| Popis: |
Absorption, fluorescence and phosphorescence maxima, singlet and triplet energies, and quantum yields of fluorescence φ F and intersystem crossing φ T of highly purified disodium salts of fluorescein dyes in alcoholic solutions are reported. The method of φ 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 O 2 -saturated solutions, mainly by energy transfer yielding singlet oxygen, 1 O 2 ( 1 Δ g ). By increasing the number and atomic weight of halogen substituents in the fluorescein molecule, the quantum yields of triplet formation φ T and 1 O 2 generation φ Δ are enhanced, whereas the efficiency S Δ , with which interactions of the triplet excited dye with 3 O 2 yield singlet oxygen, seems to decrease. Within experimental error, φ F +φ 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 1 O 2 generation in alcoholic solution. Why mercury does not exhibit the expected internal heavy atom effect on the intersystem crossing process, remains to be elucidated. |
