Perylene-derivative singlet exciton fission in water solution

Abstract

We provide direct evidence of singlet fission occurring with water-soluble compounds. We show that perylene-3,4,9,10-tetracarboxylate forms dynamic dimers in aqueous solution, with lifetimes long enough to allow intermolecular processes such as singlet fission. As these are transient dimers rather than stable aggregates, they retain a significant degree of disorder. We performed a comprehensive analysis of such dynamic assemblies using time-resolved absorption and fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, and theoretical modelling, allowing us to observe the characteristic signatures of singlet fission and develop a model to characterize the different species observed. Our findings reveal that structure fluctuations within perylene-3,4,9,10-tetracarboxylate associations are key in favoring either singlet fission or charge separation. The efficiency of triplet formation is higher than 100%, and the disordered system leads to triplets living in the nanosecond time range.

Fig. 1. (a) Room temperature absorption (solid lines), represented as molar absorption coefficient (ε), and normalized fluorescence (dashed lines) of PTC at 0.01 and 90 mM. The maxima of the vibronic peaks for absorption and fluorescence are marked by vertical black dashed lines and the exact value is described on the side by I_A1, I_A2, I_F1, I_F2. Inset: chemical structure of PTC. (b) Concentration variation of ε at 466 nm (black circles – left y-axis) and R_abs (red diamonds – right y-axis). The red line is the fitting of R_abs using eqn (5) (vide infra).

Fig. 2. Time-resolved fluorescence – single photon counting (SPC) collected at 510 nm upon 415 nm excitation for (a) 10 mM, and (b) 90 mM solution. Time-spectral 2D fluorescence matrix collected by streak camera, fluorescence decay associated spectra (FDAS), and kinetics at 510 nm for (c) and (e) 10 mM, and (d) and (f) 90 mM solution. The OD was adjusted to less than 0.1 to minimize self-absorption distortion, the excitation was at 343 nm, and the excitation energy was ∼15 nJ cm⁻².

Fig. 3. Transient absorption spectra of PTC in the fs-to-ns window: time-spectral map, gated-spectra at 1, 10, 100, 500, and 1000 ps, and kinetics at 515, 585, and 740 nm for (a) 10 mM, (b) 90 mM solution. Excitation: 415 nm; power: 100 mW.

Fig. 4. (a) Dataset of transient absorption in the ns-to-μs window, and (b) decay-associated spectra (DAS) for 90 mM solution. (c) Gated spectra, kinetics (inset), and (d) DAS for 45 mM PTC in 70/30 (v/v) water/ethanol upon sensitization with 1H-phenalen-1-one (excitation at 350 nm).

Fig. 5. (a) ¹H NMR chemical shifts showing two distinct environments for the protons (δ₁ and δ₂) (see ESI†). (b) ¹H NMR chemical shift of PTC as a function of concentration, along with the fit to dimers and oligomers with a maximum cutoff of N = 5. (c) Effective radius (r_eff) extracted from DOSY measurements fit with the same models. (d) Species populations for the modified dimer model.

Fig. 6. (a) Schematic representation of the dimer twist and tilt angles considered (see ESI for the formal definition). (b) Temporal evolution of the twist and tilt angles (light and dark blue, respectively) along the AIMD trajectory at 300 K. Combined angular/radial probability of (c) the twist angle and (d) the tilt angle with respect to the distance between the two monomers computed from the 300 K AIMD trajectory.

Fig. 7. (a) and (b) Fluctuations of diabatic state energies (in eV) and statistical dispersion parameters. CT_1→2, CT_2→1 stand for charge-transfer states from monomer 1 to monomer 2, and from monomer 2 to monomer 1, respectively. Ex₁ and Ex₂ stand for local excited states localized on monomer 1 and monomer 2, respectively. ¹(TT) stands for the correlated triplet-pair state. (c) and (d) Fluctuations of interstate couplings (in eV) computed along the AIMD dynamics of the lowest-energy PTC dimer and statistical dispersion parameters. Direct S₁/¹(TT) SF-coupling (blue); CT mediated SF-coupling (red); exciton coupling (black).

Fig. 8. (a) Target model for two parallel deactivation pathways. The electronic levels are color-coded with the associated SAS obtained from the global fitting. The system evolves as follows for DP1 (DP2): hot-S₁ – grey (black), ¹Ex – green (orange), triplet excimer – blue, and charge transfer species – red. (b) Kinetics and fitting of the isosbestic point (585 nm) for 90 mM PTC. (c) and (d) SAS for 90 mM PTC with 415 nm excitation at 100 mW for DP1 and DP2, respectively.