Chart 1.2 Molecular structures of TPP–TPA and TPP–PPI.

Table 1.1 EL performance of devices.

      ^a V _on = turn‐on voltage at 1 cd/m².

      ^b The maximum luminescence (L_max), current efficiency (η_C), power efficiency (η_P), and external quantum efficiency at the maximum values for the devices.

      ^c Data recorded at a luminescence of 1000 cd/m².

      It was later reported that TPP derivatives with a D–A structure possess a planarized intramolecular charge transfer (PLICT) effect in the excited state in the polar media. For example, the Φ_F of TPP–TPA in toluene, ethyl acetate, dichloromethane, and DMF changes from 22.7, 50.9, 80.7 to 80.9% as the polarity of the solvent increases. It is due to the formation of planarization conformation or quinone conformation with a good conjugation in the excited state to increase the probability of transition. In that work, another TPP–TPA‐based blue OLED (configuration: ITO/HATCN (5 nm)/TAPC (40 nm)/TCTA (5 nm)/TPP–TPA (20 nm)/Bepp2 (45 nm)/Liq (2 nm)/Al) is fabricated with HATCN (2,3,6,7,10,11‐hexacyano‐1,4,5,8,9,12‐hexaazatriphe‐nylene), TAPC (1,1‐bis(4‐di‐p‐tolylaminophenyl)cyclohexane), TCTA (4,4′,4′′‐tri‐9‐carbazolytriphenylamine), and Bepp2 (bis(2‐(2‐hydroxyphenyl)‐pyridine)beryllium) functioned as hole‐injecting layer, hole‐transporting layer, hole‐transporting and electron‐blocking layer, and electron‐transporting and hole‐blocking layer, respectively. However, a very good device performance with V_on, L_max, and EQE of 2.8 V, 19 170 cd/m², and 4.08%, respectively, is obtained (Table 1.1, Device II) [51].

      The PLICT effect also takes places in phenanthroimidazole derivative‐modified TPP (TPP–PPI), though the phenanthroimidazole‐based group is not so electron‐donating (Chart 1.2). TPP‐PPI shows the AIE effect in THF/water mixtures and emits at 470 nm in the film with Φ_F of 28.1%. While fabricating it into the device with a configuration of ITO/HATCN (5 nm)/NPB (40 nm)/TcTa (5 nm)/TPP–PPI (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al, an excellent device performance of V_on (2.9 V), L_max (16 460 cd/m²), and EQE (4.85%) is achieved. It is worth noting that the theoretical limit of EQE of OLEDs fabricated with typical fluorescent materials is 5%. Thus, it demonstrates the huge potential of developing OLEDs with TPP‐based luminescent materials [52].

       1.3.2 Fluorescent Sensors

Hydrogen sulfide (H₂S) is a natural gas with a rotten egg smell. It is poisonous, corrosive, and flammable. Exposure of H₂S with a small amount can give rise to headache, dizziness, and even death. On the other hand, H₂S is an indispensable endogenous gas in human body by metabolism. It is related to different physiological processes like cell growth, vasodilation, regulation of inflammation, and so on. The abnormal level of H₂S is associated with symptoms such as Alzheimer's diseases and diabetes [53].

Tang synthesized an AIEgen of malonitrile‐functionalized TPP (TPP‐PDCV) to act as a ratiometric fluorescent probe to detect H₂S with high sensitivity and good selectivity [54]. TPP‐PDCV shows an orange emission at 565 nm in the DMSO/PBS buffer mixture (v/v = 9 : 1) due to the TICT from the TPP part to the strong electron‐withdrawing malonitrile group. However, upon addition of NaHS, the orange emission disappears gradually, whereas a blue emission centered at 429 nm in the short‐wavelength region enhances accordingly. Such a fluorescent response (I₄₂₉/I₅₆₅) to H₂S changes less until 10 minutes, indicating that the detection is efficient and can be finished in 10 minutes (Figure 1.1). It is due to the activity of the double bond in the malonitrile group, which can undergo nucleophilic addition by H₂S. Thus, the double bond is easy to break to prohibit the TICT effect. On the other hand, TPP‐PDCV is transformed to a thiol‐substituted TPP derivative (TPP‐PSH) after addition and elimination reactions, which continues to oxidize to form the dithio‐containing derivative of TPP‐2PS. Because the resulting TPP‐2PS shows a lower polarity and lager rigidity, it displays a bad solubility in the DMSO/water mixture. The TPP‐2PS is easy to aggregate to produce a blue light signal contributed by the TPP unit.