layer, and cathode, respectively. The device emits at 482 nm with a turn‐on voltage (Von), a maximum luminescence (Lmax), and an external quantum efficiency (EQE) of 3.7 V, 17 459 cd/m2 and 2.88%, respectively (Table 1.1, Device I). Although a high‐lying highest occupied molecular orbital energy level (−5.04 eV) of TPP–TPA is evaluated by cyclic voltammetry, a simplified double‐layer device without the hole‐transporting layer (TPP–TPA is expected to act as both light‐emitting and hole‐transporting layers) does not show an improved device performance [50].
Chart 1.2 Molecular structures of TPP–TPA and TPP–PPI.
Table 1.1 EL performance of devices.
| λ EL (nm) | V on a (V) | Lmaxb (cd/m2) | ηCb (lm/W) | ηPb (lm/W) | EQEb (%) | CIE (x, y)b | |
|---|---|---|---|---|---|---|---|
| TPP–TPA (I) | 482 | 3.7 | 17 459 | 5.49 | 3.18 | 2.88 | — |
| TPP–TPA (II) | 472 | 2.8 | 19 170 | 6.57 | 6.55 | 4.08 | 0.15, 0.21 |
| TPP–PPI | 474c | 2.9 | 16 460 | 8.34 | 8.18 | 4.85 | 0.16, 0.23c |
a V on = turn‐on voltage at 1 cd/m2.
b The maximum luminescence (Lmax), current efficiency (ηC), power efficiency (ηP), 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 ΦF 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 Von, Lmax, 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 ΦF 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 Von (2.9 V), Lmax (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.
Figure 1.1 Fluorescent detection of H2S by TPP‐PDCV. (a) Time‐dependent PL spectra of TPP‐PDCV (25 μM) in the DMSO/PBS buffer mixture (v/v = 9 : 1) in the presence of NaHS (250 μM). (b) Plot of the relative PL intensity (I429/I565) versus the number of scan in 25 minutes, where I429 and I565 are the PL intensity at 429 and 565 nm, respectively. Inset: the photographs of the TPP‐PDCV solution before and after the addition of NaHS taken under an irradiation of a 356 nm UV light. (c) PL spectra of TPP‐PDCV in the DMSO/PBS buffer mixture (v/v = 9 : 1) with different NaHS concentrations (0–500 μM). (d) Plot of the relative PL intensity (I429/I565) versus NaHS concentration. For all the tests, the excitation wavelength is 372 nm.
The