Intersystem crossing inverse temperature

A rise in temperature increases the rate of vibrations and collisions, resulting in increased intersystem crossing, internal and external conversion. Consequently, the fluorescence intensity is inversely proportional to the temperature increase. Additionally, an increased temperature causes a red shift of the emission wavelength. 

The selection rules for intersystem crossing (ISC) were given in Section 9.2. If a higher triplet is produced, it too will obey Kasha s Rule and relax to Ti. Since its two modes of relaxation to So, phosporescence and ISC, both involve spin-inversion, Ti will be comparatively long-lived. Except when trapped at very low temperatures, there is ample time for the three components of the triplet T , and to reach equilibrium before it phosphoresces, relaxes by ISC or reacts chemically. 

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