Triplet states spectrum

The complex of tetrakis(aminomethyl)methane was studied by ESR at 77 K423 and the triplet state spectrum at —1500 G, due to AM/= 2, interpreted assuming dimeric oxovanadium(IV) with V—V of 5.7 A. 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

The latter rule is rigidly obeyed in the observed spectrum of helium. From the accurately known energy levels it is known precisely where to look for transitions between singlet and triplet states but none has been found. 

Indazoles have been subjected to certain theoretical calculations. Kamiya (70BCJ3344) has used the semiempirical Pariser-Parr-Pople method with configuration interaction for calculation of the electronic spectrum, ionization energy, tt-electron distribution and total 7T-energy of indazole (36) and isoindazole (37). The tt-densities and bond orders are collected in Figure 5 the molecular diagrams for the lowest (77,77 ) singlet and (77,77 ) triplet states have also been calculated they show that the isomerization (36) -> (37) is easier in the excited state. 

Optical detection of magnetic resonance (ODMR) was attempted for measurements of the pH effects on the triplet state of purine to investigate the protonation site of purine at low temperatures (78JA7131). The ODMR spectrum did not show the presence of more than one triplet state at liquid helium temperatures. Since the protonated tautomers 1H,9H (3a) and H,1H (3b) have similar bond structures, their triplets should have similar zero-field parameters and are thus not easy to distinguish by ODMR. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

Like TME, the diradical 15 was shown to have nearly degenerate singlet and triplet states by magnetic susceptibility [60, 61], although the early works by Dowd identified a triplet ground state on the basis of ESR spectrum [62, 63], The UCCSD(T) calculations predicted a singlet ground state with a small S-T gap of... 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Oxo-2,5-cyclohexadienylidene [83] was generated in solid argon at 9 K by irradiation of diazo compound [84] with visible light (A>495 nm) (Sander et al., 1988 Bucher and Sander, 1992 Bucher et al., 1992). The IR, UV, and esr spectra of [83] were in accord with a structure having a triplet state with one delocalized electron. In the IR spectrum of the carbene [83] the r (CO) mode was found at 1496 cm which indicates a bond order of the C—O bond considerably less than 2. The low-temperature reaction of carbene [83] with CO generated the keto-ketene [85]. Irradiation (A = 543 10 nm) of [83] led to its transformation into a very labile species, presumed to be [86], which rearranged back to [83] not only under UV or... 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

For a triplet state with a lifetime of 10 sec or longer the concentration of triplet molecules ( 10-3 mole) is sufficiently high under continuous illumination that one can obtain a UV spectrum of the excited molecules. 

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

We will see that a Hamiltonian term identical in form also arises from spin-orbit coupling, but first we will pause to see the effect of this Hamiltonian on the energy levels and ESR spectrum of a triplet-state molecule. The spin triplet wave functions can be written in the notation S,ms) ... 

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