Jingqi
Han
,
Shun-Cheung
Cheng
,
Shek-Man
Yiu
,
Man-Kit
Tse
and
Chi-Chiu
Ko
*
Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China. E-mail: vinccko@cityu.edu.hk
First published on 5th October 2021
A new class of luminescent bis(bipyridyl) Ru(II) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium have been developed. Owing to the involvement of the orbitals of the diaminocarbene ligand in the emissive excited state, the phosphorescence properties of these complexes are strongly affected by H-bonding interactions with various H-bonding donor/acceptor molecules. With the remarkable differences in the emission properties of the monomer, dimer, and H-bonded amine adducts together with the change of the dimerization equilibrium, these complexes can be used as luminescent gas sensors for humidity, ammonia, and amine vapors. With the responses to amines and humidity and the corresponding change in the luminescence properties, a proof-of-principle for binary optical data storage with a reversible concealment process has been described.
Ammonia and volatile organic amine detections are essential because of their wide applications in food industries,5 environmental monitoring,6 and even disease diagnosis.7 As a result, there is a high demand for developing reliable sensors with high sensitivity and fast response to reduce food poisoning, ocular irritation, and respiratory infection. In this context, various optical amine sensors based on metal–organic frameworks and coordination polymers have been developed.8 On the other hand, moisture level or humidity is another important indicator for food quality assurance.9 Humidity sensors have also found applications in different areas,10 such as skin-humidity level monitoring,10a breath humidity detection,10b moisture detection in organic solvents,10c and sealed packaging humidity value visualization.10d
To date, commercial semiconductor-based sensors for humidity and amines have been developed. However, their applications as sensors in food product quality monitoring, consumer products, or rapid screening tools are limited due to the need of a continuous energy supply, an electronic device for reading the signal, high cost, or poor gas selectivity. For practical applications, luminescent sensors for amine and humidity detection, particularly those with considerable changes suitable for naked-eye detection, are highly desired but have been hardly developed.8,10d,11 Compared with fluorescent sensors, phosphorescent sensors have better emission characteristics, including large Stokes shift and longer-lived lifetimes that provide methods to distinguish sensing emission signals from background fluorescence.12 Moreover, phosphorescent transition metal complexes with environmental sensitive emission properties are ideal candidates for designing luminescent sensors.13
Based on the strongly dependent phosphorescent properties of the acyclic carbene complexes towards the conformational variations and the intermolecular interactions with hydrogen-bonding donor and acceptor molecules of the acyclic carbene ligands,14 herein, we report the design of a new class of bis(bipyridyl) Ru(II) complexes with pyridyl acyclic diaminocarbene ligands as amine and humidity chemosensor. It is hypothesized that the emission energy would be strongly affected by H-bonding interactions with various H-bonding donor and acceptor molecules due to the strong involvement of the orbitals of the diaminocarbene ligand. Interestingly, the N-deprotonated diaminocarbene ligand would dimerize upon coordination to electron-deficient Ru(II) system by forming strong intermolecular H-bonding. As a result, these complexes exhibit dual phosphorescent properties. The H-bonded dimerization equilibrium and its impacts on the photophysical properties have been studied. Based on the shift of the dimerization equilibrium with various types of H-bonding donor and acceptor molecules and their different H-bonding interactions, the dimeric and monomeric forms of the complex can serve as luminescent humidity and amine sensors. With reference to its responses to amines and humidity and the corresponding luminescence responses, a proof-of-principle for binary optical data storage15 has been described.
Using the synthetic methodology for pyridyl diaminocarbene ligands,14 a series of bis(bipyridyl) Ru(II) complexes with pyridyl diaminocarbene ligands (1–3) has been synthesized by the coordination of substituted 2-aminopyridine ligands to the isocyano bis(bipyridyl) Ru(II) precursors16 (Scheme 1). The structures of all these complexes have been determined by X-ray crystallography. The crystal data and selected bonding parameters are summarized in Tables S1–S4 in ESI.† It is interesting to note that the complex units dimerize with two strong intermolecular H-bonds as evidenced by a short N⋯N distances17 in the range of 2.97–2.99 Å and N⋯H distances of ca. 2.10–2.11 Å in the crystal structures of 1 (Fig. 1a) and 2 (Fig. S1, ESI†). Unlike other structurally-related N-deprotonated carbene ligands in the iridium(III) cyclometalates,14b the N-phenyl group of acyclic carbene ligands is cis to C–Ru in these H-bond dimers. For 3, only a tiny amount of single crystals with quality suitable for X-ray crystallography in the recrystallization can be obtained. The difference of these single crystals from the bulk solid of 3 is verified by the different powder XRD patterns of the bulk solid compared to the simulated diffraction patterns from the single-crystal data of 3 (Fig. S2a, ESI†). This contrasts with the close agreement of the simulated patterns and the powder XRD patterns of the bulk solid of 1 (Fig. S2b, ESI†). The crystal structure of these single crystals of 3 is in the monomer form with the N-phenyl group trans to C–Ru (Fig. 1b). Although the bonding parameters of these complexes are within the typical bonding parameter ranges,18 a close scrutiny of acyclic carbene ligands in these complexes reveals that Ru–C(carbene) of 3 is slightly shorter (2.01 Å for 3vs. 2.03–2.04 Å for 1 and 2) with a larger N–C–N angle (116.6° for 3vs. 111.2–111.5° for 1 and 2). Besides, the positions of N-deprotonation in the diaminocarbene ligands are different. In the structure of 3, the nitrogen attached to the pyridine of the diaminocarbene is deprotonated, whereas the nitrogen attached to the chlorophenyl ring is deprotonated in the structures of 1 and 2. This may be due to the difference between trans and cis conformation. Moreover, the presence of electron-withdrawing CF3 substituent on the pyridine of 3 also renders its amine substituent more acidic.
On the contrary, the absorption features in the spectra of 3 do not change with the concentration (Fig. S5a, ESI†). The close resemblance of the lowest-energy absorption band of 3 to the dimeric form of 1 and 2 suggests that the major form of 3 in the dichloromethane solution is the dimer form. This is corroborated by the observation of the characteristic dimeric emission of 3 (see below). The much higher dimerization affinity of 3 than 1 and 2 is likely due to the presence of electron-withdrawing CF3 substituent, which can strengthen the H-bonding interaction by enhancing the proton donating ability of the N–H group of the diaminocarbene ligand.
As similar to bis(bipyridyl) Ru(II) complexes, these complexes show intense ligand-centered ππ* transitions in the UV region and moderately intense metal-to-ligand charge transfer (MLCT) transitions in the visible region. Compared to [Ru(bpy)3]2+, the MLCT absorption bands/shoulders of these Ru(II) carbene complexes are broader and of much lower energy, which can be attributed to the mixing of two different MLCT transitions [dπ(Ru) → π*(bpy)] and [dπ(Ru) → π*(N^Ccarbene)]. The lowest-energy MLCT transition in these complexes is ascribed to MLCT[dπ(Ru) → π*(N^Ccarbene)]. This is further supported by the high sensitivity and the drastic red-shift of this band upon dimerizing the acyclic carbene. The red-shifted MLCT absorption in these complexes (450–470 nm) compared to [dπ(Ru) → π*(R-bpy)] transition in [Ru(R-bpy)3]2+ (410–430 nm),21 is due to the lower-lying π*(N^Ccarbene) orbital than that of the bipyridyl ligands (R-bpy).14e,18c
As with absorption properties, the dimerization equilibrium of 1 and 2 in dichloromethane solution would result in strongly concentration-dependent emission properties (Fig. 3a), comprising two emission components characterized by two different emission lifetimes in the range of 81–88 ns and 193–264 ns. The longer-lived high-energy and the shorter-lived low-energy emission components are ascribed to the emission of the monomeric and dimeric form, respectively. The evolution of a lower-energy emission band, corresponding to the emission of the dimeric form, at higher concentrations is attributed to the increase of the ratio of the dimeric form. With distinguishable emission lifetimes, the two emission bands can be well-resolved using time-resolved emission spectroscopy (Table S5 in ESI, Fig. 3b and S3c in ESI†). Concerning the assignment of the lowest-energy absorption band and the red-shift of the emission upon dimerization, these emissions are likely originated from the 3MLCT [dπ(Ru) → π*(N^Ccarbene)] excited state. Based on the ratios of absorptivity, emission intensity, and absorbance, the luminescence quantum yields of the monomeric and dimeric forms (Table S5, ESI†) have been determined. For 3 in dichloromethane solution, the emission does not change with its concentration with a lifetime of 160 ns and emission energy similar but slightly higher than those of the dimeric form of 1 and 2 (Fig. S5b, ESI†). Therefore, it is also ascribed to the MLCT [dπ(Ru) → π*(N^Ccarbene)] phosphorescence of the dimeric form.
As intermolecular hydrogen bonding is the major driving force for dimerization, the extent of dimer formation would be strongly affected by the environments, particularly H-bonding donor/acceptor molecules. Given the drastic differences in the emission properties of the monomer and dimer, the environment-dependent dimerization is examined by the solvatochromic behavior of 1. The significant solvatochromism of 1, readily distinguishable by the naked eyes, is shown in Fig. S6, ESI.† As dimerization is concentration-dependent, the absorption and emission spectra were recorded for a fixed concentration of 23 μM in different solvent media. On top of solvatochromic shifts commonly observed in other charge transfer transitions,13,22 the intensity ratio of the monomeric and dimeric MLCT absorption of 1 in the visible region varies with the nature of the solvent (Fig. S6c, ESI†). In non-polar dichloromethane, the absorption intensity of the dimeric form (lower-energy band) relative to the monomer is much higher than that in polar or H-bonding solvents. The complex has the lowest absorption intensity of the dimeric MLCT transition in the strongest hydrogen-bonding organic solvent (methanol).
Consistent with the absorption properties, the intensity of the lower-energy emission band relative to the higher energy band also follow the order of dichloromethane > ethyl acetate > acetonitrile > ethanol ≫ methanol (Fig. S6d, ESI†). Except for methanol, the emission decays of 1 in all the studied solvents follow clear biexponential kinetics for the monomeric and dimeric forms described in the dichloromethane solution. In methanol solution, the emission decay is almost single exponential with a lifetime similar to the monomeric form observed in ethanol, suggesting that the complex is mainly the monomeric form in methanol (Table S6, ESI†). The shift of dimerization equilibrium to the monomeric form is due to the solvation stabilization by the H-bonding interaction of the acyclic carbene with methanol.
The change in the monomer/dimer ratio is also clearly observed from the emission properties in different types of low-temperature solid matrices. In 77 K EtOH/MeOH glass, 1 and 2 only exhibit the phosphorescence with maxima at 584–587 nm (Fig. S7a, ESI†), ascribed to the emission of the monomeric form. This is consistent with the results in the solvatochromic study of 1, in which only the emission from the monomeric form is observed in methanol. In contrast, 3 with the most stable dimeric form show a dual emission derived from the monomeric and dimeric forms peaking at 562 and 651 nm, respectively. Based on the relative emission intensities, the dimeric form of 3 is predominant in 77 K EtOH/MeOH glassy medium. In 77 K dichloromethane, the ratio of the monomeric and dimeric emission change considerably (Fig. S7b, ESI†). The higher-energy monomeric emission of 3 almost disappears compared with the dimeric lower-energy emission at 654 nm in 77 K dichloromethane. Similarly, the higher-energy monomeric emission of 1 becomes a minor component in the dual emission bands. For 2, the monomeric emission is still the major component (Fig. S7b, ESI†). This is consistent with the trend of dimerization affinity 3 ≫ 1 > 2.
In acetonitrile solution, 1 (100 μM) exhibits a dual phosphorescence with the dimeric emission as the major component. As with the solvatochromism induced by protic solvents, gradual addition of water to the acetonitrile solution of 1 (100 μM) would lead to a blue-shift of the MLCT absorption bands and the growth of the high-energy emission band (Fig. 4). This is due to the shift of the dimerization equilibrium to the monomeric form as a result of the H-bonding interaction of the acyclic carbene with water.
The emission changes of 1Ms towards different types of saturated organic amines, butylamine (BUA), cyclohexylamine (CYA), isopropylamine (IPA), diethylamine (DEA), diisopropylamine (DIPA), and triethylamine (TEA), have been studied (Fig. S9, ESI†). Upon exposure to amine vapor for four minutes, the emission intensity decreases with the extent showing strong dependence on the nature and bulkiness of the amines. On the other hand, the emission is also red-shifted with Δλmax following the same order as the change in the emission intensity: BUA (103 nm) > CYA (102 nm) > IPA (98 nm) > DEA (83 nm) ≫ DIPA (19 nm) ≈ TEA (17 nm) (Fig. 6b and S9, ESI†). This trend does not correlate with the vapor pressure nor the basicity of the amines. In general, the extent of the change follows the order of primary amine > secondary amine > tertiary amine and reverse order of bulkiness. For all the studied primary amines, it follows the order of BUA > CYA > IPA. Thus, the least bulky butylamine induces the most drastic change of the emission intensity and maximum amongst all the studied organic amines.
As shown in the plot of [(Iλmax-[Ru-A]/Ioλmax-[Ru-A])/(I631/Io631)] (Fig. 6c), such changes are due to the decrease of emission band at 631 nm with a concomitant growth of new emission peaking at 713–734 nm. Because of the significant changes of the emission maximum induced by ammonia, primary and secondary amines, these changes are attributed to the formation of H-bonded carbene–amine adducts (Scheme 2). The sensitivity of the new emission band to the substituent on the amine further supports the adduct formation because the strength of the H-bonding is expected to be affected by the steric properties of the amine. The formation of H-bonded carbene–amine adduct is further verified by the isosbestic absorption in the UV-vis absorption spectral change in the titration of 1 (2 μM) with isopropylamine (IPA) (Fig. S10, ESI†). In the NMR study, the adduct is characterized by an upfield shift of NH proton of the diaminocarbene upon addition of isopropylamine (IPA) (Fig. S11, ESI†). In addition, the decrease of the diffusion coefficient in DOSY (Fig. S12, ESI†) after adding 75 mol equiv. of IPA is consistent with the decrease of molecular weight due to the shift of the dimerization equilibrium to the monomeric carbene–amine adduct. As these emission changes are due to the formation of the H-bonded carbene–amine adduct, the emission can be reverted when the amine vapor is removed (Fig. S13 and video, ESI†). Compared with other reported amine sensors,8 this amine sensor shows significantly improvement in its response time, stability, and reversibility, which can be reused many times and applied for real-time monitoring.
In the case of very bulky secondary amine (DIPA) and tertiary amine (TEA), the carbene-amine adduct cannot be formed. At a result, the emission of 1Ms only shows minimal red-shift with much less decrease of the emission intensity after fuming with DIPA and TEA vapor (650 nm vs. 631 nm). As shown in Fig. 6c, the value of [(Iλmax-[Ru-A]/Ioλmax-[Ru-A])/(I631/Io631)] is close to 1 for DIPA and TEA, suggesting the quenching of the initial emission band.
As shown in Fig. 7, the initial deposit 5 × 8 dots translate into “00000”. The five ASCII codes for “CityU”, 01000011, 01101001, 01110100, 01111001, and 01010101, respectively, can be stored in the 5 × 8 dots by chemical encoding process with methanol. The data can be chemically concealed by fuming ammonia vapor for a few seconds, which converts all bright orange emissive dots into dark red emissive dots. This is due to the binding of the ammonia with the aminocarbene ligand of the emissive monomeric form (Scheme 2). After removing the NH3 vapor, the unbounded monomeric form can be regenerated, and thus the original bright orange emission can be restored to show the hidden ASCII codes. Such concealment–restoration processes are reversible for many cycles. The stored information can be cleared by fuming methanol vapor for 10 min, after which all dots show bright orange emission.
Footnote |
† Electronic supplementary information (ESI) available: Experimental details (synthetic procedures, characterization data, physical measurements and instrumentation, quantum yield determination), crystal data, powder XRD and simulated patterns of 1 and 3, selected bonding parameters for 1, 2 and 3, absorption spectra of the monomeric and dimeric form of 2, concentration-dependent absorption and emission spectra of 3, solid-state emission of the crystal sample and solvatochromic properties of 1, emission spectral changes of 1Ms upon addition of various amines, UV-vis absorption, 1H NMR, and DOSY NMR spectra of 1 after addition of IPA, and other supporting experimental data. CCDC 2076382–2076384. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc04074j |
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