Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers

Abstract

aWe show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) × 10 12 s −1, respectively, in room temperature solutions.

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

“…The S 1 lifetime is systematically shortened with increasing polarity, from 1.17 ps in hexane to ∼300 fs in the most polar solvents. As previously noted812, this direct correlation of SF time constant to solvent polarity suggests triplet formation is mediated by CT states, though these are not directly resolved. However, we highlight the intriguing spectral shape of S 1 in the NIR.…”

“…To circumvent these difficulties, recent efforts have focused on intramolecular SF in covalent dimers, most notably in various dimers of pentacene89101112. These materials provide uniquely detailed insight into the SF mechanism, particularly with regard to the influence of molecular geometry and charge-transfer (CT) states.…”

“…These are generally calculated to exhibit strong coupling to singlet and triplet-pair states, even when they are energetically inaccessible, and could drive efficient triplet formation. These CT states are difficult to observe directly, but evidence of solvent-polarity-dependent SF rates in two classes of pentacene dimer81112 and dimers of 1,3-diphenylisobenzofuran16 supports such a model. Likewise, a recent report of terrylenediimide dimers17 found direct, solvent-dependent competition between CT state formation and SF.…”

Unified model for singlet fission within a non-conjugated covalent pentacene dimer

“…These are the singlet ground state |S 0 S 0 〉, the correlated triplet pair (or multi-exciton) state | 1 (T 1 T 1 )〉, the locally excited singlet states | 1 (S 1 S 0 )〉 and | 1 (S 0 S 1 )〉 and the CT states | 1 (CA)〉 and | 1 (AC)〉 (refs 2, 3, 26, 34). To assess the relevance of the different SF mechanisms, we have calculated the effective coupling, V eff , of | 1 (S 1 S 0 )〉 to | 1 (T 1 T 1 )〉 (or | 1 (S 0 S 1 )〉 to | 1 (T 1 T 1 )〉)3536:…”

“…Moreover, it has been hypothesized that the electronic coupling between the pentacenes is critical to render CT-state-mediated SF energetically accessible. While no evidence for CT-mediated SF was observed in pentacene dimers linked directly or using p -phenylene oligomers2425, work on π-conjugated and cross-conjugated pentacene dimers2627 has shown that effective π-conjugation enables fast, CT-state-mediated SF.…”