Mechanism of Singlet Fission in Thin Films of 1,3-Diphenylisobenzofuran

Abstract

In order to elucidate the mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran (1) we have performed ultrafast transient absorption spectroscopy as a function of sample temperature and excitation fluence on polycrystalline thin films composed of two polymorphs. Our earlier investigations revealed that films enriched in a particular polymorph of 1 displayed near 200% efficiency for triplet formation at 77 K, while films composed primarily of a second polymorph had a very low triplet quantum yield. Present data confirm the triplet yield disparities in the two polymorphs and demonstrate the distinct fates of the initially prepared singlets in films of different structure. Singlet fission is inhibited in the more stable polymorph due to rapid excimer formation and trapping. The less stable polymorph undergoes highly efficient singlet fission with a dominant time constant of 10-30 ps and without strong thermal activation. Transient absorption measurements with varying excitation fluence indicate that singlet-singlet annihilation is a primary competitor of singlet fission at higher fluence and that fission from higher-lying states can also contribute to the triplet formation process. Measurements employing different excitation energies and sample temperatures reveal the role that trapping processes play in attenuating the triplet quantum yield to produce the complex temperature dependence of the singlet fission yield. The rate constants for singlet fission itself are essentially temperature independent.

Exploring Non-Condon Effects in a Covalent Tetracene Dimer: How Important Are Vibrations in Determining the Electronic Coupling for Singlet Fission?

“…Under this model, we consider BT1 a two-level system weakly coupled to a bath of harmonic oscillators (i.e., its vibrational modes), with the model Hamiltonian given by eq 15. (15) In this expression, aα † and aα are creation and annihilation operators of bath mode α, g i,α is proportional to the energy gradient of electronic state i with respect to mode α, g i,α = (ℏ/ (2m α ω α )) 1/2 (∂E i /∂q α ) and σx = |S 1 S 0 ⟩⟨ 1 TT| + | 1 TT⟩⟨S 1 S 0 |. Critical to eq 15 in terms of the general findings of this work is the term c α , which is proportional to the electronic coupling gradient with respect to mode α.…”

Intramolecular Singlet Fission in an Antiaromatic Polycyclic Hydrocarbon

“…[8] In these covalent dimers, SF is an intramolecular process referred to as iSF,which was also reported in donor-acceptor polymers. [12] On the other hand, although the unimolecular mechanism minimizes the difficulty in adjusting the solidstate molecular packing, iSF in covalent dimers and donoracceptor polymers still needs CT-mediated electronic coupling between intramolecular chromophores. [12] On the other hand, although the unimolecular mechanism minimizes the difficulty in adjusting the solidstate molecular packing, iSF in covalent dimers and donoracceptor polymers still needs CT-mediated electronic coupling between intramolecular chromophores.…”

Potential Use of Squarates and Croconates as Singlet Fission Sensitizers

“…This strategy was proposed by Michl and has been successfully used afterwards . The viability of this approach, that is, the design of SF sensitizers by means of their diradicaloid character, has been also demonstrated experimentally ,,,. In this sense, squarates and croconates appear as potential candidates to undergo SF efficiently.…”