Bridge Resonance Effects in Singlet Fission

Abstract

A major benefit of intramolecular singlet fission (iSF) materials, in which through-bond interactions mediate triplet pair formation, is the ability to control the triplet formation dynamics through molecular engineering. One common design strategy is the use of molecular bridges to mediate interchromophore interactions, decreasing electronic coupling by increasing chromophore−chromophore separation. Here, we report how the judicious choice of aromatic bridges can enhance chromophore− chromophore electronic coupling. This molecular engineering strategy takes advantage of "bridge resonance", in which the frontier orbital energies are nearly degenerate with those of the covalently linked singlet fission chromophores, resulting in fast iSF even at large interchromophore separations. Using transient absorption spectroscopy, we investigate this bridge resonance effect in a series of pentacene and tetracene-bridged dimers, and we find that the rate of triplet formation is enhanced as the bridge orbitals approach resonance. This work highlights the important role of molecular connectivity in controlling the rate of iSF through chemical bonds and establishes critical design principles for future use of iSF materials in optoelectronic devices.

Pushing the Limits of the Donor–Acceptor Copolymer Strategy for Intramolecular Singlet Fission

“…While extensive theoretical work has been made for acene dimers in the analysis of the optimal spatial arrangement that enhance the coupling between the S 1 and 1 TT states, ,,− little is known about the role of the degree of coplanarity in the iSF capabilities of copolymers. It has been shown that iSF in dimers of acene derivatives is modulated through-bond with conjugated linkers, controlled by the relative position of the orbital energies of the linker with respect to the chromophores as well as the coplanarity between them. − In this context, it is expected that coplanarity also plays an important role in the iSF capabilities of −A–D–A– copolymers in which the D unit plays the role of an active linker in the iSF process.…”

The Role of Quantum Interference in Intramolecular Singlet Fission

“…= 240 ps), which have double the bridge length. 24,25 This result underscores the significance of bridge connectivity in dictating triplet pair formation dynamics. Interestingly, we find that the magnitude of the difference between DQI and CQI effects diminishes with increasing bridge length, in the order of Ph > N > A. Analogous to Ph bridged compounds, large differences in the rate constants between DQI (27N) and CQI (26N) connectivity points are observed in naphthalene bridged dimers.…”

“…= 48 ps), and others. 25 As the FMOs of the bridging units approach resonance with those of the SF chromophores, the rates of triplet pair formation are enhanced. Here, we find that this concept can also be extended to DQI bridge connectivity, such that moving from P-13Ph-P → P-27N-P → P-27A-P yields an initial increase in 𝛕SF from 391 ps (Ph) to 882 ps (N) followed by a decrease to 253 ps (A).…”

“…With these considerations in mind, it can be reasoned that bridge resonance effects have a stronger impact on the anthracene-bridged DQI chromophores. 25,26 For example, the DQI chromophore P-27A-P undergoes faster iSF than P-13Ph-P, despite having a bridge that is longer by ~4.5 Å. Such a bridge resonance effect is even more pronounced in DQI tetracene analogs, where 𝜏 !"…”

“…The synthesis of pentacene and tetracene bridged dimers with 𝛽 = 14Ph, 26N, and 26A have been previously reported. 25,24 Similar cross-coupling conditions are used to synthesize 𝛽 = 13Ph, 27N, 15N, 16N, and 27A, as detailed in the SI. We note that TIPS (triisopropylsilylethynyl) groups are installed on all anthracene, tetracene, and pentacene chromophores for solubility and stability.…”