Stimulating and harnessing circularly polarized luminescence of helically assembled carbonized polymer dots via interfacial dynamics

2.1 Structural and optical characterization of excitation-independent CPDs

To obtain surface states-determined chiroptical property, excitation-independent CPDs with NIR emission were synthesized via the hydrothermal approach as described in Experimental Procedures and Figure S1, in which growth mechanism is consistent with previous reports.^[30-32] Transmission electron microscopy (TEM) image in Figure 2A demonstrates the monodisperse size of CPDs with spherical morphology. Typically, the as-synthesized CPDs show a statistical average size around 3.3 nm, as illustrated in Figure 2A. In addition, the lattice spacing is characterized by high-resolution TEM as inserted in the figure in which the 0.21 nm spacing refers to the graphene (100) planes. In order to clarify the chemical structure of CPDs, X-ray photoelectron spectra (XPS) analysis was utilized as shown in Figure 2B–E, which corresponds to C 1s, N 1s, O 1s, and S 2p orbitals, respectively (see Figure S2 for the survey spectrum). C 1s spectrum exhibits four subpeaks after fitting: strong C–C with red line at 284.8 eV; mild blue line for C=C at 283.8 eV as well as magenta for C–N/C–S at 285.7 eV; weak C=O at 287.6 eV (Figure 2B). N 1s also shows three peaks involving strong pyridinic N peak at 399.5 eV, two weak peaks correspond to pyrrolic N around 397 eV, and graphitic N around 402 eV (Figure 2C). Two types of O are observed in Figure 2D for O 1s spectrum, which can be assigned to S–O bond with 530.8 eV and S=O bond with 533 eV. Finally, Figure 2E investigates the S 2p orbital. There are four different peaks corresponding to S–C and C–SO₃ bonds both in 2p_1/2 and 2p_2/3 orbitals. These results confirm the abundant sulfonic acid group around surface of as-prepared CPDs. In addition, the Raman spectra exhibit two evident peaks around 1348 and 1587 cm⁻¹, which assign to disordered structures or defects (D band) and the graphitic carbon domains (G band), respectively, with intensity ratios (I_D/I_G) of 0.94. Figure 2G is the Fourier transform infrared (FTIR) spectra for an as-prepared sample. It reveals the C–H vibration mode at around 3000 cm⁻¹, as well as lower energy stretching vibrations of C=C at 1477 cm⁻¹, C=O/C=N at 1583 cm⁻¹, C–N at 1307 cm⁻¹ and C–N–C at 1060 cm⁻¹. It should be noted that the observed FTIR peaks located at 1140 cm⁻¹ for C–SO₃ and 731 cm⁻¹ for C–S vibration confirmed the presence of sulfonic acid group and thiol group onto the surface of pristine CPDs.

Besides the structural characterization, the optical properties of CPDs with NIR emission are also investigated. Steady-state UV-Vis absorption and photoluminescence (PL) measurements in Figure 2H confirm the excitation-independent emission property of the CPDs. The PL emissions invariably exhibit a strong peak at 600 nm together with a weak peak at 650 nm and a shoulder around 720 nm with respect to 290–570 nm excitation only except the PL intensity increases gradually. The emission covers the red and NIR region with a PL quantum yield as high as 86%. The magnetic circular dichroism (MCD) spectra (Figure S3) in parallel demonstrates that the featured absorption peaks around 540 and 590 nm showed much weaker response compared to the MCD signal originated from carbon core in UV region. As the strong π–π* transitions of the C=C and C=N bonds in aromatic sp² domains generally locate close to UV region, we assert the observed NIR emission is probably attributed to the surface states. In order to better investigate the carrier dynamics and emission process, further characterizations are conducted in-assisted with transient spectroscopy. Figure 2I shows the 2D contour maps of transient absorption spectra for CPDs under 540 nm laser pumps. The corresponding wavelength and temporal evolution are also exhibited in Figure 2J,K. Evident bleaching signals are shown at 586 nm, as a result of ground-state bleaching (GSB) signal in CPDs according to steady-state absorption behaviors. GSB also accompanies with the photo-induced excitation state absorption at 550 nm and around short wavelength region at ∼400 nm. There are also two bleaching peaks with the stronger one located at 552 nm and weaker one at 712 nm, which are attributed to the stimulated emission based on their PL performance. The ultrafast dynamics for GSB proclaim the multiple relaxation process for the hot carriers after excitation. The decay of bleaching peak at 586 nm could be fitted into three exponent functions. The fastest decay time of 1.2 ps is generated from optical phonon scattering within ultrafast pumps. Subsequently, the carriers would undergo an electron–phonon coupling with 125 ps for heat transfer from excited carrier to lattice by trapping from surface states. The longest decay time of 2.2 ns demonstrates a phonon–phonon coupling for heat dissipation process based on non-radiation recombination. In addition, the time-resolved photoluminescence spectra (TRPL) is employed for investigating the fluorescence lifetime for CPDs. It is easy to note that the delay time for 600, 650, and 720 emission peaks exhibits similar lifetimes, namely 2.33, 2.37, and 2.41 ns, respectively, after exponent fitting as shown in Figure 2L. These results indicate the three PL emissions all originated from the same radiative recombination channel, which is attributed to the surface states of the CPDs.

2.2 Enantiomeric synthesis of CPD helix via supramolecular co-assembly

With NIR-emissive CPDs, supramolecular co-assembly strategy is then applied for the fabrication of CPD-based nanohelix. First, the molecular assembling dynamics of chiral hydrogels (Fmoc-(L/D)Glu+(L/D)Lys) is investigated. Figure 2M shows the SEM images of typical organogels (0.4 M) consisting of Fmoc-L-Glu+L-Lys, named as L-gels hereafter, in which they form helical nanostructures in 7 days, with diameters varying from 16 to 44 nm (30.3 nm as an average) and pitch length varying from 59 to 96 nm (79.4 nm as an average) (see Figure S4 for gels made of Fmoc-D-Glu+D-Lys molecules, named as D-gels hereafter). Notably, the pitch of a single fiber is uniform in nature; therefore, the overall helicity of the molecular assembly can be finely tailored via the chirality of building molecules with nearly the same sizes.

In terms of chiroptical properties, take L-gels as an example, the overall chiral morphology could contribute a CD response in a range of 300–600 nm (with generally positive signs from 300 to 350 nm and large negative signs from 350 to 600 nm) depending on the concentration of the gels, which is overwhelmingly associated to the geometrical shapes of the organogels (Figure 2N,O). Further, PL spectra with peaks around 450 nm confirm that the assembling behavior of the organogels results in the same emission property as shown in Figure 2P. Therefore, to avoid reproducibility issues and maintain relatively strong chirality, we choose L/D gels with 0.4 M as standard concentration for co-assembly with CPDs hereafter. Then, intrinsically achiral CPDs with various amounts (0.1–1.0 mg/mL) were applied for co-assembly with forementioned organogels. As CPDs have an average size of ∼3.3 nm (Figure 2A) and only a very limited amount (<1.0 mg/mL) was added, the co-assemblies of CPDs with organogels as a result maintains the overall helical nanostructures (average diameters of 28.6/29.2 nm in a range of 21–36/19–38 nm, and average pitch lengths of 81.7/84.3 nm in a range of 62–104/65–116 nm for L and D co-assemblies, respectively) as illustrated in Figure 3A,B), indicating the addition of CPDs does not necessarily change the supramolecular assembly behavior of the organogels but endows achiral CPDs with enhanced chiroptical activities in the vicinity of their featured surface states absorption bands (550–650 nm), as exhibited in Figure 3C–F. Although some imperfections were observed as well in the CD measurements probably due to strong absorptions and inhomogeneity, the observed CD signals show basically mirrored lineshapes as L/D organogels were used, respectively. To further quantify and compare the induced chiroptical activities of L-/D-gels+CPDs co-assembly systems, noted as L-/D-CA hereafter, g_CD-factor, defined as Δε/ε = ΔA/A, where ΔA is the absorbance difference between left- and right-handed circularly polarized lights, is calculated for each wavelength accordingly. As the amount of achiral CPDs increases, the characteristic absorption peak intensity of L-/D-CA increases as expected, and the g_CD values reach a maximum of 5.4 × 10⁻³–4.2 × 10⁻³ (Figure 3G–I) for the sample of 0.7 mg/mL CPDs, respectively, indicating an optimized ratio of CPDs (0.7 mg/mL) to organogels (0.4 M) within such system, which is, so far unprecedented, the highest g_CD value originating from surface states of CPDs fabricated by supramolecular assembly strategy (Table S1).

In addition, gelation time is investigated in parallel to figure out the best reaction time for the formation of CPD helix. Take D-CA for instance, the sample could show twisted-ribbon-like shape within 1 day after mixing as indicated in Figure S6, and longer gelation time to 7 days will result in more uniform helical structures, which possesses strongest CD signal and g_CD-factor as previously observed. Further elongation of gelation time (14 and 21 days), however, does not necessarily improve the CD performances (Figure S6A–C) in which the corresponding SEM images (Figure S6D–G) point out that excess gelation time would increase the possibility for unexpected aggregations and even untwist the helices, leading to an undesired decrement on chiroptical activities. Therefore, within this synthesis, a mixture of 0.4 M chiral gels and 0.7 mg/mL intrinsically achiral CPDs with 7 days gelation can basically bring in uniform helical structures with optimized optical chirality.

To deepen further the chirogenesis, only two possible chiral origins could be intuitively involved in this system as stated in previous section: (a) ligand-induced chirality, and (b) chiral assemblies of achiral CPDs. However, it is obvious that for ligand-induced chirality, the g_CD values are generally close to 10⁻⁴ or even less (Table S1) and more importantly, such strategy can only transmit chirality of organic ligands to π–π* transitions in aromatic sp² domains of the CPDs; while chiral assemblies of achiral CPDs at nanometric or microscopic level could achieve g_CD values as high as 10⁻³. Therefore, although we did not clearly observe TEM images (probably because of poor negative staining and low contrast of CPDs, Figure S7) showing the helical attachment of CPDs onto the gels, the above-mentioned SEM images, CD/UV measurements, and CPL studies (discussed in next part) prompt us to assert that the helical co-assembly structure is the main reason for the induced chirality.

2.3 Study on CPL activities of CPD-nanohelix

We then investigated the chiroptical activities at excited state by measuring the CPL spectra of the co-assemblies. Indeed, L-/D-CA exhibited opposite CPL responses, as presented in Figure 4. Take L-CA for instance, the observed CPL signals at ∼700 nm, which are attributed to the CPDs, vary largely when tuning the excitation wavelength as indicated by the PL spectra (Figure S8). Interestingly, it starts with a very weak CPL signal along with a slightly negative bias when the excitation is 254 nm (Figure 4A–C). Then, the signal is negatively enhanced to −5 mdeg as the excitation wavelength increases to 290 nm. Afterwards, the overall line-shape would experience a series of positive increments to reach as high as 65 mdeg as the excitation wavelength varies from 310 to 460 nm, and witness a CPL signal inversion at around 320 nm. Such CPL enhancement/inversion phenomena is reproducibly observed for D-CA sample as well (Figure 4D–F) and interestingly, the measured luminescence dissymmetry ratio, defined as: g_lum = 2(I_L − I_R)/(I_L + I_R), where I_L and I_R are the luminescence intensity of left and right CPL, demonstrated such tendency again in both L-/D-CA systems with a highest g_lum close to 1.5 × 10⁻²/−1.1 × 10⁻², respectively (Figure 4G–I, Table S1), that is to say, the maximum value of |g_lum| would basically increase as the incident light moves from short wavelengths to longer ones, indicating the chirality-related emission from L-/D-CA is strongly excitation-dependent, as summarized in Figure 4I. Further, the obtained g_lum stemmed from such co-assembly's strategy, to the best of our knowledge, shows the highest g_lum value reported so far when compared to other related works about chiral carbon dots based on ligand-induced chirality or supramolecular assemblies. Therefore, we do believe such chiral CPD systems with excitation-sensitive CPL properties and highly enhanced g_lum values would become an essential highlight for next-generation chiral optoelectronics’ designs.

The above-mentioned findings spontaneously motivate us to look at the chirogenesis of the CPL signals and how excitation wavelength determines the CPL performance of such system. As L-/D gels can basically express CPL activities with an energy higher (∼465 nm) than that of CPDs (Figure S9), a rational understanding is^{[13, 28, 29, 34]} when the chiral co-assemblies are irradiated by a linear polarized (LP) light, which is generated by a xenon lamp and polarized by a polarizer from the CPL instrument, one can basically consider the incident light as two circularly polarized lights that are oppositely polarized, as illustrated in Figure 5A. Apparently, the chiral system would then be excited by the two circularly polarized lights at the same time. As both chiral gels and achiral CPDs are sensitive luminophores, there are two possible pathways that may be mutually existed and imperative to the chiral emissions, as shown in Figure 5B–E^[35]: (a) when the organogels are activated, the absorption to right-handed circularly polarized light (RCP) and left-handed circularly polarized light (LCP) would be different depending on the CD responses of the chiral co-assembly to the incident light (Figure 5B,C). Then, the excited electrons, generated by both RCP and LCP, would undergo a donor–acceptor-like interfacial energy transfer (IET) process^{[13, 34]} to arrive at the excited state of CPDs, and release corresponding CPL signals depending on the chirality of the co-assembly. (b) On the other hand, the helically assembled achiral CPDs, which possess strong chiral dipole–dipole interactions (DDI) originated from their chiral morphologies would inevitably express CPL responses as well when they were excited by the incident light, and obviously such chirality is overwhelmingly related to the helicity of the CPDs (Figure 5D,E). Therefore, the overall output will be decided by the synergistic-competition between the two phenomena. In brief, when the incident light possesses high energy (short wavelength, namely 254–310 nm), the CD signal of the co-assembly, L-CA for instance, is negative (Figure 3D), indicating the absorption of RCP is higher than LCP, the chiral emissions generated via IET effect would mainly be right-handed (RCP > LCP), suggesting negative CPL signals (Figure 5B). Then, in case for 330–375 nm irradiations, the CD signal of the co-assembly is positive and the measured CPL signals therefore show positive signs (LCP > RCP) (Figure 5C). Afterwards, when relatively low energy light source (long wavelength, such as higher than ∼400 nm) is applied, the IET-induced CPL signals should be right-handed again and then change to left-handed when incident light is higher than ∼500 nm. However, the observed CPL signals remain invariably positive for which we assert the chiral DDI effect starts to be dominated in these cases because the absorption increases rapidly for the CPDs (Figure 2B) while decreases sharply for the gels (Figure 2O), indicating less electrons are transferred from the chiral gels to the achiral CPDs via IET process, but more DDI-related chiral responses are achieved in parallel. The synergistic-competition between IET and DDI effects eventually turn out positive CPL signals, as suggested in Figure 5D. Vice versa, such synergistic-competition phenomenon is also applicable for D-CA system, which can be explained by the pathways illustrated in Figure 5C–E with just opposite CPL behaviors in return (Figure 4D,E). Notably, although the forementioned chiral light-matter interactions provide critical insights for the chiral emissions, parameters such as energy conversion efficiency during IET, radiative transition efficiency, degree of chiral DDI, and environmental variations would affect as well the overall CPL behaviors of the sample, which might be further investigated soon with more advanced experimental and theoretical support.

2.4 Theoretical understandings on CD/CPL performances of CPD helix

To further investigate the interfacial energy transfer, we employ DFT simulations to take a closer look at the interface between the CPDs and chiral gels. According to previous publication,^[36] the self-assembly is formed by linking two Fmoc glutamic acid molecules to one lysine with peptide bonds on its two ends. The large molecules then accumulate by stacking the lysine parts with the rings in the Fmoc parts exposed on the gel surface. In this case, the CPD is attached to the benzene rings via H-bond interactions with –SO₃H groups on it. With further calculation of the molecular orbitals via Gaussian 09^[37] using the B3LYP/6-31G^[38] basis set, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and LUMO+1 can be obtained, as exhibited in Figure 6. It can be observed that the excitation is from the –SO₃H group on the CPD to the hydrogel molecule, indicating that the CPL activities are relevant to this surface state of the CPD. This observation can be further confirmed from the red-edge effect^[39] observed from the sample as indicated by the fluorescence lifetime scale of the co-assemblies (Figure S10). With solvents of higher polarizability, the CPL intensity increases, which indicates that the CPL is highly relevant with polar interactions with the solvent molecules that favors the longer wavelength excitations, that is, the H-bond linkage between the gel molecule and the CPD-SO₃H here. Because of this H-bond relevant fluorescence mechanism, as the excitation wavelength becomes longer, the excited states are more relevant near the H-bond as the other requires higher energy. This preference of the H-bond states under longer wavelength excitation favors here the surface states of the CPD-SO₃H, while for shorter wavelength excitations, the CPL still prefers the gel states. Thus, as the wavelength increases, the emission states gradually shift from the gel to the CPD-SO₃H, resulting in a CPL signal inversion.

Further analysis on the excitations is demonstrated to illustrate the inversion of the chirality of the CPL. After the calculation, the orbitals were visualized according to the indices of the states involved in the calculated excitations. For the excitations with energy lower than 330 nm, the excitations are mostly from the hydrogel to carbon, while for energy higher than 330 nm, the excitations involve more of the hydrogels and end up mostly with orbitals located on the gel. Two relevant excitations are shown in Figure 8. Therefore, through simulations-based TDDFT method, an excitation energy-dependent CPL behavior is witnessed in our chiral CA system with a switch around 330 nm, demonstrating a synergistic-competition phenomenon existing between configurational chirality and intermolecular energy-transfer dynamics as proposed by IET and DDI effects.

4.1 Synthesis of the carbonized polymer dots

The CPDs were prepared from o-phenylenediamine in a one-pot solvothermal process. 0.16 g of o-phenylenediamine was added to 25 mL dilute sulfuric acid solution with a concentration of 4 mol/L, and then transferred to Teflon-lined stainless-steel autoclave after ultrasonic dissolution and dispersion, and heated at 180°C for 12 h. Then, it was cooled to room temperature in the annealing furnace, and the black powder obtained after centrifugation of the stock solution was washed five times with a mixture of deionized water and ethanol (6:1), finally dried in vacuum 40°C.

4.2 Synthesis of chiral gels

The chiral gels were prepared according to the previous approach. Typically, an equimolar mixture of (0.1, 0.2, 0.3, and 0.4 mol) Fmoc and L- or D-lysine were added to 1 mL DI water, undergoing heating and shaking treatments until all hybrids dissolved in the aqueous solution. Different concentrations of chiral gels would be produced after cooling down to room temperature naturally, following with subsequent morphology analysis and optical measurements. The vial inversion method could verify the successful preparation of gels.

4.3 Synthesis of carbon dots co-assembled gel

A certain mass of CPDs powders (0.1, 0.3, 0.5, 0.7, and 1.0 mg), 0.4 mol Fmoc, and L- or D-lysine were added to 1 mL of water, all assembled complexes would be dissolved in solution after heating treatment, following cooling to room temperature. The carbon dots co-assembled gel is thus formed (the best temperature is above 90°C).

4.4 Simulation methods

The geometric optimization, excitation states, and ECD spectrum are calculated using Gaussian 09 software under B3LYP/6-31G basis set. Ground state geometries were optimized at the DFT. Time-dependent-DFT (TD-DFT)^[41] was utilized for UV and ECD spectrum. The structures combining with carbon dots, sulfonic acid group, and gel molecules are modeled by C₇₈H₆₈N₄O₁₈S₂.