Phosphorescence triplet state

Schmidt, J., Antheunis, D. A., van der Wtials, J. H. The dynamics of populating and depopulating the phosphorescent triplet state as studied by microwave induced delayed phosphorescence. Mol. Phys. 22. 1 (1971). 

Great advances in the elucidation of electronic structure and the dynamics of optical spin polarization in organic triplet-state molecules have been made by ESR spectroscopy since the first successful experiment of Hutchison and Mangum (39) in 1958. Most of the triplet ESR studies can be grouped into two sections the photo-excited phosphorescent triplet states and the photochemical ly prepared ground triplet-state intermediates. 

Another incompletely elucidated problem is that of the configuration of the free radicals S and S". Malrieu and Pullman stated that S " is tetragonally folded and adopts the configuration H-intra (see Section 111, A, 2) the nonplanarity of the radical has been claimed as a possible cause of the disagreement between calculated and experimental ESR spectra. However, Lhoste and Tonnard, though considering that no definitive conclusion may be drawn in this question, appear to favor the hypothesis that the radical is nearly planar. Preliminary data about the phosphorescent triplet state of the phenothiazine molecule support this view. 

The assignment of the orbital symmetry of the phosphorescent triplet state of iV,A, A, jV -tetramethyl-p-phenylenediamine (TMPD) has been studied by microwave-induced delayed phosphorescence (MIDP) and the assignment of B2 for the orbital symmetry established. ... 

Vibration-vibration energy transfer in SOa has also been discussed.390 The SOa-sensitized cis-trans isomerization of but-2-ene is a complex reaction, which at low pressures can be explained as involving only the ZBX state of S02, but which at higher pressures requires the intervention of some triplet state other than the phosphorescent SBX level.391 The photolysis of S02 in the presence of acetylene and allene also involves these non-phosphorescent triplet states, yielding CO as product, and giving rise to aerosol formation.392 Aerosol formation in S02-N2-02 mixtures has been experimentally investigated,393 and the effects of N02, propylene, and water on SOa reactions have been reported.884 Interactions between S02 and atmospheric particulate matter,395 photochemical removal of SOa from the lower atmosphere,396 the use of SOa to trap 1,4-biradicals generated photo-chemically,397 and the photochlorination of S02 398 have all been discussed in recent publications. 

We begin with a brief survey of the fine structure characteristics of a ligand-centered phosphorescent triplet state. This is followed by some considerations concerning the ODMR experiment and finally we review results of recent ODMR investigations with focus on the information regarding structure and dynamics complementary to optical work. 

MHz of [Rh(bpy)3](0104)3, in the phosphorescent triplet state, upon switching on the microwave power. The oscillations occur as the microwave pulse duration is increased. Photoexcitation is near 320 nm, detection is at 456 nm temperature is 1.4 K. b Optically detected echo amplitude decay for the 2320 MHz zero-field transition of [Rh(bpy)3] (0104)3 as obtained by applying a n/2-T-n-T-nl2 pulse sequence when increasing 2r... 

From magnetic resonance spectroscopy [49] it is well-known that IB effects are adequately circumvented by the tricks of a spin echo experiment. For instance, in a two-pulse echo experiment, IB effects are averaged out and one probes spin dephasing determined by time-dependent fluctuations characteristic of HB only (and not IB). More specifically, a nll-r-n microwave pulse sequence is applied, where the first nil pulse creates a coherent superposition state for which a la = 1 and the n pulse, applied at time r after the first pulse, generates a spin coherence (the echo) at time 2r after the initial pulse. The echo amplitude is traced with r. The echo amplitude decay time is characteristic of the pure dephasing dynamics. For phosphorescent triplet states it is possible to make the echo optically detectable by means of a final nil probe pulse applied at time f after the second pulse [44]. In Fig. 3b, the optically detected echo amplitude decay for the zero-field transition at 2320 MHz of... 

As shown in Fig. 3 a, spin coherence is manifested in the optically detected transient nutation signal for [Rh(bpy)3] (0)04)3 the phosphorescent triplet state. In this experiment, one observes that the phosphorescence intensity becomes modulated as the pulse length of microwave pulses, resonant with the D - transition, is gradually increased. The modulation is evidence that the micro-wave excitation induces a spin coherence in the ensemble of molecules in the photoexcited triplet state [44]. Moreover, from the transient nutation experiment one obtains the information about the duration of the pulses needed in a spin echo experiment. In the case of the example, the n/2 pulse is 100 ns and the 71 pulse has a length of 200 ns. Similarly, transient nutation signals for the other zero-field spin resonances could be obtained. The optically detected spin echo decay as measured for the D - j j zero-field transition for [Rhlbpylj](004)3... 

In addition to the ODMR investigations of Rh +(4d )-chelates, recently similar studies have been performed for the Pd +(4d )-complexes, Pd(thpy)2 and Pd(qol)2 (with qol" = 8-hydroxyquinolinate) [80, 81]. Optical investigations of Pd(thpy)2 (with (thpy) 2,2 -thienylpyridinate, see Fig. 1) doped into an n-oc-tane Shpol skii matrix revealed highly resolved emission spectra and showed that the phosphorescent triplet state decays with three hfetime components of T] = 1200 ps, T]] = 235 ps, and rm = 130 ps characteristic of the triplet state sub-levels [82 - 84]. The emission data of Pd(qol)2 in an n-octane ShpoTskii matrix have been reported recently [81,85]. Two distinct emissive sites in the matrix were found,with electronic origins at 16,090 cm (77%) and 16,167 cm (23%), respectively. From the Zeeman splittings of the optical hne transitions in magnetic fields up to 12 T, the emission for the two sites was assigned as Tj Sg. 

T = 80 ms, T = 180 )xs, and riu= 100 ps, has been obtained. Recall that the D+E zero-field ODMR transition remained unobserved. Thus from the time-re-solved experiments it follows that this transition is between the triplet state sub-levels with the shortest lifetimes. In this respect the situation is similar to Pd(thpy)2 in the phosphorescent triplet state. For Pd(thpy)2 the resonance between the sublevels with lifetimes of 134 ps and 235 ps remained unobserved in zero-field ODMR [80]. Most likely, the two sublevels with the shortest lifetimes show little population difference and therefore this will largely affect the detectability of the ODMR transition between them. 

The phosphorescence lifetimes are on the order of several seconds in each case which is consistent with a st,n phosphorescent triplet state. The highly forbidden nature of the phosphorescence transition Sn - To) as indicated by the long phosphorescence lifetime makes direct optical excitation of the triplet states not feasible with anything less than a high power laser source. 

The UV absorption spectra of tr3q)tophan, tyrosine and phenylalanine are virtually identical to those of indole 35), phenol 86) and benzene 27) respectively. Close similarities also exist between the fluorescence and phosphorescence spectra of each pair, as well as between the phosphorescence lifetimes 8.28,38). This and the electron spin resonance observations to be discussed in a later section indicate that the addition of a methyl group or other substituents of the type —CH2R does not have a large effect on the measurable properties of the phosphorescent triplet state of the compounds being discussed here. 

Detection of the phosphorescent triplet states in frozen solutions of aromatic molecules by electron paramagnetic resonance (EPR) was not accomplished until somewhat over a decade ago 56,66) shortly after... 

Azizova et al. 7 > showed that the production of free radicals involved a mechcmism in which the triplet level of the aromatic chromophore appears as an intermediate state. The mechanism proposed electron photoejection after absorption of a second photon by an aromatic molecule already excited to its phosphorescent triplet state 77,78). In acidic media the photoejected electron can be trapped by a proton and the EPR signal of hydrogen atoms is observed. In basic media the EPR signal from 0(—) ions is observed 79). In some instances secondary excitation with visible light induces radical recombination by freeing the trapped electrons 79). 

Fig. 9. Energy level diagram of a phosphorescent triplet state in zero field illustrating the dynamic processes which determine the steady-state populations (JV ) of the magnetic sublevels Tt). The symbols are defined in the text. Broken arrows represent radiationless processes while normal arrows represent radiative ones. ki=h2+ h ...

The temperature dependence of the PL spectra of 1 is shown in Figure 3.8. There was a strong iuCTease in triplet emission intensity with decreasing temperature. At 11K, the emissive peak occured at about 508 nm for 1. The luminescence lifetime (Tp) values for 1 were determined to be about ll.Ops at 290K and 40.2 ts at 11K (versus Tp = 30 s at lOK for I), which was in line with the Jt-Jt character of the phosphorescent triplet state of the emission. The reduced conjugation in 1 shifted the phosphorescence to the blue by 0.06 and 0.38 eV, respectively, as compared with the related Pt(ll) polyynes I and 30. 

Typically, phosphorescence quantum yields are very low unless the sample is in a solid matrix, usually at low temperature, and/or is vigoronsly deoxygenated. These precautions are important in order to eliminate radiationless deactivation of the phosphorescing triplet state. 

Stemlicht H, McConnell HM (1960) Effect of deuterium substitution on the lifetime of the phosphorescent triplet state of naphthalene. J Chem Phys 33 302-303... 

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