Lowest triplet states

Because of this spin selection rule, atoms which get into the lowest triplet state, 2 Si, do not easily revert to the ground 1 state the transition is forbidden by both the orbital and spin selection rules. The lowest triplet state is therefore metastable. In a typical discharge it has a lifetime of the order of 1 ms. 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

Figure 7. Potential energy diagram of CH2O. After excitation to specific rovibrational levels of Si, internal conversion leads to highly excited molecules in the ground electronic state So, whereas intersystem crossing populates the lowest triplet state Ti.

The shape of these orbitals does not change strongly between the lowest triplet state ( 2g) with the configuration. .. b2) 3e) (2b f and the lowest quintet state ( Aig) with the configuration. .. (Ifi2) 3e) (2fii) ... 

To obtain more information on this point, let us examine the data given in Table 3.6<42-47> for some substituted benzophenones. The data in Table 3.6 indicate that benzophenone derivatives having lowest triplet states of n->TT character undergo very efficient photoreduction in isopropyl alcohol. Those derivatives having a lowest it- -it triplet, on the other hand, are only poorly photoreduced, while those having lowest triplets of the charge-transfer type are the least reactive toward photoreduction. In additon, in some cases photoreduction is more efficient in the nonpolar solvent cyclohexane than in isopropanol. This arises from the solvent effect on the transition energies for -> , ir- , and CT transitions discussed in Chapter 1 (see also Table 3.7). 

For compounds having lowest triplet states, factors that in-... 

As stated in Chapter 1, transitions involving a change in multiplicity are spin forbidden. However, for reasons which we will consider later, such transitions do indeed occur although with very low transition probabilities in most cases. The intensity of an absorption corresponding to a transition from the ground state S0 to the lowest triplet state Tx is related to the triplet radiative lifetime t ° by the following equation[Pg.114]

The previous sections have shown that cis-trans isomerization of stilbene can take place via the lowest triplet state of stilbene. The question to be considered now is whether the isomerization upon direct photolysis takes place via the singlet state, the triplet state, or a vibrationally excited ground state.a 7 81 50)... 

PPP calculations indicate the lowest triplet state of o-xylylene to be 8000 cm-1 above the ground state singlet. 

More recent investigations of the photodimerizations of cyclopentenone and cyclohexenone<134) have revealed that both reactions result from the lowest triplet states of the enones (Et > 70 kcal/mole), which is probably 77- 77 in character/1331 Both photodimerizations occur via reversibly... 

We conclude that the Norrish type II rearrangement in PET proceeds, for the most part, via the lowest triplet state. 

Fig. 22k. The reasonably good fit between the measured points and the triplet-triplet (T i -> Tn) absorption spectrum of flavin (dotted lines, 174>117)) again suggests a flavin photoreceptor. These authorsIS4) assume an effective decrease of the lifetime of the lowest triplet state by quick triplet-triplet (Tj Tn) turnovers, thereby inhibiting photochemistry (i.e. initiation of phototropism).

Delbriick et al., (1976) have recently sought to determine whether or not the stimulation of Phycomyces involves the excitation of the lowest triplet state of riboflavin. They determined an action spectrum of light-growth response between 575 and 630 nm using a tunable laser beam and taking advantage of the null method described above. This action spectrum was compared with an action spectrum obtained by computer extrapolation of a phototropic action spectrum covering 445—560 nm. 

Comparison of the computed and the measured relative cross sections between 575 and 630 nm shows a difference which is indicative for the lowest triplet state of a riboflavin. The action spectrum has a small peak at 595 nm and a cutoff at 585 nm and a tail extending beyond 630 nm. The peak at 595 nm may be regarded, within the accuracy of this system, as the lowest triplet state of riboflavin and can be compared to the value of 600 nm obtained by phosphorescence emission for this state (Song and Moore, 1974). 

Subsequent to the formation of a potentially chemiluminescent molecule in its lowest excited state, a series of events carries the molecule down to its ground electronic state. Thermal deactivation of the excited molecule causes the molecule to lose vibrational energy by inelastic collisions with the solvent this is known as thermal or vibrational relaxation. Certain molecules may return radia-tionlessly all the way to the ground electronic state in a process called internal conversion. Some molecules cannot return to the ground electronic state by internal conversion or vibrational relaxation. These molecules return to the ground excited state either by the direct emission of ultraviolet or visible radiation (fluorescence), or by intersystem crossing from the lowest excited singlet to the lowest triplet state. 

The two lowest triplet states are the 1 3BU and 1 kg states. The former is mainly described by the HOMO —> LUMO single excitation while the latter is a mixture of single excitations of proper symmetry, i.e. HOMO—1 —> LUMO and HOMO -> LUMO + 1. 

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

The lifetime of the singlet excited state (the fluorescence lifetime TF) is of the order of picoseconds to 100 nanoseconds (10—12 - 10-7 seconds) and can now be measured accurately using pulsed laser excitation methods and other techniques. Since the radiative transition from the lowest triplet state to the ground state is formally forbidden by selection rules, the phosphorescence lifetimes can be longer, of the order of seconds. 

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