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

complex 1 onto screen‐printed electrodes (SPEs) after its blue‐emitting powder was mechanical pressed to generate the desired interaction and therefore an orange solid photoluminescence (PL). The same light could be collected upon electrochemical oxidation in presence of C₂O₄²⁻ obtaining an enhancement of a factor 20 from the blue powder (Figure 4.4c). It is worthful to say that C₂O₄²⁻ acts as a better coreactant because it stabilizes the platinum complex when Pt²⁺ is oxidized to Pt⁴⁺ changing the geometry from square planar to tetrahedral [56]. The change in coordination geometry could be a limitation in the use of these complex‐types for multiple scans and further studies are required to understand deeper mechanism behind the generation of the MMLCT excited state by electrochemistry, since a structural change is happening during the process.

Figure 4.4 (a, d) Chemical structures of the investigated complexes; (b) Photographs of luminescence of Pt‐PEG onto SPE under top‐bench UV lamp (365 nm) and cartoons showing the principle of its mechanocromism before and after grinding with a pestle; (c) ECL emission recorded during a cyclic voltammetry at scan rate 0.2 V/s in 0.1 M LiClO₄, aqueous solution of Pt‐PEG physically transferred onto SPE before (blue trace) and after (red trace) mechanical stress using 10 mM Na₂C₂O₄ as coreactant; (e) Cyclic voltammetry at scan rate 0.1 V/s in 0.1 M LiClO₄ aqueous solution of 1 (blue trace) and 2 (green trace); (f) ECL intensity during a potential scan of 10⁻⁵ M aqueous solution of [Ru(bpy)₃]²⁺ (red trace) and Pt‐PEG₂ (blue trace) using 0.1 M LiClO₄ as supporting electrolyte upon addition of 10 mM of Na₂C₂O₄. Scan rate 0.05 V/s. GC working electrode, Ag/AgCl reference electrode, and Pt wire counter electrode.

      Source: Reproduced from Ref. [28].

This contribution has allowed Ye and coworkers to develop an iriudium(III) complex with AI‐ECL properties[57].) Complex 3 in Figure 4.5a, namely [Ir(tpy)(bbbi)] where tpy is 2,2′ : 6′,2″‐terpyridine and bbbi is 1,3 bis(1H‐benzimidazol‐2‐yl)benzene, contains these two planar and aromatic ligands that can easily generate π–π interactions and that can form hydrogen bonds. Therefore, as for complexes 1 and 2 (Figure 4.4), the self‐assembly of complex 3 in an aqueous solution could form nanoaggregates of 120–160 nm, according to the ratio of water/DMSO (Figure 4.5b). PL was centered at 643 nm and the φ_PL increased according to the amount of water in the mixture with DMSO. The formation of aggregates was also elucidated by ¹H NMR spectra. ECL emission arose cycling at +1.23 V vs Ag/AgCl using TPrA as coreactant. Since the oxidation potential of the latter was at +1.3 V vs Ag/AgCl, we hypothesized that the mechanism follows the “catalytic route” as described by Miao and Bard for metal complexes with lower oxidation potential (Figure 4.5) [29, 58].

Figure 4.5 (a) Chemical structure of complex 3; (b)TEM analysis of nanoparticle size of complex 3 in a DMSO/H₂O (10/90 v/v) mixture ; (c) Cyclic voltammogram (dark line) and ECL trace (blue line) of complex 1 (c = 200 μM) in a DMSO/H₂O (20/80, v/v) mixture containing 1 mM TPrA, 100 mM NaCl, and 10 mM PBS (pH 7.4). Inset: CV curves of a blank solution (excluding 1 and TPA) and complex 1 in the absence of TPrA; (d) ECL intensity changes of complex 1 (c = 200 μM) upon variation of the H₂O fraction of a DMSO/H₂O mixture. The scan rate was 0.1 V/s.

      Source: Reproduced from Ref. [57].

      4.2.2 Polymers and Polymeric Nanoaggregates

      Conjugated polymers based on metal complexes or organic moieties have attracted remarkable interest in the field of fluorescent and electrochemiluminescent materials for their excellent photostability, brightness, and fast emission rate [4759–63].

Their nanoaggregated form has been, nowadays, widely exploited for different aims, like biosensing [64], bioimaging [65], controllable drug [66] and it was found to be an excellent ECL emitter [59, 67, 68]. Compared to quantum dots (QDs) [5, 21, 69], they are less toxic but they suffer from low ECL intensity due to their size and the slow diffusion in solution [70]. Therefore, much effort has been employed to prepare small polymeric nanoparticles (PNPs) with improved luminescence, in particular, choosing highly luminescent moieties and to improve it in aggregated form (AIE‐gens).

      In order to achieve the high ECL intensity, Quan and coworkers have designed a three‐component polymer containing 9‐(diphenylmethylene)‐9H‐fluorene (DPF) that is known to be an important AIE‐gen [71]. They not only succeeded in preparing 10 nm nanoparticles in water with high AIE intensity at 543 nm but also demonstrated that such Pdots could give high ECL emission onto electrode surface upon addition of TPrA as coreactant [72]. The ECL spectrum showed the same AIE emission at 543 nm of DPF, clearly different from the PL peak at 415 nm of the other moiety 9,9‐dioctyl‐9H‐fluorene (DOF). The excited state of DPF is generated by Förster resonance energy transfer (FRET) mechanism from DOF moieties to DPF, since the latter has electrochemical inactivity at anodic scan. Both FRET and AIE properties improved their overall AI‐ECL emission.

The same group designed Pdots with donor‐acceptor AIE‐moieties in a three‐component structure obtained through Suzuki reaction (Figure 4.6) [31]. The performance of both designed polymers (P1 and P2 in Figure 4.6a) was assured by the different functions of the moieties. The D–A electronic components were represented by fluorene (P‐1)/carbazole (P‐2) as electron donor and the BODIPY as electron acceptor. The first intramolecular FRET pair was composed by the fluorene (P‐1)/carbazole (P‐2) unit, which was acting as a FRET donor, while the tetraphenylethene (TPE) AIE‐moiety as a FRET acceptor. The second intramolecular FRET pair was characterized by TPE as FRET donor and BODIPY as acceptor. TPE was essential for the enhancement of luminescence intensity after aggregation. Both annihilation and coreactant mechanisms were chosen to detect their ECL light in phosphate‐buffered saline (PBS) solution once immobilized onto the electrode surface. Results showed that P‐2 is more difficult to be oxidized in these conditions than P‐1 and their ECL emission happens at anodic potentials suggesting more stable radical anions than the cations. P‐2 ECL happens at a lower potential than P‐1 of 553 mV with a significant enhancement (Figure 4.6c). Their full photophysical study suggests that P‐2 contains a stronger electron‐donating unit such as the carbazole with a subsequent lower lowest unoccupied molecular orbital (LUMO) level that shifts the emission (25 nm) and makes weaker AIE intensity due to intramolecular charge transfer (ICT) phenomena. However, the lower LUMO facilitates the electron injection into the polymer, therefore, P‐2 ECL results higher in intensity than the P‐1 one. The coreactant ECL obtained by adding TPrA in PBS solution clearly enhanced both emissions, displaying always a higher emission for P‐2 than P‐1 for the aforementioned reason. The proposed mechanisms follow the annihilation pathway and the coreactant pathway initiated by the oxidation of TPrA presented by Bard and Miao [29]. The coreactant experiment was then allowed to record the ECL spectra, centered at 578 nm (P‐1) and 598 nm (P‐2).