Actinometry in Monochromatic Flash Photolysis: The Extinction Coefficient of Triplet Benzophenone and Quantum Yield of Triplet Zinc Tetraphenyl Porphyrin

The extinction coefficient ε T , of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, Δ D 0 T , under demonstrably linear conditions where incident excitation energy, E 0 , and ground state absorbance, A 0 , are both extrapolated to zero. The result, 7220 ± 320 M ‐1 cm ‐1 at 530 nm, validates and slightly corrects many measurements relative to benzophenone of triplet extinction coefficients made by the energy transfer technique, and of triplet yields obtained by the comparative method. As E 0 and A 0 both decrease, Δ D 0 T becomes proportional to their product. In this situation, the ratio R = (1/ A 0 )(dΔ D 0 T /d E 0 ) = (ε T ‐ ε G )φ T . Measurements of R , referred to benzophenone, give (ε T ‐ ε G )φ T for any substance, without necessity for absolute energy calibration. Both absolute and relative laser flash measurements on zinc tetraphenyl porphyrin (ε T ‐ ε G at 470 nm = 7.3 × 10 4 M ‐1 cm ‐1 ) give φ T = 0.83 ± 0.04.