Phosphorescence radiative deactivation

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

The observed phosphorescence lifetime t neglecting quenching mechanisms is determined by a radiative deactivation path (Ijrr) and by a radiationless one (l/vri)... 

We now consider hydrogen transfer reactions between the excited impurity molecules and the neighboring host molecules in crystals. Prass et al. [1988, 1989] and Steidl et al. [1988] studied the abstraction of an hydrogen atom from fluorene by an impurity acridine molecule in its lowest triplet state. The fluorene molecule is oriented in a favorable position for the transfer (Figure 6.18). The radical pair thus formed is deactivated by the reverse transition. H atom abstraction by acridine molecules competes with the radiative deactivation (phosphorescence) of the 3T state, and the temperature dependence of transfer rate constant is inferred from the kinetic measurements in the range 33-143 K. Below 72 K, k(T) is described by Eq. (2.30) with n = 1, while at T>70K the Arrhenius law holds with the apparent activation energy of 0.33 kcal/mol (120 cm-1). The value of a corresponds to the thermal excitation of the symmetric vibration that is observed in the Raman spectrum of the host crystal. The shift in its frequency after deuteration shows that this is a libration i.e., the tunneling is enhanced by hindered molecular rotation in crystal. 

Phosphorescence the emission of light in a radiative deactivation involving initial and final states of different multiplicities. 

However, it is possible, to record emission spectra of individual substates by applying the methods of time-resolved spectroscopy. This has been shown, to our knowledge for the first time for transition metal complexes, by Yersin et al. in Ref. [58]. Having these time-resolved spectra available, it becomes possible for example, to elucidate individual vibronic radiative deactivation paths, as will be shown in this section. Interestingly, results that are deduced from a complementary method, namely from phosphorescence microwave double resonance (PMDR) studies [61], provide a nice agreement with the results deduced from time-resolved investigations. (Compare also Sect. 3.1.5.)... 

Electronically excited dye molecules can undergo a number of decay processes including radiative deactivation by fluorescence or phosphorescence and nonradiative deactivation by internal conversion and intersystem crossing. Because the marking event is so fast (<50 ns), triplet-state processes can be ignored and only the singlet-state manifold need be considered. 

ES2 has its own decay processes including radiative (k, termed phosphorescence) and non-radiative deactivation (k ), unimolecular reaction to product(s) (kp) and bimolecular quenching by energy or electron transfer (kq) to another species. For this model the ES2 lifetime is defined by... 

In contrast to PAS this method makes use of the radiative deactivation upon excitation by fluorescence or phosphorescence, as illustrated in Fig. 3. According to the Kasha rule fluorescence starts from the first excited singlet state Si, while phosphorescence is the result of ISC followed by a spin-forbidden Tj Sq transition, both reflecting the vibrational structure of the electronic ground state. 

Luminescence processes can further be categorized as fluorescence or phosphorescence. This distinction is based on the multiplicity of the two energy levels involved in radiative deactivation fi om the excited state. If the electron spin state of the two energy levels are the same, the process is referred to as fluorescence. In contrast, if the spin states are not the same, the process is appropriately called as phosphorescence. 

Luminescence includes phenomena such as fluorescence and phosphorescence. It comes from the radiative deactivation of excited matter following an excitation (the mechanism of the excitation, as well as fluorescence and phosphorescence is explained below). The excitation can come from light (photoliuninescence), electricity (electroluminescence), a chemical reaction (chemoluminescence or bioluminescence, if the reaction takes place in a biological system), or a mechanical stress (triboluminescence). We focus on photoluminescence, because most of the other excitation sources require special conditions and are, with the exception of electroluminescence, quite rare, especially when dealing with the luminescence of the lanthanides. 

Some other important characteristics of the emission are the rate of the deactivation of the excited state and the rate of the radiative deactivation. If we measure a time-resolved emission spectrum of the emission, we will observe that the emission spectrum loses some intensity as a function of time after a pulsed excitation. This emission decay is usually monoexponential and corresponds to the rate constant of the deactivation of the excited state, or observed deactivation rate constant kobs- It is important here not to confuse this rate constant with the rate constant of the radiative deactivation (in Figure 8, k and for the fluorescence and phosphorescence rate constants of the ligand, respectively, for the radiative rate constant of the lanthanide). Despite the fact that this method measures the decay of the emission, between each time step, the nonradiative processes (the k deactivation rate constants in Figure 8) also deactivate the excited state. To better visualize the decay rates, some equations are helpful. 

But both the ligand and the lanthanide can also be deactivated. The ligand can emit a photon by fluorescence from its singlet state or by phosphorescence from its triplet state, or can be deactivated (also from either its singlet or triplet excited state) by internal conversion, quenching, or any otiier nomadiative possibility. Even if the sensitization is efficient, i.e., if the isc and et is quick relative to the undesired deactivations, the lanthanide can also experience some nomadiative deactivations, so that the rate of the radiative deactivation of the lanthanide is in competition with nomadiative processes. 

Figure 1.10 Modified JablonskI diagram Illustrating the antenna effect. Abs - absorption, FI -fluorescence, Ph -phosphorescence, L - luminescence, ISC-intersystem crossing, ET - energy transfer, BT - back energy transfer, NR - non-radiative deactivation, - first excited singlet state, T - lowest excited triplet state, GS - ground state, f - emissive f excited state...

In the absence of triplet quenchers two processes compete for triplet deactivation radiative (phosphorescence) and nonradiative decay of the triplet to the ground state ... 

The various mechanisms which affect the Ti-<- Sq intensity and thus the radiative lifetime have been discussed earlier. For the class of aromatic hydrocarbons spin-orbit couphng is small and a t5q3ical value of about 30 sec for Tr seems appropriate However, the observed phosphorescence lifetimes vary greatly, demonstrating in most cases a dominating influence of the radiationless contribution. Siebrand and Williams > have noticed a very interesting correlation (Fig. 27) between the radiationless deactivation rate = 1 /rri and the triplet... 

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