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Open AccessCCS ChemistryRESEARCH ARTICLES20 Jan 2025

Isoelectronic Substitution-Induced Intrachromophoric Symmetry-Breaking Charge Separation

    https://doi.org/10.31635/ccschem.024.202404982

    Symmetry-breaking charge separation (SB-CS) originating from natural photosynthesis is important for improving the efficiency of energy conversion from solar to electrical or chemical energy. However, SB-CS commonly occurs between two or more identical chromophores and is highly sensitive to the π–π stacking or bridging modes, posing significant challenges for their applications. In this study, we demonstrate that SB-CS can occur within a single chromophore by replacing a specific naphthalene unit with an isoelectronic 10,9-borazaronaphthalene unit. Unlike the mechanisms of interchromophoric SB-CS dominated by electronic coupling, intrachromophoric SB-CS is realized by aromaticity reversal and the pseudo Jahn–Teller effect. Distinctive mechanisms endow intrachromophoric SB-CS with unique properties, such as broad absorption, tolerance to different chromophores, and distinctive dependence on solvent dielectric constants. Most surprisingly, the time-resolved circular dichroism spectra revealed that intrachromophoric SB-CS is accompanied by symmetry breaking of the chirality of the chromophore. These findings provide a new perspective on the development of SB-CS systems and their applications in optoelectronic and photochemical devices.

    Introduction

    The photosynthetic reaction center (RC) accepts solar energy and converts it into electrochemical energy, a process with nearly 100% energy conversion efficiency that almost sustains the survival of all life forms on Earth.1–3 The key mechanism enabling this extraordinary efficiency is symmetry-breaking charge separation (SB-CS) in the photosynthetic RC.4–6 When the light-harvesting antenna system absorbs and transfers solar energy to the photosynthetic RC, the symmetrical chlorophyll chromophore pairs are disrupted by environmental and structural fluctuations, resulting in the formation of the charge-separated state.7,8 Mimicking the SB-CS system of the photosynthetic RC is regarded as the “holy grail” in modern science because of its ability to facilitate charge transfer with minimal energy loss.9–11 This capability makes high-efficiency photovoltaic conversion in solar cells and hydrogen evolution in photocatalytic water splitting possible.12–15

    The model that adjusts the intermolecular π–π stacking or intramolecular bridging methods between two chromophores, referred to as the “chromophore pairs model,” is currently the most common method to realize artificial SB-CS.16–19 Based on the molecular exciton theory established by Kasha and Davydov and short-range charge transfer coupling in molecular aggregates emphasized by Spano and colleagues,20–22 reported intramolecular or intermolecular chromophore pairs with SB-CS properties are typically null or weak exciton-coupled systems.7,9–11,23–25 In rare cases, they are strong exciton-coupled systems with significant interchromophoric vibrational coupling.26 Owing to the partial overlap of the electronic wave functions in reported SB-CS chromophore pairs (the connection modes of two chromophores can be π–π interaction, rotatable single-bond connection, or multiple-bond connections with nearly orthogonal angle),7–11 when the chromophore pairs are excited by a photon, the charges are primarily localized within one of the chromophores.9 Subsequently, interchromophoric SB-CS occurs under the influence of the external environment (Figure 1a).11 However, the SB-CS in the chromophore pair model is extremely sensitive to slight variations in electronic and vibrational coupling between the chromophores, leading to many unpredictable challenges for practical applications.10 Therefore, it is imperative to seek methods for achieving SB-CS that diverge from the classical chromophore pair model, prompting the investigation of the feasibility of designing SB-CS within a single chromophore.

    Figure 1

    Figure 1 | Mechanistic sketch map for interchromophoric and intrachromophoric SB-CS. (a) Typical interchromophoric SB-CS in chromophore pairs. The black curve represents the connection modes of two chromophores, which may be π–π interaction, rotatable single-bond connection, or rigid bond connection with nearly orthogonal angle. (b) Mechanism of intrachromophoric SB-CS.

    Since Colin Baird’s pioneering prediction of aromaticity reversal using perturbative molecular orbital theory in 1972,27 it has been widely confirmed that polycyclic aromatic hydrocarbons (PAHs), which are aromatic in the ground state, undergo aromaticity reversal in the S1 and T1 states and exhibit antiaromatic properties.28–32 If a PAH chromophore exhibits strong antiaromaticity33–35 or has close energy between the adjacent excited states,36 it can lead to the pseudo Jahn–Teller effect (PJTE). A strong PJTE induces spontaneous symmetry breaking in the chromophore, resulting in the formation of a less symmetrical low-energy configuration. Introducing isoelectronic B/N units into PAH (B/N PAH) chromophores has become an effective strategy for modulating the properties of aromatic π-electron systems.37–44 As is well known, the aromaticity of B/N PAH chromophores is slightly lower than that of isoelectronic C/C PAH chromophores in the ground state.45 This is likely to cause B/N PAH chromophores to exhibit stronger antiaromaticity compared with C/C PAH chromophores in the S1 state.a Consequently, the stronger PJTE of B/N PAH chromophores generates a powerful driving force for symmetry breaking. This symmetry-breaking process, which inevitably involves charge migration, provides the necessary conditions for the formation of intrachromophoric SB-CS (Figure 1b).

    Because of their robustness,46 tunable absorption spectra,47 significant extinction coefficients,48 and strong propensity for π–π interactions,49 perylene and terrylene diimides ( PDI and TDI) are among the most actively studied chromophores in the SB-CS field.50–53 In this study, 10,9-borazaronaphthalene-fused BN-PDI and BN-TDI and their isoelectronic naphthalene-fused CC-PDI were synthesized (the 10,9-borazaronaphthalene and naphthalene units are highlighted in blue and red, respectively, in Scheme 1). Their completely delocalized aromatic π-system was confirmed experimentally, and the UV–vis absorption spectra, anisotropy of the induced current density (ACID), and nuclear independent chemical shift (NICS(1)_ZZ) were calculated. The results indicate that the two PDI and TDI units in BN-PDI, CC-PDI, and BN-TDI do not belong to intramolecular null, weak, or strong exciton-coupled chromophore pairs but rather surpass the strong exciton-coupling limit to form a new chromophore. Theoretical calculations show an aromaticity reversal from the S0 to the S1 state and greater antiaromaticity in BN-PDI than in isoelectronic naphthalene-fused CC-PDI. Consequently, the C2 symmetric BN-PDI relaxes to a C1 symmetric configuration in its adiabatic S1 state owing to the strong PJTE. This symmetry-breaking process was accompanied by directed charge migration, ultimately providing the first example of intrachromophoric SB-CS in polar solvents. Through time-resolved circular dichroism (TR-CD) spectroscopy, we discovered that intrachromophoric SB-CS is accompanied by symmetry breaking in the chirality of the chromophore in the excited state. These findings not only provide new insights into the development of SB-CS and its applications in photonic or photochemical energy conversion devices but also offer potential avenues for studying the origin of chirality in nature.

    Scheme 1

    Scheme 1 | Synthetic routes to 10,9-borazaronaphthalene-fused BN-PDI and BN-TDI and molecular structure of Ref-PDI, Ref-TDI, and isoelectronic naphthalene-fused CC-PDI. The 10,9-borazaronaphthalene and naphthalene units in BN-PDI, BN-TDI, and CC-PDI are highlighted in blue and red, respectively.

    Experimental Methods

    All solvents were reagent grade, which were dried and distilled prior to use according to standard procedures. All commercially available reagents were used without further purification unless otherwise noted. All deuterated solvents were purchased from Cambridge Isotope Laboratories (Massachusetts, United States). Column chromatography was generally performed on silica gel (200–300 mesh). The 1H nuclear magnetic resonance (NMR) (400 or 600 MHz) and 13C NMR (100 or 150 MHz) spectra were measured on Bruker ADVANCE 400 and Bruker AV-600 spectrometers (Bruker Corporation, Switzerland). High resolution mass spectra (HRMS) were determined on a SolariX 7.0T Fourier transform ion cyclotron resonance mass spectrometer (Bruker Corporation, Switzerland). The UV–vis-NIR absorption spectra data were documented by a Shimadzu UV-3600 spectrophotometer (Shimadzu Corporation, Japan). Fluorescence measurements were measured with Shimadzu RF6000 spectro fluorophotometer (Shimadzu Corporation, Japan) at room temperature. Photoluminescence quantum yields (ФPL) were determined by a Hamamatsu absolute PL quantum yield spectrometer (C13534-12) (Hamamatsu Corporation, Japan) with an integrating sphere. Cyclic voltammetry (CV) was performed with a CHI720E electrochemical workstation (CH Instruments, Shanghai, China) using glassy carbon discs as the working electrode, Pt wire as the counter electrode, and a Ag/AgCl electrode as the reference electrode at a scanning rate of 50 V/s. 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) dissolved in anhydrous dichloromethane or tetrahydrofuran (THF) was employed as the supporting electrolyte. The plot includes the signal of the ferrocene as an internal potential marker. Femtosecond transient absorption (fs-TA) spectrometer (Helios) adopts pump-probe technology produced by Ultrafast Systems (Florida, United States). Further experimental data, computational details, and characterizations of products are available in the Supporting Information, including the syntheses and characterizations of the compounds, steady-state spectroscopy, time-resolved spectroscopy, CV, and density functional theory (DFT) calculations.

    Results and Discussion

    Design and synthesis

    The synthetic routes to BN-PDI and BN-TDI are shown in Scheme 1. NH2-PDI, NH2-TDI, Br-PDI, and Br-TDI were synthesized according to previously reported methods.54 While Wu and Xia recently prepared BN-PDI using a Pd-catalyzed Buchwald–Hartwig coupling reaction,55 in this study, a more effective method was employed using Cu-catalyzed Ullmann condensation followed by borylation. In addition, BN-TDI was synthesized for the first time. NH-PDI and NH-TDI were synthesized with yields of 67% and 34%, respectively, starting from NH2-PDI, Br-PDI, NH2-TDI, and Br-TDI. These intermediates were subsequently reacted with boron trichloride to yield BN-PDI and BN-TDI with yields of 68% and 50%, respectively. Furthermore, Ref-PDI and Ref-TDI chromophores were synthesized in high yields by reacting phenylboron dichloride with NH2-PDI or NH2-TDI ( Supporting Information Scheme S1).56 To elucidate the role of the B–N bond in facilitating intrachromophoric SB-CS, the isoelectronic species CC-PDI of BN-PDI was synthesized for comparison.

    Evidence for the formation of a new chromophore

    Whether the two intramolecular chromophores maintain their relative independence is crucial for determining the formation of a new chromophore. There are three pieces of evidence indicating that BN-PDI and BN-TDI form a new chromophore, rather than weak, null, or strong exciton-coupled chromophore pairs: (1) Two relatively independent intramolecular chromophores may exhibit absorptive characteristics of H or J aggregates, with their absorption spectra showing slight red or blue shifts but no significant changes in characteristic absorption peaks compared with their monomers (except for variations in the intensity of the 0–0 and 0–1 vibrational peaks).57 However, in comparison to Ref-PDI and Ref-TDI, the BN-PDI and BN-TDI in this study clearly display new electronic absorption peaks, leading to entirely different UV–vis absorption spectra (Figure 2a,d). (2) Two relatively independent intramolecular chromophores typically have degenerate or near-degenerate S1 and S2 states (i.e., similar energy and oscillator strength parameters for S1 and S2 states).18,23 However, BN-PDI and BN-TDI show distinct energies and vastly different oscillator strengths for their S1 and S2 states (Figure 2b,e, Supporting Information Tables S2 and S3). (3) ACID calculations indicate that the ground states of BN-PDI and BN-TDI exhibit global aromatic ring currents (Figure 2c,f), suggesting delocalization of π-electrons throughout the intramolecular π-system. This is further supported by NICS(1)_ZZ calculations for the ground states of BN-PDI and BN-TDI (Figures 5e and 6e). Therefore, the two BN-fused PDI and TDI units in BN-PDI and BN-TDI clearly constitute a new chromophore.

    Figure 2

    Figure 2 | Evidence for the formation of a new chromophore. (a) Normalized UV–vis spectra of Ref-PDI and BN-PDI in Tol. (b) Calculated absorption wavelengths and oscillator strengths of Ref-PDI and BN-PDI at different excited states. (c) ACID of BN-PDI. (d) Normalized UV–vis spectra of Ref-TDI and BN-TDI. (e) Calculated absorption wavelengths and oscillator strengths of Ref-TDI and BN-TDI at different excited states. (f) ACID of BN-TDI.

    SB-CS hypothesis (photophysical data analysis)

    To analyze the influence of solvent dielectric constants on photophysical properties of BN-PDI, BN-TDI, and referenced Ref-PDI, Ref-TDI, and CC-PDI, toluene (Tol), THF, and benzonitrile (BCN) (ε = 2.38, 7.58, and 25.20, respectively) were employed as solvents. The experimental data are summarized in Supporting Information Table S1. The UV–vis absorption spectra of all the compounds exhibited no significant wavelength shifts and aggregation in Tol, THF, and BCN ( Supporting Information Figures S1 and S2), indicating rigid molecular configurations. The fluorescence emission spectra of Ref-PDI, Ref-TDI, and CC-PDI showed insignificant redshifts with increasing solvent dielectric constant, yet maintained well-resolved vibronic fine structures, suggesting that these reference compounds possess negligible charge transfer properties. In contrast, the wavelength shifts of BN-PDI and BN-TDI did not correlate clearly with the solvent dielectric constant, and the fine vibronic structures in their fluorescence emission spectra disappeared in THF and BCN ( Supporting Information Figure S3). Given that the S1 states of BN-PDI and BN-TDI are localized excited states (as shown in Supporting Information Figures S19 and S20), the observed changes in their fluorescence spectra in THF and BCN are not attributed to charge transfer properties but rather to the involvement of other photophysical processes.

    Fluorescence quantum yield (φFl) and transient fluorescence lifetime (τ) measurements further confirmed the formation of new photophysical processes ( Supporting Information Figure S4). The φFl of Ref-TDI and CC-PDI exhibited negligible variation in Tol, THF, and BCN. To rule out the impact of the notable change in φFl in THF for Ref-PDI, its fluorescence intensity in various solvents was tested under the same conditions ( Supporting Information Figure S5). The results suggest that Ref-PDI does not exhibit new photophysical processes with changes in solvent polarity. In contrast, BN-PDI exhibited dual τ with reduced φFl below 1% in THF and BCN. BN-TDI showed similar behavior in BCN. This suggests that BN-PDI and BN-TDI undergo nonradiative decay pathways competing with radiative transitions in solvents with high dielectric constants.

    SB-CS confirmation (fs-TA data analysis)

    To confirm the presence of SB-CS in BN-PDI and BN-TDI in THF and BCN, fs-TA measurements were conducted for the two compounds as well as three reference compounds: Ref-PDI, Ref-TDI, and CC-PDI ( Supporting Information Figures S10, S11, and S13). To simplify the discussion, the following abbreviations are used: local excited singlet state (LE), relaxed local excited singlet state (LE′), excited triplet state (T), singlet emission (SE), charge-separated state (CS), and ground state (GS).

    The fs-TA spectra of BN-PDI are shown in Figure 3. In Tol solution, the fs-TA spectrum of BN-PDI exhibited LE and LE′ bands in the range of 560–800 nm (Figure 3a). The LE and LE′ bands subsequently evolved into a dominant T state with a main absorption peak at 550–600 nm, as confirmed by nanosecond transient absorption spectra and a triplet sensitization experiment ( Supporting Information Figures S6 and S7). In THF, the spectral evolution of BN-PDI was completed within 148 ps, in sharp contrast to the microsecond-scale evolution observed in Tol, indicating a rapid physical decay process competing with the T state in THF. As shown in Figure 3b, the LE band at 660 nm relaxed to a lower-energy state at 641 nm in the LE′ band within 2.1 ps. Subsequently, within 23 ps, the LE′ band evolved into two bands at 550–630 nm and 650–760 nm. Comparing the UV–vis-NIR absorption spectra of the electrochemically obtained anion and cation of Ref-PDI ( Supporting Information Figure S9) made it possible to identify these two bands as corresponding to the absorption of the cation and anion of Ref-PDI. Therefore, the spectral evolution process from the LE′ state to the CS state of BN-PDI in THF was confirmed. Unlike the clear resolution of each excited state observed in THF, the spectral evolution of the LE′ and CS states in BN-PDI in BCN coexists within 1.0 ns (Figure 3c). This phenomenon challenges the conventional understanding that solvents with higher dielectric constants facilitate the formation of SB-CS.

    Figure 3

    Figure 3 | fs-TA data of BN-PDI. (a) fs-TA spectra and evolution associated different spectra (EADS) of BN-PDI in Tol. (b) fs-TA spectra and EADS of BN-PDI in THF. (c) fs-TA spectra and EADS of BN-PDI in BCN.

    To elucidate the importance of the B–N bond in achieving intrachromophoric SB-CS, fs-TA experiments were conducted on CC-PDI ( Supporting Information Figure S10), the isoelectronic species of BN-PDI. The results demonstrated spectral evolution of LE → LE′ → T → GS in Tol, THF, and BCN for CC-PDI. The generation of the T state was confirmed by nanosecond transient absorption spectra ( Supporting Information Figure S8). This indicates that CC-PDI does not have SB-CS characteristics. The data obtained for CC-PDI are consistent with the reported photophysical data.58 These experiments underscore the critical role of the B–N bond in facilitating the SB-CS within BN-PDI chromophores.

    In the chromophore pair model, changes in the torsional angles can easily lead to the disappearance of the SB-CS. Analyzing the fs-TA data of BN-TDI revealed that the intrachromophoric SB-CS is resilient to variations in chromophore structure. As shows in Supporting Information Figure S17, BN-PDI and BN-TDI have different torsional angles. The fs-TA data of BN-TDI reveal a spectral evolution of LE → LE′ → CS → GS in both THF and BCN (Figure 4a–c). Using cobaltocene and tris(4-bromophenyl)ammoniumyl hexachloroantimonate as the reducing and oxidizing agents, respectively, the UV–vis-NIR absorption spectra of Ref-TDI cations and anions were obtained ( Supporting Information Figures S12a,c). It was confirmed that the absorption peaks at 700–810 nm and 790–1000 nm generated during the spectral evolution of BN-TDI in THF and BCN correspond to the cation and anion absorption peaks of Ref-TDI ( Supporting Information Figure S12b). The CS → GS lifetime of BN-TDI in THF is 2.0 ns, which is relatively close to its fluorescence lifetime of 4.12 ns in THF. Therefore, the dual fluorescence lifetimes of BN-TDI in THF could not be fitted, as mentioned in the photophysical data analysis section.

    Figure 4

    Figure 4 | Fs-TA data of BN-TDI. (a) fs-TA spectra and EADS of BN-TDI in Tol. (b) fs-TA spectra and EADS of BN-TDI in THF. (c) fs-TA spectra and EADS of BN-TDI in BCN.

    SB-CS mechanism (theoretical computational analysis)

    To confirm that aromaticity reversal and PJTE are the driving forces of SB-CS in BN-PDI and BN-TDI chromophores, the ground states, vertical S1 states (S1(FC)), and adiabatic S1 and S3 states (S1(adi) and S3(adi)) of CC-PDI, BN-PDI, and BN-TDI were calculated using the (TD)ωB97XD/6-31g(d). The specific calculation graphics and tables can be found in Supporting Information Tables S2–S15 and Figures S17–S31.

    The optimized configuration and NICS(1)_ZZ calculations show that the ground state of BN-PDI is C2 symmetry (Figure 5a) and exhibits strong aromaticity (Figure 5e). This is consistent with the ACID calculations, which show fully delocalized aromatic ring currents (Figure 2c). However, the S1(FC) state of C2 symmetric BN-PDI shows pronounced antiaromaticity (Figure 5f). The NICS(1)_ZZ values indicate antiaromaticity in all six-membered rings, with the values of the BN-containing six-membered rings reaching as high as +68. To confirm the strong antiaromaticity of the BN-PDI S1(FC) state, NICS(1)_ZZ values for its isoelectronic CC-PDI were calculated under the same conditions ( Supporting Information Figure S18). The NICS(1)_ZZ values show that only a few central six-membered rings exhibit antiaromaticity in the CC-PDI S1(FC) state. Moreover, the maximal NICS(1)_ZZ value of CC-PDI in the S1(FC) state is only +32. Therefore, the S1(FC) state of BN-PDI exhibits significantly higher antiaromaticity than CC-PDI. Moreover, it was found that the energy gap between the S1(FC) and S2(FC) states of BN-PDI is 0.09 eV ( Supporting Information Table S2), while the energy gap between the S1(FC) and S2(FC) states of CC-PDI is 0.23 eV ( Supporting Information Table S4). The strong antiaromaticity in the S1(FC) state and the narrow energy gap between the S1(FC) and S2(FC) states in BN-PDI induce a pronounced PJTE, resulting in a robust driving force for symmetry breaking from C2 symmetric configuration to C1 symmetric configuration (Figure 5b–d). This explains why the charge distributions of the BN-PDI S1(adi) state are predominantly situated on one side of the chromophore (Figure 5c,g), unlike the D2 symmetrylike S1(adi) state of CC-PDI ( Supporting Information Figure S18c–e). The NICS(1)_ZZ calculations for BN-TDI (Figure 6a–g) closely resemble those for BN-PDI, with NICS(1)_ZZ values of up to +50 in its S1(FC) state (Figure 6f) and a small energy gap of 0.11 eV between the S1(FC) and S2(FC) states ( Supporting Information Table S3), thereby leading to charge distributions of the BN-TDI S1(adi) state predominantly on one side of the chromophore (Figure 6c,g).

    Computational studies of the adiabatic configurations of the different excited states of BN-PDI and BN-TDI indicate that the charge separation state configuration is likely to be S3(adi) (Figures 5d, 6d and Supporting Information Figures S22 and S23). This is because compared with the S1(adi) and S2(adi) states, which exhibit distinct localized excited properties, S3(adi) demonstrates high charge transfer characteristics (D values = 3.007 and 4.749 for BN-PDI and BN-TDI, respectively).

    Figure 5

    Figure 5 | Mechanistic studies and DFT calculations. (a) Optimized configuration of BN-PDI in the S0 state. (b) Calculated electron and hole distribution of BN-PDI in the vertical S1 state (S1(FC)). (c) Calculated electron and hole distribution of BN-PDI in the adiabatic S1 state (S1(adi)). (d) Calculated electron and hole distribution of BN-PDI in the adiabatic S3 state (S3(adi)). (e) Calculated NICS(1)_ZZ values and ACID of BN-PDI at S0 state. (f) Calculated NICS(1)_ZZ values and ACID of BN-PDI in the S1(FC) state. (g) Calculated NICS(1)_ZZ values and ACID of BN-PDI in the S1(adi) state. Owing to the lack of effective methods for calculating the aromaticity of large PAHs in the S1 state, T1 state NICS(1)_ZZ and AICD calculations were conducted based on the molecular configurations of S1(FC) and S1(adi) to illustrate the aromaticity of the S1 state.

    Unique SB-CS (TR-CD data analysis)

    The SB-CS mechanism in the chromophore pair model primarily relies on electronic coupling between chromophores, whereas it was confirmed that the intrachromophoric SB-CS mechanism is driven by aromaticity reversal combined with PJTE in the S1 state. This significant mechanistic difference imparts unique properties to the interchromophoric SB-CS. For instance, the UV–vis absorption spectra of most chromophore pairs closely resemble those of their monomeric chromophores. In contrast, the intrachromophoric SB-CS system can achieve larger π-electron delocalization, resulting in more redshifted and broad absorption wavelengths. Furthermore, whereas the chromophore pair model is highly sensitive to the stacking model between chromophores, intrachromophoric SB-CS requires only the S1 excited state to exhibit strong antiaromaticity, significantly reducing the dependency on configuration and stacking. In addition, BN-PDI exhibits a more pronounced SB-CS process in THF than in BCN, challenging the conventional understanding that solvents with higher dielectric constants facilitate SB-CS.

    Figure 6

    Figure 6 | Mechanistic studies and DFT calculations. (a) Optimized configuration of BN-TDI in the S0 state. (b) Calculated electron and hole distribution of BN-TDI in the vertical S1 state (S1(FC)). (c) Calculated electron and hole distribution of BN-TDI in the adiabatic S1 state (S1(adi)). (d) Calculated electron and hole distribution of BN-TDI in the adiabatic S3 state (S3(adi)). (e) Calculated NICS(1)_ZZ values and ACID of BN-TDI at S0 state. (f) Calculated NICS(1)_ZZ values and ACID of BN-TDI at S1(FC) state. (g) Calculated NICS(1)_ZZ values and ACID of BN-TDI at S1(adi) state. Owing to the lack of effective methods for calculating the aromaticity of large PAHs in the S1 state, T1 state NICS(1)_ZZ and ACID calculations were conducted based on the molecular configurations of S1(FC) and S1(adi) to illustrate the aromaticity of the S1 state.

    The intrachromophoric SB-CS and the interchromophoric SB-CS exhibit an unexpected difference in addition to the aforementioned three points. TR-CD experiments indicate that the intrachromophoric SB-CS is accompanied by symmetry breaking of the chirality of the chromophore in the excited state. As Figure 7a,b shows, BN-PDI and BN-TDI chromophores exhibit CD signals in BCN. Best fitting of the TR-CD signals of BN-PDI and BN-TDI at selected wavelengths reveal that the evolution processes of these circular dichroism signals correspond to the LE → LE′ → CS → GS processes in fs-TA (Figure 7c,d). TR-CD data for CC-PDI show no signal throughout the entire TR-CD testing period, indicating that the LE → LE′ → T → GS process does not generate circular dichroism signals ( Supporting Information Figure S14). This validates the hypothesis that chiral symmetry breaking arises from the LE′ → CS process. Unfortunately, the chiral symmetry breaking in the excited states of BN-PDI and BN-TDI cannot be preserved upon return to the ground state. This is because the structures of BN-PDI and BN-TDI include only two [4]helicenes, and the chiral configurations of [4]helicenes can rapidly interconvert at room temperature ( Supporting Information Figure S15).59

    Figure 7

    Figure 7 | TR-CD data of BN-PDI and BN-TDI. (a, b) TR-CD spectra of BN-PDI and BN-TDI in BCN. (c, d) TR-CD kinetics at selected wavelengths of BN-PDI and BN-TDI in BCN. As the pump for the TR-CD experiments of BN-PDI and BN-TDI, 541- and 649-nm femtosecond pulse light with 100-μW power (pulse width of 100 fs) were utilized, respectively.

    Conclusion

    SB-CS, the sole means by which plants convert solar energy into electrochemical energy, is the result of billions of years of natural evolution. With the development of fs-TA technology, it is now possible to understand the physical mechanisms of this process and apply it to the fabrication of artificial solar energy conversion devices. However, the existing SB-CS chromophore pair model is highly sensitive to stacking models, posing numerous challenges in practical applications. In this study, the first intrachromophoric SB-CS was achieved by combining aromaticity reversal with the PJTE. The detailed intrachromophoric SB-CS mechanism is shown in Figure 8. BN-PDI and BN-TDI chromophores are vertically excited to the S1 or Sn states (Sn states return to the S1 state via internal conversion, Process 4). In the S1 state, aromaticity reversal occurs compared to the S0 state, exhibiting strong antiaromaticity (Process 1). Because of the PJTE, chromophores with strong antiaromaticity spontaneously undergo symmetry breaking to reduce their antiaromaticity and achieve a more stable, low-energy S1(adi) state (Process 2). This symmetry-breaking process is accompanied by directed charge transfer, leading to charge accumulation primarily on one side of the BN-PDI and BN-TDI chromophores. Ultimately, under external environmental influences, SB-CS is achieved (Process 3). In elucidating the mechanism, the importance of the B–N bonds in BN-PDI and BN-TDI for promoting SB-CS was explained, and the differences between single chromophore and chromophore pair SB-CS were revealed. These differences manifest themselves in broadened absorption spectra, increased tolerance to different chromophores, and unprecedented chiral symmetry breaking. In planned future work, the reasons behind chirality symmetry breaking in single-chromophore SB-CS will be investigated, and the possible connections between SB-CS and the origins of natural chirality will be explored.

    Figure 8

    Figure 8 | Schematic potential energy diagrams for intrachromophoric SB-CS of BN-PDI and BN-TDI in THF and BCN. (1) Aromaticity reversal between the S0 and S1 states of BN-PDI and BN-TDI. (2) PJTE-induced symmetry breaking, leading to charge accumulation primarily on one side of the BN-PDI and BN-TDI. (3) Forming CS state in the polar solvent. (4) Returning the S1 state from Sn states via internal conversion.

    Footnote

    a Calculated NICS(1)_ZZ values of benzene (S0 = −27.2, S1 = 93.4), 1,2-dihydro-1,2-azaborine (S0 = −15.8, S1 = 135.6), naphthalene (S0 = −25.1, S1 = 82.2), and 10,9-borazaronaphthalene (S0 = −17.4, S1 = 171.2) in the S0 and S1 states using B3LYP/6-311+G(d,p). Also see Figure S31.

    Supporting Information

    Supporting Information is available and includes synthetic methods, theoretical computational data, additional steady-state spectroscopy, additional time-resolved spectroscopy, NMR spectra, and HRMS spectra.

    Conflict of Interest

    There is no conflict of interest to report.

    Funding Information

    This work was financially supported by the National Natural Science Foundation of China (grant nos. 22235005, 22122503, 22461160284, 22401094, 92156024, and 92356307) and the Natural Science Foundation of Shanghai (grant no. 24ZR1416100).

    Acknowledgments

    We gratefully acknowledge Prof. He Tian from East China University of Science and Technology for his constructive guidance.

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