Emission of light in the form of fluorescence often accompanies deactivation of an electronically excited species. Fluorescence is defined as the radiative transition between two electronic states of the same spin multiplicity. Most organic molecules have “paired” electrons in their ground state molecular orbital configuration. The spins are balanced (e.g., s 1 = + 1 2 https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203912096/65d60b20-996e-4c7a-8cd9-766bfff12068/content/eq1.tif"/> and s 2 = − 1 2 https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203912096/65d60b20-996e-4c7a-8cd9-766bfff12068/content/eq2.tif"/> , S = Σs = 0) and the spin multiplicity (Ms = 2S + 1 = 1) is singlet. Alternatively, inversion of the spin of the excited electron results in the two unpaired electrons having the same spin orientation. The overall spin S is 1 ( s 1 = + 1 2 , s 2 = + 1 2 ) https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203912096/65d60b20-996e-4c7a-8cd9-766bfff12068/content/eq3.tif"/> , the spin multiplicity (Ms = 2S + 1) is 3, and a triplet state results.