Construction of an Anion Photoelectron Spectrometer for Investigating Singlet Fission in Molecular Clusters PDF Download

Author: Steven John Kregel
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Languages : en
Pages : 172

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Book Description
The electronic structure of molecular systems is responsible for a wide range of phenomena including fluorescence, phosphorescence, and many effects in semiconductors. One of the most remarkable is singlet fission, in which a singlet excitation on one molecule is split into two triplet excitations on neighboring molecules. These two triplets relax independently, and if integrated into a solar cell, singlet fission would raise the theoretical efficiency of solar cells from 30% to 44%. Singlet fission is difficult to study due to its dependence on the interplay of singlet and triplet states, between which transitions are strongly forbidden. This work details the construction of an instrument capable of performing anion photoelectron spectroscopy, one of the few techniques with access to singlet and triplet states on equal footing. The spectrometer is a new design, capable of providing constant resolution over its entire range, and was used to observe the ground and first excited states of the polyaromatic hydrocarbons anthracene and fluoranthene, along with the simple aromatic oxide radicals phenoxy, 1-naphthoxy, and 2-naphthoxy. The spectra provide highly precise values for these species electron affinities and excited state term energies, along with vibrational frequencies for the Franck-Condon active modes. Presented here is the first anion photoelectron study of the fluoranthene molecule, and the highest-resolution study to date of the anthracene molecule. Results show the clear presence of vibronic coupling in the singlet and triplet manifolds of both molecules, and we determine the electron affinities and T1 term energies of these species with great precision. The aromatic oxide radicals contain an oxygen atom bonded to the aromatic ring, which imparts a significant dipole moment to the neutral system. This dipole moment is strong enough to weakly bind an electron and produces photoelectron spectra which are strongly dependent on photon energy. While interesting, we acquired spectra at wavelengths designed to minimize this effect, and assign the active vibrational modes, some of which have been predicted but unseen until now due to their low intensity. We also determine the electron affinities for the 1-naphthoxy and 2-naphthoxy radicals.