Singlet-triplet absorption

When D and A are similar molecules emission-reabsorption cannot be very important due to the usually small overlap of the emission and absorption spectra. Also, this mechanism should not be important for triplet-triplet energy transfer because of (a) low phosphorescence quantum yields in fluid solutions and (b) the low oscillator strengths for singlet-triplet absorption. 

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... 

Itoh, M. and Mulliken, R.S. (1969). Singlet-triplet absorption bands of methyl-substituted ethylenes. J. Phys. Chem. 73, 4332-A334... 

Population of the triplet manifold by direct singlet-triplet absorption is a very inefficient process, being spin-forbidden. Instead, the triplet manifold is populated indirectly by excitation into the singlet manifold followed by intersystem crossing. 

Since the photon emitted by D is absorbed by A, the same rules will apply to radiative energy transfer as to the intensity of absorption. Because singlet-triplet transitions are spin-forbidden and singlet-triplet absorption coefficients are usually extremely small, it is not possible to build up a triplet state population by radiative energy transfer. For this... 

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. ))...

Fig. 22 B. Exeitation spectrum at room temperature showing the intensity of delayed fluorescence of a naphthalene crystal as a function of the wavelength of the exciting light. The ordinate is proportional to the square of the singlet-triplet absorption coefficient. (From Avakian and Abramson, Ref.52))...

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). 

Hirota, N. Use of triplet-state energy transfer in obtaining singlet-triplet absorption in organic crystals. J. Chem. Phys. 44, 2199 (1966). 

Values of e span a very wide range, from near 0 to nearly 106. The former are characteristic of forbidden transitions such as singlet-triplet absorptions which are spin-forbidden the latter can be found in the absorption spectra of some dyes which have very strong colour in dilute solutions. Since values of e can vary between 0 and 106 for certain molecules, absorption spectra are often given on a logarithmic scale. 

A molecular orbital treatment of the UV spectrum of benzo[6]thio-phene has been given.68,72,74,75 The luminescence166,175 and phosphorescence176,177 spectra of benzo[6]thiophene have been recorded its singlet-triplet absorption spectrum has been measured.178... 

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

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