Simon
Felder
a,
Marie-Leonie
Delcourt
a,
Damian
Contant
a,
Rafael
Rodríguez
b,
Ludovic
Favereau
b,
Jeanne
Crassous
*b,
Laurent
Micouin
*a and
Erica
Benedetti
*a
aUniversité Paris Cité, CNRS, Laboratoire de Chimie et de Biochimie Pharmacologiques et Toxicologiques, F-75006, Paris, France. E-mail: laurent.micouin@u-paris.fr; erica.benedetti@u-paris.fr
bUniv Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, F-35000, Rennes, France. E-mail: jeanne.crassous@univ-rennes1.fr
First published on 16th January 2023
Due to their unique three-dimensional framework and intriguing electronic properties, [2.2]paracyclophanes (pCps) have been employed over the years as building blocks in materials science for the development of organic light-emitting diodes and non-linear optical systems. In addition, depending on their substitution patterns, [2.2]paracyclophanes can display planar chirality and are nowadays considered as useful scaffolds for the development of original circularly poliarized luminescence (CPL) emitters. This perspective gives an overview on the synthesis and characterization of different families of compact luminescent compounds derived from planar chiral [2.2]paracyclophanes. The chiroptical properties of these small molecules are described, with a particular focus on their ability to emit circularly polarized luminescence in solution. Some future prospects on the design and potential applications of CPL emitters derived from pCps are finally presented.
Originally discovered in a serendipitous fashion by vapor phase pyrolysis of p-xylene,10 [2.2]paracyclophane (pCp) and its derivatives have rapidly gained popularity amongst chemists due to their unique three-dimensional architecture.11 These compounds incorporate two distorted benzene rings covalently fixed in a stacked geometry by two ethylene bridges at their para positions. The conformational stiffness and proximity of the two π systems favours transannular through-space and through-bond interactions that confer on pCps intriguing electronic properties.12,13 These molecules show a unique reactivity14–20 and an unusual spectroscopic behaviour.21,22 The absorption spectrum of commercially available pCp is characterized by a low-intensity band with a maximum around 302 nm which is rarely observed for more common benzene derivatives. Non-substituted pCp also displays a broad emission band with a maximum at 356 nm. [2.2]Paracyclophanes have been successfully exploited in material science for the development of organic light-emitting diodes (OLEDs) and nonlinear optical materials.23–26 Differently functionalized pCps have also been employed as starting materials to produce through-space conjugated polymers and functionalized surfaces using chemical vapor deposition (CVD) methods.27
Another fascinating feature of [2.2]paracyclophanes is their planar chirality.28 Since their rigid structure precludes any rotational motion of their aromatic rings, all mono-substituted pCps are optically active compounds. Different planar chiral molecules can be rapidly generated by increasing the number of substituents on the pCp core.29 Chiral pCps are characterized by high configurational stability (up to 200 °C), they can be stored for long periods of time and are generally easy to handle. These compounds also show chemical stability towards light and oxidants, as well as a good tolerance to the action of acids and bases. Thanks to their advantageous physico-chemical properties, pCps have gradually emerged as powerful ligands or catalysts in stereoselective synthesis and asymmetric catalysis.30–33 Enantiopure paracyclophanes are also increasingly employed as building blocks to access new families of optically active luminescent compounds. Various dyes showing strong electronic circular dichroism (ECD) and CPL have been reported in the literature. As attested by recent reviews,34,35 most of the CPL active pCp-based chromophores developed so far present “branched” π-extended structures or complex three-dimensional shapes.35–40 In this short account of results obtained in our laboratories,‡ we will give an overview on the synthesis and (chir)optical properties of more compact planar chiral dyes (Scheme 1).
Different families of small organic luminophores derived from [2.2]paracyclophanes will be described: naphthalenes, coumarins, PRODAN derivatives and boron-β-diketonates. We will present practical methods to control the planar chirality of these three-dimensional analogues of well-known “flat” aromatic dyes. The spectroscopic properties of the optically active luminophores will be described, with an emphasis on their chiroptical behaviours. The potential use of pCps as building blocks for the development of innovative CPL emitters will be discussed. Finally, prospects on the design and possible applications of compact CPL emitters derived from pCps will be outlined.
Among the different possibilities,50–56 the kinetic resolutions and desymmetrization reactions of racemic/meso pCp-based aldehydes developed by us present several advantages, including easy implementation, good repeatability, and cost-effective scalability.57–59 These procedures, which involve asymmetric transfer hydrogenations (ATH) promoted by commercially available Noyori's catalysts,60–62 readily afford enantioenriched pCps decorated with various reactive groups (Scheme 2). As highlighted in the next sections of this perspective, such compounds have been successfully engaged in a variety of late-stage derivatisation processes and can be considered as key precursors for the preparation of several compact pCp-based luminescent compounds.
Scheme 2 Access to optically active pCp key intermediates through kinetic resolution and desymmetrization reactions. |
Recently, we have employed an intramolecular dehydrogenative Diels–Alder reactions as key step to construct functionalized naphthalenes motifs onto one of the two benzene rings of [2.2]paracyclophane (Scheme 3).68
The precursors of these compounds (4, Scheme 3) have been obtained starting from aldehydes intermediates (1, Scheme 3) in only three synthetic steps. The structure of the pCp-based naphthalenes could be decorated with a variety of functional groups, including aromatic and aliphatic moieties, electron-rich or electron-poor motifs, and halogen atoms.
A photophysical study revealed that the pCp-based luminophores display red-shifted absorption and emission bands in comparison with analogous “flat” naphthalene derivatives (Table 1). The “out-of-plane” ring of pCps can therefore be considered as a non-conventional electron-donating substituent and highlights the influence of through-space conjugation.69 Interestingly, amine-containing pCp-based naphthalenes display intriguing solvatochromic properties.68
Entry | Compound | R1, R2 | λ absmax (nm) | λ emmax (nm) | θ (%) |
---|---|---|---|---|---|
a 10−5 M solutions of the dyes in CH2Cl2. b 10−5 M solutions of the dye in DMSO. c Absorption maximum of the most red-shifted band (putative cyclophane band). d Relative fluorescence quantum yields (θ) were determined using anthracene and 9,10-diphenylanthracene as standards. | |||||
1a | 5a | H, Ph | 376 | 440 | 62 |
2a | 5b | Br, Ph | 371 | 430 | 17 |
3a | 5c | NHBn, Ph | 365 | 555 | 6 |
4a | 5d | H, Me | 373 | 420 | 39 |
5a | 5e | H, CH2OH | 373 | 415 | 16 |
6a | 5f | H, p-Cl-Ph | 375 | 440 | 41 |
7a | 5g | H, p-OTf-Ph | 378 | 445 | 43 |
8a | 5h | H, p-OMe-Ph | 378 | 443 | 41 |
9a | 5i | H, p-NMe2-Ph | 400 | 510 | 23 |
10a | 5k | H, p-NHBn-Ph | 385 | 485 | 30 |
11b | 5l | H, p-OH-Ph | 374 | 454 | 16 |
12a | 6 | — | 345 | 380 | 22 |
Starting from enantiopure aldehydes Rp-1a and Sp-1a, optically active pCp-based naphthalenes Sp-5a and Rp-5a have been prepared in good overall yields (∼37% over 4 steps). These luminophores exhibit interesting ECD and CPL activities. The ECD spectra of the enantiomeric pair recorded in CH2Cl2 display mirror-image relationships (Fig. 1). Two main ECD-active bands of moderate strength can be observed, the first at 250 nm (Δε = +12 M−1 cm−1 for Sp-5a) and the second at 300 nm (Δε = −16 M−1 cm−1 for Sp-5a). Naphthalenes Sp-5a and Rp-5a also possess reliable mirror-image CPL signatures in CH2Cl2 (Fig. 1). An absorption dissymmetry factor (gabs) of 6.6 × 10−4 at 350 nm and a luminescence dissymmetry factor (glum) of 6.7 × 10−4 at 450 nm (Table 3) have been calculated for these compounds, which sets them in the typical range for CPL-active pCp derivatives.70,71
Fig. 1 Chiroptical properties of pCp-based naphthalenes Sp-5a and Rp-5a. UV-Vis, PL, ECD and CPL spectra were recorded in CH2Cl2 (10−5 M solutions, λex = 365 nm). |
Interestingly, in addition to the classical Stokes shift often used to describe the structural and electronic reorganization of a fluorophore excited-state prior to the emission process, the gabs/glum ratio for a chiral emitter appears as a complementary value to discuss the impact of that reorganization on the chirality of the ground- vs. excited states. Here, the glum/gabs ratio of 1 reveals that no structural/electronic reorganization occurred during the excitation/emission processes.
The influence of the pCp motif and its “phane” interactions on the spectroscopic properties of coumarins 10a–o has been studied by unpolarized UV-vis absorption and fluorescence emission spectroscopy. As a general trend, the 3D coumarins showed red-shifted absorption and emission bands when compared to paracyclophane-deprived model compounds (11a, b, Table 2). The extension of the π-conjugation between the pCp and coumarin motifs in the novel luminophores is clearly responsible for this behaviour. It is worth mentioning that the UV-vis absorption profiles of the pCp-based coumarins are significantly blue-shifted in comparison with commercially available flat dyes which display a substantial intramolecular charge transfer (coumarin 6, Table 2).
Entry | Compound | R1, R2 | λ absmax (nm) | λ emmax (nm) |
---|---|---|---|---|
a 10−4 M solutions in DCE. b 10−5 M solutions in DCE. c 10−5 M solution in CH2Cl2. d No significant effects were observed on the emission spectra while changing the excitation wavelength or the concentration of the samples. | ||||
1a | 10a | H, Ph | 265, 300, 330 | 460 |
2a | 10b | H, Me | 301, 319 | 435 |
3a | 10c | H, i-Pr | 300, 320 | 432 |
4a | 10d | H, H | 304, 320 | 445 |
5a | 10e | Br, Ph | 307, 325 | 450 |
6a | 10f | Br, Me | 306, 320 | 425 |
7a | 10g | H, p-MePh | 315 | 455 |
8a | 10h | H, p-OMePh | 300, 330 | 455 |
9a | 10i | H, p-ClPh, | 288, 310, 335 | 465 |
10a | 10j | H, p-OTfPh, | 280, 330 | 470 |
11a | 10k | H, p-CF3Ph | 270, 335 | 475 |
12a | 10l | NHBoc, Ph | 317 | 450 |
13a | 10m | NH2, Ph | 300, 330 | 560 |
14a | 10n | H, p-NHBocPh | 317 | 450 |
15a | 10o | H, p-NH2Ph | 330 | 470 |
16b | 11a | —, Ph | 294 | 418 |
17b | 11b | —, Me | 286 | 413 |
18c | Coumarin 6 | — | 455 | 498 |
The introduction of different functional groups on the “out-of-plane” deck of the pCp core significantly influences the luminescence of the [2.2]paracyclophanepyranones. For example, halogen-containing derivatives show blue-shifted emission bands in comparison with their non-substituted analogues (Table 2, entries 5 and 6 vs. entries 1 and 2). On the contrary, for compounds incorporating electron-donating amino groups, strong bathochromic shifts of the emission maxima are observed (Table 2, entry 13 vs. entry 1). These results clearly demonstrate that the photophysical properties of this series of compounds can easily be tuned simply by playing with the nature and the relative position of their substituents. Note that all coumarins 10a–o showed a good photostability,79 as well as extremely large Stokes shifts (up to 43478 cm−1 for compound 10m) compared to their “flat” analogues. Low fluorescence quantum yields were however observed for these molecules (0.1% < θ < 5%). When optically active aldehydes Rp-1a and Sp-1a (Scheme 2) are employed as synthetic precursors, enantiopure planar chiral coumarins can be obtained in an analogous manner as previously described for the pCp-based naphthalene derivatives. The two enantiomers of coumarin 10b have been prepared as a proof of concept. The chiroptical behaviour of these dyes has been investigated by ECD and CPL spectroscopies. An interesting mirror-image Cotton effect can be observed when ECD spectra are recorded in CH2Cl2 (10−5 M solutions, Fig. 2). Coumarin Sp-10b displays a quite intense positive response at 240 nm (Δε = +64 M−1 cm−1), as well as a more moderate negative one at 320 nm (Δε = −23 M−1 cm−1). As expected, opposite signals are detected for compound Rp-10b (Fig. 2). The CPL spectra of Rp-10b and Sp-10b also display clear mirror image signals in CH2Cl2 (10−5 M solutions, λex = 330 nm). The pCp-based coumarin dyes show higher absorption and luminescence dissymmetry factors (gabs ∼ 6.2 × 10−3 at 360 nm; glum ∼ 4 × 10−3 at 440 nm, see Table 3) in comparison with pCp-based naphthalenes. Such values are quite remarkable especially if one considers the very compact structure of these purely organic luminophores, as compared to former π-extended systems.35–40 Here again, the glum/gabs ratio of 0.7 highlights rather weak reorganization upon excitation/emission.
Fig. 2 Chiroptical properties of pCp-based coumarins Sp-10b and Rp-10b. UV-Vis, PL, ECD and CPL spectra were recorded in CH2Cl2 (10−5 M solutions, λex = 320 nm). |
Entry | g abs/λabs (nm) | g lum/λlum (nm) | g lum/gabs |
---|---|---|---|
a The lowest absorption energy values have been considered. b Values are too low to be determined accurately. | |||
S P-5a | −2.6 × 10−3/305 | +6.7 × 10−4/450 | 1.02a |
+6.6 × 10−4/350 | |||
S P-10b | +8.4 × 10−3/240 | −4.3 × 10−3/440 | +0.7a |
−6.2 × 10−3/360 | |||
S P-15 | +1.3 × 10−3/350 | +4 × 10−3/486 | −2.9 |
−1.7 × 10−3/380 | |||
S P-21a | −2 × 10−3/335 | −7 × 10−4/550 | n.d.b |
6 × 10−4/380 | |||
∼10−4/∼480b | |||
S P-21b | −2 × 10−3/330 | −8 × 10−4/550 | n.d.b |
9 × 10−4/380 | |||
∼10−4/∼480b |
Planar chiral PRODAN derivatives in which the naphthalene core of the parent molecule has been replaced with the pCp scaffold have recently been synthesized and spectroscopically characterized in our laboratory.93
Notably, a pseudo-para disubstituted analogue was obtained in five steps starting from enantiopure intermediates Rp-1d and Sp-1d (Scheme 5). The propionyl group was introduced first, via a Grignard addition and subsequent double oxidation reaction. The dimethylamino motif was then obtained through the conversion of key oxime 13 to the corresponding aniline 14, followed by dimethylation under standard conditions.
Optically active para disubstituted analogues of PRODAN based on the pCp core have been prepared as well starting from key intermediate 16 (Scheme 6). Note that this compound can be obtained in its enantiopure form from aldehydes Rp-1a and Sp-1a in 3 steps.20 After the addition of a vinyl Grignard reagent leading to alcohol 17, an isomerization reaction afforded both enantiomers of product 18 in good overall yields (Scheme 6).
A photophysical study was again conducted to evaluate the impact of the pCp core and its “phane” interactions on the spectroscopic properties of compound 15 and 18 compared to “classical” PRODAN. When dissolved in dichloromethane, the pseudo-para disubstituted pCp-based dye 15 showed a blue-shifted absorption spectrum, as well as a red-shifted emission band (Fig. 3). As a result, an extremely large Stokes shift (46730 cm−1) was observed for this compound. Theoretical calculations highlighted that such an extreme value can arise from the large geometrical relaxation of the strongly dipolar excited state of the molecule.93
On the contrary, the para disubstituted pCp analogue 18 displayed absorption and emission profiles close to those of PRODAN (Fig. 3). The pCp-based dyes showed low luminescence quantum yields in dichloromethane (<0.1% and ∼1% for 15 and 18 respectively). The lower fluorescence quantum yields as compared to the parent PRODAN were supposed to originate from the increased flexibility of the dyes, which may favour non-radiative decay.94
The solvatochromic behaviour of pCp derivatives 15 and 18 was investigated and compared with that of commercially available PRODAN. As expected, increasing solvent polarity resulted in clear bathochromic shifts of the emission bands for both pCp-based PRODAN analogues. The para disubstituted compound 18 showed a slightly less pronounced solvatochromism in comparison with the parent compound (Table 4). For the pseudo-para disubstituted derivative 15 a strongly red-shifted emission band was observed in dichloromethane compared to toluene (Table 4). An improved solvatochromic behaviour is therefore displayed by pCp 15 in comparison with both 18 and PRODAN. The pseudo-para di substituted compound however suffered from fluorescence quenching in polar solvents, such as acetonitrile (MeCN) and ethanol. Interestingly, PRODAN analogue 15 exhibited enhanced luminescence at the solid state (Fig. 4). An absolute fluorescence quantum yield of 9% was measured in this case. The observed aggregation-induced emission enhancement was supposed to originate from the stiffening occurring at the solid state.
Solvent | λ emmax PRODANa (nm) | λ emmax pCp 15b (nm) | λ emmax pCp 18b (nm) |
---|---|---|---|
a Fluorescence emissions of PRODAN in different solvents have been described before, see ref. 95. b 10−5 M solutions of the pCp dyes were used to perform the analyses (λex = 350 nm for both compounds 15 and 18). | |||
Toluene | 416 | 486 | 424 |
1,4-Dioxane | 422 | 506 | 425 |
CH2Cl2 | 446 | 542 | 437 |
MeCN | 455 | — | 440 |
EtOH | 485 | — | 458 |
Fig. 4 Emission spectra of pseudo-para disubstituted pCp 15 at the solid state (λex = 360 nm, λem = 480 nm). |
These effects could indeed decrease the vibrational motion of the molecule and therefore reduce the efficiency of non-radiative decay routes.
Optically active compounds 15 and 18 were found to display quite different chiroptical properties. Indeed, while several ECD-active bands of moderate strength with mirror-image relationships were observed for the para disubstituted derivative Rp-18 and Sp-18 in CH2Cl2,93 this molecule was found to be CPL inactive. pCps Rp-15 and Sp-15 also displayed clear ECD signals in CH2Cl2 with bands of moderate strength at 250 (Δε = −18 M−1 cm−1), 272 (Δε = +15 M−1 cm−1), 304 (Δε = −5 M−1 cm−1), 334 (Δε = +2 M−1 cm−1), and 365 nm (Δε = −1 M−1 cm−1) (Fig. 5). However, contrary to pCps Rp-18 and Sp-18, compounds Rp-15 and Sp-15 were found to possess clear CPL activity in toluene (Fig. 5). Fluorescence dissymmetry factors glum of ∼4 × 10−3 were obtained for these planar chiral PRODAN analogues (λexc = 350 nm, λem = 486 nm, Table 3). The glum/gabs ratio is negative and different from 1, which highlights strong reorganization of the excited state prior to the emission process, most probably through charge reorganization due to strong charge transfers.93 It is worth mentioning that mirror-imaged spectra were recorded in CD2Cl2 through vibrational circular dichroism spectroscopy (VCD) for optically active pCps Rp-15 and Sp-15, and clear fingerprints were detected between 1025 and 1625 cm−1 (Fig. 5). Note that the CO stretching mode observed in the IR spectra was too strong to enable VCD measurements due to saturation. When compounds Rp-18 and Sp-18 were analysed under the same conditions, less resolved VCD signals of low intensity were detected. This set of spectroscopic data may therefore reveal useful in future endeavours to distinguish between different pCp regioisomers. These results show that the (chir)optical properties of the pCp-based PRODAN analogues are significantly influenced by through-space interactions. The spectroscopic behaviour of these dyes can indeed be strongly modulated by changing the relative position of the substituents onto the two aromatic rings of the pCp core. To observe promising CPL activity in these donor-acceptor derivatives, within this series, it appears important to decorate both decks of [2.2]paracyclophane, and maintain a pseudo-para relationship between the electron-donating and electron-withdrawing groups on the molecule.
Racemic [2.2]paracyclophane-based boron-β-diketonates have been described in the literature and showed promising aggregation induced emission and solvatochromic properties.102,103 More recently, enantiopure analogues of these dyes have been synthesized by our group in four steps starting from optically active aldehydes Rp-1a and Sp-1a (Scheme 7). In fact, two enantiopure boron-difluoride derivatives were isolated, incorporating either a phenyl or a 4-methoxyphenyl group on their β-diketonates moieties (compounds 21a and 21b, respectively). The main spectroscopic characteristics of these dyes are reported in Scheme 7. Note that the more electron-rich derivative 21b shows a slightly reduced Stokes shift and a higher fluorescence quantum yield compared to 21a. The chiroptical properties of these compounds have been studied in dichloromethane.
As previously observed for the other families of compact pCp-based planar chiral dyes, boron-β-diketonates displayed clear ECD signatures with several mirror-image signals of moderate intensity. The pCp-based boron-β-diketonates dyes also exhibited mirror-image CPL bands, with luminescence dissymmetry factors glum around 7.8 × 10−4 at their maximum emission (Fig. 6 and Table 3). Absorption dissymmetry factors gabs ∼ 2 × 10−3 at 330 nm and ∼6-9 × 10−4 at 380 nm were determined for Rp-21a, b and Sp-21a, b, respectively. Note that the low-energy gabs values are too low to be determined accurately. Nevertheless, they are of same sign but much lower than the glum suggesting strong electronic reorganization of the excited state prior to the emissive process through intramolecular charge transfer (ICT) as illustrated by important solvatochromism associated with a strong conformational change (see ref. 102). Overall, these results pave the way for the design of new planar chiral CPL emitters with finely tuned photophysical properties.
Footnotes |
† Electronic supplementary information (ESI) available: Synthetic procedure for the synthesis of pCp-based boron diketonates, characterization data, copies of 1H and 13C NMR spectra. See DOI: https://doi.org/10.1039/d2tc04885j |
‡ Synthesis and spectroscopic characterization through unpolarized absorption and emission spectroscopies of different families of compact pCp-based luminophores have been performed at the Laboratoire de Chimie et de Biochimie Pharmacologiques et Toxicologiques (Université Paris Cité). Investigation of the chiroptical properties of all enantiopure pCp-based compact dyes has been realized at the Institut des Sciences Chimiques de Rennes (Univ Rennes). |
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