Electronic levels triplet

If the cross-coupling is strong enough this may include a transition to a lower electronic level, such as an excited triplet state, a lower energy indirect conduction band, or a localized impurity level. A common occurrence in insulators and semiconductors is the formation of a bound state between an electron and a hole (called... 

This raises the question as to why one should observe any photoreduction at all with those compounds in which the lowest triplet clearly is of it -> rr character. The answer to this question lies in the fact that the electronic states are not pure in character. That is, an n - it state has some character obtained from a mixing of other electronic levels. This mixing results from vibronic and spin-orbit coupling/51-52 Likewise a it - w state has a degree of n —> w character obtained by mixing. It is this degree of n—> ir character which is responsible for the observation of photoreduction... 

As seen in the previous section, one characteristic of the triplet state is its paramagnetism. This alone would of course not suffice as a definition of the triplet since there are many odd-electron species that also exhibit paramagnetism but do not exist as triplets. Thus we might state that a triplet is a paramagnetic even-electron species. This still does not constitute a limiting definition since compounds containing even numbers of electrons may exhibit two, three, or even five distinct electronic levels. For example, when in a biradical the radical centers are separated by several carbon atoms as below, no interaction between the electron spins occurs and the radicals appear as two doublet states ... 

Since we have two unpaired electrons, 5=1, and the number of states is three. If we should have other even numbers of unpaired electrons, such as four or six, we could have three, five, or even as many as seven states. Thus we arrive at a definition for the triplet as a paramagnetic species possessing an even number of unpaired electrons and existing in a set of three energetically similar electronic levels which result from interaction of the electronic spin. Generally these three distinct electronic levels, between which transitions may be observed under certain conditions, are collectively referred to as the triplet state. 

Figure 11.2—Energy diagram showing the rotational, vibrational and electronic states of a molecule. On the scale of this diagram, the rotational and vibrational levels should be comparatively closer than the electronic levels because their relative values are in the order of 1000 50 1. The framed section shows, in the form of an energy diagram, the absorption of a photon. This figure represents the promotion of an electron from an occupied orbital (HOMO) to an unoccupied orbital (LUMO) giving rise to a singlet, which rapidly evolves into a more stable triplet state.

It should also be emphasized that in certain molecules the electrophilic character of the energy-transfer process may be sharply changed by resonance of the triplet mercury with a close-lying electronic level (NO) or by spin-orbit relaxation due to the presence of a suitable vibrational level in the substrate molecule (N2, CO, and partly H20, NH3, etc.). 

Triplet excited states of four derivatives of ubiquinone-6 (256), in which various ring substituents are progressively altered, have been studied by laser flash photolysis (265 nm) and pulse radiolysis (9—12MeV electrons). The triplet absorption spectra, extinction coefficients, lifetimes, energy levels, and quantum efficiencies of formation were determined.117... 

Jablonski tvas the first to suggest a simple set of three electronic levels to explain many of the phenomena of fluorescence and phosphorescence. Lewis and Kasha later identified one of these states as a triplet state. We do noli reproduce a Jablonski diagram because it has appeared so often. 

Upon collisional deactivation tra i-azoalkanes isomerize to the cis form. (This reaction is the only known synthetic route to m-azoalkanes.) The process is reversible, but — at least in the liquid phase — occurs only in direct photolysis or singlet sensitized photolysis and not in triplet sensitization. Decomposition in the condensed phase appears to exhibit similar behavior. These kinetic features cannot be rationalized in terms of the two electronic levels predicted by mo calculations and various other alternatives have been suggested as will be discussed below. 

The triplet low lying electronic level of 4000 cm" is estimated by analogy with that of CP (g). The other level of — 1 ... 

Mathews (1 ) from the fine structure of the 2500A absorption band. Mathews also reported Wg, and Herr and Pimentel (18) reported Wg, both from gas phase studies. Milligan et al. (19) have observed all three vibrations in matrix-isolated CPg 1222, 668 and 1102 cm". The electronic levels are from Mathews ( ) with the triplet level estimated by Simons ( ) to... 

The structure and electronic levels of the molecule have been established by Merer and Travis ( ) from an analysis of the rotational structure of the visible absoorption bands. The estimated triplet state is based on the electronic levels of CClg and CHg as given in these tables. The vibrational levels are from Jacox and Milligan (2 ) who investigated the infrared spectrum of matrix-isolated CHCl, and also estimated the missing frequency. 

The X and A electronic levels, ground state structure, and rotational constants are from Herzberg and Travis (4), th< level is estimated from the work of Schoen ( ). The vibrational frequencies have been observed in the matrix-isolated sp< by Milligan and Jacox (6, 7 ). Wasserman et al. (8) have confirmed the ground state is linear and triplet by electron paramagnetic resonance. 

Vibrational frequencies and quantum weights are estimated by comparison with those of WCl (g). The molecular configuration is assumed to be tetrahedral similar to those of WCl (g), SiCl (g), TiCl (g) and PbCl (g). The bond distance is estimated to be the same as that of MoClg(g). The principal moments of inertia are 1 Ig Iq 80.8964 x 10" g cm. The low lying electronic levels are estimated to be 0, 10000, and 30000, all triplets, by comparison with those of WCl (g). 

The electronic levels, bond angle and stretching frequencies are from infrared and emission spectra of TiOg isolated in rare-gas matrices ( 14). We assume the emission at 5295 A to be a forbidden triplet-singlet transition due to its similarity with CeOg... 

The orbital triplet states (transforming as the Ti or Ty irreducible representations) interact with crystal lattice vibrations, and this interaction affects also the SO splitting of these states. From the computational point of view, the effects of interaction between the electronic levels arising from the orbital triplet and surrounding ions can be modeled easier if each orbital triplet would be considered individually, without explicit treatment of mixture with electronic states from other... 

Figure 10-1. Energy scheme of the relevant electronic levels that are affecting stimulated emission in an isolated dye molecule in solution (a) and in a conjugated polymer (b). In both systems absorption of a photon induces a transition from the singlet ground state Sq to a vibronically excited state within the Si-manifold. After vibronic relaxation, a red-shifted emission can be observed. This transition can occur in a spontaneous as well as in a stimulated manner. A photoinduced absorption may occur as competing processes in both systems (PIA) due to the population of the triplet state T. A pair of oppositely charged carriers (GP) can be generated in the conjugated polymer due to dissociation of the 5i-state. This can lead to additional photoinduced absorption bands that compete with the stimulated emission.

Fig. 4. Relative position of lower excited singlet state (S,), triplet state (Tt) and metal ions (Mi), electronic levels, and transitions between them in metal ion complexes with organic ligands, (A) luminescence (fluorescence and phosphorescence) with levels of metal ion located above Sr levels, (B) phosphorescence with metal ion levels located between S,- and Trlevels, (C) sensitized (native) luminescence with metal ion levels located below Ti-levels...

The thermocliromism of dianthrone and its 2,2 -dicarboxylic acid45, which change from yellow through green to blue-green, has been explained as due to the existence of a distinct low triplet electronic level situated 3-5 kcal above the ground state. 

The singlet and triplet electronic levels are labeled S and T, respectively. Subscripts indicate the order of increasing energy the superscript v indicates that a molecule has an excess vibrational energy absence of a superscript indicates that the vibrational energy of the molecule is in thermal equilibrium a zero superscript indicates that a molecule is in the lowest vibrational level. For clarity, the vibrational and rotational levels are shown equally spaced. 

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