Triplet-singlet interconversion

Data on duorescence, phosphorescence, excited-state lifetimes, transient absorption spectra, and dye lasers are tabulated in Ref. 16. The main nonduorescent process in cyanine dyes is the radiationless deactivation Sj — Sg. Maximum singlet-triplet interconversion ( 52 ) methanol for carbocyanines is about 3% (maxLgrp > 0.03), and the sum [Lpj + st] I than 0.10. 

Fig. 13.6. Energy diagram showing potential energy curves for interconversion of ground-state (5 o) and first and second singlet (,Sj and S2) and first triplet (7"]) excited states. The angle is the C—C—C bond angle at C-2 and C-3. [From D. Grimbert, G. Segal, and A.

J.R. Bolton We have not carried out any experiments as yet on metalloporphyrins linked to quinones. The spin selection rules should be operative in the radical pair. The singlet state of the radical pair should be able to return to the ground state with no spin inhibition however, the triplet state of the radical pair can return to the ground state only via spin interconversion or via the triplet state of the porphyrin. 

The 3Fe(CO)3(H2) species is the one that can be formed most readily from the separated reactants, but there are several other minima on the [Fe,C3,03,H2] singlet and triplet potential energy surfaces that could conceivably also be formed in the reaction. Most notably, we located isomers 1Fe(CO)3(H2), 3Fe(CO)3(H)2, and 1Fe(CO)3(H)2, located, respectively, at 4.8, 14.9, and —1.4 kcalmol-1 with respect to 3Fe(CO)3(H2) (see Table III and Scheme 5, and Ref. (24)). The triplet dihydride species lies higher in energy than separated H2 and Fe(CO)3, and hence is unlikely to be formed in this system. However, the two singlet species are lower in energy and might be formed hence, we have also explored the pathways for interconversion. 

Quantum-chemical calculations of the transition state of the enantiomerization of 6, that is, for the interconversion of (M)-6 and (P)-6, support the diradical 6-D [13, 18] (Scheme 6.4). Most probably, its singlet state is a few kcal mol-1 more stable than the triplet state. The zwitterions 6-Zi and 6-Z2 are excited states [1], However, if a methylene group directly attached to the allene system in 6 is replaced by an appropriate heteroatom, either 6-Za or 6-Z2 may be strongly stabilised (see Section 6.3.5). 

Although interconversions of the two spin states under matrix-immobilized spectroscopic conditions have not yet been observed in the cases of the pyrrole derivative 46f and the bis(phenoxyl) 53, reversible thermal isomerizations of the singlet and triplet states have been reported in the cases of the bis(aminoxyl) 54 and the carbene 55. 

The hydrogen nucleus is classified as a Eermi particle with nuclear spin I = 1/2. Because of Pauli exclusion principle, hydrogen molecule is classified into two species, ortho and para. Erom the symmetry analysis of the wave functions, para-hydrogen is defined to have even rotational quantum number J with a singlet nuclear spin function, and ortho-hydrogen is defined to have odd J with a triplet nuclear spin function. The interconversion between para and ortho species is extremely slow without the existence of external magnetic perturbation. 

Other probes of the initial distribution have been tried. A few studies of the time-dependent ion-pair recombination probability have been made recently with picosecond pulse radiolysis equipment. A magnetic field alters the rate of interconversion between a triplet and singlet ion-pair. If this rate is fast enough to compete with the recombination rate of ions, the yield of recombined ion-pairs is markedly affected by the magnetic field. 

Only a few examples of these planar tetrahedral equilibria have been successfully investigated and there remain unanswered questions about the dynamics. Nevertheless, it seems reasonable to regard these equilibria as intramolecular isomerizations in which the spin state change is not an important factor in the dynamics. Electron spin affects the bonding, geometry, and thermodynamics of the two isomers, but there is apparently sufficient mixing of the singlet and triplet states to allow their interconversion to be adiabatic. 

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