Singlet Fission in Tetraaza-TIPS-Pentacene Oligomers: From fs Excitation to μs Triplet Decay via the Biexcitonic State

Abstract

Generating two long-living low-energy excitations after absorption of a single high-energy photon has stoked interest in singlet fission (SF) to enhance solar energy conversion in photovoltaics. To this end, survival of the triplet states is critical. This process is investigated in diethynylbenzene-linked tetraaza-triisopropylsilylethynylpentacene dimers, for which SF is energetically feasible and facilitated by the close distances between the azapentacenes. The ortho and meta connectivities are explored and compared with the tetraazapentacene molecule and the (1,3,5) trimer. Efficient SF (potential Φ T ≥ 160%) is demonstrated in all oligomers by quantitative kinetic analysis of broadband transient absorption and fluorescence signals. Together with dynamics of the starting singlet, the triplet pair, and the final free triplet state, our results show an intermediate component with spectral properties compatible with a biexcitonic state. Long-living triplets represent only a fraction of the high number of transient triplet pair intermediates, which undergo triplet−triplet annihilation as well as fusion between neighboring pentacenes. Therefore, our work provides new insight into the SF in covalent dimers and paves the way for the application of these materials for carrier multiplication.

The Photophysical Properties of Triisopropylsilyl-ethynylpentacene—A Molecule with an Unusually Large Singlet-Triplet Energy Gap—In Solution and Solid Phases

“…Matching bleaching of the ground state with the corresponding absorption spectrum leads to a value for ∆ε 455 of 37,200 M −1 cm −1 , which is well within experimental error of the value obtained earlier by pulse radiolysis (∆ε 455 = 38,250 M −1 cm −1 ) [22]. The transient differential absorption spectrum recorded here is in good agreement with that reported by Wasielewski et al [20] in both fluid solution and thin films, where the derived molar absorption coefficient at the band maximum of 481 nm (in a thin film!) was 33,300 M −1 cm −1 .…”

“…Iodoethane quenches fluorescence from TIPS-P and promotes population of the excited-triplet state by way of the external heavy atom effect [20,39]. A non-linear Stern-Volmer plot is recovered for the addition of iodoethane to TIPS-P in 2-methyltetrahydrofuran solution.…”

“…The natural lifetime calculated from the Strickler-Berg expression [38] is 24 ns. Literature reports for τ S in toluene solution include values of 17 ns [20] and 13 ns [19]. Excitation of TIPS-P in de-aerated toluene with a short laser pulse (FWHM = 0.2 ps) at 590 nm populates the corresponding excited-singlet state.…”

“…The TIPS group is used primarily to improve solubility but it also affects the electronic properties of the chromophore [15]. It has been demonstrated repeatedly that pentacene derivatives are excellent materials for SEF in devices [16], thin films [17] and highly concentrated solutions [18] and also as components of covalently-linked molecular bichromophores [19][20][21][22]. Despite this inherent popularity, the photophysical properties of isolated TIPS-pentacene (TIPS-P) have not been so well documented and, in particular, the triplet excitation energy remains a matter of some uncertainty.…”

“…These include isolated triplets, spin coherent pairs, quintets and so on and an increasing variety of spectroscopic techniques, including vibrational [30] and magnetic [31] probes, are being applied to monitor the triplet state. Additional studies are addressing aspects of competition between SEF and light-induced electron transfer [20]. An independent assessment of the spectroscopic properties appears to be in good order.…”