Читать книгу Handbook of Aggregation-Induced Emission, Volume 1 - Группа авторов - Страница 52

It is known in the textbook of organic chemistry that a carbon–carbon double bond is unable to rotate in general conditions. However, under irradiation or at high temperature, the π‐bond of the double bond can undergo homolytic cleavage and the double bond can rotate. The most well‐known example is molecular photoswitches or even molecular motors in which the Z‐isomer and E‐one can be reversibly transformed into each other (E/Z isomerization, EZI) through double bond rotation under irradiation or heating [38, 39]. Among AIEgens, most of them such as TPE, cyanostilbene, and their derivatives possess one essential double bond. Whether the double bond undergoes a rotation under irradiation of ultraviolet (UV) light and arouses nonradiative decay of these AIEgens in solution is brought to concern even from the moment the AIEgens start to appear. Because of the relatively simple synthesis method, easy to modify, and stable AIE effect, TPE and derivatives become the most extensively studied AIEgens. Meanwhile, RIR mechanism is rightly concluded from the investigation of TPE and its derivatives. Whether the double bond rotation of the TPE unit is involved in the AIE process is always in controversy. Due to the symmetrical structure of TPE, it is difficult to observe the obvious effect of double bond rotation without proper modification. In the beginning, it was thought that the double bond had little effect on the AIE mechanism.

In 2012, Tang et al. prepared a dimethylated TPE derivative called 1,2‐diphenyl‐1,2‐di(p‐tolyl)ethylene (DPDTE) [40]. To conduct an EZI experiment, a pure E or Z conformation is needed. Therefore, they used an unconventional way, a Pt‐catalyzed reaction rather than the normal McMurry coupling reaction, to provide a Z‐rich product. In ¹H NMR spectra, E‐isomer of DPDTE has two classes of protons that appear at δ 6.89 and 2.24 ppm, while the corresponding protons have signals at δ 6.91 and 2.26 for the Z‐isomer. By comparing the integral areas of these two protons of the two isomers, it is revealed that the as‐prepared product is Z‐rich and has a Z/E ratio of 93 : 7. Through recrystallization, pure Z‐isomer was obtained. By using a UV lamp of spectrofluorometer as a light source to irradiate a solution of Z‐DPDTE for a long period of time (29 minutes), they hoped to get EZI result. However, throughout the whole radiation process, no change of ¹H NMR spectra could be observed. But when they used a UV lamp with a much stronger power to irradiate the DPDTE solution, EZI happened in a much shorter period. The NMR spectra showed that the peaks of E‐isomer were raised after the strong UV irradiation. In the same year, they reported similar results of TPE derivative named BPHTATPE [41]. By attaching large and polar moieties to a TPE core, they prepared BPHTATPE whose difference in the shape and polarity between E‐ and Z‐isomers can allow these two isomers to be separated just by making use of simple column chromatography. They performed EZI experiments on both the two isomers. It turned out that BPHTATPE could undergo E/Z isomerization only under a powerful UV light and high temperature (>200 °C) (see Figure 3.13). Consequently, they deduced that the EZI was not involved in its AIE process under the normal photoluminescence spectral measurement conditions.

But in 2016, the possible involvement of a double bond rotation in the AIE process of the TPE‐based luminogen was found by Tang et al. [42] They designed a TPE derivative 1,2‐bis(4‐fluorophenyl)‐1,2‐bis(4‐methoxyphenyl) ethylene (TPE‐FM), which possessed more polarity due to the strong electron‐attracting fluorine atoms and the electron‐donating methoxyl groups (see Figure 3.14). Therefore, the E‐ and Z‐isomers are easily separated and single crystals even grow well due to the large molecular polarity. The obtained crystal structure can make sure the identity of E‐ and Z‐isomers. With the identified isomers in hand, they carried out more careful test on the EZI of TPE‐FM in solution. Due to the difference of ¹H NMR spectra of E‐ and Z‐isomers, the EZI process of TPE‐FM was observed in the solution at a high concentration of 10 mM under a UV radiation of high power (321 nm, ~0.47 mW/cm²) for a 15‐minutes irradiation. But emission spectrum was generally measured at a highly diluted solution (~10 μM). The EZI process at high concentration did not guarantee its occurrence at highly diluted solution. Considering the low sensitivity of ¹H NMR spectroscopy and unable to measure NMR spectra at very low concentration, they prepared three cuvettes of solutions (10 μM), which were irradiated by a 312‐nm UV light from the xenon lamp with a low power (321 nm, ~0.012 mW/cm²) in the spectrofluorometer for specific periods of time. The solutions from the three cuvettes were combined and evaporated to dryness in a dark room. The obtained solid was dissolved in a small amount of dichloromethane‐d₂ for NMR measurement. By this effort, ¹H NMR spectra with high quality showed that EZI truly occurred even at very low concentration under low‐power excitation. Whereas it is difficult to determine whether the double bond rotation played a major role or minor role on the emission process of the TPE AIEgens, compared with the phenyl ring rotation.

Figure 3.13 The molecular structures of DPDTE and BPHTATPE.

Figure 3.14 Structure of TPE‐FM and TPE‐Fl. (a) Absorption spectra of TPE, TPE‐Fl, and fluorescein. (b) Emission spectra of TPE‐Fl in solution (λ_ex: excitation wavelength). (c) Relative fluorescence quantum yields of TPE‐Fl.

Source: Reproduced with permission from Ref. [42]. Copyright 2016, Royal Society of Chemistry.

In order to disclose the different roles of the double bond rotation and phenyl ring rotation on the AIE process, Tang et al. elaborately designed a TPEgen‐bearing fluorescein unit (TPE‐Fl). This TPEgen displays two main absorption bands at 340 and 450 nm, which are from TPE and fluorescein units, respectively (see Figure 3.15). In solution, TPE emits a weak fluorescence at 490 nm, which is overlapped with the main absorption band of the fluorescein unit. Therefore, the emission wavelength of the TPE unit can act as the excitation wavelength of the fluorescein unit through a fluorescence resonance energy transfer process. Therefore, by using a two‐color excitation, the contribution of the double bond rotation and phenyl ring rotation can be determined. When TPE‐Fl was excited by a long wavelength of 480 nm, the decrease of the fluorescence intensity of TPE‐Fl compared to that of the fluorescein only came from the phenyl ring rotation because the double bond rotation would not occur under so long wavelength of light. When TPE‐Fl was excited by a short wavelength of 330 nm, the decrease of the fluorescence intensity of TPE‐Fl compared to that of the fluorescein came not only from the phenyl ring rotation but also from the double bond rotation because the double bond would break under a high energy of light. By taking the fluorescence quantum yield (Φ_f) of fluorescein to be 100%, the Φ_f with excitation wavelengths of 480 and 330 nm were 19.2 and 9.2%, respectively. It suggested that the fluorescence was decreased by 10% due to double bond rotation but the fluorescence decrease due to the phenyl ring rotation was 100–19.2% = 80.8%. From this indirect experimental evidence, they concluded that the double bond rotation played a minor role on the AIE effect.

Figure 3.15 The synthetic route of cis‐TPE dicycles 3–5 and TPE tetracycle 6.

Indeed, in the ground state, phenyl rotation is much easier than the C═C bond, but during the fluorescent course, the double bond can be excited and break to generate radical or charged species (radical, cation, and/or anion), its twist or torsion become available, which make the true radiation‐less relaxation pathway become quite complicated.