Parallel triplet formation pathways in a singlet fission material

Abstract

Harvesting long-lived free triplets in high yields by utilizing organic singlet fission materials can be the cornerstone for increasing photovoltaic efficiencies potentially. However, except for polyacenes, which are the most studied systems in the singlet fission field, spin-entangled correlated triplet pairs and free triplets born through singlet fission are relatively poorly characterized. By utilizing transient absorption and photoluminescence spectroscopy in supramolecular aggregate thin films consisting of Hamilton-receptor-substituted diketopyrrolopyrrole derivatives, we show that photoexcitation gives rise to the formation of spin-0 correlated triplet pair 1(TT) from the lower Frenkel exciton state. The existence of 1(TT) is proved through faint Herzberg-Teller emission that is enabled by vibronic coupling and correlated with an artifact-free triplet-state photoinduced absorption in the near-infrared. Surprisingly, transient electron paramagnetic resonance reveals that long-lived triplets are produced through classical intersystem crossing instead of 1(TT) dissociation, with the two pathways in competition. Moreover, comparison of the triplet-formation dynamics in J-like and H-like thin films with the same energetics reveals that spin-orbit coupling mediated intersystem crossing persists in both. However, 1(TT) only forms in the J-like film, pinpointing the huge impact of intermolecular coupling geometry on singlet fission dynamics.

Singlet Fission in Perylene Monoimide Single Crystals and Polycrystalline Films

“…However, the temperature-dependent TA results for dp-PMI thin films show that correlated triplet pair formation and dissociation are significantly slower at lower temperatures (Figure S12), which may introduce triplet formation via SO-ISC as a competitive decay pathway. Such temperature-dependent switching between SF and SO-ISC has previously been observed in tetracene single crystals, while parallel SF and ISC decay mechanisms have been reported for diketopyrrolopyrrole thin films …”

“…Following 580 nm excitation, the TREPR spectrum exhibits an eae / aea spin polarization pattern (low to high magnetic field), where ( e ) and ( a ) denote emissive and enhanced absorptive signals, respectively. Simulating the spectrum gives zero-field splittings of | D | = 1350 MHz and | E | = 80 MHz, similar to those of PDI derivatives with relative sublevel populations [ p x , p y , p z ] = [0.24, 0.71, 0.04]. , Interestingly, the observed TREPR spectrum suggests triplet formation is induced via spin–orbit intersystem crossing (SO-ISC) at 85 K since it does not exhibit the unique polarization pattern typically observed for SF-born decorrelated triplets ( aee / aae when D > 0) . While it is possible that the polarization pattern is consistent with SF-born triplets that undergo selective spin relaxation through the T z sublevel, as with some tetracene derivatives, we observe no evolution of the triplet spectrum within the time window of the experiment that would suggest this spin relaxation (Figure S19).…”

“…20,41 Interestingly, the observed TREPR spectrum suggests triplet formation is induced via spin−orbit intersystem crossing (SO-ISC) at 85 K since it does not exhibit the unique polarization pattern typically observed for SF-born decorrelated triplets (aee/aae when D > 0). 42 While it is possible that the polarization pattern is consistent with SF-born triplets that undergo selective spin relaxation through the T z sublevel, as with some tetracene derivatives, 43 we observe no evolution of the triplet spectrum within the time window of the experiment that would suggest this spin relaxation (Figure S19). Therefore, the observed polarization pattern indicates that the mechanism of triplet formation at 85 K is likely SO-ISC, which disagrees with the observation of SF in optical experiments at room-temperature.…”

“…Such temperaturedependent switching between SF and SO-ISC has previously been observed in tetracene single crystals, 44 while parallel SF and ISC decay mechanisms have been reported for diketopyrrolopyrrole thin films. 42 It is well-known that ISC rates in isolated or solution-phase rylene imides are very slow, leading to very low triplet yields. 45 This is evident for dp-PMI based on the near-unity fluorescence quantum yield observed when it is dissolved in DCM.…”

Surface energetics mediated charge transfer and exciton transfer in cation-adsorbed rubrene/PbS nanocrystal hybrids

“…Considering the diffusion model of SF in solution, the triplet exciton related dynamics should occur at a relatively later temporal region [39,40]. Therefore, by applying ns-TA spectroscopy, we compare the decay dynamics of DBE and triplet PIA at 510 nm in corresponding spectra, as shown in figures 5(a) and (b), respectively.…”