Fluorescence spin-allowed

Fluorescence Photon emission. Fluorescence involves a radiative transition between states of the same multiplicity (spin allowed), usually from the lowest vibrational level of the lowest excited singlet state, Si. Si(v = 0) —> S0 + hv... 

A distinction is made between fluorescence and phosphorescence according to the change in the spin quantum number between the initial and final states. When these quantum numbers are the same AS = 0 and the transition is spin-allowed (but there may be other factors to be taken into account as well, as we shall see) this emission is then defined as a fluorescence. If there is any change in the spin quantum number, AS = 0, the transition is spin forbidden and is defined as a phosphorescence. The important difference between these two forms of luminescence resides in their kinetics a fluorescence is shortlived, with emission lifetimes in the range 1 ns to 1 ts, while phosphorescence lifetimes go from 1 ms to many seconds or even minutes. 

Delayed Fluorescence. By definition the fluorescence emissions are spin-allowed radiative transitions of atoms or molecules they have short lifetimes, of the order of ns to a few hundred ns. There are however some cases where molecules emit the very same fluorescence spectra but with much longer decays and often with complex non-exponential kinetics. These... 

In absorption spectra, the probability of a transition is expressed by its oscillator strength, /. While / is close to unity for allowed transitions, it is of the order of 10 -10 and 10 -10 respectively for spin-allowed transitions within the 3d and 4f configurations. Efficient conversion of a UV radiation to light (as needed in fluorescent lamps) requires strong absorption properties at the wavelength of incident photons. Figure 1 shows a comparison of the luminescence intensity for 4f- 5d (A/ = 1) and 4f- 4f (A/ = 0) excitations in the case of a terbium compound. 

Similar to other d -d systems, the drnuclear iridium(I) complex [Ir(/x-pz)(COD)]2 (23) showed spin-allowed and spin-forbidden (da — pa) absorption bands at 498 and 585 nm, respectively. Under ambient conditions, the complex displayed fluorescence at 564 nm and phosphorescence at 687 nm, which were assigned to singlet and triplet excited states of (da — pa) character. The triplet excited state of the complex was a powerful reductant with an excited-state reduction potential E° (Ir2+ ) of-1.81 V vs. SSCE. Facile electron transfer reactions occurred between the excited complex and methyl viologen and other pyridinium acceptors. The absence of an inverted effect for the forward electron transfer reactions, and the presence of such inverted behavior for the back-electron-transfer reactions were observed and explained. ... 

In the electron-transfer process generalized in Eq. 1, one of the components of the reactant state may be fluorescent. This spin-allowed radiative process will thus be in competition with the nonradiative electron-transfer reaction and the two processes will contribute to the overall decay of the reactant state. The intrinsic lifetimes of fluorescent molecular states range typically from 10 to longer than 10 s. The occurrence of electron transfer involving the fluorescent state will shorten its lifetime and measurement of this quantity will therefore allow computation of the rate constant for electron transfer. 

Emission or luminescence is referred to as fluorescence or phosphorescence, depending on whether it corresponds to a spin-allowed or a spin-forbidden transition, respectively. Similarly, radiationless transitions between states of the same multiplicity and of a different multiplicity are known as internal conversion (1C) and intersystem crossing (ISC), respectively. 

Emission of a photon from an electronically excited state is referred to as luminescence. Fluorescence and phosphorescence can be differentiated depending on whether the transition is between states of equal or different multiplicity and hence spin-allowed or spin-forbidden. (Cf. Section 5.1.1.) Thus, for molecules with singlet ground states fluorescence constitutes a pathway for deactivating excited singlet states whereas phosphorescence is observed in the deactivation of triplet states. 

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