Unveiling Singlet Fission Mediating States in TIPS-pentacene and its Aza Derivatives

Abstract

Femtosecond pump-depletion-probe experiments were carried out in order to shed light on the ultrafast excited-state dynamics of triisopropylsilylethynyl (TIPS)-pentacene and two nitrogen-containing derivatives, namely, diaza-TIPS-pentacene and tetraaza-TIPS-pentacene. Measurements performed in the visible and near-infrared spectral range in combination with rate model simulations reveal that singlet fission proceeds via the extremely short-lived intermediate (1)TT state, which absorbs in the near-infrared spectral region only. The T1 → T3 transition probed in the visible region shows a rise time that comprises two components according to a consecutive reaction (S1 → (1)TT → T1). The incorporation of nitrogen atoms into the acene structure leads to shorter dynamics, but the overall triplet formation follows the same kinetic model. This is of particular importance, since experiments on tetraaza-TIPS-pentacene allow for investigation of the triplet state in the visible range without an overlapping singlet contribution. In addition, the pump-depletion-probe experiments show that the triplet absorption in the visible (T1 → T3) and near-infrared (T1 → T2) regions occurs from the same initial state, which was questioned in previous studies. Furthermore, an additional ultrafast transfer between the excited triplet states (T3 → T2) is identified, which is also in agreement with the rate model simulation. By applying depletion pulses, which are resonant with higher vibrational levels, we gain insight into internal vibrational energy redistribution processes within the triplet manifold. This additional information is of great relevance regarding the study of loss channels within these materials.

(Electro)chemical Oxidation of 6,13‐Bis[tri(isopropyl)silylethynyl]pentacene to its Radical Cation and Dication

“…[24] Acene-based devices now indeed demonstrate higherf ield-effect mobilities than those based on amorphous silicon. [12] For example, starting from the parent pentacene scaffold, av ariety of molecules with substituents (mostly in the reactive positions 6a nd 13 of the parent structure [10,11,25,26] )o r with ring carbon atoms replaced by hetero atoms,such as bis(benzothio)quinodimethanes, for which the dications are isoelectronic to acenes, [27] or di-a nd tetraazapentacenes, [15,[28][29][30][31] have been investigated. [12] For example, starting from the parent pentacene scaffold, av ariety of molecules with substituents (mostly in the reactive positions 6a nd 13 of the parent structure [10,11,25,26] )o r with ring carbon atoms replaced by hetero atoms,such as bis(benzothio)quinodimethanes, for which the dications are isoelectronic to acenes, [27] or di-a nd tetraazapentacenes, [15,[28][29][30][31] have been investigated.…”

“…[14] Organic semiconductors provide furthera dvantages over their inorganic counterparts because their properties can be tuned through different molecular structures and functionalization. [12] For example, starting from the parent pentacene scaffold, av ariety of molecules with substituents (mostly in the reactive positions 6a nd 13 of the parent structure [10,11,25,26] )o r with ring carbon atoms replaced by hetero atoms,such as bis(benzothio)quinodimethanes, for which the dications are isoelectronic to acenes, [27] or di-a nd tetraazapentacenes, [15,[28][29][30][31] have been investigated.…”

The entangled triplet pair state in acene and heteroacene materials

“…22). To elucidate the behaviour of 1 (TT) in photovoltaic blend films and devices, we have studied bulk-heterojunction films of F 2 -TES ADT and [6,6]-phenyl C71 butyric acid methyl ester (PC 71 BM) in 4:1 molar ratio, in which we expect significant demixing.…”

“…Beyond applications of singlet fission and TTA, a clearer understanding of the 1 (TT) state will shed light on the nature of the interaction between triplet excitons and may open the way to new physics involving entangled bosons1011. Despite significant theoretical effort1011121314151617181920, few experiments have succeeded in directly probing the 1 (TT) state in any system2122232425, particularly in solid films relevant to eventual singlet fission devices. We are aware of a recent study of the 1 (TT) state in solid films of 6,13-Bis(triisopropylsilylethynyl) (TIPS)-tetracene, which crystallizes into a structure unique among singlet fission materials26.…”

Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission

“…Near the time origin of the experiment we observe a population of excitons exhibiting transient spectra similar to that expected for photogenerated singlet excitations ( Figure S4), whereas at longer time delays the transient spectra are assigned to triplet excitons. 16 While this particular triplet transition is formally forbidden in isolated pentacene molecules, 26 it has been observed in concentrated solutions 27 and solids 14,16 of pentacene and its derivatives. Aggregates of each pentacene derivative exhibit similar overall qualitative features and trends ( Figure S5).…”