Symmetry breaking charge separation in linked violanthrone dimers

Abstract

Symmetry-breaking charge separation (SBCS) has the potential to significantly enhance energy conversion efficiency. However, the role of molecular structure in mediating SBCS remains incompletely understood. Here, we report SBCS in novel violanthrone dimers where charge transfer (CT) and charge separation (CS) is tunable through molecular design. We show that in polar environments CT readily occurs even at large interchromophore distances and with flexible linkers. While CT character is observed in all the dimers studied, CS occurs most readily for the dimer with the shortest interchromophore separation.

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Symmetry-breaking charge separation (SBCS) is a critical mechanism in energy conversion technologies, enabling the separation of charge carriers in otherwise symmetric molecular systems. This process can be important in applications such as organic photovoltaics (OPVs) and artificial photosynthesis, where efficient charge separation (CS) enhances device performance by minimizing losses.^1,2 One of the biggest hurdles in OPV performance is their low open circuit voltage (V_oc), moderated in part by the driving force of exciton splitting at the donor–acceptor interface.³ Materials undergoing SBCS are therefore of significant interest as they provide a pathway to improve V_oc by utilizing a small driving force of charge separation.^4,5

Nature evolved highly efficient SBCS in photosynthetic reaction centers, with near-unity quantum efficiency.¹ These systems inspire the design of synthetic analogues, yet several aspects remain unresolved regarding the exact mechanism of SBCS and the role of molecular structure architecture dictating SBCS efficiency.^6,7 A well-established understanding is that strong electronic coupling between closely spaced chromophores facilitates SBCS, as seen in conjugated dimers with rigid linkers.^8–10 However, recent studies have challenged the assumption that close chromophore proximity is a strict requirement for efficient CS. It has been demonstrated that SBCS can occur even in systems with flexible linkers or large chromophore separations and weaker electronic interactions.^1,11,12 Therefore, understanding the interplay between chromophore separation, orientation and linker flexibility is essential to optimize SBCS in synthetic systems.

Herein we investigate SBCS in linked violanthrone dimers, previously unexplored chromophores. Violanthrones are a class of polycyclic aromatic hydrocarbons that recently garnered attention due to their favorable energetics, large conjugated system, and exceptional thermal, chemical and photostability, making them interesting candidates for application in OPVs and other molecular electronics.^13–15

We report on SBCS in three novel violanthrone dimers with varying chromophore separations and linker flexibility by probing their electronic interactions and charge transport properties. Our results shed light on how structural factors, such as chromophore orientation, distance and linker flexibility modulate SBCS efficiency, providing new insights into design principles of energy conversion materials.

Fig. 1 shows the chemical structure of the three violanthrone dimers, comprising identical chromophores, bridged by different linkers. The linkers were designed to vary the interchromophore separation, relative orientation and structural flexibility. Their synthesis, together with a model violanthrone-bridge compound (V79-Ar), is described in the ESI.†

Fig. 1 Chemical structures of a violanthrone monomer (V79), dimer I, dimer II, and dimer III. Structures on the right show DFT optimized ground state geometries of the three dimers.

The optimized structures of the dimers, obtained via DFT calculations (D3-B3LYP/6-31G(d,p)), are shown in Fig. 1 (aliphatic chains omitted for clarity). Dimer II has the shortest interchromophore distance (3.3 Å) linked through a dibenzosilole bridge (through space distance between closest chromophore carbon atoms). Dimer III lacks a C–C bond between the two aryls of the bridge, giving the largest interchromophore separation (10.4 Å) and resulting in a very flexible motif, allowing for dynamic reorientation in solution. Dimer I has a rigid fluorene linker connecting the two chromophores at a distance of 7.2 Å. In this dimer, the two violanthrone components are attached at a site closer to the aliphatic chains on the fluorene bridge, resulting in a more sterically hindered structure with a large interchromophore distance.

The UV/Vis absorption maxima of the violanthrone dimers in both polar (benzonitrile ε = 26) and non-polar (toluene ε = 2.4) solvents show only small red shifts compared to the monomer (Fig. S14 and Table S1, ESI†). This suggests little ground state electronic interaction between the two chromophores. Similarly, the emission spectra only show small spectral shifts between the monomer and the dimers in polar and non-polar solvents, suggesting excimer emission is not present (Fig. 2).

Fig. 2 Fluorescence spectra and decay traces of violanthrone monomer and dimers in toluene (a) and (b) and benzonitrile (c) and (d).

In dimer II, the two chromophores are in close proximity in a co-facial, stacked conformation, forming an X-type dimer (Fig. 1 and Fig. S3, ESI†).¹⁶ The lack of hypsochromic or bathochromic shifts, despite its close chromophore proximity, indicates only weak π–π stacking interactions in the ground state.

Significant fluorescence quenching was observed across all three dimers when the solvent environment was switched from non-polar to polar (Table S1, ESI†). The reduction in quantum yield strongly suggests the activation of non-radiative decay channels (such as SBCS). The extent of quenching varies between dimers, with the dimer I (rigid linker) showing the most efficient quenching, followed by dimer II. Dimer III, with a flexible and longer linker, showed slightly less pronounced quenching.

Fluorescence decay kinetics further supported this quenching behavior. In toluene, the decay traces obtained for two of the dimers (I and II) showed a good fit with only a single exponential component and are comparable to violanthrone monomer (τ = 5.5–6 ns). Interestingly a multi-exponential fit was required for dimer III in toluene (τ₁ = 4.36 and τ₂ = 6.18 ns), with the two components contributing equally to the fit (Table S1, ESI†). This suggests that multiple conformations of dimer III may be present in solution, possibly due to the flexible nature of the linker.

In benzonitrile the fluorescence lifetime is significantly reduced in all three dimers and multi-exponential decays are observed. This is particularly evident in dimer I, where the fastest component (0.33 ns) contributed over 80% of the signal. In dimer II the signal was dominated by a component with τ = 0.99 ns (61%), while dimer III displayed the slowest dynamics with the majority of the excited state population decaying within 2.2 ns (Table S1, ESI†).

Femtosecond transient absorption (TA) spectroscopy provided key insights into the excited-state dynamics and SBCS in violanthrone dimers. In non-polar toluene, the TA spectra of all three dimers exhibit features typical of locally excited (LE) states, including ground-state bleach (GSB) at 600–750 nm, stimulated emission (SE) at 650–750 nm, and transient absorption at 750–1000 nm.¹⁷ The dynamics follow a simple sequential process: (1) internal conversion (IC) S_n → S₁, within hundreds of femtoseconds (not observed at 550 nm or 720 nm excitations) (Fig. S16–S20, ESI†); (2) vibrational cooling (tens of picoseconds) evident by a blue shift of the LE state spectra; (3) ground state recovery (GSR), ∼5 ns for V79, dimer I and II, while ground state recovery in dimer III is multiexponential with ∼5 ns and ∼3 ns, likely due to multiple structural conformations present in solution due to more structural flexibility in this dimer (Table S2, ESI†).

The TA spectra of V79 monomer (Fig. 3) and model violanthrone/bridge compound (Fig. S18, ESI†) in benzonitrile are very similar to those in toluene. These spectra indicate that in both non-polar and polar environments no SBCS occurs. However, faster ground state recovery is observed in benzonitrile (τ = 3.2 ns) compared to toluene (τ = 5.2 ns), consistent with fluorescence decay measurements (Table S2, ESI†). In contrast, the TA spectra in benzonitrile of all three dimers reveal the formation of a new transient species at λ_max = 910 nm. A very strong signal and relatively narrow bandwidth is observed in dimer II while only a weak signal, overlapped with broad LE spectra, is evident in dimers I and III (Fig. 3 and Fig. S17, S19 and S20, ESI†). The transient spectra show a strong ground state bleach, followed by a rapid rise of this new transient species. The strong 910 nm feature in dimer II is assigned as a violanthrone anion signature forming upon CS due to SBCS. The identity of this spectral signature was confirmed with spectroelectrochemical measurements (Fig. S21, ESI†).^12,18 In addition, a positive absorption feature, overlapped with ground state bleach, is observed in the 700–750 nm region which evolves concurrently with the 910 nm feature and that may be associated with violanthrone cation formation.

Fig. 3 Femtosecond transient absorption spectra at select time delays and the kinetic traces at representative wavelengths of violanthrone 79 monomer (a) and the three dimers: dimer I (b), dimer II (c), and dimer III (d) in benzonitrile at 410 nm excitation.

In dimer II, SBCS is the most efficient with CS occurring within 225 picoseconds, as indicated by a slow rise in the new transient signal at 910 nm, followed by a slow (1.2 ns) charge recombination (CR) to the GS. The efficient CS in dimer II is likely due to its shorter interchromophore distance (3.3 Å), facilitating stronger electronic coupling compared to dimers I and III.

Interestingly, dimer I, despite having a large chromophore separation and a rigid linker, exhibits a new transient at 910 nm in benzonitrile. This signal is weaker and is significantly overlapped with the LE signature. The appearance of the 910 nm signal in this dimer is much faster (26 ps) and CR to the GS occurs within 350 ps. This is consistent with the measured main component of the radiative relaxation in this dimer. The small contribution to the fluorescence signal, with slower decays of ∼1 ns and ∼3 ns, are not observed in the TA data, most likely due to the relative insensitivity of the TA method. We suggest that a small portion of the population undergoes relaxation through a competing pathway, likely relaxation from the LE directly to the ground state. The dominant deactivation process, however is though symmetry breaking charge transfer (SBCT), where partial charge transfer occurs (V^δ+V^δ−). We suggest that SBCT in dimer I is distinct from SBCS (V⁺˙V⁻˙), observed in dimer II, where full charge separation occurs and strong anion spectral character is observed. The rigid, conjugated linker in dimer I could be playing a role in facilitating interchromophore coupling that allows this partial CT. In this case rapid CR from this intermediate CT state seems to compete efficiently with full charge separation, CS (V⁺˙V⁻˙) (Fig. 4). The CT state shares the spectral characteristics of the LE and CS state. Global analysis of the TA spectra supported this model where a sequential fit with 3 exponential components (S_n → S₁, S₁ → CT, CT → GS) was sufficient to explain the data. Proposed kinetic schemes and global analysis are shown in Fig. S22–S25 (ESI†).

Fig. 4 Jablonski diagram for SBCT and SBCS processes with different solvent polarities for violanthrone monomer (V79) and the three dimers (dimer I, II and III).

Dimer III exhibits similar behavior to dimer I, where symmetry breaking with CT character is observed very rapidly after excitation (τ_CT = 14 ps), followed by rapid CR (τ_CR = 135 ps). This suggests that even with reduced electronic coupling and the flexible linker, sufficient interchromophore interaction occurs to induce symmetry breaking charge transfer (SBCT) in polar solvents. In addition to the CT and CR signatures, the TA spectra of dimer III is complicated by an additional component evident at longer delay times. This component decays with a lifetime of 1.7 ns and is consistent with recovery of the LE ground state. This result, in combination with the results obtained from fluorescence lifetime measurements, is explained by the presence of multiple structural conformations of dimer III in solution. We expect that only a portion of the population of dimer III with a structurally favorable conformation undergoes rapid CT and CR after excitation into a LE state. This population is in equilibrium with dimer conformations which do not allow SBCT and relax to the GS from a LE state (Fig. S22, ESI†).

The presence of this CT state is also evident in dimer II, where at 14 ps we observe a spectral profile similar to that of the CT state in dimer I and III that subsequently evolves into CS state after 225 ps (Fig. 3c). We assign this CT state as an intermediate which precedes full electron transfer to form a CS state which then, in dimer II, undergoes slow CR (1.2 ns) to the GS (Fig. 4).^19,20 No evidence for triplet mediated CR was observed.

In summary, our results demonstrate that SBCT and SBCS are major processes in these linked violanthrone dimer systems that compete with other relaxation processes. SBCS/SBCT is heavily influenced by solvent polarity, with polar solvents facilitating CT and CS. In the case of large chromophore separations, dimer I and III, SBCT occurs rapidly and CR from this state to the GS outcompetes CS. In dimer III, linker flexibility results in a mixture of structural conformations in solution, with only a portion of the population undergoing SBCT. SBCS is observed only in dimer II where a rigid linker and chromophore attachment site results in closely spaced chromophores, stacked in an X orientation to each other. We thus show that SBCS and SBCT in violanthrone dimers can be controlled through linker design, providing an attractive platform for further system design in energy conversion applications.

This research was supported by the ARC Centre of Excellence in Exciton Science (CE170100026). Responsibility for the views, information, or advice expressed herein is not accepted by the Australian Government.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare there are no conflicts of interest.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cc00768b

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