Singlet fission in TIPS-anthracene thin films

Abstract

Singlet fission is an exciton multiplication process that allows for the conversion of one singlet exciton into two triplet excitons. Organic semiconductors, such as acenes and their soluble bis(triisopropylsilylethynyl) (TIPS) substituted counterparts, have played a major role in elucidating the understanding of the underlying mechanisms of singlet fission. Despite this, one prominent member of the acene family that has received little experimental attention to date is TIPS-anthracene, even with computational studies suggesting potential high singlet fission yields in the solid state. Here, time-resolved spectroscopic and magneto-photoluminescence measurements were performed on spin-cast films of TIPS-anthracene, showing evidence for singlet fission. A singlet fission yield of 19% (out of 200%) is estimated from transient absorption spectroscopy. Kinetic modeling of the magnetic field effect on photoluminescence suggests that fast rates of triplet dissociation lead to a low magnetic photoluminescence effect and that non-radiative decay of both the S₁ and ¹(TT) states is the cause for the low triplet yield.

Fig. 1. The structure and some possible fates for the singlet state (S₁S₀) in TIPS-anthracene. Upon excitation (hν), coupled chromophores (S₁S₀) can undergo singlet fission to a strongly-coupled triplet pair (¹(TT)) before dissociating (D) into two free triplets (2 × T₁). Possible loss mechanisms from S₁S₀ include (1) intersystem crossing (ISC) to the T₂ state facilitated by a virtual charge transfer (CT) state, followed by internal conversion (IC) to a T₁ state, and (2) IC to the ground-state (S₀S₀).

Fig. 2. Normalised absorbance of a 1 mM toluene solution (black) and thin film (red) of TIPS-Ac. Thin film spectrum was corrected for scattering.

Fig. 3. Transient absorption spectroscopy of TIPS-Ac thin films. (a) Experimental TA spectrum of thin films excited with a 380 nm pump (13 μJ cm⁻²). (b) A tri-exponential global fit of the experimental TA results. (c) Decay-associated spectra (DAS) along with their corresponding lifetimes (τ), the singlet excited state absorption (ESA), and stimulated emission (SE) are labeled. Inset depicts the region between 400 and 450 nm, highlighting the anti-correlation between σ₁ and σ₂. (d) The species-associated spectra derived from applying the displayed sequential rate model to the DAS.

Fig. 4. Overlay of species C (black) and time slices of the long-time spectrum of TIPS-Ac thin films at 1 μs (red) and the sensitized spectrum in solution at 15 μs (blue). The triplet state was sensitized with PdOEP. The time slices are scaled to the GSB at 442 nm.

Fig. 5. Magnetic field effect on the PL of TIPS-Ac films. The MPL of TIPS-Ac (red) is compared to TIPS-Tc (blue) which is efficient at singlet fission. Error bars represent the standard error of the mean.

Fig. 6. Time-correlated single photon counting of TIPS-Ac thin films. Films were excited at 405 nm (1 μJ cm⁻²). All emission was collected after the 450 nm short-pass filter. The red line represents the fit of the data with τ values displayed in the fit. The values with their corresponding errors and integration in parenthesis are τ₁ = 115 ± 3 ps (0.14), τ₂ = 1.14 ± 0.05 ns (0.07), τ₃ = 6.23 ± 1.21 ns (0.07).

Fig. 7. Kinetic modeling of the magnetic field effect on photoluminescence. (a) Simplified schematic of the model used to investigate dissociation's (k_D) effect on the MPL. (b) The effect of dissociation on the MPL, the numbers above the lines indicate the free triplet yields. (c) Population dynamics of the involved species with k_D set at 3 × 10¹⁰ s⁻¹. (d) Simplified schematic of the model used to investigate intersystem crossings (k_ISC) effect on the MPL. The model is the same as (a), but with the addition of the T₂ state. (e) The effect of ISC on the MPL with k_D set at 3 × 10¹⁰ s⁻¹. MPL curves are overlayed as changing the rate of ISC has no effect. (f) Population dynamics of the involved species with k_ISC set at 3 × 10¹⁰ s⁻¹. More information on the kinetic model may be found in the ESI with Fig. S8 showing the complete model. The rates applied to the model are summarised in Table S1.