Emission property and the electronic configuration

The ground states of most molecules are singlet and contain a pair of electrons with antiparallel spins. On electronic excitation, the ground and the excited electronic configurations are delinked and singlet and triplet states are possible. 

This is equivalent to coulombic repulsional interaction between two equal charge densities The S T energy split AEst is equal to 

A small value of AEst facilitates intersystem crossing. We expect singlet state to be fast depleted along this pathway if the lowest excited state is of (n, it ) type. This pathway is further promoted due to the fact that t y by a factor of ten, due to the forbidden character of 

In aromatic hydrocarbons, short-lived ](it, it ) is the lowest excited state and energy gap between (n, it ) and 8(it, it ) states is large. Both these factors are conducive to fluorescence emission and in general aromatic hydrocarbons are good fluorescer. Sometimes, the prediction may not come true if a higher triplet state T2 is available near the St state such as in anthracene. In such cases, fluorescence and phosphorescence both are observed at low temperatures in suitable solvent medium specially when S, and T, are states of different symmetry type. Some data correlating AEst and 4 isc are given in the Table 5.2. 

Rate constants of unimolecular processes can be obtained from spectral data and are useful parameters in photochemical kinetics. Even the nature of photoproducts may be different if these parameters change due to some perturbations. In the absence of bimolecular quenching and photochemical reactions, the following reaction steps are important in deactivating the excited molecule back to the ground state. 

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