SENSITIZED TRIPLET FORMATION OF CHLOROPHYLL‐A AND ß‐CAROTENE

The naphthalene‐sensitized formation of triplet excited chlorophyll‐ a (Chl‐ a ) and all‐trans ß‐carotene has been studied by pulse radiolysis. The rate constants for transfer of triplet energy from naphthalene to Chl‐ a and all‐trans ß‐carotene in benzene at 25°C are (3.6 ± 0.6)·10 9 M ‐1 s ‐1 and (10.7 ± 1.2)·10 9 M ‐1 s ‐1 , respectively. The decays of the excited triplet states of naphthalene, Chl‐ a and all‐trans ß‐carotene all follow a mixed first‐and second‐order mechanism. The first‐order rate constant for triplet decay is strongly dose dependent for naphthalene but only slightly dependent and independent of dose for Chl‐ a and all‐trans ß‐carotene, respectively. The rate constants for triplet‐triplet annihilation are (1.4 ± 0.3)·10 9 M ‐1 s ‐1 for Chl‐ a and (3.6 ± 0.4)·10 9 M ‐1 s ‐1 for all‐trans ß carotene. The nearly constant ratio k(ß‐carotene)/k(Chl‐ a ) for the bimolecular triplet energy transfer rate constants is discussed in terms of the molecular shapes of the two molecules. The energetics of the triplet‐triplet annihilation of all‐trans ß‐carotene are discussed, and it is proposed that production of the excited 1 A B state may be a major route in the annihilation process.