INTRODUCTION
Electrochemiluminescence (ECL) is a light-emitting process, involving the radiative transition of excited species (R*) to their ground states (S0), in which R* is generated via electrochemical reactions between high-energy species.[] According to spin statistics, the ratio of the lowest excited singlet states (S1) and triplet states (T1) of R*, 1R* and 3R*, is calculated to be 1:3.[] To obtain high-efficiency ECL emitters, the full utilization of 1R* and 3R* will be considered as a promising way. Generally, transition metal-based phosphorescent compounds, such as iridium (III) complexes[] and ruthenium (II) complexes,[] can achieve full utilization of R* to emit phosphorescence owing to the strong spin-orbit coupling (SOC) effects, thus possessing excellent ECL efficiency (ΦECL). Nevertheless, the cost of these metal complexes and their potential environmental pollution remain as limitations in these systems.
With continued demand in developing ECL emitters of low biotoxicity and cost, organic fluorescent dyes are attracting increasing attention in recent years.[] Especially, all-organic aggregation-induced emission (AIE) gens exhibit potential for promising and efficient ECL emitters, since De cola et al.[] reported aggregation-induced electrochemiluminescence (AIECL) behaviors of Pt (II) complex under assembly. Various organic dyes have been confirmed as valid AIECL emitters, including polymer dots,[] conjugated microporous polymer,[] and covalent–organic framework (COF),[] as well as organic small molecules-based organic nanocrystals[] and organic dots (ODs).[] Unfortunately, these fluorescent AIECL-type emitters still intrinsically suffer from low ECL efficiency due to the utilization of only 1R* to produce light, limited by the weak SOC effects.
The theoretical limitation of photoluminescence from 3R* can be overcome through reverse intersystem crossing (RISC), where RISC represents that 3R* undergo uphill spin-flip to 1R*.[] For molecules of thermally activated delayed fluorescence (TADF) activity, maximum ECL efficiency approaching 100% can be expected. In principle, when the energy bandgap (ΔEST) between S1 and T1 is sufficiently small (≈0.1 eV), RISC from 3R* to 1R* can occur in aromatic organic compounds.[] By constructing molecules of electron donor and acceptor (D-A) pair, a small overlap between the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs) can be conveniently achieved to realize the small ΔEST required for TADF.[] For example, molecules containing carbazole and cyanobenzene as a D-A pair were recently reported as efficient ECL emitters (with ΦECL of around 50%) in MeCN solutions, which exceed the 25% spin-statistics limitation.[] Despite the much improved ECL efficiency, the aggregation-caused quenching effects of these TADF emitters have largely restricted their applications in an aqueous medium.
On the basis of these facts, the construction of molecules integrating both AIE and TADF superiorities, also called aggregation-induced delayed fluorescence (AIDF), should be a promising approach in developing bright ECL emitters in aqueous media. Aiming at easy synthesis, precise molecular composition, and tunable optoelectronic properties, we synthesized an AIDF-active small molecule, p-BP-DMAC, which is comprised of dimethylacridine (DMAC) and benzophenone (BP) unit as a D-A pair (Scheme ). The RISC-dependent ECL of the resulting p-BP-DMAC ODs was also identified by comparing the ECL behaviors between p-BP-DMAC ODs and the exclusively AIE-active p-BP-Ph-DMAC ODs in the aqueous solution. Owing to the efficient RISC, the p-BP-DMAC ODs showed strong ECL signals of yellow visualization with a 12-fold enhancement (in terms of the ratio value of ΦECL and fluorescence quantum yield, that is, ΦECL/ΦPL, also expressed as “K” value) in comparison with that of p-BP-Ph-DMAC ODs. Similar to AIECL, we coined such AIDF-enhanced ECL as aggregation-induced delayed electrochemiluminescence (AIDECL). More significantly, by regulating molecular structures, such as dual D-A pairs and its relative position, we demonstrated that two AIDECL-type emitters of m-BP-di-DMAC ODs and p-BP-di-DMAC ODs can achieve higher ECL efficiency, with a 1.7-fold and 6.8-fold increase, respectively, compared to p-BP-DMAC ODs. Single-crystal data combined with density functional theory (DFT)/time-dependent DFT (TD-DFT) calculations reveal that this enhancement stems from the effective intermolecular weak interaction (i.e., C─H···π and C═O···H) facilitating radiative decay, and the fast RISC rate triggered by enhanced SOC effects between S1 and excited triplet states.
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RESULTS AND DISCUSSION
All compounds (p-BP-DMAC, p-BP-Ph-DMAC, m-BP-di-DMAC, and p-BP-di-DMAC) were synthesized by Pd-catalyzed Buchwald coupling reactions according to previous reports using similar strategy[] (Scheme ). These compounds were characterized by 1H NMR, 13C NMR, ESI-MS, and single crystal X-ray (Figures and Table ). The corresponding ODs were prepared easily using these compounds as luminous precursors by a nanoprecipitation method, in which poly(styrene-co-maleicanhydride) was employed as functional reagents to favor the formation of carboxyl sites on the ODs surface via the maleic anhydride hydrolysis[] (Scheme ). Additional experimental details are also presented in the Supporting Information.
Photophysical property of p-BP-DMAC and p-BP-Ph-DMAC
As a class of D-A type compounds, p-BP-DMAC and p-BP-Ph-DMAC, the weak absorption bands (333–450 nm) associated with intramolecular charge transfer (ICT) from DMAC donor to BP acceptor were observed, except for the intense absorption peaks around 285 nm from the π–π* transition of molecules (Figure ). The red-shifted fluorescence emission shown in Figure , ranging from nonpolar to polar solvent, have also confirmed their ICT nature.
We also examined their photophysical properties of crystals. As can be seen from the excitation-emission correlation spectra of p-BP-DMAC and p-BP-Ph-DMAC crystals (Figure ), the maximum emission wavelength of them remains unchanged at 507 and 455 nm, respectively. Meanwhile, both p-BP-DMAC crystals and p-BP-Ph-DMAC crystals were also observed under an optical microscope. The blocky-shape p-BP-DMAC crystals and strip-like p-BP-Ph-DMAC crystals show an intense green luminescence and a strong blue emission (Figure ), respectively, under ultraviolet light irradiation, which coincide well with 3D excitation-emission correlation spectra.
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Additionally, we collected the transient photoluminescence (PL) spectra of p-BP-DMAC crystals and p-BP-Ph-DMAC crystals. The temperature-dependent transient PL spectra of p-BP-DMAC exhibit biexponential decay route including prompt and delayed components (Figure ). The ratios of delayed components increase obviously as temperature rises and the emission spectra between prompt fluorescence and delayed fluorescence are the same (the inset in Figure ). These results suggest that the delayed fluorescence springs from the RISC process, in which the rate increases with temperature. The prompt emission could be explained as the fluorescence from the S1 to S0 transition, whereby S1 is generated directly from photoexcitation. The delayed emission is from the intersystem crossing of S1 to T1, then up-conversion back to S1, finally radiative decay to S0. On the contrary, for p-BP-Ph-DMAC, the time-resolved decay spectrum at 298 K reveals only the presence of short-lived species with lifetime of nanosecond scale (Figure ).
The PL behaviors of p-BP-DMAC and p-BP-Ph-DMAC were also investigated in THF/H2O mixtures with different H2O fractions (fw) and exhibit clear AIE characteristics (Figure and Figure ). With the poor solvent (H2O) addition, the emission intensity of both luminophores exhibits a first decrease (fw ≤ 10%), which might be caused by the twisted intramolecular charge transfer process.[] Both remain weak emissions as the increase of fw (p-BP-DMAC: fw ≤ 80% and p-BP-Ph-DMAC: fw ≤ 70%). Enhanced and blue-shifted PL emission is then observed and reaches a maximum at fw = 99%. Taking p-BP-DMAC as an example, the PL peak is blue-shifted from 532 nm (fw = 0%) to 516 nm (fw = 90%), and 504 nm (fw = 99%) (Figure ). This can be ascribed to the formation of nanoaggregates under the large fw because of their hydrophobic nature. The formation of aggregates, on one hand, can restrict the intramolecular rotational and vibrational motions for blocking the nonradiative decay,[] and on the other hand, can give rise to lowered reorganization energy to facilitate the blue-shifted emission relative to that in pure THF solution.[] The aggregation process has been evidenced from TEM images of p-BP-DMAC from fw = 70% to fw = 99% (Figure ). The morphology of the corresponding particle sizes gets smaller as the increase of fw, for example, a representative size of 1.1 μm, 122 nm, 83 nm, and 63 nm for an fw of 70%, 80%, 90%, and 99%, respectively (Figures and ). More interestingly, the lifetimes of DF components (i.e., p-BP-DMAC) evidently increase with nanoaggregate formation (Figure ), suggesting the superiorities of AIE and DF are not independent of each other, namely AIDF.
Photophysical property of p-BP-DMAC ODs and p-BP-Ph-DMAC ODs
To investigate the ECL performance of these lumiphores in aqueous media, we prepared the ODs of p-BP-DMAC and p-BP-Ph-DMAC. We first characterized the morphology, size, and photophysical properties of p-BP-DMAC ODs and p-BP-Ph-DMAC ODs (Figures , and generalized in Table ).
TABLE 1 Photophysical properties of p-BP-DMAC ODs and p-BP-Ph-DMAC ODs.
| ODs | λem) (nm) | ΦPL) (%) | τprompt (ns) | τdelayed (μs) | kRISC (106 s−1) | dH ± SD (nm) | ζ-pot. ± SD (mV) |
| p-BP-DMAC | 503 | 29.9 | 27.2 | 1.76 | 1.95 | 23.93 ± 0.56 | −16.73 ± 3.87 |
| p-BP-Ph-DMAC | 492 | 15.0 | 15.7 | – | – | 24.64 ± 0.69 | −14.90 ± 1.01 |
Both ODs exhibit an essential spherical shape with uniform size distribution of ≈24 nm (Figures and ). They also show strong absorption bands below 340 nm and weak CT absorption bands from 340 to 500 nm (Figure ). The maximum PL emission is located at 503 and 492 nm for p-BP-DMAC ODs and p-BP-Ph-DMAC ODs, respectively (Figure ). Notably, p-BP-Ph-DMAC ODs demonstrate a red shift of 37 nm compared to p-BP-Ph-DMAC crystals (Figure ), while p-BP-DMAC ODs are close to p-BP-DMAC crystals (Figure ). This is probably due to that the extra phenyl ring in p-BP-Ph-DMAC can cause easier deformation in the crystalline phases and facilitate the crystalline-amorphous phase transition under external pressure, thus promoting planar conformation.[] For further gaining insights into the unique phenomenon, we collected the PL spectra of p-BP-Ph-DMAC crystals and p-BP-DMAC crystals before and after grinding (Figure ). As expected, red-shifted emission is observed for p-BP-Ph-DMAC after grinding compared to that before grinding (Figure ), indicating the red-shifted emission from crystalline phases to amorphous phases and the reasonability of a red shift in p-BP-Ph-DMAC ODs. While p-BP-DMAC crystals demonstrate the same PL peaks at 502 nm before and after grinding (Figure ), which further verifies the role of an extra phenyl ring in p-BP-Ph-DMAC.
In addition, the p-BP-DMAC ODs demonstrate a prompt lifetime of 27.2 ns and a delayed lifetime of 1.76 μs, while the p-BP-Ph-DMAC ODs only own the short-lived fluorescence of 15.7 ns at room temperature (Figure ). Moreover, owing to the large negative surface charge (zeta potential over −10 mV), both ODs possess excellent stability without obvious precipitation in several months.
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Electrochemistry and ECL property of p-BP-DMAC ODs and p-BP-Ph-DMAC ODs
Next, we examined the ECL behaviors of p-BP-DMAC ODs- and p-BP-Ph-DMAC ODs-modified glass carbon electrodes (GCEs) in 0.1 M pH 7.4 phosphate buffer saline (PBS) containing 25 m tri-n-propylamine (TPrA) as the co-reactant. p-BP-DMAC ODs show strong ECL signals of yellow visualization (the inset in Figure ). It is important to note that the ECL intensity of p-BP-DMAC ODs is 12.8 folds that of p-BP-Ph-DMAC ODs under the same conditions (1448 a.u. vs 113 a.u., Figure ). The ФECL values of p-BP-DMAC ODs and p-BP-Ph-DMAC ODs are calculated to be 7.2% and 0.3%, respectively, relative to tris(2,2′-bipyridyl)ruthenium (Ru(bpy)32+)/TPrA system (according to Equation ) in the Supporting Information); that is, the ФECL of p-BP-DMAC ODs is 24 times that of p-BP-Ph-DMAC ODs. Notably, the ECL signals were observed with an onset oxidation potential of ≈+0.95 V (Figure ), which follows the oxidation of TPrA at +0.8 V (Figure ) and is in line with the oxidation behaviors of p-BP-DMAC ODs and p-BP-Ph-DMAC ODs (Figures and , see below). This indicates that the ECL emission conforms to the typical oxidative-reductive ECL mechanism.[] In addition, both ODs exhibit excellent stability of ECL signals under continuous cyclic voltammogram (CV) scans, with relative standard deviation (RSD) of 1.37% and 2.87% for p-BP-DMAC ODs and p-BP-Ph-DMAC ODs, respectively (Figure ). The ECL emission peaks are observed at 569 nm for p-BP-DMAC ODs and 605 nm for p-BP-Ph-DMAC ODs (Figure ), with a red shift of 66 and 113 nm in comparison with the PL emission (503 nm for p-BP-DMAC ODs and 492 nm for p-BP-Ph-DMAC ODs), respectively. This might be due to that the ECL emission undergoes the surface-states route with lower energy.[] It is worth noting that, similar to the PL behaviors during aggregation (Figure ), the ECL signals of p-BP-DMAC are also enhanced with the increase of fw (Figure ), implying their similar radiative decay.
In consideration of the distinct difference in ΦECL between p-BP-DMAC ODs and p-BP-Ph-DMAC ODs due to minor structural change, we then probed such enhanced behaviors of ECL. As is well known, the ΦECL is dependent on the synergy between electrochemical excitation and subsequent radioluminescence.[] Subsequently, we measured the CV behaviors of p-BP-DMAC ODs/GCE and p-BP-Ph-DMAC ODs/GCE in 0.1 Bu4NPF6 CH3CN solution (Figure ). Under positive scanning, quasi-reversible redox peaks are detected at the formal potential of +1.04 V and +1.02 V for p-BP-DMAC ODs and p-BP-Ph-DMAC ODs, respectively, with an approximate onset potential at +0.93 V. In addition, the CV curves of both ODs/GCE also demonstrate similar shapes in PBS without TPrA (Figure ). The almost identical oxidation behaviors between the two luminophores suggest essentially the same electrochemical excitation process. Therefore, we speculate that the difference in ΦECL mainly originates from the radiative luminescence process.
It is easily found that the ΦPL of p-BP-DMAC ODs demonstrates a nearly twofold increase relative to that of p-BP-Ph-DMAC ODs (Table ), which can be also considered as an account for the enhancement of ECL to some extent. However, the enhanced K value between p-BP-DMAC ODs and p-BP-Ph-DMAC ODs is well above 1, reaching 12 (Figure ). Hence, we further assume that the remarkably enhanced effects of ΦECL might be attributed to the radiative transition of electrogenerated 3R* through RISC in p-BP-DMAC ODs.
Single-crystal structure and theoretical calculations
To better understand the occurrence of the RISC process, we first estimated the luminophores’ ΔEST value, which is a prerequisite for maintaining a valid up-conversion of 3R*.[] Through the fluorescence and phosphorescence spectra in toluene at 77 K, their energies of S1 and T1 states are obtained (Figure ). As a result, the ΔEST values are calculated to be 0.019 eV for p-BP-DMAC and as high as 0.323 eV for p-BP-Ph-DMAC. The small ΔEST of p-BP-DMAC ensures a plausible possibility of AIDF property, enabling efficient up-conversion from 3R* to 1R*.
Next, to further decode the enhanced ECL effects arising from AIDF, we dissected the single crystal structure of p-BP-DMAC and p-BP-Ph-DMAC. As shown in Figure , they adopt a highly twisted conformation. Specifically, the carbonyl and phenyl rings give rise to small torsion angles (φ) of 34.59°/19.05° and 20.78°/34.84° for p-BP-DMAC and p-BP-Ph-DMAC, respectively, but the phenyl group is attached vertically at DMAC (φ = 97.62° for p-BP-DMAC, and 91.63° for p-BP-Ph-DMAC), which is generally conducive to reducing electronic coupling and separating frontier orbitals of HOMO and LUMO.[] However, a small φ of 31.29° between BP and the adjacent benzene ring is also observed in p-BP-Ph-DMAC, which will cause electron delocalization to the benzene unit.
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Meanwhile, the spatial distributions of HOMOs and LUMOs, as well as ΔEST values of these luminogens were also calculated by DFT and TD-DFT (Figure ). Consistent with the above-mentioned deductions, the HOMOs are dominantly concentrated on the DMAC units while the LUMOs are mainly located on the BP cores, which is favored to reduce ΔEST. However, for p-BP-Ph-DMAC, the LUMOs extend to the adjacent benzene ring, resulting in the increase of overlap between HOMO and LUMO. Consequently, the small ΔEST value of p-BP-DMAC is calculated to be 0.013 eV, in favor of the occurrence of the AIDF property, while the ΔEST value of p-BP-Ph-DMAC is as high as 0.216 eV, which well reproduced the experimental values. Furthermore, we noticed that they share similar HOMO energy levels (−5.04 eV and −4.99 eV for p-BP-DMAC and p-BP-Ph-DMAC, respectively), representing nearly identical electrochemical oxidation. This is also consistent with the CV results, with the same HOMO energy levels of −5.27 eV for p-BP-DMAC and p-BP-Ph-DMAC (Figure ).
Additionally, we calculated their hole-electron distribution of S1 and T1 (Figure ), and found that the hole and the electron are basically separated for both p-BP-DMAC and p-BP-Ph-DMAC. This indicates that they exhibit charge transfer characteristic for both S1 and T1 excited state, which is consistent with the experimental results of red-shifted PL spectra from nonpolar to polar solvent (Figure ).
Moreover, the twisted conformation can also effectively impede close packing and weaken intermolecular π–π interactions. Abundant C─H···π and C═O···H hydrogen bonds while no apparent π–π interactions are clearly observed in both p-BP-DMAC and p-BP-Ph-DMAC single crystals (Figure ). These weak interactions contribute to rigidifying the molecular conformation and blocking the non-radiative pathway by restricting the molecular motions, thus promoting the AIE effects.[]
AIDECL mechanism
Under essentially the same electrochemical excitation, p-BP-DMAC ODs demonstrate a 12-fold higher K value than p-BP-Ph-DMAC ODs. We thus concluded that the AIDF effects can enhance ECL efficiency. Herein, the possible AIDECL mechanism is also proposed (Scheme ): the TPrA and ODs are oxidized sequentially to produce TPrA•+ and ODs•+ (Equations (1) and (2)). It is worth noting that the electrogenerated short-lived p-BP-DMAC•+ was also identified by mass spectrometry (m/z 389.1789, theoretical m/z 389.1780, Δm = +0.9 mDa, see Figure ); After deprotonation, the resulting radicals TPrA• (Equation (3)) could reduce ODs•+ via electron transfer (ETPrA• ≈ −1.7 V,[] EODs•+/ODs = +1.04 V, see Table ), eventually generating the excited states ODs*; The ODs* are redistributed by spin statistics, 1ODs* at S1 and 3ODs* at T1 (Equation (4)); Subsequently, 3ODs* could undergo fast RISC process to arrive 1ODs* (Equation (5)), thus increasing the amounts of excitons for radiative transition from S1 to S0 (Equations (6) and (7)), which ultimately promotes ECL emission. The AIDECL mechanism is also illustrated in Figure .
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AIDECL-based structure-property relationships
Armed with the AIDECL, we then synthesized two new ODs of m-BP-di-DMAC and p-BP-di-DMAC for achieving higher ECL efficiency, where both are composed of dual pairs of D-A units (Figure ). It is apparent that both m-BP-di-DMAC ODs and p-BP-di-DMAC ODs demonstrate delayed fluorescence nature, with a delayed lifetime of 1.51 μs and 1.54 μs, respectively (Figure ). Additionally, their energy levels diagram, crystal structures, DFT/TD-DFT data, electrochemical data, and the corresponding photophysical properties of ODs as well as ECL behaviors are demonstrated in the Supporting Information (Figures , and Tables and ).
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With respect to both m-BP-di-DMAC and p-BP-di-DMAC, on one hand, the dual D-A pairs are expected to be beneficial to twist the molecular conformation and strengthen the intermolecular interactions to further improve ФPL.[] As evidenced by the single crystal data, clear C─H···π hydrogen bonds of a distance of 2.598–2.906 Å and a distance of 2.652–2.678 Å were observed for m-BP-di-DMAC and p-BP-di-DMAC, respectively (Figure ). Both are shorter than that of p-BP-DMAC (2.995–3.053 Å). Just as expected, the ODs prepared from m-BP-di-DMAC and p-BP-di-DMAC exhibit high ФPL of 32.6% and 46.7%, respectively (Table ). It is also confirmed by the calculation results that the rate constants of radiative decay increase in the order of p-BP-DMAC ODs (3.19 × 106 s−1) < m-BP-di-DMAC ODs (3.26 × 106 s−1) < p-BP-di-DMAC ODs (3.72 × 106 s−1) (Equations ). On the other hand, the introduction of multiple heteroatoms (i.e., nitrogen atom) will be conducive to degenerating frontier molecular orbitals and facilitate the generation of multiple excited states.[] It can trigger multiple channels for uphill spin-flip transformation of 3R* for promoting RISC process. As can be seen, two degenerate HOMOs (HOMO and HOMO-1) (Figure and Table ) and two excited triplet states (T1 and T2) with close adiabatic energy (Figure and Figure ) are observed, implying the efficient RISC process. According to the above analysis, it is anticipated that luminophores with higher ФECL can be obtained on the basis of AIDECL effects.
Inspiringly, both m-BP-di-DMAC ODs/GCE and p-BP-di-DMAC ODs/GCE show strong ECL signals of visualization (Figure ), and possess high ФECL of 12.3% and 48.9%, respectively, with their ФECL values increasing by 1.7-fold and 6.8-fold compared to p-BP-DMAC ODs/GCE. Particularly, the ФECL of p-BP-di-DMAC ODs stands out among state-of-the-art pure organic nanomaterials (Table ).
To gain further insight into the structure-related RISC process in AIDECL, we quantified the contribution of RISC in m-BP-di-DMAC ODs and p-BP-di-DMAC ODs. It is obvious that higher K values of 0.38 and 1.05 for m-BP-di-DMAC ODs and p-BP-di-DMAC ODs are achieved (Figure ), respectively, in comparison with that of p-BP-DMAC ODs (K = 0.24, Figure ), suggesting the higher RISC efficiency of both m-BP-di-DMAC ODs and p-BP-di-DMAC ODs. This is likely due to the fact that the 3R* at T1 and T2 of both m-BP-di-DMAC and p-BP-di-DMAC can participate in the RISC (Figure ), improving the utilization efficiency of R* at triplet excited states.[] In accordance with the above results, the rate constants of RISC (kRISC) also increase in the order of p-BP-DMAC ODs (1.95 × 106 s−1) < m-BP-di-DMAC ODs (2.54 × 106 s−1) < p-BP-di-DMAC ODs (3.06 × 106 s−1) (Equations ).
More significantly, to guide the rational design of AIDECL-type ODs in the future, we explored the difference of kRISC between m-BP-di-DMAC and p-BP-di-DMAC. It is well known that both energy gaps and SOC effects between the triplet and S1 excited states play a crucial role in accelerating RISC process.[] As shown in Figure , both m-BP-di-DMAC and p-BP-di-DMAC possess small and close ΔEST values of 0.035 eV and 0.033 eV, respectively, which are also close to the experimental results (Figure ). In addition to that, the tiny and nearly equal energy gap between S1 and T2 is evaluated to be 0.015 eV and 0.013 eV for m-BP-di-DMAC and p-BP-di-DMAC, respectively (Figure ), which indicates an extra RISC pathway. Given this, we deduce that the difference in kRISC can arise from distinct SOC effects. As revealed in Figure , both of them own similar SOC matrix elements of ξ(S1, T2) (0.173 cm−1 for m-BP-di-DMAC, and 0.145 cm−1 for p-BP-di-DMAC), while p-BP-di-DMAC possess ξ(S1, T1) value of 0.375 cm−1, markedly larger than that of m-BP-di-DMAC (ξ(S1, T1) = 0.091 cm−1), thus endowing p-BP-di-DMAC with a more effective RISC channel. Through the analysis of molecular stacking (Figure ), it is evident that m-BP-di-DMAC, DMAC units meta to a carbonyl group, own large steric hindrance effects with DMAC units crowding. As a result, the larger folding dihedral angle between two phenyl rings of DMAC unit is measured to be 177.87°/178.83° (Figure ), close to the plane, which might result in the decrease of the SOC value.[] This offers more clues for us to design high-efficiency AIDECL-type molecules. To the best of our knowledge, this is the first paradigm of the exploitation of AIDECL-type ODs at the molecular levels.
CONCLUSIONS
To fully utilize 3R* for improving the ECL efficiency in aqueous media, a novel AIDECL-active emitter, p-BP-DMAC ODs, was developed for the first time by integrating D-A pair of dimethylacridine-benzophenone. Benefiting from the effective spin-up conversion of non-emissive 3R* to the emissive 1R* through RISC, the p-BP-DMAC ODs demonstrate high ΦECL and strong ECL signals of visualization. Compared to p-BP-Ph-DMAC ODs with only AIE activity, a much higher K value of p-BP-DMAC ODs confirmed the dependence of RISC on ECL enhancement. More significantly, through elaborate structure design, we developed new ECL dyes consisting of dual D-A units, allowing for the acceleration of both radiative decay and RISC, and achieving a high ECL efficiency of ≈50%. The enhanced AIDECL due to effective intermolecular weak interactions and promoted SOC effects between S1 and excited triplet states is supported by single-crystal studies and theoretical calculations. Our work opens an avenue for developing next-generation ECL dyes and gains insights into the structure-property relationships at the molecular structure levels.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China (Grant nos. 22034003, 22204075, 22275085), the Natural Science Foundation of Jiangsu Province (BK20220769), the Excellent Research Program of Nanjing University (ZYJH004), and State Key Laboratory of Analytical Chemistry for Life Science (5431ZZXM2203).
CONFLICT OF INTEREST STATEMENT
The authors declare no conflicts of interest.
DATA AVAILABILITY STATEMENT
Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC 2191756 (p-BP-DMAC), CCDC 2191757 (p-BP-Ph-DMAC), CCDC 2129394 (m-BP-di-DMAC), and CCDC 2191755 (p-BP-di-DMAC). Copies of the data can be obtained free of charge via .
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Abstract
Full utilization of the excited species at both singlet states (1R*) and triplet states (3R*) is crucial to improving electrochemiluminescence (ECL) efficiency but is challenging for organic luminescent materials. Here, an aggregation‐induced delayed ECL (AIDECL) active organic dot (OD) containing a benzophenone acceptor and dimethylacridine donor is reported, which shows high ECL efficiency via reverse intersystem crossing (RISC) of non‐emissive 3R* to emissive 1R*, overcoming the spin‐forbidden radiative decay from 3R*. By introducing dual donor‐acceptor pairs into luminophores, it is found that nonradiative pathway could be further suppressed via enhanced intermolecular weak interactions, and multiple spin‐up conversion channels could be activated. As a consequence, the obtained OD enjoys a 6.8‐fold higher ECL efficiency relative to the control AIDECL‐active OD. Single‐crystal studies and theoretical calculations reveal that the enhanced AIDECL behaviors come from the acceleration of both radiative transition and RISC. This work represents a major step towards purely organic, high‐efficiency ECL dyes and a direction for the design of next‐generation ECL dyes at the molecular level.
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Details
; Shi, Shen‐Yu 2 ; Wang, Shu‐Min 1 ; Tao, Qian‐Qian 1 ; Ma, Hui‐Li 2 ; Hu, Jun 3 ; Chen, Hong‐Yuan 1 ; Xu, Jing‐Juan 1
1 State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, China
2 Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing, China
3 School of Life Science and Health Engineering, Jiangnan University, Wuxi, China