Emission decay time

Emission Decay Time of Rare-Earth-Activated Lithium Magnesium Aluminum Silicate Glasses... 

Directly Excited Emission Decay Times of Glass Powders at 300°K... 

Distinct differences between the two compounds are also found in the emission decay times and the emission quantum yields. The phosphorescence of Pt(4,6-dFppy)(acac) decays with 0.3 ps and the quantum yield amounts only to 0pL= 2% in... 

Despite the remarkable quantum yield and the relatively short emission decay time of Ir(4,6-dFppy)2(acac), much less research work has been published than for the related famous compound Ir(4,6-dFppy)2(pic) (FTrpic, pic = picolinate), which exhibits a 15 nm blue shifted emission compared to Ir(4,6-dFppy)2(acac) [50, 53], Therefore, Ir(4,6-dFppy)2(pic) is a more suited dopant for highly desired blue-emitting OLEDs [17, 54-56], It is noted that by the implementation of strongly electron-withdrawing ancillary ligands, further shifts towards a deep blue emission could be achieved [45, 57, 58],... 

Other significant spectral changes are also observed. The total splitting increases from 8.3 cm-1 at B=0 T to 24 cm-1 at 5= 12 T. Moreover, due to the field induced mixings of the wave functions, the radiative allowedness of the transitions from the T substates to the ground state is strongly redistributed. The emission from the lowest B-field disturbed substate 1(B) becomes dominant, while the transitions 11(B) —> 0 and III(B) — 0 lose intensity. This is also displayed in the emission decay time of substate I at 1.5 K, which becomes as short as 12 ps at 12 T, while it amounts to 85 ps at zero-field (see next section). Due to this B-field induced increase of radiative allowedness, it also becomes possible to tune magnetically other important properties like the mechanisms of vibrational deactivation [78-82]. 

Fig. 5 Thermalized emission decay time of Pt(4,6-dFppy)(acac) in -octane vs temperature. For temperatures of T<2 K, the emission was detected at the energy of the 0-0 transition I — 0, while for T> 2 K, detection at the energy of the 0-0 transition II/III — 0 was chosen. The solid line represents a fit of (3) to the experimental data. The results obtained from the fit are shown in the inset (compare [71])...

The properties of the lowest triplet state of Pt(4,6-dFppy)(acac) in n-octane are nearly independent of the site chosen. An investigation of two other discrete sites reveals ZFS values which do not deviate significantly from the values observed for the main site. Furthermore, even different host materials do not lead to remarkable changes. Corresponding data are summarized in Table 1. For CH2C12 the splitting could be measured directly by site-selective spectroscopy of one discrete site, while for THF only a broadband spectrum was obtained. In this case, the ZFS was obtained from the temperature dependence of the thermalized emission decay time by a fit of (3) as described in Sect 3.2. 

As a most remarkable difference, the total zero-field splitting AE(ZFS) = AE is more than one order of magnitude larger for the Ir(III) than for the Pt(II) compound. Further, the decay time rm is significantly shorter. Moreover, the AE(ZFS) values and individual emission decay times are found to exhibit distinct site- and matrix-dependences for Ir(4,6-dFppy)2(acac), while these parameters are much less affected for Pt(4,6-dFppy)(acac). 

Furthermore, the derived models also allow us to explain the different sensitivities of ZFSs and radiative rates on the host environment. Usually the sensitivity is less distinct for square planar Pt(II) than for octahedral Ir(III) compounds. Also for Ir(4,6-dFppy)2(acac), the ZFSs and emission decay times strongly depend on the... 

The emission decay time increases by about three orders of magnitude (Fig. 22 and Sect. E.IV.). 

At low temperatures (e.g. T = 1.9 K) magnetic fields induce a blue shift by 200 to 300 cm-1 of the low energetic components and further the emission decay time is reduced by about three orders of magnitude. At temperatures above about 10 K the emission properties do not depend on magnetic fields94,151). (These effects are further discussed in Sect. G.)... 

Fig. 5. Energy level diagram for Pd(2-thpy)2 dissolved in n-octane. The Tj state at 18,418 cm is zero-field split on the order of 0.2 cm. The emission decay times refer to the individual triplet suhstates I, II, and III, respectively, at T = 1.3 K. (Compare Fig. 6.) These suhstates are radiatively deactivated as purely electronic transitions, as well as by Franck-Condon (FC) and Herzberg-Teller (HT) vibrational activity, respectively. This leads the different vibrational satellites. (Compare also Sects. 4.2.2 and 4.2.3.) The lifetime of the S, state is determined from the homogeneous linewidth of the spectrally resolved Sq —> S, electronic origin. (Sect. 3.2) The electronic state at 24.7 x 10 cm is not yet assigned...

Table 2. Emission decay times ( is) and relative intensities for different radiative decay paths of Ti of Pd(2-thpy)2 in n-octane at T = 1.2 K for selected excitation and detection energies...

Emission decay time of sublevel Relative emission intensities ... 

The complementary studies of decay properties of the triplet substates at T = 1.2 K (Sect. 3.1.3, Table 2) show that the disparities between the triplet substates I and II as well as between I and III are significant, while the states II and III might exhibit too little disparities with respect to 2. and 3. Thus, one would expect to detect at least two ODMR transitions, but only one is observed. On the other hand, the relatively large transition probability of the transition So Tj (excitation spectra can be recorded, emission decay times are relatively short, see Sects. 3.1.1 and 3.1.3) show that spin-orbit coupling is not unimportant. 

Figure 11 shows that deuteration of the matrix material has an interesting and not anticipated effect on the temperature dependence of the emission decay of Pd(2-thpy)2. At r = 1.3 K, one observes the three individual decay components of the three triplet substates I, II, and III, as discussed in Sect. 3.1.3. Within limits of experimental error, deuteration of the n-octane matrix does not alter this decay behavior. (Compare Fig. 11a to Fig. 6 a.) With temperature increase, the emission decay times are reduced in both matrices due to effects of thermaliza-tion between the three triplet sublevels, i.e. due to the growing in of spin-lattice relaxation (e.g. see Sects. 3.1.3 and 4.2.6). These processes are particularly important for the long-Hved components. For example, already at T = 2.0 K this component is reduced to 950 ps and 840 ps for the deuterated and the protonated matrix, respectively (decay curves not reproduced). This trend continues, as is shown in Fig. 11b. At T = 3.0 K, the long-decaying components are determined to 460 ps and 320 ps, respectively. Finally, near T=5 K, thermal equilibration between the three substates is reached for both matrices, and the decays become monoexponential. (Fig. 11c, compare also Sect. 3.1.3.) At this temperature, the emission decay of Pd(2-thpy)2 is again almost equal in n-octane-hig and n-octane-dig.

Fig. 21. Temperature dependence of the rate k and time ijir of spin-lattice relaxation of state II of Pt(2-thpy)2 dissolved in n-octane. The experimental data (points) result from the emission decay times of state II, but they are corrected according to Eq. (21). The solid line represents a fit according to Eq. (22), while the broken and dotted lines give the contributions of the respective processes. The inset shows the triplet sublevels and depicts schematically the three different processes of spin-lattice relaxation. (Compare Ref. [24])...

Fig. 26. Energy level diagram for the triplet sublevels of (a) perprotonated, (b) partially deu-terated, and (c) perdeuterated Pt(2-thpy)2 dissolved in an n-octane matrix (Shpol skii matrix). Emission decay times and spin-lattice relaxation times are given for T= 1.3 K. Several vibrational satellites are specified, HT = Herzberg-Teller active vibration, FC = Franck-Condon active vibration. The emission spectrum of the partially deuterated compound (b) exhibits vibrational satellites of the two different ligands. (Compare Fig. 27b.) The data given for Pt(2-thpy-hg)(2-thpy-d6) (b) refer to the lower lying site A. (Compare Ref. [23])...

Finally, we discuss briefly the emission decay behavior. At T = 1.3 K, the emission decay time is mainly determined by radiative and non-radiative processes of state I. For Pt(2-thpy-hg)(2-thpy-dg) one finds a value of (120 3) ps, which thus lies between (110 3) ps and (140 3) ps of the perprotonated and per-deuterated compounds, respectively (see also Fig. 26). Apart from the effects of spin-lattice relaxation occurring in the first microseconds, the decay is strictly monoexponential, at least over five lifetimes. It is important that the decay is exactly equal, when measured on a vibrational satellite, which is related to the protonated part of the molecule (e.g. 713 cm satellite) and to the deuterated part (e. g. 685 cm satellite), respectively. (Fig. 27b) This result also strongly supports the assignment to a delocalized excited state. A similar behavior has also been observed for [Oslbpylj], for which the lowest triplet states are also delocalized [47]. 

The emission decay time of the long-living triplet sublevel I decreases by more than a factor of ten from 1200 ps in Pd(2-thpy)2 to 110 ps in Pt(2-thpy)2. 

Compound Lowest electronic origin 1 Zero-field splittings Emission decay times [ps] AEni Ti Til Tin sir time Tsir [ns] Assignments Remarks, References... 

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