Chromophore Multiplication To Enable Exciton Delocalization and Triplet Diffusion Following Singlet Fission in Tetrameric Pentacene.

A tetrameric pentacene, PT, has been used to explore the effects of exciton delocalization on singlet fission (SF). For the first time, triplet decorrelation through intramolecular triplet diffusion was observed following SF. Transient absorption spectroscopy was used to examine different decorrelation mechanisms (triplet diffusion versus structural changes) for PT and its dimeric equivalent PD on the basis of the rate and activation barrier of the decorrelation step. Charge-separation experiments using tetracyano-p-quinodimethane (TCNQ) to quench triplet excitons formed through SF demonstrate that enhanced intersystem crossing, that is, spin catalysis, is a widely underestimated obstacle to quantitative harvesting of the SF products. The importance of spatial separation of the decorrelated triplet states is emphasized, and independent proof that the decorrelated triplet pair state consists of two (T1 ) states per molecule is provided.

Singlet fission in pentacene dimers.

Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley-Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.

Two-Photon Absorption in Pentacene Dimers: The Importance of the Spacer Using Upconversion as an Indirect Route to Singlet Fission.

In this proof of concept study, we show that intramolecular singlet fission (iSF) can be initiated from a singlet excited state accessed by two-photon absorption, rather than through a traditional route of direct one-photon excitation (OPE). Thus, iSF in pentacene dimers 2 and 3 is enabled through NIR irradiation at 775 nm, a wavelength where neither dimer exhibits linear absorption of light. The adamantyl and meta-phenylene spacers 2 and 3, respectively, are designed to feature superimposable geometries, which establishes that the electronic coupling between the two pentacenes is the significant structural feature that dictates iSF efficiency.

Influence of the heavy-atom effect on singlet fission: a study of platinum-bridged pentacene dimers.

The process of singlet fission (SF) produces two triplet excited states (T1 + T1) from one singlet excited exciton (S1) and a molecule in its ground state (S0). It, thus, possesses the potential to boost the solar cell efficiency above the thermodynamic Shockley-Queisser limit of 33%. A key intermediate in the SF mechanism is the singlet correlated triplet pair state 1(T1T1). This state is of great relevance, as its formation is spin-allowed and, therefore, very fast and efficient. Three fundamentally different pathways to formation of 1(T1T1) have been documented so far. The factors that influence which mechanism is associated with which chromophore, however, remain largely unknown. In order to harvest both triplet excitons independently, a decorrelation of the correlated triplet pair state to two individual triplets is required. This second step of the SF process implies a change in the total spin quantum number. In the case of a dimer, this is usually only possible if the coupling between the two pentacenes is sufficiently weak. In this study, we present two platinum-bridged pentacene dimers in which the pentacenes are coupled strongly, so that spin-decorrelation yielding (T1 + T1) was initially expected to be outcompeted by triplet-triplet annihilation (TTA) to the ground state. Both platinum-bridged pentacene dimers undergo quantitative formation of the (T1T1) state on a picosecond timescale that is unaffected by the internal heavy-atom effect of the platinum. Instead of TTA of (T1T1) to the ground state, the internal heavy-atom effect allows for 1(T1T1)-3(T1T1) and 1(T1T1)-5(T1T1) mixing and, thus, triggers subsequent TTA to the (T1S0) state and minor formation of (T1 + T1). A combination of transient absorption and transient IR spectroscopy is applied to investigate the mechanism of the (T1T1) formation in both dimers. Using a combination of experiment and quantum chemical calculations, we are able to observe a transition from the CT-mediated to the direct SF mechanism and identify relevant factors that influence the mechanism that dominates SF in pentacene. Moreover, a combination of time-resolved optical and electron paramagnetic resonance spectroscopic data allows us to develop a kinetic model that describes the effect of enhanced spin-orbit couplings on the correlated triplet pair state.

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

When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron-hole pairs, leading to a predicted ∼50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.