Direct Observation of Correlated Triplet Pair Dynamics during Singlet Fission Using Ultrafast Mid-IR Spectroscopy

Abstract

Singlet fission is an exciton multiplication mechanism in organic materials whereby high energy singlet excitons can be converted into two triplet excitons with near unity quantum yields. As new singlet fission sensitizers are developed with properties tailored to specific applications, there is an increasing need for design rules to understand how the molecular structure and crystal packing arrangements influence the rate and yield with which spin-correlated intermediates known as correlated triplet pairs can be successfully separateda prerequisite for harvesting the multiplied triplets. Toward this end, we identify new electronic transitions in the mid-infrared spectral range that are distinct for both initially excited singlet states and correlated triplet pair intermediate states using ultrafast mid-infrared transient absorption spectroscopy of crystalline films of 6,13-bis­(triisopropylsilylethynyl) pentacene (TIPS-Pn). We show that the dissociation dynamics of the intermediates can be measured through the time evolution of the mid-infrared transitions. Combining the mid-infrared with visible transient absorption and photoluminescence methods, we track the dynamics of the relevant electronic states through their unique electronic signatures and find that complete dissociation of the intermediate states to form independent triplet excitons occurs on time scales ranging from 100 ps to 1 ns. Our findings reveal that relaxation processes competing with triplet harvesting or charge transfer may need to be controlled on time scales that are orders of magnitude longer than previously believed even in systems like TIPS-Pn where the primary singlet fission events occur on the sub-picosecond time scale.

Electron spin polarization generated by transport of singlet and quintet multiexcitons to spin-correlated triplet pairs during singlet fissions

“…6) is much shorter than L D and thus exciton migration is free from the excitontrapping 31 in the crystalline phase. The above SF-induced 1 T/T / T + T spin-transportation with s T ¼ 20 ps is compatible with the previous reports of the corresponding dissociation dynamics (3.2 ps) in the crystalline TIPS-Pn lm observed by the ultrafast spectroscopic measurements 18,19 and with a recent magnetic eld effect studied by Yago and co-workers 20 on SFinduced geminate triplet hopping kinetics (9 ps) in the uorinated diphenyl-hexatriene crystals that form parallel stacking structures. From these reported picosecond dissociations at room temperature and from the present ps-dissociation at 80 K, the SF-induced geminate hopping dynamics is roughly temperature independent.…”

“…3b at 80 K. In a previous report, the triplet diffusion motion was investigated in the TIPS-Pn lms by monitoring the triplet-triplet annihilation (TTA) kinetics. 19 In this, D T ¼ 2.7 Â 10 À5 cm 2 s À1 was obtained for a high crystallinity lm at room temperature. The TTA rate constant may become smaller at 80 K to result in D T < 10 À5 cm 2 s À1 .…”

“…This is because, for the TTA, the exciton migration occurs via threedimensional motion and thus is hindered in the minor disordered area during the long-distance diffusion (L D > 30 nm, as the diffusion length). 19 In contrast, the exciton-exciton separation (ca. 1 nm) in the primary T/T state (Fig.…”

“…Dissociation time constants ranging from subpicosecond to microsecond regions are reported. [16][17][18][19][20][21][22][23][24][25] However little is known about how the T + T dissociation or the decoupling 12,16,24 occurs from the strongly coupled TT state.…”

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

“…Dichloromethane solutions of mono-, trans-, and cis-Pt were excited at 650 nm and probed between 1900 and 2300 cm À1 to focus on the alkyne stretching region between 2000 and 2200 cm À1 . 56 Overall, mono-Pt shows a decay in the fsIR experiments that is similar to that observed in the TA experiments (vide supra). Transient (S 1 ) and (T 1 ) state spectra were identied based on a correlation with the TA data.…”