Singlet - Triplet Absorption Spectra

Singlet-Triplet Absorption Spectra.—Spin-orbit interactions and relative intensities of forbidden transitions in I2 and IX molecules (X = other halogen) 

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 So to the lowest triplet state is related to the triplet radiative lifetime Tp° by the following equation 

Since the transition is thus mixed with the CT state, the resulting So - Ti absorption appears somewhat diffuse and is slightly red-shifted 

In contrast to the technique of oxygen perturbation to obtain enhanced singlet-triplet absorption spectra, the use of heavy-atom perturbation results in no significant changes in the position or energy of the singlet - singlet 

---

Among the many excited singlet and triplet levels, 5i and Ti have distinct properties. They are in general the only levels from which luminescence is observed (Kasha rule) also most photochemical reactions occur from Sr or Ti. Here we discuss the characterization of the lowest triplet state by electronic spectroscopy. First we treat the theoretical background that allows the absorption spectra of conjugated systems to be described, and then we discuss the routes that lead to phosphorescence emission and Ti- - Sq absorption intensity. Details of the experimental methods used to determine triplet-triplet and singlet-triplet absorption spectra, as well as phosphorescence emission spectra are given in Chapters III, IV, and V. Representative examples are discussed. 

Fig. 22 A. The singlet-triplet absorption spectra of naphthalene in ethyliodide, ether, ethanol, toluene (FEET 2 2 1 1 and in several halogcnated benzenes at 77 K. (From Marchetti and Kearns, Ref. ))...

Rothman, W., Case, A., Kearns, D. R. Determination of singlet-triplet absorption spectra from phosphorescence excitation spectra a-bromonaphthalene. J. Chem. Phys. 43, 1067 (1965). 

Marchetti, A. P., Kearns, D. R. Investigation of singlet-triplet transitions by the phosphorescence excitation method. IV. The singlet-triplet absorption spectra of aromatic hydrocarbons. J. Am. Chem. Soc. 89, 768 (1967). 

The more convenient method of phosphorescence excitation was introduced by Kearns in 1965. Because emission spectroscopy is much more sensitive than absorption, particularly for phosphorescence that can efficiently be separated from stray light and fluorescence, the determination of singlet triplet absorption spectra by monitoring the resulting phosphorescence emission has often been successful with comparatively little effort. 

Since modern structural determination depends heavily upon H and C NMR analysis, information from IR and UV spectroscopies is less abundant than formerly. However, these spectroscopies are still useful to study photophysical properties, chemical behavior, and electronic orientation of the substituents in heteroaromatics. The pH dependence of both electronic and IR spectra of pyrazine has been examined to obtained p. values and information about the solute-solvent interactions, respectively <84JST(i 14)367). Electronic absorption taken in conjunction with fluorescence spectra indicate that 1,4-dihydro-2,3-quinoxalinedione exists in its diketo form irrespective of the nature of the solvent, by comparison with the spectra of the parent quinoxaline and 1,4-dimethyl-2,3-quinoxalinedione <87MI 603-01). New measurements of the singlet-triplet absorption spectra of pyrazine and pyrazine-d 4 at longer pathlengths and higher vapor pressures have been recorded <93aci537). 

Figures 3 and 4 give the singlet-triplet absorption spectra of E-hexatrlene and Z-hexatrlene in methylene iodide. The 0-0 band probably is situated at V 610 (46.9 kcal/mole) and 600 nm (47.7 kcal/mole), respectively In both cases It Is followed by a vl500-cm l progression.

Triplet excitation energies refer to the 0-0 bands of phosphorescence or singlet-triplet absorption spectra (Herkstroeter et al., unpublished results Saltiel, 1964). 

As was pointed out in the previous section, determination of curves such as that shown in Fig. 5 may lead to fairly precise assignments for So"-To energy gaps for many olefins. Since the precision, with which triplet excitation energies of olefins can be measured spectroscopically, is not good due to poor resolution of vibrational structure in singlet-triplet absorption spectra, as well as to extremely small extinction coefficients for such transitions, even in ethyl iodide, information gleaned from chemical spectra (e.g.. Fig. 5) can be extremely useful. 

---