Singlet fission in pentacene dimers

Abstract

Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photonto-current conversion efficiencies beyond the 30% ShockleyQueisser 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.acene oligomers | excited states | singlet fission | multireference perturbation theory | time-resolved spectroscopy S inglet fission (SF) is a spin-allowed process to convert one singlet excited state into two triplet excited states, namely a correlated triplet pair (1). The ability to effectively implement SF processes in solar cells could allow for more efficient harvesting of high-energy photons from the solar spectrum and allow for the design of solar cells to circumvent the Shockley-Queisser performance limit (2). Indeed, several recent studies have demonstrated remarkably efficient solar cell devices based on SF (3-6).One requirement that needs to be met to achieve SF is that the photoexcited chromophore in its singlet excited state must share its energy with a neighboring ground-state chromophore. As such, the potential of coupled chromophores to afford two triplet excited states via SF has been elucidated in, for example, a tetracene dimer with an SF yield of around 3% (3, 7). Additionally, past experiments in single-crystal, polycrystalline, and amorphous solids of pentacene have documented that the efficiency of SF relates to the electronic coupling between these two chromophores (8, 9). Hence, molecular ordering in terms of crystal packing, that is, proximity, distances, orbital overlap, etc., is decisive with respect to gaining full control over and to finetuning interchromophoric interactions in the solid state (10, 11). Of equal importance are the thermodynamic requirements, namely that the energy of the lowest-lying singlet absorbing state must match or exceed the energy of two triplet excited states (S 1 ≥ 2T 1 ) (11). In light of both aspects, hydrocarbons such as acenes--tetracene, pentacene, hexacene--and their derivatives are at the forefront of investigations toward a sound understanding and development of molecular building blocks for SF. In tetracenes, the singlet-and triplet-pair energy levels are nearly degenerate (S 1 = 2T 1 ), leaving no or little standard enthalpy of reaction for SF (12). In solution, the latter is, however, offset by sizable entropy rendering the process rather slow and, thus, inefficient (13). In addition, the low SF yield relates to the dimer geometry. Its nature hinders electronic coupling through space, leaving only thro...

Tuneable Singlet Exciton Fission and Triplet–Triplet Annihilation in an Orthogonal Pentacene Dimer

“…[ 2,8 ] Interestingly, a recent study of pentacene dimers separated by a phenyl spacer unit achieved triplet yields above 100% in spite of using the same symmetric bonding motifs of the earlier tetracene dimers. [ 9 ] In this work, we report highly effi cient intramolecular SEF in a new type of covalent dimer, with triplet yields of up to 192 ± 3%. The molecule used in this study, 13,13′-bis(mesityl)-6,6′-dipentacenyl (DP-Mes, Figure 1 a), consists of two pentacenes directly bonded through a single C C bond with two bulky mesityl groups at the meso -positions.…”

“…By contrast, the recent study of pentacene dimers separated by a phenyl spacer showed a doubling of extinction coeffi cient upon dimerization. [ 9 ] Likewise, earlier studies of 9,9′-bianthryl, diphenylisobenzofuran dimers and tetracene dimers all reveal weakly coupled chromophores with extinction coeffi cients twice those (1) and reaction conditions for DP-Mes formation. b) DFToptimized (B3LYP/6-311G**) ground-state geometry of DP-Mes, demonstrating the orthogonality of the pentacenes (details in the Supporting Information).…”

Triplet transport in thin films: fundamentals and applications

“…Current investigations into the intermediates in singlet fission, e.g. bound triplet pair excimers 99 or strongly interacting triplet pairs, 136 etc., enabled by various monomer or dimer configurations 97,[137][138][139][140][141][142] will shed light on the dielectric environment needed to stabilize triplet excitons. Such interdisciplinary approaches will reveal the fundamental structure-property relationships affecting triplet diffusion in thin films, and ultimately enable the engineering of optoelectronic devices making use of multi-excitonic processes.…”

Inducing Multiple Reactions with a Single Photon

“…This means that the quantum yield ϕ ¼ N prod =N phot of the reaction, which describes the percentage of molecules that end up in the desired reaction product per absorbed photon, has a maximum value of 1. This limit can be overcome in some specific cases, such as in photochemically induced chain reactions [2-4], or in systems that support singlet fission to create multiple triplet excitons (and thus electron-hole pairs) in solar cells [5,6].Polaritonic chemistry, i.e., the potential to manipulate chemical structure and reactions through the formation of polaritons (hybrid light-matter states) was experimentally demonstrated in 2012 [7], and has become a topic of intense experimental and theoretical research in the past few years [8][9][10][11][12][13][14][15][16][17][18]. However, existing applications and proposals have been limited to enhancing or suppressing the rates of single-molecule reactions.…”

“…This means that the quantum yield ϕ ¼ N prod =N phot of the reaction, which describes the percentage of molecules that end up in the desired reaction product per absorbed photon, has a maximum value of 1. This limit can be overcome in some specific cases, such as in photochemically induced chain reactions [2-4], or in systems that support singlet fission to create multiple triplet excitons (and thus electron-hole pairs) in solar cells [5,6].…”