Singlet/triplet states, distinction between

Fig. 2.2. Distinction between singlet and triplet states, using formaldehyde as an example.

For organic molecules, the lifetime of the singlet state ranges from tens of picoseconds to hundreds of nanoseconds, whereas the triplet lifetime is much longer (microseconds to seconds). However, such a difference cannot be used to make a distinction between fluorescence and phosphorescence because some inorganic compounds (for instance, uranyl ion) or organometallic compounds may have a long lifetime. 

Figure 4 Correlation between Pariser-Parr-Pople bond orders (35), and Parameter Method 3 bond lengths of anthracene. AP is the bond-order change between the ground state and the singlet, triplet and charged states discussed in the text for the five distinct bonds of anthracene AR is the corresponding bond-length change. The line has slope AP/AP = -0.36[58].

Phosphorescence is a photoluminescence phenomenon that is quite similar to fluorescence. Understanding the distinction between these two phenomena requires an understanding of electron spins and the difference between a singlet state and a triplet state. Ordinary molecules that are not free radicals exist in the ground state with their electron spins paired. A molecular electronic state in which all electron spins are paired is said to be a singlet state. The ground state of a free radical, on the other hand, is a doublet state, because the odd electron can assume two orientations in a magnetic field. 

Ultra-Violet Spectra Lack of Distinction Between Singlet and Triplet States... 

Total bond-order. 62. 63. 64, 65. 68 Total jr-electron energy. 35-38 Transition state. 123 Triplet states. 33, 76, 78. 79-81. 134-137 distinction between singlet and, 134-137... 

At the beginning of this section the crucial difference between radicals and diradicals needs to be discussed. Whereas the former has only one spin state, the latter may exist in two distinct spin states, the singlet and the triplet state. The following discussion is limited to diradicals that are formally derived from cr-bond homolysis and are not concerned with broken (twisted) 7c-bonds. 

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