Triplet sublevels

The phosphorescence from a triplet sublevel will thus be uniquely polarized the calculated oscillator strengths and transition moments are given in Table 7. 

In the experiment done under conditions of inefficient spin-lattice relaxation the triplet spin states originally produced are retained during internal conversion and vibrational relaxation. Important conclusions about the routes of intersystem crossing can thus be obtained from a study of the population of the triplet sublevels, the so-called "spin alignment . 

Zero field splitting (zfs) values in photoexcited triplets of primary donor bacteriochlorophyll a in photosynthetic bacteria are much lower than those found for vitro BChla triplets. There is a pronounced difference in kinetics of population and depopulation of the triplet sublevels as well. The differences have been attributed to the effect of BChla dimerization and it is now generally accepted that the primary electron donor in photosynthetic bacteria consists of a BChla dimer (special pair)(l- ). 

J >> D. The triplet characteristics will be affected by a charge transfer contribution to the triplet electronic state. It is predicted that this will reduce the values of D and E, and increase the rate constants of decay from the triplet sublevels (5,8,24). 

The transitions in the X-band ESR spectra of triplet species occur in two regions. The so-called Anis = 1 region represents transitions between energetically adjacent pairs of the three triplet sublevels. These are characterized by two so-called zero-field splitting parameters, D and E. The parameter D is inversely proportional to the cube of the average separation of the electron spins, and E is related to the molecular symmetry. The number of lines depends on the molecular symmetry. If all three magnetic axes of the molecular carrier of the spectrum are distinct, the spectrum in the Anis = 1 region will show six major resonances, plus any hyperfine lines that may be visible. If two of the principle axes are equivalent by symmetry, only four lines will be observed. In the latter case, the parameter E has the value of... 

If there is a second-order spin-orbit splitting, can we predict which of the triplet sublevels is lowered in energy due to spin-orbit coupling ... 

Fig. 1.13. Energy levels of a pair of polarons in singlet and triplet states in a magnetic field. If 2J = 0, the triplet sublevel ms = 0 will degenerate with the singlet state and can be (de-)populated via intersystem crossing...

Second, it is conceivable that amines quench triplet ketones before spin-lattice relaxation takes place within the three triplet sublevels 159>, which are populated unevenly 160>. In that event, radicals can be produced with their electron spins polarized. The CIDEP phenomenon 161>, whereby EPR emission is observed upon irradiation of ketones and very reactive substrates, may involve this mechanism. In fact, certain CIDNP observations may depend on rapid quenching of spin-polarized triplets 162>. 

The utility of CIDEP in photochemistry was greatly enhanced when it was realized (131) that the radical-pair mechanism is not the exclusive spin polarization mechanism. Initial triplet spin polarization produced by the different intersystem-crossing rates to the excited triplet sublevels can be "transferred" to radicals formed by the photochemical reaction of the polarized triplet. 

Here s> is the singlet state function 2 l/2(ag ga), Tq> is the the triplet sublevel function 2 l/2 (ag+got). <]>ab is a time-independent nuclear spin function which denotes the nuclear spin states of radicals 1 and 2 as a and b, respectively. Normally,... 

The phenomenon of electron spin polarization in photoexcited triplet molecules in solids has been known for quite some time (39,51,52). The mechanism is associated with the unequal populations of the triplet sublevels induced by the spin-selective nature of the spin-orbit coupling interactions which couple the excited singlet and triplet states during the intersystem crossing (ISC) process. In the presence of an external magnetic field the spin polarization in the molecular frame can be transferred to the laboratory frame for esr observation. Kim and Weissman (83,84) have recently demonstrated beautifully that the initial polarization following photoexcitation of the triplet molecules such as pentacene in dilute solid solution can be readily observed up to a temperature of 275°K. 

The qualitative features of the photoexcited triplet CIDEP of the radical pair can be summarized in terms of two quantities the sign of the zero-field splitting, D, and the relative populations of the triplet sublevels, A ... 

We reiterate our belief that there are still other classes of radical reactions which may produce electron polarization and which are awaiting to be discovered. For example, a possible mechanism that would produce polarized radicals is one in which the three triplet sublevels of a photoexcited molecule react at different rates. This concept was suggested by El-Sayed (51,53) in connection with some solid-state photochemical systems. Also, the observations reported by Gupta and Hammond (66) on the small change in the initial quantum yields in the benzophenone-sensi-tized isomerization of stibenes and piperylene in the presence of an external magnetic field have not been completely explained (14). 

Satisfaction of the third condition above depends on the rates of the spin-lattice relaxation processes between the spin sublevels. These rates are highly temperature dependent and depend also on the environment within which the molecular system is placed. In order to maintain a steady-state triplet sublevel population imbalance, the rates of spin-lattice relaxation must be slower than the rates at which the sublevels decay to the ground state. To reduce the spin-lattice relaxation rates to this level requires temperatures of the order of 4.2°K or lower, in the proper solvent. Whether or not spin-lattice processes can be "frozen" at liquid helium temperatures may even depend on the solid phase of the... 

In the preceding sections, properties of Pd(2-thpy)2 were discussed on the basis of time-integrated spectra. Due to the fact that the three triplet sublevels I, II, and III of T, could not be resolved spectrally by the methods discussed in Sects. 3.1.1 and 3.1.2, the observed properties were ascribed to Tj as one state. However, it is known from other investigations of the triplet state, in particular, from investigations of organic compounds that the three substates are zero-field split on the order of 0.1 cm and that these substates exhibit partly very different properties. (Compare, for example Refs. [ 105 -112]). In several respects the situation is similar for organometallic compounds like Pd(2-thpy)2 and related complexes. This important behavior is not well known. Therefore, it is subject of the present and the next two sections to focus on individual properties of the substates I, II, and III of the Tj state. 

After pulsed excitation into a higher lying singlet state (e. g. at Aexc = 337,1 nm A 29,665 cm-1, pulse width 3 ns), the three triplet sublevels of Pd(2-thpy)2 are populated individually and emit at low temperature, also individually. In Fig. 6, a typical emission decay curve is depicted employing a detection at the electro-... 

The studies presented in the previous Sect. 3.1.3 demonstrate that the three triplet sublevels I, II, and III of Tj are zero-field split by less than 1 cm and that they are thermally not equilibrated below T 2K. Thus at lower temperature, the three states emit independently with significantly different decay constants of Ti = 1200 ps. Til = 235 ps, and im = 130 ps, respectively. Moreover, the time-... 

Figure 8 shows further that the time-integrated spectrum is mainly determined by the fast-decaying triplet sublevels (compare Fig. 8 a to 8b), while the information about the slowly decaying sublevel I is very poor in the time-inte-... 

The results discussed above have shown that time-resolved emission spectroscopy can provide detailed insight into vibronic deactivation paths of triplet substates, even when the zero-field splitting is one order of magnitude smaller than the obtainable spectral resolution (= 2 cm ). This is possible at low temperature (1.3 K), because the triplet sublevels emit independently. They are not in a thermal equilibrium due to the very small rates of spin-lattice relaxation between these substates. In the next section, we return to this interesting property by applying the complementary methods of ODMR and PMDR spectroscopy to the same set of triplet substates. 

Pd(2-thpy)2 is dissolved in an n-octane matrix and is excited at low temperature (T < 2 K) by a c.w. source (non-pulsed, e.g. at A = 330 nm [61]). Additionally, microwave irradiation is applied and scanned in frequency. The microwave radiation can cause transitions between the triplet sublevels in the case of resonance, and thus, the previously different and non-thermalized steady state populations of the substates are usually altered. Under suitable conditions (see below) a change of the phosphorescence intensity will result due to microwave perturbation. Usually, this effect is very weak. Therefore, microwave pulse trains are applied, for example, with a repetition rate of 150 Hz. Thus, one can monitor the microwave-induced intensity changes by a phase-sensitive lock-in technique [90]. 

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