MERBROMIN (MERCUROCHROME) AND OTHER XANTHENE DYES - QUANTUM YIELDS OF TRIPLET SENSITIZER GENERATION AND SINGLET OXYGEN FORMATION IN ALCOHOLIC SOLUTIONS

Absorption, fluorescence and phosphorescence maxima, singlet and triplet energies, and quantum yields of fluorescence PHI(F) and intersystem crossing PHI(T) of highly purified disodium salts of fluorescein dyes in alcoholic solutions are reported. The method of PHI(T) determination by "external heavy atom enhanced photo-oxygenation" is described. Oxygen exerts no measurable effect on the short-lived excited singlet states but quenches the triplet states quantitatively in O2-saturated solutions, mainly by energy transfer yielding singlet oxygen, 1O2((1)DELTA(g)). By increasing the number and atomic weight of halogen substituents in the fluorescein molecule, the quantum yields of triplet formation PHI(F) and 1O2 generation PHI(DELTA) are enhanced, whereas the efficiency S(DELTA), with which interactions of the triplet excited dye with 3O2 yield singlet oxygen, seems to decrease. Within experimental error, PHI(F) + PHI(T) is equal to unity for the xanthene dianions. For the cationic sensitizer rhodamine B, however, this sum equals about 0.7, indicating that deactivation of the singlet excited state by internal conversion plays an appreciable role. Merbromin, a HgOH-substituted dibromofluorescein used world-wide as an inexpensive antiseptic (mercuchrome(TM), mercurochrome(TM)), shows quantum yields of 0.16 +/- 0.04 for triplet formation and 0.14 +/- 0.02 for 1O2 generation in alcoholic solution. Why mercury does not exhibit the expected internal heavy atom effect on the intersystem crossing process, remains to be elucidated.