Handbook of Aggregation-Induced Emission, Volume 3. Группа авторов

      (4.12)

      (4.13)

      (4.14)

      (4.15)

followed by Equation 4.9.

      When the concentration of TPrA is high and the concentration of AS is low, as is generally the case when the luminophore is being used as a probe in a bioanalytical context, an alternative pathway emerges [29].

      (4.16)

(4.17)

Here, TPrA^• generates the reduced form of the luminophore via homogeneous electron transfer (Equation 4.11), following which, the excited state may be formed via reaction, according to Equation 4.3 (annihilation), or by reaction with the radical cation TPrA^•+ (Equation 4.17).

      In general, there is a strong dependence of ECL intensities on pH according to analytes. Oxalates show virtual independence of ECL intensities on sample pH but are mostly detected at pH 6.0 [47]. Alkylamines produce maximum intensities between pH 4.5 and 6.0 [48, 49].

The reductive‐oxidative mostly employed mechanism concerning K₂S₂O₈, which has been firstly explored by White and Bard in 1982 [50], and used in AI‐ECL of siloles [51]. K₂S₂O₈ offers some advantages with respect to amines. Because S₂O₈²⁻ does not react appreciably with water or oxygen, it appears particularly promising for aqueous ECL, where the decomposition of water generally prohibits the direct production by electrolysis of the reactant that generates the excited state. The pathways are summarized by the Equations 4.18–4.21:

(4.18)

(4.19)

(4.20)

(4.21)

At first, AS gets reduced to generate the radical anion AS^•− (Equation 4.18). Then, the reaction between this radical anion and S₂O₈²⁻ produces the strong oxidant species SO₄^•− (Equation 4.19). Following the electron transfer between the AS^•− and SO₄^•− that generates the excited state (Equation 4.20) that finally emits light (Equation 4.21).

4.2 Classification and Properties of AI‐ECL luminophores

      From our discovery in 2017 [28], a huge attention has been paid to the development of different AI‐ECL luminophores, considering the background knowledge in both ECL and AIE. Since the aggregation form can be obtained in aqueous systems, ECL properties have been mostly evaluated by following the coreactant mechanism. Up to today, the reported samples based on inorganic materials or polymeric aggregated forms belong to the oxidative‐reductive pathway, while few of the organic materials show reductive‐oxidative pathway. Following, AI‐ECL luminophores are classified by typology of material or molecule. Metal transition complexes include platinum and iridium complexes, polymers and polymeric nanoaggregates can be entirely organic or can include metal transition complex moieties. The polymer has the advantage of easy‐forming dots or nanoparticles by reprecipitation a microemulsion. The same fate is observed for organic molecules while different hybrid materials have been synthesized in other ways to obtain them with various sizes and shapes.

      4.2.1 Metal Transition Complexes

The first investigation of AI‐ECL was reported by our group on square‐planar platinum(II) complexes and their supramolecular assemblies in solution and solid‐state (Figure 4.4) [28]. The two complexes presented, 1 and 2 in Figures 4.4a–d, maintain the same tridentate ligand 2,6‐bis(3‐[trifluoromethyl]‐1H‐1,2,4‐triazol‐5‐yl)pyridine (pyC₅‐CF₃‐tzH₂), and differ on the number of triethylene glycols chain attached to a 4‐amino pyridine ancillary ligand. It was demonstrated from the same group that this class of complexes exhibits very poor structured blue emission in a nonpolar solvent like dichloromethane with photoluminescence quantum yield (φ_PL) around 1% and short lifetimes [52, 53]. When they were dissolved in aqueous media, photoluminescent properties showed a significant bathochromical shift to 600 nm and an enhancement of both φ_PL and lifetime, up to 70–80%. The group attributed these results to the formation of aggregates in water, which approached single metal complexes from a distance shorter than 3.5 Å. The Pt⋯Pt metallophilic interactions at that distance caused the destabilization of the d_z² orbitals and the population of a new metal‐based highest occupied molecular orbital (HOMO) leading to metal‐metal‐to‐ligand charge transfer (MMLCT) transitions. While 1 formed spherical aggregates not thermodynamically stable in water, complex 2 displayed a stable aqueous suspension of nanoaggregates of 20 nm, which were relatively small to diffuse at the electrode surface and got oxidized. As we said, the Pt⋯Pt interaction destabilizes the HOMO and from an electrochemical point of view it decreases the oxidation potential of complex 2, which was not falling in the electrochemical window of dichloromethane but it is visible at +1.33 V vs Ag/AgCl in water (Figure 4.4e). In fact, upon oxidation and in presence of a coreactant, these nanoaggregates resulted in extremely active ECL compared to the molecularly dissolved complex in dichloromethane or acetonitrile with an ECL efficiency compared to Ru(bpy)₃²⁺ of Equation 4.2 using TPrA as coreactant and Equation 1.14 with C₂O₄²⁻. Despite the impossibility to use complex 1 in solution, it was possible to obtain the same kind of Pt⋯Pt interaction in solid‐state. It is in fact well known that these complexes manifest similar