Unveiling the concentration dependent direct triplet formation via singlet fission in a tetracene derivative

: Singlet fission is slowed down on diffusional timescales in highly concentrated solutions compared to thin films for a tetracene-like molecule. The underlying mechanism suggests competing intersystem crossing and singlet fission, where in the latter singlets are converted directly to triplets without an intermediate excimer state.


Introduction
Singlet fission (SF) is the process by which one excited singlet state yields two triplet states upon close interaction of two chromophores. This ultrafast photoreaction was first observed in solid state and has strong implications in organic photovoltaics [1]. SF was also reported in concentrated solutions, where the diffusion of the reaction partners extends the dynamics to longer timescales [2]. Thus, the rate of the SF process can be on the same order of magnitude as other triplet forming processes such as intersystem crossing (ISC). In this work, ultrafast transient absorption spectroscopy as well as time-correlated single photon counting (TCSPC) is used to investigate concentrated solutions of a tetrachlorinated phenazinothiadiazole (TDCl4) in toluene. Measurements at different concentrations shows a correlation between singlet decay and triplet formation and an acceleration of the reaction dynamics with increasing concentration. Both observations together speak for a diffusion mediated SF which occurs on the order of several nanoseconds. Within this process, no intermediate state can be identified, which points towards very efficient, almost instantaneous SF.

Experimental
Broadband transient absorption measurements are carried out using fs pump pulses and continuum probing with 4 kHz repetition rate. Pump pulses (<100 fs) at 620 nm are generated in a two stage femtosecond optical parametric amplifier of white-light continuum (TOPAS Prime). The mechanically delayed white light continuum for sub-ns measurements ranges from 440-740 nm and is generated within a sapphire crystal inside a HELIOS transient absorption spectrometer (Ultrafast Systems). For sub-µs measurements, an electronically delayed white light laser acts as the source for the probe pulse within an EOS transient absorption spectrometer (Ultrafast Systems). The synthesis of TDCl4 is described elsewhere [3]. TDCl4 was dissolved in toluene and the concentration determined by the extinction coefficient at 670 nm.

Spectral Evolution
Analysing the change of TA spectra over time shows a blueshift of the excited state absorption (ESA) within the first 20 ps, centred around 545 nm for all concentrations. This behaviour is typical for a vibrational relaxation process. Afterwards, an initial, uniform decay of the ESA is followed by a spectral change on the order of ns (Fig. 1a, b). Spectral similarity with solid state measurements as well as the lifetime of the emerging ESA (≈ 600 ns) allows for the assignment of this process to a triplet formation.

Concentration Dependence
Long-time dynamics show an increased triplet yield for highly concentrated solutions (Fig.  1c). This is in accordance with the concept of SF, which inherently implies a higher quantum yield for triplet formation than pure ISC. Additionally, TCSPC experiments display a monoexponential decay of the singlet state in good agreement with results obtained by TA.
A comparison with a diffusion model for bimolecular reactions [4] shows, that the triplet yield at low concentrations is higher than expected. This leads to the conclusion that two competing reaction pathways are taking place on a similar timescale: The direct formation of the triplet species via concentration independent ISC and SF. Thus, the triplet formation rate kT can be formulated as kT = kISC + c × kSF. A linear fit of the experimental data leads to a SF c) rate of kSF = 7.43 ± 0.43 × 10 9 l mol -1 s -1 , which lies within 40 % of the calculated diffusion limited association rate of 1.17 × 10 10 l mol 1 s -1 .

Global Target Analysis
Applying a global target analysis with a three-step sequential model allows for the identification of the species associated spectrum of the species involved in the process within the first 8 ns (Fig. 2b). It can be seen that the hot S1* state has the same lineshape as the one assigned to the vibrationally relaxed S1 state. The slight blue shift of 7nm is assigned to the increase in energy needed to excite the S1 state due to the loss of vibrational energy. The third SADS is almost identical and merely has a lower amplitude compared to the one assigned to the S1 state, suggesting an intramolecular rearrangement process within the singlet manifold (S'). The offset of the GTA represents the final and thus triplet state. This model explains the dynamics over a broad concentration range (10 -1 -10 2 mmol l -1 ) whereas no excimer state can be identified. This points towards a singlet fission process, where the initial singlet states are converted very fast and efficient to triplet states. Fig. 2: a) Triplet formation rates obtained with TA and TCSPC with corresponding linear fit (red) and the diffusion-limited association rate (green). The value for kSF (7.43 ± 0.43 × 10 9 l mol -1 s -1 ) is within 40 % of the calculated diffusion limited association rate of 1.17 × 10 10 l mol 1 s -1 . b) Normalized species associated decay spectra (SADS), obtained for a three-step sequential model. The first three SADS only slight deviations, suggesting that they belong to the same electronic state.

a) b)
ISC