Vibronically coherent ultrafast triplet-pair formation and subsequent thermally activated dissociation control efficient endothermic singlet fission

Abstract

Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S) into two spin-triplet excitons (T), could provide a means to overcome the Shockley-Queisser limit in photovoltaics. SF as measured by the decay of S has been shown to occur efficiently and independently of temperature, even when the energy of S is as much as 200 meV less than that of 2T. Here we study films of triisopropylsilyltetracene using transient optical spectroscopy and show that the triplet pair state (TT), which has been proposed to mediate singlet fission, forms on ultrafast timescales (in 300 fs) and that its formation is mediated by the strong coupling of electronic and vibrational degrees of freedom. This is followed by a slower loss of singlet character as the excitation evolves to become only TT. We observe the TT to be thermally dissociated on 10-100 ns timescales to form free triplets. This provides a model for 'temperature-independent' efficient TT formation and thermally activated TT separation.

Site-selective measurement of coupled spin pairs in an organic semiconductor

“…In keeping with previous assignments, the first spectral peaks at 560 nm are attributed to 0-0, i.e. zerophonon, transitions [22]. This is also consistent with the greater overlap of low-energy modes on the higherorder vibrational transitions described in detail below (Fig.…”

Manipulating molecules with strong coupling: harvesting triplet excitons in organic exciton microcavities

“…Transient absorption spectroscopy (solid line) has shown that triplet photoinduced absorption signatures generated by singlet fission occur well within our instrument response and do not decay significantly until >10µs 44 . The same sample shows emission with 10ns lifetime, attributed to delayed fluorescence during the S1-1 (TT) equilibrium 44 . This decay closely matches our prompt emission decay (open circles).…”

“…This is a critical distinction from inorganic excitonpolaritons that has not yet been addressed in the literature. The existence of this state mixing is a critical driver in photophysical processes such as ultrafast intersystem crossing 45 , thermally activated delayed fluorescence 46 , singlet exciton fission 42,44 and its reverse, triplet-triplet annihilation. For example, in singlet fission the nominal S1 state contains both CT and TT character, in some cases such as pentacene films to a surprisingly high degree (circa 50% CT) 47 .…”

“…Importantly, the combination of these two data sets shows that it is the character of the triplet-pair states, rather than a loss in population, that causes a decay in delayed fluorescence. This change in character is attributed to spin evolution 38,44 . Indeed, time-resolved electronic paramagnetic resonance spectroscopy shows the presence of quintet states 38 , reproduced by the dashed line.…”

Optical Projection and Spatial Separation of Spin Entangled Triplet-Pairs from the S1 (21Ag-) State of Pi-Conjugated Systems

“…In synthetic molecular materials developed for organic electronics they have transformed transistor and lightemitting diode technology [6][7][8] , as well as becoming promising components for next-generation photovoltaic (PV) devices with the potential to overcome the Shockley-Queisser limit via singlet fission (SF). In SF the absorption of a single photon ideally results in the formation of two spatially separated triplet excitons [9][10][11][12] , although a concerted sequence of ultrafast electronic and vibrational dynamics must compete with both radiative and non-radiative losses for this exciton multiplication to be efficient enough for applications [13][14][15][16][17] . Observation, understanding and control of these many-body quantum dynamics is therefore essential for optimisation of conjugated molecules for technologies such as SFenhanced PVs [18][19][20][21] .…”