Theory of Singlet—Triplet Exciton Fusion

Förster's theory of resonant transfer of energy has been used to calculate the probability per unit time of transfer of energy from an excited singlet to a triplet, raising the latter to a higher triplet. The magnitude of the effect is computed in applications to chlorophyll, rhodamine 6G, and anthracene, and in all cases it is found that the corresponding Förster radius R   0 STis of the order of 40 to 50 Å. The rate constant k ST for singlet—triplet exciton collisions resulting in quenching of the singlet is then calculated. It is found that the theoretical value of R   0 ST , for the case of crystalline anthracene, which is much larger than that predicted by Babenko et al., leads to better agreement with experiment and is to be preferred. Assuming a joint singlet—triplet diffusion constant of 10 −5 cm 2 /s, the rate constant k ST for chlorophyll in solution is predicted to be 4.5 × 10 −11 cm 3 /s.