Vacancy control in acene blends links exothermic singlet fission to coherence

Abstract

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet pair energy alignments. For pentacene, where fission is exothermic, coherent mixing between the photoexcited singlet and triplet-pair states is promoted by vibronic resonances, which drives the fission process with little sensitivity to the vacancy concentration. Such vibronic resonances do not occur for endothermic materials such as tetracene, for which we find fission to be fully incoherent; a process that is shown to slow down with increasing vacancy concentration.

Distinguishing between Triplet-Pair State and Excimer Emission in Singlet Fission Chromophores Using Mixed Thin Films

“…13,20,30 Importantly, we can not only conclude that SF is the main decay path of the free exciton, but our results for ADT blends also allow us to elucidate its microscopic mechanism. The linear dependence of the SF rate on the chromophore concentration has been observed before for TET and referred to as the replacement effect, 52,53 indicating that SF occurs via incoherent population transfer from S 1 to 1 (TT). The deviation from this linear dependence found for TET blends with low ADT fractions can be explained by the similar band gaps of ADT and TET, which makes an excitation of solely ADT impossible and can also enable an energy transfer from ADT to TET.…”

“…50 However, the admixture of these states can be considered small for several reasons. From a theoretical standpoint, calculations on pentacene 37,55 and tetracene 56 only showed a small admixture of these states to the 1 (TT) state and since SF is occurring via the coherent mechanism in pentacene, 52 while, in contrast, we showed that in ADT the incoherent SF mechanism is driving SF, it can be expected that the admixture of these states is even smaller in ADT than in pentacene. 57 Additionally, from an experimental viewpoint, a large admixture of such states to 1 (TT) would make the 1 (TT) state visible in absorption and photoluminescence spectroscopy without the need for Herzberg−Teller coupling, 55 which, however, contrasts with our results.…”

“…An additional noteworthy finding is the decrease of the intensity of the RSL relative to the free exciton emission with decreasing ADT concentration. For processes which strongly depend on interactions between neighboring molecules, like excimer formation or triplet-pair state formation, such an intensity decrease results from the replacement of nearest ADT neighbors by TET as well as 6PH . Importantly, as the probability for a given number of ADT neighbors follows a binomial distribution and is thus independent of the molecule (TET or 6PH) replacing ADT, this intensity decrease is identical in ADT blends with TET or 6PH.…”

“…For processes which strongly depend on interactions between neighboring molecules, like excimer formation or triplet-pair state formation, such an intensity decrease results from the replacement of nearest ADT neighbors by TET as well as 6PH. 52 Importantly, as the probability for a given number of ADT neighbors follows a binomial distribution 52 and is thus independent of the molecule (TET or 6PH) replacing ADT, this intensity decrease is identical in ADT blends with TET or 6PH. To summarize these first results, the coexistence of the RSL with emission of the free exciton makes these ADT based blends an ideal model system to investigate the origin of the RSL and its impact on SF using TRPL measurements.…”

Distinction of charge transfer and Frenkel excitons in pentacene tracedviainfrared spectroscopy

“…Compared to the Frenkel exctions, CT excitons usually possess only small oscillatory strength making these hard to excite and observe experimentally. 3 The CT-like excitonic contributions are considered as an important ingredient in photovoltaic applications and fast singlet exciton fission rates in molecular solids, 28–30 which has a potential to improve the power conversion efficiency of solar cells. Therefore, identifying the CT-like nature of the excitations at the absorption edge is crucial.…”