Tuning the role of charge-transfer states in intramolecular singlet exciton fission through side-group engineering

Abstract

Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. The challenge of controlling molecular packing and energy levels in the solid state precludes clear determination of the singlet fission pathway. Here, we circumvent this difficulty by utilizing covalent dimers of pentacene with two types of side groups. We report rapid and efficient intramolecular singlet fission in both molecules, in one case via a virtual charge-transfer state and in the other via a distinct charge-transfer intermediate. The singlet fission pathway is governed by the energy gap between singlet and charge-transfer states, which change dynamically with molecular geometry but are primarily set by the side group. These results clearly establish the role of charge-transfer states in singlet fission and highlight the importance of solubilizing groups to optimize excited-state photophysics.

Using Structurally Well-Defined Norbornyl-Bridged Acene Dimers to Map a Mechanistic Landscape for Correlated Triplet Formation in Singlet Fission

“…formation of 1 TT based on results from a growing number of pentacene-based systems in the literature. [5][6][7][10][11][12]14,15,[17][18][19] It is noted that in the communication of our synthetic methodology, we showed preliminary spectral evidence for 1 TT at 10 ps following photoexcitation. 18 However, that work did not establish time constants or yields.…”

Pentacene Dimers as a Critical Tool for the Investigation of Intramolecular Singlet Fission

“…Furthermore, the authors demonstrate that iSF can be directed to the direct mechanism, by incorporating dimer 44 into av iscous polymer matrix (polystyrene, polyvinyl acetate, polyethylene oxide)t hat impedes intramolecular rotation along the 6-,6'-bond. The combinedr esults for 44 and 45 are in good agreement with the other 6-,6'-linked pentacene dimers 27-30, 34-35 and 41,w hich also suggest the typical involvement of chargetransfer states in the iSF mechanism.I nc ontrastt o6 ,6'-linked pentacene dimers, [20,78,83,87,88] however,2 ,2'-linked pentacene dimers such as 4-6 do not show solvent dependency of the SF kinetics,t riplet decay,o rt riplet quantum yield, which supports ad irect mechanism in these cases (i.e.,Step 2, Figure 2).…”

“…In two consecutive reports, Lukmane tal. [87,88] adopted a structural motif that linked two pentacene units directly at the 6-position to give dimers 44 and 45 (Scheme 8). The syntheses began with the formation of quinone 46 via the reactiono f pentacenone 47 with pyridine-N-oxide (PNO) in the presence of FeSO 4 ,p yridine,a nd piperidine.…”

“…The authors report highly efficient iSF in solutions of pentacene dimers 44 and 45 with triplet quantum yields as high as 197 %. [87,88] Spectroscopic analysisr eveals that SF kinetics and efficiency of both dimers 44 and 45 can be adjusted via solvent polarity,a llowing for the energetic fine-tuning of locally excited states in relation to the charge-transfers tate. Furthermore,t he study suggests that the iSF mechanism is affected by the corresponding endgroup.…”

Liquid Crystallinity as a Self‐Assembly Motif for High‐Efficiency, Solution‐Processed, Solid‐State Singlet Fission Materials

“…[27] Theoretical calculations show that a slippage of only 2-3 Å along the main acene axis can significantly reduce orbital overlap reducing SF yields. [31][32][33][34] However, most of these studies were carried out in solution and often no mention was made of their SF yield in the solid state, which is important for practical exploitation. [25] For example, no SF in pentacene is reported for films grown on a polymer substrate, whereas the vapor deposited pentacene films show efficient SF.…”