Direct observation of slow intersystem crossing in an aromatic ketone, fluorenone

Abstract

Direct measurements of Single vibronic Level InterSystem Crossing (SLISC) have been performed on the fluorenone molecule in the gas phase, by time resolved photoelectron and photoion spectroscopy. Vibronic transitions above the S₁ nπ* origin were excited in the 432–420 nm region and the decay of S₁ and growth of T₁³ππ* could be observed within a 10 ns time domain. The ionization potential is measured as 8.33 ± 0.04 eV. The energy of the first excited triplet state of fluorenone, T₁ has been characterized directly at 18 640 ± 250 cm⁻¹. The internal conversion of S₁ to S₀ is found to amount to ∼15% of the population decay, thus ISC is the dominant electronic relaxation process. ISC, although favored by the S₁¹nπ*–T₁³ππ* coupling scheme, is 3 orders of magnitude less efficient than in the similar molecule benzophenone. Thus, the planarity of the fluorenone molecule disfavors the exploration of the configuration space where surface crossings would create high ISC probability, which occurs in benzophenone through surface crossings. The time evolution of S₁ fluorenone is well accounted for by the statistical decay of individual levels into a quasi-continuum of T₁ vibronic levels.