Ground-State Triplet Molecules

The existence of ground-state triplet carbenes, nitrenes, and their derivatives as intermediates (291) in 

2 transition and its hyperfine structure (b) Computer simulated spectrum. Taken from reference (43). 

Observed Zero-Field Splitting Parameters of Some Aromatic Hydrocarbons in their Phosphorescent Triplet State3 

Experimental Zero-Field Splittings of Some Nitrogen-Containing Aromatic Compounds in Their Phosphorescent Triplet State3 

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Molecules with two or more unpaired electrons may be divided into two classes by far the most common examples are molecules where the unpaired electrons are contained in a set of degenerate atomic or molecular orbitals with qualitatively similar spatial distributions, e.g., an octahedral Cr(m) (4A2g) or Ni(n) (3A2g) complex, a ground state triplet molecule like 02, or the excited triplet states of naphthalene or benzophenone. 

As Lewis recognized, this was true for organic molecules, but in other cases spin labeling might be reversed, as shown by the case of ground state triplet molecules, such as oxygen. 

Another case of interest is the influence of paramagnetic quenchers, which are fast relaxing paramagnetic species, on the nitroxide spectrum. Such quenchers are the ground-state triplet molecule oxygen, transition metal complexes with group spin S > 1/2, and lanthanide ions. If the longitudinal relaxation time of the quencher is much shorter than the lifetime of the collisional encounter complex, the effect on the nitroxide is a pure relaxation enhancement. 

The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

Minima in Ti are usually above the So hypersurface, but in some cases, below it (ground state triplet species). In the latter case, the photochemical process proper is over once relaxation into the minimum occurs, although under most conditions further ground-state chemistry is bound to follow, e.g., intermolecular reactions of triplet carbene. On the other hand, if the molecule ends up in a minimum in Ti which lies above So, radiative or non-radiative return to So occurs similarly as from a minimum in Si. However, both of these modes of return are slowed down considerably in the Ti ->-So process, because of its spin-forbidden nature, at least in molecules containing light atoms, and there will usually be time for vibrational motions to reach thermal equilibrium. One can therefore not expect funnels in the Ti surface, at least not in light-atom molecules. 

A related phenomenon has been observed in the benzophenone sensitized isomerization of c/y-piperylene.150 The measured quantum yield of cis to trans isomerization increased from 0.55 to 0.90 as the concentration of piperylene increased from 0.08 to lOAf. This observation can be rationalized as arising from addition of the piperylene triplet to a ground state diene molecule to give a biradical intermediate which can either cyclize to the dimer151 or dissociate to give two molecules of the more thermodynamically stable trans-isomer. This mechanism predicts that the quantum yield for the isomerization of /runs-piperylene to cw-piperylene should decrease with increasing diene concentration, an experiment that has not yet been reported. 

The mechanic. u proposed for this biphotonic delayed emission is that encounter between two triplet molecules gives rise to an intermediate species X. which subsequently dissociates into an excited and a ground state singlet molecule. The excited singlet molecule finally relaxes by emission of radiation. The rate constant of emission is not that of fluorescence but is governed by the rate of formation of triplet molecules. The various steps leading to emission process are ... 

Depending on the amount of thianthrenium perchlorate, /V,/V,/V, /V, /V",/V"-hexakis (anisyl)-l,3,5-triaminobenzene gives its cation radical, dication diradical, and trication triradical as well (Stickley et al. 1997). These species are stable in methylene chloride at low temperatures (at 298 K they can exist for several days). Spin and charge are localized at each oxidized nitrogen atom. The dication diradical and trication triradical structures are ground-state triplet and quartet molecules (Sato et al. 1997). 

The problem of the decay of exciplexes arises directly from their stability. There is no simple and unambigous relation between the electron donor ability of an excited complex A, electron withdrawing property of its reaction partner Q, and the decay rate constant kq. This experimental observation may be understood on the basis of two deactivation pathways of exciplexes (A — Q) a spin-forbidden conversion to the singlet ground state (GS) molecules A and Q, e.g. for a triplet exciplex 3(A — Q)... 

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