Molecular Design Strategy, UV-Vis Absorption and Fluorescence Emission Properties
In general, PDIs can be chemically modified in the imide-region, bay-region, or ortho-region. The substituents at the imide-region are primarily used to improve the solubility of PDIs in various solvents while minimally altering the optical and electronic properties. The modifications in the bay-region result in a twist of the two naphthalene half units in PDIs, which can tune the optical and electronic properties. The functionalization in the ortho-region can tune the optical and electronic properties without affecting the planarity of the perylene core.40 PDI is a typical electron acceptor. Thus, the attachment of an electron donor to the bay- or ortho-region of PDI core can effectively induce an intramolecular charge-transfer and finely tune the 2PA properties.
The selection of electron donor is based on two main points. Firstly, the intramolecular rotation (IMR) of donor "rotor" against the PDI core "stator" can deactivate the excited state of the molecule. For the largely planar PDI "stator", phenyl or biphenyl "rotors" are not enough to convert the emission behavior from ACQ to AIE due to the small volume.39 Therefore, a donor "rotor" with a larger size should be considered. At the same time, to avoid a big rotational barrier, the molecular weight of "rotor" should not be too large. Secondly, it is recognized that a good conjugation between building blocks as well as conformational rigidity are favored for 2PA properties. So, the donor "rotor" should effectively prolong the molecular conjugation without introducing more rotatable aromatic rings. As one kind of polycyclic aromatic hydrocarbons (PAH), anthracene (ANT) is selected as the "rotor" because of the above reasons. Herein, PDI substituted with ANT at bay- or ortho-position (B-PDI-ANT and O-PDI-ANT) have been designed and synthesized, and their structures are shown in Figure 2a.
For comparison, the UV-vis absorption spectra of PDI, B-PDI-ANT and O-PDI-ANT dyes in their dilute tetrahydrofuran (THF) solution (10−5 M) were measured and the results are displayed in Figure 2b. O-PDI-ANT and PDI show very similar absorption features: the maximum absorption bands range between 400 and 550 nm with a series of well-resolved vibronic structures. This spectral feature indicates that the ortho-substitution by ANT has little influence on the planarity of the PDI core. The above results are quite different from that observed for the PDI dye with a substituent at bay-position. Due to the steric hindrance, the perylene core is twisted in bay-substituted PDI dye. In comparison with the spectrum of O-PDI-ANT, the line-shape of the spectrum for the B-PDI-ANT is broader and displays less vibronic structures due to the loss of the planarity of the perylene core. According to the literature, the symmetrically forbidden S0-S2 transition band of bay-substituted PDI is enhanced due to the core-twisting induced broken symmetry.41 In this case, the spectral features of B-PDI-ANT are typical of PDI derivatives with modification at the bay-positions.
The fluorescence emission spectra of PDI, B-PDI-ANT and O-PDI-ANT dyes in THF solution (10−6 M) are shown in Figure 2(c). Upon excitation at 525 nm in THF, PDI displays fluorescence emission with a peak and a pronounced shoulder at 530 nm and 571 nm, respectively. The emission and absorption spectra of PDI display a mirror image relationship, denoting that no aggregation occurs at this concentration in THF at room temperature.42 The fluorescence quantum yields (ΦF) of PDI in THF solution is 70% using rhodamine B (ΦF = 97% in ethanol) as the calibration. At the same condition, B-PDI-ANT and O-PDI-ANT show significantly red-shifted emission bands and drastically reduced emission efficiencies (ΦF = 0.9% and 0.1%, respectively). The fluorescence red-shift effect might be attributed to the expansion of the π-conjugated system by introducing ANT to the PDI core (bay- or ortho-position). The fluorescence quenching effect may be due to the intramolecular charge-transfer between ANT and PDI moieties, which is frequently observed in the D-π-A systems constructed by one electron-donating unit and one electron-withdrawing unit. Also, upon UV illumination, we noted that the wet spots of both B-PDI-ANT and O-PDI-ANT on TLC plates were nonemissive. However, they became emissive when they became dried spots. This result implies that both B-PDI-ANT and O-PDI-ANT would be AIE-active. Thus, the fluorescence quenching of B-PDI-ANT and O-PDI-ANT in their dilute THF solutions can be tentatively associated with the intramolecular rotations of the ANT groups, which could exhaust the energy of the excited state.
To verify the AIE property, fluorescence emission spectra were tested to monitor the emission intensity fluctuation of the molecules in THF-water mixtures at various water volume fractions (fw). As shown in Figure 3a and 3c, the emission intensities are intensified for these two molecules when fw increases from 0 to 90%, exhibiting an AIE behavior. The comparison of their characteristic AIE properties discloses important information. Firstly, the evolution of the AIE process for their aggregates is different. To have a quantitative picture of their AIE process, we estimated their ΦF values in THF-water mixtures, using rhodamine B as the reference. As shown in Figure 3b and 3d, for B-PDI-ANT and O-PDI-ANT, the ΦF of the THF solution are very low (ΦF=0.93% and 0.13%) and are almost unchanged when water is added up to 50%, but start to increase swiftly upon addition of water to 80% and 50%, respectively. The trajectory of the ΦF change for B-PDI-ANT suggests that the molecules start to aggregate at a water fraction of >50%, but the highly twisted PDI core might provide enough room for the intramolecular rotation (IMR) of the ANT in B-PDI-ANT. Thus, the channel for consuming the excitation energy cannot effectively be switched off. As a result, B-PDI-ANT shows subdued emission at the fw of 60-80%. However, only when the molecules are packed much more tightly at the fw>80%, the aggregation-induced emission would be realized. A similar situation has also been reported by previous literature.43 Unlike B-PDI-ANT, O-PDI-ANT exhibits accelerated AIE behavior. When the fw reaches 50%, aggregation occurs, resulting in a rapid increase in the ΦF based on the restriction of the intramolecular rotation (RIR) mechanism. The planar conformation of the PDI core could effectively diminish the molecular rotation to improve the efficiency of the radiative process. Accompanying the further increase of fw to 70%, the ΦF decreases, probably due to the transformation from ordered to random aggregates. Secondly, the degree of the AIE enhancement for their aggregates is different. Their aggregates formed in THF-water mixtures (fw = 60%) display increased emission with the ΦF values of 2.8 and 3.9%, respectively, resulting in the AIE amplification factors (αAIE = ΦF,aggr/ΦF,soln) of 3 and 30, respectively. Their highest ΦF values are achieved at fw of 90%, with the αAIE of 18 and 34, respectively. These data suggest that O-PDI-ANT has a better AIE performance than B-PDI-ANT.
Femtosecond Transient Absorption and Theoretical Calculation
In order to study the excited state dynamics and especially the AIE mechanism of B-PDI-ANT and O-PDI-ANT, the femtosecond transient absorption (fs-TA) spectroscopy studied on PDI dyads were performed. The pump wavelength was 525 nm. Figure S5-S6 and Figure 4a show the fs-TA spectra of B-PDI-ANT. As shown in Figure S5, at the earlier delay time from 0 to 0.39 ps, the intensity of the broad band (465-625 nm) with centered wavelength at 528 nm increases, which is attributed to the ground-state bleaching (GSB). Two bands at short wavelength (centered at 350 and 420 nm, respectively) and one band at long wavelength (centered at 680 nm) also increase at this delay time range, which are attributed to the excited-state absorption (ESA) associated with the S0→S1. As shown in Figure S6, at later delay time from 0.39 to 6.58 ps, the ESA bands show apparent hypsochromic shift with the center wavelength emerging at 325 nm, 396 nm, and 675 nm, respectively. This process is mainly associated with the twisted intramolecular charge-transfer (TICT) from ANT to PDI groups. As shown in Figure 4a, at the last delay time from 6.58 to 925 ps, the GSB and ESA bands gradually drop to zero. The rotation of the ANT group in B-PDI-ANT is the primary channel to relax its excited state energy, which completely overwhelms the radiative process. As shown in Figure S7, global fitting analysis of the decay kinetics at all wavelengths yields three time constants: 0.43 (τ1), 1.92 (τ2) and 250 ps (τ3), which is consistent with the three excited-state decay processes observed in Figure S5-S6 and Figure 4a. O-PDI-ANT is a positional isomer to B-PDI-ANT. Nevertheless, the fs-TA spectra results show that the excited state of O-PDI-ANT undergoes a process apparently different from that of B-PDI-ANT. As shown in Figure S9, at the first stage (0-0.36 ps), the GSB band (450-550 nm) is not a broad peak but multiple peaks with fine structure (457, 488, and 528 nm), which is in agreement with the steady-state absorption spectrum. At the same time, a stimulated emission (SE) band is observed at 572 nm, which corresponds to the steady-state fluorescence emission. As shown in Figure S10, at the second stage (0.36-2.87 ps), the decrease of the SE peak at 572 nm is seen, accompanied by the blue-shift of the ESA peaks at 335, 402 and 678 nm. This reveals the conversion from fluorescence emission to ICT. Therefore, the excited state of O-PDI-ANT is mainly consumed by the nonradiative process rather than the radiative process. As shown in Figure 4b, in the third stage (2.87-7510 ps), the GSB and ESA bands completely disappears, which is attributed to the rotation of the ANT group in O-PDI-ANT. As shown in Figure S11, global fitting of the kinetics for O-PDI-ANT also obtains three-time constants: 0.36 (τ1), 3.00 (τ2) and 39 ps (τ3). These values denote the procedures of excitation, TICT and rotation. The τ3 for O-PDI-ANT is significantly shorter than that of B-PDI-ANT, indicating the rotation process dramatically accelerates in O-PDI-ANT. It is worthy to note that two new positive peaks at 470 and 504 nm begin to increase at this stage, suggesting an alternative relaxed pathway for O-PDI-ANT. An intersystem crossing may take place to generate a triplet state. The formation of the triplet state of O-PDI-ANT was confirmed with nanosecond transient absorption (ns-TA) spectroscopy (Figure S13-S14). The results and discussion are available in the ESI†.
Next, fs-TA spectra (Figure S16-S27) of B-PDI-ANT and O-PDI-ANT were recorded on their aggregated states in THF and H2O of different proportions. An increase in the proportion of H2O leads to the eventual aggregation of molecules, and hence, a corresponding change is observed in the fs-TA spectra. The study above shows that the ΦF of B-PDI-ANT and O-PDI-ANT do not show an obvious change until the proportion of H2O increase to 60%. Thus, the measurements were conducted at the fw = 60 %. For B-PDI-ANT, the measurement is also conducted at the fw = 90 % because a further significant increase in the ΦF of B-PDI-ANT is observed at the fw = 90 %. It can be easily seen that the GSB and ESA bands show an apparent redshift upon aggregation. To quantitatively describe the difference, the decay dynamics are fitted at the selected wavelength. In comparison to the global fitting results, the similar time constants of B-PDI-ANT and O-PDI-ANT can be obtained by using the wavelength fitting at 430 nm. Therefore, these two wavelengths are selected. As shown in Figure S28, for non-aggregated B-PDI-ANT in THF, the rotation amplitude is 38.5%, but this value shows a slight decrease to 31.3% upon aggregation in THF/H2O (fw = 60%). A larger extent of decline (from 30.4 to 7.95%) is measured for the rotational amplitude of O-PDI-ANT between the non-aggregated state in THF and the aggregated state in THF/H2O (fw = 60%). Such a decrease in the rotational amplitude could explain the increase of the ΦF values of aggregated molecules. These results provide evidence of rapid ΦF growth for O-PDI-ANT within the aggregates as the rotation can be constrained much quicker than that of B-PDI-ANT.
To obtain a deep insight into the different ground- and excited-state properties between B-PDI-ANT and O-PDI-ANT, TD-DFT calculation using a CAM-B3LYP/6-31G(d) method based on Gaussian 16 is performed.44-45 As the crystal structures were rather challenging to obtain, the optimized molecular structures are examined at the ground state. As shown in Figure S29 and S31, the perylene core of O-PDI-ANT is more planar than that of B-PDI-ANT, as also indicated by their UV/Vis spectra. Similar results are observed in the optimized molecular structures at the excited state (Figure S30 and S32). The more twisted conformation of perylene core in B-PDI-ANT should be ascribed to the introduction of the ANT group into the bay-position of PDI. We also calculated the rotational barriers (potential energy surface, PES) of ANT group in the ground state and excited state for isolated B-PDI-ANT and O-PDI-ANT. As shown in Figure 4c and 4d, the PESs of O-PDI-ANT exhibit "single-well", but those of B-PDI-ANT display "double-well". Not surprisingly, the intramolecular rotation of the ANT group in O-PDI-ANT is free because no energy barrier exists. In contrast, the intramolecular rotation of the ANT group in B-PDI-ANT should overcome an energy barrier of 1.1-1.8 kcal/mol at the ground state and 4.1-6.4 kcal/mol at the first excited state. Overcoming these energy barriers will cause a longer lifetime for the ANT group in B-PDI-ANT to rotate. Therefore, the PES calculations provide some evidence to explain the dynamic differences of the solution-phase results. As shown in Figure S33, B-PDI-ANT and O-PDI-ANT exhibit the charge-transfer nature for the spatially separated HOMOs and LUMOs. Their HOMOs are restricted on the ANT groups, while LUMOs are distributed to the PDI core. The TICT process can be suppressed by the physical constraint in the packing mode, thus the amplitude of TICT shows obvious decrease for B-PDI-ANT (from 27.6 to 9.08%) and O-PDI-ANT (from 28.6 to 10.8%) upon aggregation in THF/H2O (fw = 60%).
Two-photon Absorption Properties
As shown in Figure 5a, the 2PA spectrum of PDI parent shows a weak absorption with a maximum 2PA cross-section of 95 GM at 800 nm, which is also consistent with previously reported 2PA data for unsubstituted PDI parents.46 In the measuring range of 800–900 nm located at the ﬁrst near-infrared (NIR-I) window, B-PDI-ANT exhibits the 2PA cross-sections ranging from 404 GM at 800 nm to 332 GM at 900 nm, which are 4 and 12 folds that of parent PDI, respectively. Within the same scanned wavelength range, O-PDI-ANT exhibits a quick increase in the δ values from 117 GM at 800 nm to 2159 GM at 900 nm, which are 1 and 77 folds of parent PDI, respectively. Particularly, in the measuring range of 900–1100 nm located at the second near-infrared (NIR-II) window, B-PDI-ANT exhibits an increase in the δ values from 332 GM at 800 nm to 969 GM at 1100 nm, which is similar to the most cases of small molecules based on bay-substituted PDI. In sharp contrast to B-PDI-ANT, O-PDI-ANT shows very large δ values (2159-4787 GM) across the same measuring wavelength range of 900-1100 nm. And the maximum δ is 4787 GM at 1100 nm. The significant enhancement of the δ values for O-PDI-ANT could be attributed to its structural features: that is a more extensive conjugated system because of the planar perylene core. This structural feature of O-PDI-ANT can be beneficial to π-electronic delocalization and afford more effective coupling channels to increase δ values. We also summarized the previously reported 2PA data for small organic molecules containing one or two PDI unit in recent ten years from 2011-2021 (Figure 5b and Table S3).47-58 Herein, to make the δ values (measured under different experimental conditions) comparable, all of the δ values presented here are measured in the femtosecond regime, which can prevent the contribution from the excited-state absorption known to lead to artificially enhanced δ values when conducted in the nanosecond regime.59 It is worth mentioning that there are very few reports on ortho-substituted PDI and our result is among the highest δ values reported for small organic molecules containing one or two PDI units. And inspired by the excellent 2PA performance, this work would provide critical guidelines for the design of highly effective 2PA dyes.