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

Abstract

Donor–acceptor (D–A) copolymers have shown great potential for intramolecular singlet fission (iSF). Nonetheless, very few design principles exist for optimizing these systems for iSF, with very little knowledge about how to engineer them for this purpose. In recent work, a fundamental trade-off between the main electronic ingredients required for iSF capable D–A coplanar copolymers was revealed. Still, further investigations are needed to understand these limitations and learn how to bypass them. In this work, we propose to induce torsion as an effective way to circumvent the limits of the coplanar approach. We disclose the potential of noncoplanar copolymers with inherently low triplet energies that encompass all the characteristics required for iSF beyond the limiting values associated with fully coplanar systems. Our findings shed some light on the electronic structure aspects of D–A copolymers for iSF and offer a new avenue for the rational design of novel promising candidates.

Optimizing the Thermodynamics and Kinetics of the Triplet-Pair Dissociation in Donor–Acceptor Copolymers for Intramolecular Singlet Fission

“…These hold inherently low triplet energies and have been identified as promising building blocks for SF. 29 , 32 , 34 , 35 , 50 , 51 The results show that 1 TT-to-2T 1 reaction energies follow opposite trends with respect to the vertical 5 TT– 1 TT splitting in a way that larger splitting results in less favorable 1 TT-to-2T 1 thermodynamics ( Figure 4 a) in agreement with the literature. 36 , 47 Thus, TT splitting can be used as the reference of TT dissociation energies.…”

“…This is usually the case in extended conjugated systems such as polymers where the characteristic low-lying multi-excitonic singlet is strongly mixed with the lowest mono-electronic transition of the same symmetry. 48 , 49 In a previous work, we showed how the TT binding energy can be reduced in A–D–A trimers by means of inducing dihedral torsion between the D and A molecular building blocks, 35 therefore potentially accelerating the triplet-pair dissociation rates in agreement with the model established for SF dimers. 36 , 47 Still, it is unknown whether this model applies to extended D–A copolymers and more importantly which features control the intrachain triplet-pair dissociation process in these systems.…”

“…This D–A copolymer displays a significant dihedral torsion (∼60°) between the donor and acceptor units due to the steric repulsion between their functional groups and, as a consequence, reduces the 5 TT– 1 TT splitting down to ∼0 eV. 35 In agreement with this result, the computed TT-to-2T 1 reaction energies of the extended chain are close to 0 eV regardless of the spin state ( Table S6.1 ). The dissociation barrier, however, increases up to ∼0.5 eV for the minimum energy intermediate that connects both structures ( Figure S6.1 ).…”

“… 33 , 34 A comprehensive structure–property analysis of D–A copolymers allowed us to recognize the role of dihedral torsion in modulating and pushing the CT and local character of the excitations beyond the limit of the coplanar approach. 32 , 35 On the one side, a larger CT resulted in larger S 1 / 1 TT electronic coupling in a series of modified TDO-based copolymers, promoting fast S 1 -to- 1 TT dynamics. 28 On the other side, localized T 1 in the acceptor reduces the 1 TT binding energy (evaluated as the vertical energy difference with the 5 TT state), which is traditionally associated with better triplet–triplet dissociation rates.…”

Heteroatom oxidation controls singlet–triplet energy splitting in singlet fission building blocks

“…S14, ESI†), which may have consequences on the SF decay pathway and overall mechanism. 15 Less impact is expected on the photophysical properties of the S 1 state of S , S - and N -oxidized compounds.…”