Flavanthrene derivatives as photostable and efficient singlet exciton fission materials

Singlet exciton fission (SF) is believed to have the potential to break the Shockley–Queisser limit for third-generation solar cell devices, so it has attracted great attention. Conventional linear acene based SF materials generally suffer from low triplet energy and poor photostability. We report herein two flavanthrene derivatives, EH-Fla and TIPS-Fla, as new photostable singlet exciton fission materials. These N-doped two-dimensional angular fused acenes have three sets of aromatic Clar sextets, making them significantly more stable than linear acenes with only one sextet. Time-resolved spectroscopy characterization reveals that the SF process occurs in the polycrystalline films of EH-Fla and TIPS-Fla, with maximal triplet yields of 32% and 159%, respectively. The SF processes of these two molecules are mediated by excimer states. In EH-Fla, the low-lying excimer prevents the SF process from occurring effectively, resulting in a low triplet yield. In contrast, the excimer state in TIPS-Fla is mixed with strong CT coupling, which prompts efficient SF and results in a high triplet yield. Our results show that flavanthrene is a promising SF chromophore for photoenergy conversion applications, while a fine-tune of the intermolecular interaction is crucial for achieving high SF efficiency.


Film preparation and Characterization
The TIPS-Fla film was prepared by a self-made tube furnace. The sample was placed in the high-temperature area of the tube furnace, the temperature was set to 265 °C, the quartz was placed in the low-temperature area (room temperature), and the sample was deposited at 510 -5 Pa for one hour. The EH-Fla films were prepared by spin coating.
15 mg EH-Fla was dissolved in chloroform and spin-coated on quartz at 1500 rpm. The crystallinity of the film was further improved by thermal annealing at 120 °C for 10 minutes.

Steady-state spectroscopy characterization
The steady-state absorption spectrum is measured by a T6 UV-Vis spectrometer (Purkinje General, China). Photoluminescence spectrum was obtained by FLS920 (Edinburgh, UK). Phosphorescence spectra of EH-Fla and TIPS-Fla were done on PerkinElmer (American) in argon-saturated Methyl-tetrahydrofuran containing MeI (10 mM) at 77 K.

Femtosecond transient absorption spectroscopy (fs-TA)
A Coherent Chameleon oscillator seeded a Ti/sapphire regenerative amplifier (Coherent Legend Elite) to produce pulses of about 100 fs duration at a repetition rate of 1 kHz and a wavelength centred at 800 nm. The 400 nm pump pulse was generated from a Light Conversion OPerA-Solo optical parametric amplifier (285-2600 nm). The white probe pulse was produced by focusing a small portion of the 800 nm beam on the sapphire plate for measurements in the visible region. All characterization was performed at room temperature unless otherwise specified.

Global Target Analysis (GTA)
Glotaran was used to perform a GTA in this study, an approach that starts with a simple model with a defined number of components and a set of starting values for rate constants that describe the kinetic connections between the components. The entire TA S3 dataset was fitted by generating a time-dependent population evolution of spectral intensities from a set of first-order differential equations. The parameters are then optimized by re-tuning the starting values based on the resulting model. The quality of the fit can be judged by the fitted spectral and time traces and the residual matrix. Any residual structure in the singular vector shows a potential mismatch and the need for another component in the proposed model. After examining and accepting the fitted model, the program generates kinetic trajectories, population evolution, and spectra for each component. GTA was used to separate overlapping features and generate population profiles versus time for each component.
Before performing global fitting, we performed singular value decomposition (SVD) on the experimental data, and the results of SVD showed that there are three states in EH-Fla. For EH-Fla, the branched model does no give a good fit, and the best fit of EH-Fla is obtained using a 3-state sequential model.
For TIPS-Fla, the SVD results show that there are two states. We observed that with the decay of the ESA signal at 700 nm, a new ESA signal was produced at 540 nm, showing a clear sequential process. The best fit to the fs-TA of TIPS-Fla is obtained using a 2-state sequential model.

Nanosecond transient absorption spectroscopy (ns-TA)
Nanosecond transient absorption spectroscopy was used to further characterize the dynamics of long-lived species. The pump pulse was generated by a frequency-tripled sub-nanosecond laser (Picolo AOT MOPA, InnoLas) at 355 nm (duration ~0.8 ns). The probe beam was a broadband chirp-corrected supercontinuum light source generated by focusing a small portion of the femtosecond laser beam onto a 3 mm-thick sapphire plate for the visible range. The laser was synchronized to the probe pulse with a desired delay by an electronic delay generator (SRS DG645, Stanford Research System).   However, our fs-TA spectra do not show the signal of PIA around 650nm. Therefore, the fs-TA spectra can rule out the direct involvement of the real CT state.  The experimental results of temperature dependent fs-TA spectra are shown in Figure S10 and S11. With the increase of temperature, the peak is gradually red-shifted and its intensity weakened, accompanied by an increase in the intensity of the peak between 450 nm and 550 nm. Further, increase the temperature to 350 K, no triplet was     in about 13ns. Fitting to the curve indicates a lifetime of 1.62 ns for this signal. There is no signal in red/NIR region on longer time scales, therefore we truncated the comparison in Figure S17 at 600nm (EH-Fla) or 650nm (TIPS-Fla) for clarity.

Triplet sensitization
To obtain the triplet spectra of EH-Fla and TIPS-Fla, the sample and sensitizer   Figure S14). ii) The evolution of the triplet pair to free triplets is strongly temperature-dependent in endothermic singlet fission. From our data, the kinetic curves ( Figure S13) at different temperatures are not significantly different, indicating that the kinetic process at the given time scales is temperature-independent. The result indicates that there is no evolution of triplet pairs to free triplet states in the dynamic process in EH-Fla and TIPS-Fla.
In the TIPS-Fla film, the spectra are matched reasonable well between 450 and 650 nm ( Figure S17b). We note that, in the sensitization experiments of TIPS-Fla, a much higher concentration of PdPc(OBu)8, than that of EH-Fla, is necessary to obtain an acceptable signal-to-noise ratio for the sensitized triplet state, indicating that the triplettriplet absorption intensity in TIPS-Fla is much weaker. This weak triplet-triplet absorption in TIPS-Fla makes the sensitization more difficult and can be the reason for the observed small spectral differences shown in Figure S17b.

Triplet yield calculation
The triplet yield is calculated by the method of energy transfer. 2 Here, we converted T/T to OD. First, the extinction coefficient  of the flavanthrene triplet excitons can be calculated by a triplet sensitizer with a known triplet molar extinction coefficient.
Here  The excitation density can be calculated according to the following formula. package. 4 The calculation details were described by Bae et al. 5 Electronic couplings were calculated using density functional theory (DFT) method. The triple ζ with two polarization functions (TZ2P) basis set and the B3LYP exchange-correlation functional 6, 7 were employed. The effective coupling between orbitals i and f, Vif, was calculated the using the fragment orbital approach implemented in the ADF package: 8 Where Jif is the Fock matrix element between a pair of monomers, Sif is the overlap integral; ei and ef are the Fock matrix elements within a monomer. In order to reduce the computational cost, we simplified the molecular models in the calculations. We used OC2H5 groups and Si(CH3)3 groups instead of OCH2CH(C2H5)C4H9 groups in EH-Fla and Si(C3H7)3 groups in TIPS-Fla, respectively.
The monomer geometries of EH-Fla and TIPS-Fla were optimized and then placed in their dimer arrangement as determined by their crystal structures.