Singlet-Fission-Born Quintet State: Sublevel Selections and Trapping by Multiexciton Thermodynamics

Abstract

Singlet fission (SF) is expected to exceed the theoretical limit of the solar cell efficiency. Quintet (Q) state generation in triplet-triplet pair is essential for preventing the unwanted loss of SF-born multiexciton through singlet channels, although little is known on the primary multiexciton spin dynamics following the intermolecular SF. In this study, time-resolved EPR revealed the intermolecular multiexciton dynamics, energetics and geometries in aggregated 6,13-bis(triisopropylsilylethynyl)pentacene and 2-phenyl-6,11-bis(triisopropylsilylethynyl)tetracene in diluted frozen solution. We have demonstrated sublevel selective generations of excited quintet states (|Q⟩, |Q⟩ and |Q⟩) by singlet-quintet (SQ) mixings during triplet-exciton diffusions within geminate multiexcitons. The present fundamental characteristics of the quintet generations shows strong impact of coexistence of molecularly ordered "hot spot" and disordered regions for exergonic SQ mixings driven by entropy, thereby paving a new avenue for rational designs of organic devices with controlled multiexciton dynamics by optimizing film morphologies.

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

“…This was explained by a strong chromophore coupling originating between the TIPS-Pn molecules indicating that the self-organized crystalline phase of TIPS-Pn is predominant, as reported previously. 25,[43][44][45] TREPR spectra Fig. 3 shows the TREPR data obtained by the 532 nm-laser excitation of the TIPS-Pc lms at T ¼ 80 K. At 0.2 ms aer the laser irradiation, an A/E/A/E spin polarization was detected at the resonance eld positions from the ne structures for the isolated triplet excitons of TIPS-Pn in 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.…”

“…1, the strongly coupled singlet TT state is known to be initially generated from the S 1 S 0 state. Generation of the quintet state in the triplet-triplet pair 17,22,23,[25][26][27][28] has been reported and is thought to be essential for preventing the loss of the SF-born multiexciton through the singlet channels. Using the timeresolved electron paramagnetic resonance (TREPR) method for TIPS-tetracene thin lms, Weiss et al 26 characterized the SFinduced strongly-coupled quintet of 5 (TT) in Fig.…”

“…Quintet electron spin polarization (ESP) was detected as the microwave absorption (A) and emission (E) in 5 (TT) for frozen aggregates of SF-materials. 25 ESP was interpreted using the sublevel-selective quintet conversion by the zero-eld splitting (ZFS) interaction at the singlet-quintet level-crossing in the presence of negative exchange coupling (J) during triplet exciton-diffusion and subsequent re-encounter in the highly disordered region, causing modulation of the J-coupling to result in spontaneous 5 (TT) generation. 25 Interestingly, transport of spin-entangled multiexcitons 20,[29][30][31] by the dissociative exciton diffusion has been observed using ultrafast transient absorption imaging of pentacene crystals 32 and by the magnetic eld effect on the uorescence time-proles.…”

“…25 ESP was interpreted using the sublevel-selective quintet conversion by the zero-eld splitting (ZFS) interaction at the singlet-quintet level-crossing in the presence of negative exchange coupling (J) during triplet exciton-diffusion and subsequent re-encounter in the highly disordered region, causing modulation of the J-coupling to result in spontaneous 5 (TT) generation. 25 Interestingly, transport of spin-entangled multiexcitons 20,[29][30][31] by the dissociative exciton diffusion has been observed using ultrafast transient absorption imaging of pentacene crystals 32 and by the magnetic eld effect on the uorescence time-proles. Although such transportation of spin-entanglements is key to elucidate the dissociation mechanism and to apply in quantum information science, [33][34][35] no direct evidence for the transportation of spin-entanglements has been found in the separated T + T state with the singlet and quintet characters.…”