Читать книгу Handbook of Aggregation-Induced Emission, Volume 3 - Группа авторов - Страница 54

Over the past decade, polycyclic aromatic hydrocarbons (PAHs) have been considered a fantastic class of emitters thanks to their low cost of production, structural tailorability, and excellent optoelectronic properties [78, 79]. Also, ECL was evaluated for PAHs with not satisfying results for their possible application, in particular, in biosensing [18, 70, 80]. These PAHs emitters were investigated usually in organic phase in nonaggregated form. The effect of an aggregation has been, therefore, studied with the advent of the AI‐ECL phenomenon.

The first organic molecule investigated for AI‐ECL was made from coumarin derivatives in an aqueous solution [81]. A donor‐acceptor structure based on 6‐[4‐(N,N‐diphenylamino)phenyl]‐3‐ethoxycarbonyl coumarin (DPA‐CM) was reprecipitated in water from a THF solution in order to form nanoparticles of 5.82 nm. The resulting particles revealed a red‐shifted absorption and a blue‐shift in the emission spectra in water compared with the one in THF with an enhancement of the φPL in water of 6 rather than 0.8% in THF. The low φPL in organic solvent is ascribed to the structural flexibility of DPA‐CM, which is lost as nanoparticle [82] and to the intramolecular photoinduced electron transfer from DPA to CM [83]. By coating a GC electrode with DPA‐CM nanoparticles, the corresponding ECL emission during anodic scan was significantly enhanced with an outstanding reproducibility and lower standard deviation. The system was then employed for the detection of ascorbic acid (AA), uric acid (UA), and dopamine (DA) in an aqueous solution, which will be discussed in the applications and outlooks (Section 4.4).

The first observation of AI‐ECL of hexagonal tetraphenylethylene microcrystals (MCs) in water was done by Yuan and coworkers recently (Figure 4.7) [42]. This enhancement was explained by the restriction of intramolecular motions limited by the crystal formation. In particular, it shows anodic ECL behavior with a great enhancement comparing the dispersed molecule in organic solution (Figure 4.7b). From the study, it is also clear that there is a notable redshift by almost 235 nm between the PL and ECL emission spectra, typical of aggregation events in solution. The AI‐ECL of these microcrystals then was employed to build an ultrasensitive «off‐on» ECL biosensor for Mucin1 (MUC1), which is a transmembrane glycoprotein expressed in human malignancies and therefore used as a biomarker for cancer.

Figure 4.7 (a) TPE molecular structure; (b) ECL‐potential profile for bare GCE in 0.1 M TBAPF₆ THF solution containing 1 mg/ml TPE monomers and 20 mM TEA (curve a, blue trace), and in 0.1 M PBS containing 1 mg/ml TPE MCs and 20 mM TEA (curve b, red trace). The inset of panel (b) shows a schematic diagram of the relationship between emission intensity and molecular states for the TPE; (c) ECL transients obtained for bare GCE in 0.1 M TBAPF₆ THF solution containing 1 mg/ml TPE monomers and 20 mM TEA (blue trace), and 0.1 M PBS containing 1 mg/ml TPE MCs and 20 mM TEA by stepping the potential between 0 and 1.6 V (red trace); (d) Schematic illustration of the preparation of TPE NCs by self‐assembly and the NIR aggregation‐induced enhanced ECL properties; (e) TEM images of TPE NCs.

Source: Reproduced from Refs. [42, 43] The Royal Society of Chemistry.

TPE nanocrystals (NCs) were also obtained by the same group, showing NIR enhanced ECL emission upon aggregation in aqueous solution, while it is invisible as a molecular entity in organic solvents (Figure 4.7d) [43]. The synthesis involved desolvation method for obtaining crystals of 450 nm (Figure 4.7e). For both MCs and NCs, triethylamine (TEA) was employed as a coreactant. From the spectral characterization and ECL studies, they give two possible options to explain the ECL enhancement: the first one follows the idea of a possible reduced energy gap when TPE molecules aggregate to form NCs, which allow highly efficient electron–hole recombination to obtain the excited state; the second possibility is that the restriction of intramolecular free rotation of phenyl ring reduces the nonradiative relaxations to obtain further ECL enhancement. Interestingly, the ECL emission peak falls at 678 nm while the PL and EL at 440 and 442 nm, respectively. This suggests a high degree of aggregation and a narrow energy gap « surface‐state type » via ECL route, which is different from the « core band‐gap » characteristic of PL [21, 22].

Very recently, TPE was integrated into a conjugated microporous polymer (CMP) by Zhang and co‐workers to give 1,1,2,2‐tetrakis(4‐bromophenyl)ethane (TBPE‐CMP) derivatives (1, 2, and 3) [84]. The structure is surrounded by four phenyl rings, which are highly twisted because of the strong hindrance via the rotatable C–C single bond. This leads to a 3D charge‐transporting network that facilitates hole transport [85]. Through DFT calculation, positions of HOMO and LUMO were evaluated to lay onto the tris(4‐ethynylphenyl)‐amine (TEPA) and TBPE, respectively. Coreactant TPrA mechanism was evaluated by potentiodynamic measurements in solid‐state onto GC electrode, showing the best result for TBPE‐CMP 1 at +1.35 V comparing with the other two polymers. It also shows good reproducibility for 27 consecutive cycles with a standard deviation of 1.15%. The ECL spectrum at 627 nm results in a redshift of 67 nm compared with its PL spectrum at 560 nm and it reflects the intrinsically different electron‐hopping pathway between them. The deprotonated intermediated of TPrA can supply the conversion from the oxidized CMP to its excited state at a relatively lower refined oscillatory level than the normal LUMO of TBPE. They further demonstrated the construction of an ECL sensor for the detection of dopamine, which can quench the emission of TBPE‐CMP 1.

Lu and co‐workers focused their investigation on another class of AIE luminophores based on 1,1‐distributed 2,3,4,5‐tetraphenylsiloles, which emit AI‐ECL by a coreactant approach. Siloles (see Figure 4.8a) show unique chemical and optical properties for their low‐lying LUMO levels because of the electronic interaction between the π * orbital of the butadiene moiety and the σ * orbital of the exocyclic Si‐R bond[86]. Their properties as AIE luminophores were then discovered by Tang and co‐workers and studied intensively [82]. However, few attempts of using it as ECL emitters have brought poor results [67, 87, 88], since they have not considered their important properties in aggregated form. Three siloles were synthesized in an easy way, such as 1,1′‐dimethyl‐2,3,4,5‐tetraphenylsilole (DMTPS), 1‐methyl‐1,2,3,4,5‐pentaphenylsilole (MPPS), and 1,1,2,3,4,5‐hexaphenylsilole (HPS), resulting simple structure, nontoxic, and stable. They show the typical characteristics of an AIE luminophore, weak emitter in solution but strongly fluorescent in aggregated form. Coreactant approach has been selected rather than annihilation because of the inefficiency of the latter. Results showed excellent cathodic ECL emission by using K₂S₂O₈. Also, the findings indicated that siloles are superior for forming good aggregates onto GC electrode surface, since there is an interaction between the silicon atoms and the oxygen functional groups on the electrode. The results show an influence of the substituents in position 1,1 with an intensity order DMTPS < MPPS < HPS. The 1,1‐position effectively enhance higher σ *– π * conjugation interactions between exocyclic carbon atoms and endocyclic silicon atom. Additionally, larger substituents also restrict the rotation of the phenyl groups at the silole ring, reducing energy loss and increasing the ECL efficiency. They demonstrate the aggregation effectively reduces the nonradiative energy decay by suppressing intramolecular motions and are largely responsible for the generation of such high ECL emissions, comparing to the dissolved molecule in organic systems. At last, ECL and PL profiles suggest that the excited state is the same for photoexcitation and electrochemical excitation (Figure 4.8c).

Figure 4.8 (a) Molecular structure of 1,1‐disubstituted 2,3,4,5‐tetraphenylsiloles; (b) ECL intensities versus different siloles/K₂S₂O₈ systems: siloles in the aggregated state modified at the electrode surface; (c) PL and ECL normalized spectra of HPS in solid‐state; (d) Molecular structure of carboranyl carbazoles; (e) ECL intensity of 1 mM T‐3 in 20, 60, 85 and 95% H₂O with 10 mM tetraoctylammonium bromide‐modified GC electrode. PMT: 800 V. Inset: 1 × 10⁻⁵ M T‐3 in pure THF and 95% H₂O; (f) ECL (red trace) and PL (black trace) emission spectra of 1 × 10⁻⁵ M T‐3 in 95% H₂O modified GC electrode.

Source: Readapted from Refs. [51, 89].

A series of boron clusters using carbazolyl and carboranyl moieties as electron donor and acceptor, respectively, have been explored as well as reductive‐oxidative ECL by Xu and co‐workers [89]. They were the first to deal with AI‐ECL properties of these unique aggregated organic dots in aqueous media, although extensive studies on carborane‐based aggregation‐induced PL molecules or assemblies have been reported [90, 91]. All the synthesized compounds displayed AI‐ECL in solid‐state onto a GC electrode starting from a solution of clusters obtained in a mixture 5/95 THF/H₂O. The results show a notable increase compared to the molecule dispersed in THF. The compound T‐3 was investigated in detail for PL and ECL properties in different water fractions. From the emission spectra, it is clear that ECL excited state might be generated from surface state transition, and therefore its result differ from the PL one. This study opened the way for further studies in the reductive‐oxidative ECL pathway which is still in its infancy.