h. The solvent was evaporated under reduced pressure, the crude mixture was taken up with ethyl acetate (100 mL). The resulting organic phase was treated in an iced bath with a 10 % w/v solution of HCl up to neutrality, and then washed twice with distilled water (2x150 mL), dried with anhydrous sodium sulfate and evaporated to dryness under reduced pressure. The chromatographic separation on silica gel (hexane/ethyl acetate = 50/50) afforded 18 (78 %) as a yellow solid. M.p. = 70.8 - 71.1°C.1H NMR (300 MHz, CDCl3): δ (ppm) = 7.91 (4H, d,3J = 8.9 Hz), 7.01 (br.s, d, 4H,3J = 7.8), 4.05 (q, 4H,3J = 6.4 Hz), 3.69 (t, 2H, 3J = 6.5 Hz), 3.61 (t, 2H,3J = 6.4 Hz), 3.24 - 3.12 (m, 4H), 1.93 - 1.77 (m, 4H), 1.70 - 1.45, 1.44-1.25 and 1.47 (2m, s, 37H), 0.91 (s, 9H), 0.06 (s, 6H).13C NMR (100 MHz, CDCl3):
δ (ppm) = 161.2, 146.9, 124.4, 114.7, 68.2, 63.2, 62.9, 47.0, 32.8, 32.7, 29.2, 28.5, 26.7 (two resonances), 26.0, 25.9, 25.7, 25.6, 5.2. ESI-MS(+): m/z = 728.
Azobenzene derivative (19) In a 50 mL round bottomed flask, 7 (0.90 g, 1.24 mmol), triethylamine (0.155 mL, 1.57 mmol) and DMAP (cat.) were dissolved in 30 mL of dichloromethane. The reaction mixture was cooled using an external ice bath and 4-methylbenzenesulfonylclhoride (0.20 g, 1.07 mmol) was added. The reaction mixture was stirred for 24 h at RT, then the organic phase was washed with distilled water (2x100 mL), dried with sodium sulfate and evaporate to dryness under reduced pressure. The residue was purified by column chromatography on silica gel (hexane/ethyl acetate = 9/1) to yield 0.97 g of 19 (89 %) as a yellow solid. M.p. = 81.9 - 83.2 °C. 1H NMR (300 MHz, CDCl3): δ (ppm) = 7.89 (d, 4H, 3J = 8.8 Hz), 7.82 (d, 2H, 3J = 8.2 Hz), 7.36 (d ,2H, 3J = 7.4 Hz), 7.01 and 6.98 (2d, 4H, 3J = 8.8 Hz), 4.06 (q, 4H, 3J = 6.4 Hz), 4.02 (t, 2H, 3J = 6.4 Hz), 3.61 (t, 2H, 3J = 6.4 Hz), 3.2 3.1 (br.s, q, 4H), 2.46 (s, 3H), 1.92 -1.66 (2m, 6H), 1.62 - 1.22 and 1.47 (m and s, 30H), 0.91 (s, 9H), 0.06 (s, 6H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 161.4, 161.2, 155.7, 146.5, 144.7, 133.2, 129.8, 127.9, 124.5, 114.7 (two resonances), 79.1, 77.2, 70.5, 68.2, 68.0, 46.9, 32.7, 29.2, 29.0, 28.8, 28.5, 26.6, 25.9, 25.7, 25.5, 25.2, 21.7.
Pyridylpyridinium derivative (20) In a 50 mL round bottomed flask, 19 (0.30 g, 0.34 mmol) and 4,4‘-bipyridyl (0.05 g, 0.35 mmol) were dissolved in 20 mL of dry acetonitrile. The resulting reaction mixture was refluxed for
48 h. After this period the solvent was evaporated under reduced pressure and the crude residue purified by precipitation from ethyl acetate yielding 0.23 g of 20 (65 %) as a yellow solid. M.p. = 116.6-117.2 °C. 1H NMR (300 MHz, CDCl3): δ (ppm) = 9.05 (d, 2H, 3J = 6.7 Hz), 8.79 (d, 2H, 3J
= 5.9 Hz), 8.44 (d, 2H, 3J = 6.7 Hz), 7.93 (d, 2H, 3J = 6.1 Hz), 7.84 (d, 2H,3J = 8.6 Hz), 7.82 (d, 2H, 3J = 8.6 Hz), 7.7 (d, 2H,3J = 8.1 Hz); 7.21 (d, 2H, 3J = 8.1 Hz), 7.04 (d, 2H,3J = 8.4 Hz), 7.02 (d, 2H, 3J = 8.4 Hz), 4.68 (t, 2H, 3J = 7.4 Hz), 4.07 (t, 4H,3J = 6.2 Hz), 3.64 (t, 2H, 3J = 6.2 Hz), 3.21 (q, 4H, 3J = 6.8 Hz), 2.37 (s, 3H), 2.16-2.01 (m, 2H), 1.9 - 1.74 (m, 4H), 1.69 - 1.22 and 1.47 (m and s, 29H), 0.91 (s, 9H), 0.06 (s, 6H).
ESI-MS (+): m/z = 866.5 [M-TsO]+.
Azobenzene derivative (21) In a 25 mL round bottom flask, 19 (0.30 g, 0.34 mmol) was dissolved in 10 mL of acetone / water = 95/5. CuCl2 (0.03 mmol) was added and the mixture was heated at 50°C for 2 hours.
Upon cooling to room temperature, the crude reaction mixture was filtered trhough a PAD of silica gel to remove the copper salts and dried under reduced pressure to yield 21 quantitatively.
Axle (22) In a sealed glass autoclave, a solution of 4a (0.10 g, 0.23 mmol) and 19 (0.21 g, 0.23 mmol) in acetonitrile (10 mL) was refluxed for 7 days.
After cooling to room temperature, the reaction mixture was evaporated to dryness under reduced pressure. Recrystallisation of the solid residue from acetonitrile afforded 22 (53 %) as an orange solid. M.p = 180-184°C. 1H NMR (300 MHz, CDCl3): δ (ppm) = 9.28 and 9.24 (2d, 4H,3J = 6.7 and 6.5 Hz), 8.66 (d, 4H, 3J = 6 Hz), 7.87 and 7.85 (2d, 4H, 3J = 7 and 6.9 Hz), 7.72 (d, 4H, 3J = 8.1 Hz), 7.26 (d, 4H, 3J = 8.1 Hz), 7.09 and 7.05 (2d, 4H,3J = 8.1 Hz), 4.79 (t, 2H, 3J = 7.6 Hz), 4.71 (t, 2H,3J = 7.6 Hz) 4.15 - 4.05 (m, 4H), 3.63 - 3.54 (m, 4H), 3.24 (br.s, q, 4H), 2.39 (s, 6H), 2.2 - 2.0 (m, 4H), 1.94 - 1.80 (m, 4H), 1.72 - 1.28 and 1.49 (m and s, 32H), 0.95 (s, 9H), 0.11 (s, 6H).
Pyridylpyridinium derivative (23) . In a 50 mL round bottom flask, 21 (0.12 g, 0.152 mmol) and 4,4‘-bipyridyl (0.04 g, 0.23 mmol) were dis-solved in 20 mL of dry acetonitrile. The resulting reaction mixture was refluxed for 48 h. After this period the solvent was evaporated under
re-duced pressure and the crude residue purified by precipitation from ethyl acetate yielding 0.08 g of 23 (59 %) as a yellow solid. 1H NMR (300 MHz, CDCl3): δ (ppm) = 9.09 (d, 2H, 3J = 6.8 Hz), 8.79 (d, 2H,3J = 6.0 Hz), 8.45 (d, 2H, 3J = 9.8 Hz), 7.95 (d, 2H,3J = 5.8 Hz), 7.86 - 7.81 (m, 4H), 7.70 (d, 2H, 3J = 8.2 Hz), 7.22 (d, 2H,3J = 8.0 Hz), 7.06 - 7.01 (m, 4H), 6.69 (t, 2H, 3J = 7.5 Hz), 4.08 (t, 4H,3J = 7.5 Hz), 3.55 (t, 2H, 3J = 6.5 Hz), 3.24 - 3.18 (m, 4H), 2.34 (s, 3H), 2.13 - 2.08 (m, 2H), 1.88 - 1.81 (m, 4H), 1.7 - 1.2 (m, 27H).
Rotaxane (24) In a 25 mL round bottomed flask, WetOEt (0.040 g, 0.02 ml) and 22 (0.030 g, 0.02 mmol) were dissolved in 5 mL of dry dichloromethane. The resulting solution was stirred for 2 h at RT, then, tert-butyl-di-methyl-silyl-chloride (0.044 g, 0.03 mmol), triethylamine (0.03 mL, 0.03 mmol), and DMAP (cat.) were added. The reaction mixture was stirred for 24 h at RT, then diluted with dichloromethane and the result-ing organic phase washed twice with distilled water (2x50 mL), dried over sodium sulfate and evaporate to dryness under reduced pressure. The solid residue was purified by column chromatography on silica gel (DCM/MeOH
= 96:4) to afford 0.025 g of rotaxane 24 (44 %) as a reddish solid. HR-MS calculated for [C153H211N11O18Si2]2+ m/z = 1273.2731 (60 %), 1273.7747 (100.0 %), 1274.2764 (82 %), 1274.7781 (26 %); Found: 1273.2693 (55 %), 1273.7710 (100 %), 1274.2721 (90 %), 1274.7730 (60 %), 1275.2745 (30 %).
Rotaxane 25 In a sealed glass autoclave, 20 (0.124 g, 0.012 mmol) and WetOEt (0.263 g, 0.018 mmol) were equilibrated in dry toluene for 24 h at 60°C, then 6-hydroxyhexyl-p-toluensulfonate (0.033 g, 0.12 mmol) was added and the reaction was stirred at 70 °C for 6 days. Upon cooling to RT, triethylamine (0.03 mL, 0.29 mmol), tert-butyl-dimethyl-silylchloride (0.044 g, 0.29 mmol) and DMAP (cat.) were added and the mixture was stirred for 24 h at RT. After this period, the solvent was evapo-rated under reduced pressure, and the solid residue was solubilised in 50 mL of dichloromethane. The organic phase was washed twice with dis-tilled water (2x50 mL), dried with sodium sulfate and evaporated to dry-ness under reduced pressure. The solid residue was purified by column chromatography on silica gel (Dichloromethane / Methanol = 95:5) to af-ford 0.162 g of rotaxane 25 (47 %) as a reddish solid. HR-MS calculated
for C153H211N11O18Si2]2+ m/z = 1273.2731 (60 %), 1273.7747 (100.0 %), 1274.2764 (82 %), 1274.7781 (26 %); Found: 1273.2716 (55 %), 1273.7731 (100.0 %), 1274.2749 (90 %), 1274.7760 (60 %), 1275.2770 (30 %).
Catenane 28 In a 5 mL round bottom flask, WEtOEt (0.36 g, 0.24 mmol), 23 (0.15 g, 0.16 mmol) and 3a (0.05 g, 0.18 mmol) were equili-brated in 3 mL of toluene. The reaction was heated at 70°C for seven days. After evaporation of the solvent, the mixture was taken up with dry dichloromethane to obtain a 10−3M solution and 27 was added. HR-MS calculated for C157H191N11O20]2+ m/z = 1275.21279 (59 %), 1275.71447 (100 %), 1276.21615 (83 %), 1276.71783 (46 %), 1277.21950 (20 %), 1277.72118 (8 %); Found: 1275.21387 (59 %), 1275.71558 (100 %), 1276.21680 (91 %), 1276.71790 (53 %), 1277.21924 (23 %), 1277.7207 (9 %).
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Encapsulation of
Stilbazolium Guests in
Calix[6]arene Hosts: a New Tool for Tunable
Fluorescent-devices
4.1 Introduction
The first member of the class of cyanine dyes was discovered more than one century ago by G. Williams.[1] Cyanine dyes are characterised by the presence of two nitrogen centers: the first, positively charged, is spanned from the second, neutral, by a conjugated chain of carbon atoms (see fig-ure 4.1).[2] These dyes generally have an all-trans geometry in their stable form, but occasional photoisomerisation can take place. Because of their highly conjugated structure, cyanine dyes have high extinction coefficients (>105M−1cm−1). Their optical properties, such as extinction and fluores-cence, can be tuned through a careful choice of the substituents attached to the nitrogen and by changing the length and the rigidity of the polymethine chain. The non-linear optical (NLO) properties, the solvatochromism, the photoisomerisation and the photodimerisation of this class of dyes[3] have
93
Figure 4.1: General structure of cyanine (1) and hemicyanine (2) dyes; the derivatives of 2 with the shortest bridge (n = 1) are often indicated either as styryl dyes or styryl cyanine dyes (see text)
been studied for more than 20 years, leading to various applications such as spectral sensitisers in large band-gap semiconductor materials, laser gain materials, probes for biological systems and light-harvesting systems.[2]
Stilbazolium salts are organic dyes belonging to the class of cationic styryl cyanines (see figure 4.1), which are usually obtained via condensa-tion of 2/4-methyl pyridinium salt with a properly substituted benzalde-hyde derivative in the presence of a base. Stilbazolium salts are widely studied thanks to their two-photon absorption in the near IR range (NIR).
Optical materials with this feature are of great interest for application in 3D fluorescence microscopy, up-converted lasing, data processing, and bio-imaging. Micro/nanolasers are of great importance in science and technol-ogy thanks to their ability to deliver intense coherent light signals at the micro/nanoscale.[4, 5]In this context, the development of innovative optical materials to be used as gain media plays a fundamental role in expand-ing the micro/nanolasers capabilities and performances.[6] Up to now, the most important gain materials for the micro/nanolasers construction are based on inorganic semiconductors thanks to their high stability and opto-electronic properties. The drawbacks presented by these materials derives from complicated and costly processing, limited spectral tunability, brittle-ness and the lack of flexibility.[7] Organic semiconductor-based lasers have attracted great interest in the last decades since they allow to overcome some of the drawbacks listed above. They are prepared with straightfor-ward fabrication processes and, most important, they allow femtosecond pulse generation, the broadband optical amplification and can be used to tune the lasing frequency.[8] The principal limitation in the use of organic dyes as gain media, including the styryl cyanines, is the aggregation-caused
quencher (O2) solvent
fluorescent dye
kFRETα 1/n2 krα n2 kpα f(n)
kIC kISC
Figure 4.2: Illustration of the role played by the complexation into a molecular container on the deactivation of singlet excited states. The container protects the dye from solvent induces quenching and quenchers and influences the rate constants of other deactivation pathways.[9]
quenching (ACQ) effect that avoids their direct use as solid-state gain me-dia.
To overcome the ACQ problem, the development of organic solid-state laser is nowadays based on the encapsulation of the dye molecules into a matrix able to spatially separate them.[4, 7] Initially, the dyes were encap-sulated as guests in polymers, sol-gel glasses and molecular/ionic crystals.
However, due to the absence of specific interactions between the matrix and the dye molecules, these latters are spatially separated, and thus the quenching mechanism is avoided, only in dilute systems. In recent years, ordered porous compounds and macrocyclic hosts are attracting great in-terest as new host materials. Their ability to spatially confine the guest through non-covalent intermolecular interactions allowed the use of higher dyes concentrations without quenching problems. This led to higher optical gain intensity and hence increased laser output power.[4] The confinement of a chromophore into a host cavity changes its environment drastically, and this inevitably affects the physical/chemical properties of the guest (see figure 4.2). The inclusion of a dye molecule into a host may be-come an important tool to modulate its optical properties. Indeed, the de-excitation pathways, as well as the excited states, are really sensitive to the microenvironment. Internal conversion (IC), one of the most important non-radiative excitation pathways, together with many other similar de-excitation pathways, is slowed down upon the inclusion of dyes into hosts
addition of a molecular container
τ ~104ns τ ~ 10 ns
Figure 4.3: schematic representation of the role played by the complexation inside an opportune macrocyclic host on the enhancement of the life-time (τ ) of the excited state of a dye in the presence of a quencher.
molecules. Indeed, the geometrical confinement of the guest inside the host cavity restricts the rotational and vibrational motions and often reduce the polarity of the microenvironment with respect to the bulk (the majority of organic dyes are water-soluble).[9, 10]
Recently, host-guest composite materials have been exploited to con-struct high-performance organic microlasers since they combine the prop-erties of both the host and the guest with new functionalities carried by the supramolecular interactions. The more common host materials present in the literature are the ordered porous materials such as silica, zeolite, and MOF. An alternative is to encapsulate the dyes in synthetic organic macro-cycles such as cyclodextrins, calixarenes or cucurbiturils.[4, 9, 11]The encap-sulation of dyes in macrocyclic compounds has been exploited in applica-tions for microlasers, fluorescent sensors, biomedical applicaapplica-tions, catalysis, functional materials, electronic devices, pharmaceuticals, drug formulations and delivery, nanomedicines, and many others.[10]A really important bioas-say is the time-resolved fluorescence (TRF). Differently from the rare earth metal ions used so far, the TRF assays based on organic dyes are com-pletely compatible with the solid-phase synthesis of oligonucleotides and peptides. Improvement in TRF measurements can be achieved by exploit-ing supramolecular radiative decay engineerexploit-ing to lengthen the life-time of the excited state (see figure 4.3).[12]
orientation have not been studied yet.
Figure 5-4: complexes between variably substituted cyclodextrins and methyl orange a) βCD/MO, b) anionic sodium heptakis[6-deoxy-6-(3-thiopropionate)]-βCD (bpsp)/MO, c) cationic heptakis(6-deoxy-6-amino)-βCD hydrochloride (βpNH2)/MO.[13]
The calix[6]arene wheel WEtOEt, introduced in the previous chapters, like the cyclodextrins, is a non-palindrome host, and the directionality of the threading process of viologen and pyridylpyridinium-based guests has been widely studied.[14,15] To verify whether the orientation of a monocationic axle with respect to the two chemically different rims of WEtOEt could be exploited as a tool to address specific spectroscopic responses, the complexation properties of this host toward a series of stilbazolium dyes will be addressed in this chapter.
Figure 4.4: complexes between variably substituted cyclodextrins and methyl orange a) βCD/MO, b) anionic sodium heptakis[6-deoxy-6-(3-thiopropionate)]-βCD/MO, c) cationic heptakis(6-deoxy-6-amino)-βCD hydrochloride/MO.[13]
Relatively less explored and of fundamental importance is the tuning of the spectroscopic properties of a dye in function of its geometrical ar-rangement inside the cavity of the host material. Almost all the examples reported in the literature are about systems in which only one orientation of the guest is taken into account; however, the different orientations of a guest inside the same host cavity could originate assemblies with different spectroscopic behavior. In this context, the possibility to employ a hollow and non-palindrome synthetic macrocyclic receptor that possesses different functionalities on the accesses of its cavity can be potentially exploited to orient the dye inside the macrocycle itself. A few examples of this approach are known in the literature. Among them, Mourtzis et al.[13] reported an interesting example of cyclodextrin (CD) hosts in which the functionalities present on the macrocycle rims are modified to tune the directional thread-ing of Methyl Orange (MO) (see Figure 4.4). Although the complexation properties of these systems have been studied using NMR, the spectroscopic properties of the enclosed dye as a function of its orientation have not been studied yet. The calix[6]arene wheel WEtOEt, introduced in the previous chapters, like the cyclodextrins, is a non-palindrome host, and the direc-tionality of the threading process of viologen and pyridylpyridinium-based guests has been widely studied.[14, 15] To verify whether the orientation of a monocationic axle with respect to the two chemically different rims of
WEtOEt could be exploited as a tool to address specific spectroscopic responses, the complexation properties of this host toward a series of stil-bazolium dyes will be addressed in this chapter.