Our previous work has proved that G1 and phCB[6] can form a highly encapsulated host-guest complex[30] with the whole pyridinium unit and part of the benzene ring of G1 entering into the host cavity (Scheme 1). In this work, one (G2) and two (G3) methyl were introduced onto the ortho-position of C-Br bond on G1 skeleton respectively, with an aim to regulate the depth of skeleton into the cavity of phCB[6] by the steric hindrance, so as to achieve host-guest complexes with different enwrapping degrees and explore its influence on supramolecular RTP performance. Due to the weak electron-donating ability and long distance away from pyridine unit, methyl was adopted to reduce the influence on the π-electron distribution of the guest backbone, thereby ensuring that the three guests exhibit similar photophysical properties in molecule state.
3.1 The photophysical properties of the three guests in solution state (including water and methanol) at RT
The three guests were prepared and their photophysical properties in diluted aqueous solution (2×10− 5 mol/L) were then investigated. The absorption and photoluminescence (PL) spectra (Fig. S11) of the three guests show slight bathochromic shifts (λmax abs at 303, 307 and 310 nm for absorption, and λmax PL at 380, 390 and 400 nm for PL, respectively) with the increasing number of the methyl substituents, verifying that the weak electron-donating nature of the methyl group has relative weak impact on the photophysical properties of these guests. Furthermore, the photophysical properties of the three guests in methanol solution (2×10− 5 mol/L) are basically identical to those in aqueous solution (Fig. 1a and Fig. S12a), indicating no intramolecular charge transfer character for the three guests. The time-resolved PL decay curves show nanosecond-level lifetimes (0.15, 0.16 and 0.56 ns) for the three emission maximums in methanol (Fig. S12b-12d), which demonstrates that there is no phosphorescence emission in solution state for all the guests because of drastic non-radiative triplet dissipation.
3.2 The photophysical properties of the three guests in methanol at 77K
Further photophysical measurements of above methanol solutions were carried out at cryogenic temperature (77 K) to activate the phosphorescence (Fig. 1b-1d). In the PL spectrum of G1, there are two emission bands with maximums at approximate 350 and 495 nm respectively. The delayed (0.5 ms) PL spectrum exhibits only one emission band and overlaps with the latter emission band in PL spectrum, thereby implying a phosphorescence nature for 495 nm. Subsequent fitted lifetimes can adequately ascribe 350 nm (1.1 ns) and 495 nm (20.1 ms) to be fluorescence and phosphorescence emission respectively (Fig. S13a and 13b). The dual emission peaks share a same excitation band with a maximum at 300 nm, demonstrating that the fluorescence and phosphorescence come from a same luminescent backbone. It is worth noting that the fluorescence band at 77 K displays a hypochromatic shift of 30 nm as compared to that at RT, which is a common phenomenon at cryogenic condition in previous reports due to the confined molecular conformation[8, 31]. With regard to G2 and G3 (Fig. 1c and 1d), their PL spectra both basically show one emission band located at a maximum of 500 nm. The delayed (0.5 ms) PL spectra and lifetimes can clearly prove its phosphorescence nature (Fig. S13c and S13d), which meantime demonstrates that G2 and G3 have the similar triplet energy with G1 in molecular state.
3.3 The photophysical properties of the three guests in film state at RT
The phosphorescence efficiency at RT of the three guest molecules were then measured in PVA (polyvinyl alcohol). PVA is a kind of water-soluble polymer with a strong ability to construct rigid hydrogen-bonding network and has been widely used to active RTP[32, 33]. The three guests were added into PVA aqueous solution with a very low weight percentage of 0.01% with respect to PVA. After removing water, three transparent films abbreviated by G1@PVA, G2@PVA and G3@PVA were obtained. The three films resemble the above cryogenic methanol solution in photophysical properties. As shown in Fig. 2a, G1@PVA exhibits a dual-emissive PL spectrum, including a fluorescence peak (353 nm, 0.48 ns, Fig. S14a) and a phosphorescence peak (497 nm, 8.74 ms, Fig. 2d). The PL spectra of G2@PVA and G3@PVA are dominated by phosphorescence peaks at 502 nm (8.24 ms) and 508 nm (8.10 ms), respectively (Fig. 2b, 2c and 2d). The triplet energies of the three films are similar with that in diluted methanol solution, which demonstrate that the three guests are fully dispersed in PVA matrix and engender molecular RTP. The absolute phosphorescence quantum yields are 18.0% for G1@PVA, 20.1% for G2@PVA and 13.5% for G3@PVA respectively (Fig. S15), thereby indicative of relatively similar molecular phosphorescence efficiency.
3.4 The photophysical properties of the three guests in aggregated state at RT
The study on the guest’s photophysical properties in aggregated state were also conducted to compare with that in molecular state discussed above. In general, the photophysical properties in aggregated state are very complicated because of too many impact factors, including molecular configuration, packing modes and intermolecular interaction, and as a result, the luminescent properties in aggregated state are generally different from that in molecular state[34, 35]. In our case, G1, G2 and G3 are all fluorescent emitters in aggregated state (420, 415 and 438 nm respectively) and no RTP was detected. (Fig. S16).
3.5 The binding behaviors between phCB[6] and the three guests
Our previous work has revealed an clear 1:1 binding stoichiometry between G1 and phCB[6], therefore G2 and G3 were also expected to form 1:1 complex with phCB[6]. UV-Vis Job’s plot method was then carried out to determine the binding stoichiometry between the two guests and the host. When the mole ratio is 1 to 1, the change in absorbance of G2-phCB[6] system reaches a maximum (0.04) with respect to the sole G2, thereby demonstrating a 1:1 stoichiometry (Fig. S17a). G3 and phCB[6] also bind together in 1:1 mole ratio, however, with the maximal change in absorbance (0.03) lowering than that of G2-phCB[6] system (Fig. S17b), which implies a relatively weak binding force. The 1:1 binding mode of G2-phCB[6] and G3-phCB[6] were also confirmed by MALDI-TOF MS measurements, whose results match well with the simulated data (Fig. S18 and S19).
The 1H NMR titration was then conducted to gain more insights into the host-guest binding behaviors. This method has been performed on phCB[6] and G1 in our previous report[30], and the enwrapping degree between them is presented in Scheme 1. The G1 is enwrapped by phCB[6], except for part of benzene ring and the C-Br tail. For G2 and G3, the enwrapping degree would be changed owning to the introduction of methyl substituents. With the help of 1H–1H COSY spectra in Fig. S20 and S21, all protons of G2 and G3 can be clearly and properly attributed. As shown in Fig. 3a, proton H1 and H2 on G2 both exhibit upfield shift (ΔδH1 = -0.54 ppm, ΔδH2 = -1.04 ppm), while proton H4, H5, H6 and H7 all move downfield (ΔδH4 = + 0.83 ppm, ΔδH5 = + 0.21 ppm, ΔδH6 = + 0.14 ppm, ΔδH7 = + 0.80 ppm) with the increasing amount of phCB[6]. It is a common rule that the protons inside the hydrophobic cucurbituril cavity undergo shielding effect while the outside ones conduct deshielding effects, and those near the carbonyl rim are scarcely affected[36]. Consequently, it is obviously that the pyridine section and the adjacent methyl are encapsulated by phCB[6], while the remaining moiety locates outside. Proton H3 is just near the carbonyl portal of phCB[6] owing to its almost unaltered chemical shift. As for G3 in Fig. 3b, all protons show similar change in chemical shifts except for H3 that exhibits downfield shift (ΔδH3 = + 0.19 ppm), which means that only a part of pyridine moiety and adjacent methyl are enwrapped by phCB[6]. Based on the above discussion and the previous work, we conclude that the three guest molecules can all enter into the cavity of phCB[6] in a 1:1 stoichiometry spontaneously in water, however, the enwrapping degrees (G1 > G2 > G3) are various on account of different methyl substitution.
The nonlinear least-squares curve-fitting method[37, 38], which would give more accurate result when compared to the Hildebrand-Benesi equation method, was conducted to determine the host-guest binding constants. The absorption spectra of the three guests in aqueous solution all decrease in intensity gradually with increasing amount of phCB[6], accompanied by bathochromic-shift, thereby further validating the occurrence of host-guest interaction. Three binding constants are determined as 8.5×104, 3.1×104 and 2.6×104 L/mol for phCB[6]/G1, phCB[6]/G2 and phCB[6]/G3 respectively (Fig. 4).
3.6 The photophysical properties of the host-guest complexes in solid state
The three guests and phCB[6] were then mixed respectively in 1:1 mole ratio in water (0.1 mmol in 4 mL) and three white powders were obtained after lyophilization, which are denoted as phCB[6]/G1, phCB[6]/G2 and phCB[6]/G3 respectively. The 1H NMR of the three complexes with clear protons attribution are listed in Fig. S22-S24. The PL spectrum of phCB[6]/G1 is occupied by a phosphorescence peak (526 nm, 10.2 ms, Fig. 4a and 4d), accompanying with a small fluorescence peak (394 nm, 0.45 ns, Fig. 4a and Fig. S25), while those of phCB[6]/G2 and phCB[6]/G3 are dominated by phosphorescence peaks (522 nm, 4.8 ms and 524 nm, 5.9 ms, as shown in Fig. 4b, 4c and 4d ). The three PL spectra are relatively similar to those of G1, G2 and G3 in cryogenic methanol or in PVA films, which is reasonable because phCB[6] provides an individual microenvironment for every guest molecular to avoid possible intermolecular interaction. Under the excitation of 365 nm lamp, the three powders all emit yellow-green light, and phCB[6]/G1 is more brilliant than phCB[6]/G2 and phCB[6]/G3 by naked eyes, implying a higher phosphorescence efficiency. The obtained absolute phosphorescence quantum yields are 78%, 36% and 18%, respectively (Fig. S26), showing obvious dependence on the binding behaviors between host and guest - the higher enwrapping degree, the higher RTP efficiency. It is rational that higher enwrapping degree would result in stronger restraint of molecular motion and more effective shielding of oxygen, high RTP performance thus originates. The phosphorescence properties involved in this work are summarized and listed in Table 1.
Table 1
The phosphorescence properties involved in this work
| λphos (nm) | τphos (ms) | Φphos (%) | K (104 L/mol) |
G1 in 77 K | 495 | 20.1 | | |
G2 in 77 K | 500 | 18.5 | | |
G3 in 77 K | 500 | 18.0 | | |
G1@PVA | 497 | 8.74 | 18.0 | |
G2@PVA | 502 | 8.24 | 20.1 | |
G3@PVA | 508 | 8.10 | 13.5 | |
phCB[6]/G1 | 526 | 10.2 | 78.0 | 8.6 |
phCB[6]/G2 | 522 | 4.80 | 36.0 | 3.1 |
phCB[6]/G3 | 524 | 5.90 | 18.0 | 2.6 |