Concentration dependence of quenching and excimer formation

The fluorescence intensity F per unit volume is proportional to the fractional light absorption I per unit volume per unit time. 

VM vd monomer and excimer emission frequency. A excimer, AHa enthalpy of excimer formation. 

In some molecules, the interaction can develop into a stronger force and the interplanar distance further reduced to form stable photodimers through covalent bonds. For example, anthracene forms a photodimer and no excimer emission is observed, whereas some of its derivatives with bulky substituents which hinder close approach give excimer fluorescence. In 9-methylanthracene both photodimer formation and excimer emission is observed. 9, 10-diphenylanthracene neither forms a photodimer nor emits excimer fluorescence due to steric hindrance. These observations are tabulated in the Table 6.3, which shows that the nature of the excited state is also important. 

The nature of the lowest energy excited state and the probability of excimer emission 

Besides the prompt excimer emission, delayed excimer emission has been observed. The mechanism is through triplet-triplet annihilation process (Section 5.9B)  

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