Tuning Singlet Fission in π-Bridge-π Chromophores

Abstract

We have designed a series of pentacene dimers separated by homoconjugated or nonconjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling between the singlet exciton and triplet pair states. Using transient absorption spectroscopy to investigate photophysics in these molecules, we find that homoconjugated dimers display desirable excited-state dynamics, with significantly reduced recombination rates as compared to conjugated dimers with similar singlet fission rates. In addition, unlike conjugated dimers, the time constants for singlet fission are relatively insensitive to the interplanar angle between chromophores, since rotation about σ bonds negligibly affects the orbital overlap within the π-bonding network. In the nonconjugated dimer, where the iSF occurs with a time constant >10 ns, comparable to the fluorescence lifetime, we used electron spin resonance spectroscopy to unequivocally establish the formation of triplet-triplet multiexcitons and uncoupled triplet excitons through singlet fission. Together, these studies enable us to articulate the role of the conjugation motif in iSF.

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

“…The first section will deal with 2,2'-linked pentacene dimers, and this is followed by a section describing 6,6'-linked pentacene dimers (see Figure 3 for pentacene numberings cheme). Many of these syntheses rely on metal-catalyzed cross-coupling protocols such as the Suzuki and Sonogashira reactions, althoughi nnovatives ynthe-ses based on, for example, Diels-Alder cycloadditions [53] have recently appeared. Beyond pentacene, av ariety of other carbon-rich oligomeric chromophores have been explored for iSF and contribute to unraveling the underlying mechanism, these reports are outside the scope of this Minireview,a nd for current examples, see references [54][55][56][57][58][59][60][61][62][63][64][65][66][67].…”

Azaacene Dimers: Acceptor Materials with a Twist

“…Campos et al. investigated rigid homoconjugated pentacenes in which a spiro‐bridge is incorporated for intramolecular SF (Figure ) …”

“…Campos et al investigated rigid homoconjugated pentacenes in which a spiro-bridge is incorporated for intramolecular SF (Figure 1). [8] Connecting arenes using 1 gives rigid polymers of intrinsic microporosity, [9] which, although being soluble, are porous due to nonefficient packing of the polymer strands.Azaacenes are tailor-made and electronically modulated by substitution, yet their strong tendency to p-stack results in crystalline domains in the solid-state and has hindered their widespread use as active layer acceptors in bulk-heterojunction [a] L.…”

“…Campos et al investigated rigid homoconjugated pentacenes in which a spiro-bridge is incorporated for intramolecular SF (Figure 1). [8] Connecting arenes using 1 gives rigid polymers of intrinsic microporosity, [9] which, although being soluble, are porous due to nonefficient packing of the polymer strands.…”

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.…”