. In polar solvents 15 is stabilized with respect to 14, and photoinduced ET is irreversible. Similar, but attenuated, solvent dependent luminescence is observed for 12c and 12d. The effect is attenuated in these complexes because the amine donors are easier to oxidize and thus the charge separated state is stabilized with respect to the MLCT state. 

A considerable amount of data exists on the luminescence properties of excited benzyl [6,83-90,94-97], arylmethyl [91-93,100-108], diphenylketyl [7,109-116], and heteroradicals [117-123] in low-temperature matrices and in room temperature solution. Tables 6 through 11 tabulate both spectral and lifetime information. Where data is available, these tables also include information on the absorption spectra of the excited radicals. Due to the fact that numerous radicals listed in the tables are partially or totally deuterated, structures indicate the number of hydrogen atoms and/or deuterium atoms present. 

Europium(III) exchanged zeolites have been studied by a number of research groups. Arakawa and coworkers (20, 21 ) report the luminescence properties of europium(III)-exchanged zeolite Y. Emission spectra were measured under a variety of conditions and bands for europium(II) were observed after thermal treatment of the europium(III) Y zeolites. A mechanism was proposed for the thermal splitting of water which involved the cycling of europium between the two different oxidation states. Europium MSssbauer experiments (22 ) also show that on thermal treatment of europium-(III) zeolites that europium(II) is formed. Stucky and coworkers (23, 24) studied the phosphorescence lifetime of these europium-(lll) zeolites and showed that the inverse of the lifetime (the decay constant) was linearly related to the number of water molecules surrounding the europium(III) ion in the zeolite supercages. These studies involved zeolites A, X, Y and ZSM-5. 

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