Silica covering driven intensity enhancement and handedness inversion of the CPL signals of the supramolecular assemblies

Article abstract of DOI:10.1039/d1nj01327k

A pair of Phe-Phe dipeptides with a tolane group were synthesized by introducing tolane with high emission efficiency to phenylalanine dipeptide. The sodium salts of the dipeptides were able to self-assemble into straight belts in a mixture of methanol and water. The supramolecular assemblies exhibited circularly polarized luminescence (CPL) properties. They can be used as templates to prepare dipeptide-based hybrid silicas through a supramolecular templating approach. The hybrid materials formed coiled nanoribbons, which showed a distinct and different morphology compared to supramolecular assemblies. Moreover, since silica covering can affect the packing structures of the tolane groups, the intensity of the CPL was enhanced and its handedness was inversed. This journal is

Full text of DOI:10.1039/d1nj01327k

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Silica covering driven intensity enhancement and
handedness inversion of the CPL signals of the
supramolecular assemblies†
Cite this: DOI: 10.1039/d1nj01327k
ab
a
Pan Jiang,a Hongkun Li,
*
Wei Liu,
Yi Li, *a Baozong Lia and
a
Yonggang Yang
*
A pair of Phe–Phe dipeptides with a tolane group were synthesized by introducing tolane with high
emission efficiency to phenylalanine dipeptide. The sodium salts of the dipeptides were able to self-
assemble into straight belts in a mixture of methanol and water. The supramolecular assemblies
exhibited circularly polarized luminescence (CPL) properties. They can be used as templates to prepare
dipeptide-based hybrid silicas through a supramolecular templating approach. The hybrid materials
Received 18th March 2021,
Accepted 20th April 2021
formed coiled nanoribbons, which showed
a distinct and different morphology compared to
DOI: 10.1039/d1nj01327k
supramolecular assemblies. Moreover, since silica covering can affect the packing structures of the
tolane groups, the intensity of the CPL was enhanced and its handedness was inversed.
rsc.li/njc
resonance energy transfer,6 plasmon-resonance,7 supramolecu-
lar assembly,8 and liquid crystals.9 In addition, CPL inversion is
Introduction
Circularly polarized luminescence (CPL) refers to the phenomenon also a research hotspot in this field. Sign inversion of CPL in
that a chiral luminescence system emits circularly polarized light chiral systems shows enormous potential for the development of
with left or right rotation.1 CPL-active materials have attracted a lot novel and well-defined functional optical devices. Plenty of
of attention in recent years for their potential applications in factors, such as temperature,10 stirring direction,11 pH value12
3D displays,2 photocatalytic asymmetric synthesis,3 optical and solvent polarity,13 may result in CPL inversion during the
information storage,4 and optical switches.5 As is known, CPL fabrication of CPL-active materials.
activity is measured by the luminescence dissymmetry factor
Supramolecular self-assemblies relying on weak noncovalent
(glum), glum = 2(IL ꢀ IR)/(IL + IR), where IL and IR are the intensities interactions, including hydrogen bonding, hydrophobic inter-
of the left- and right-handed circularly polarized emission, actions, electrostatic interactions and p–p stacking, provide an
respectively. For practical applications, the luminescence effective solution for accurately controlling the arrangement of
efficiency, especially in the condensed phase, is another building blocks and endowing the products with special
important parameter for evaluating the CPL performance. An functions.14 By regulating external conditions intelligently, the
exploration of efficient CPL-active materials with large glum size, shape and even handedness of self-assemblies are
values and high luminescence efficiency in the solid state is adjustable.15 Sanchez and co-workers applied the sergeants-
´
thus highly desirable. In recent years, various approaches and-soldiers principle to the self-assembly of very simple N,N0-
¨
have been reported to boost glum values, including Forster 1,2-ethanediylbisbenzamides and realized an amplification of
chirality.16 Stupp et al. reported that the pH value, monomer
concentration and proper functionalization of building blocks
a State and Local Joint Engineering Laboratory for Novel Functional Polymeric
can result in different morphologies.17
Materials, College of Chemistry, Chemical Engineering and Materials Science,
Therefore, supramolecular self-assembly is a simple and
efficient approach which can significantly amplify the circular
Soochow University, Suzhou 215123, P. R. China. E-mail: hkli@suda.edu.cn,
liyi@suda.edu.cn, ygyang@suda.edu.cn
polarization and realize CPL inversion.8,18 Liu et al. reported
enhanced and inversed CPL from photogenerated radical
anions in dipeptide naphthalenediimide assemblies.18d
Zheng et al. used a new concept of further assembly of helical
self-assemblies to further boost the glum factor of CPL.19 Zhang
et al. synthesized a new chiral fluorescent molecule which can
self-assemble into left-handed twist nanoribbons with a CPL
b State Key Laboratory of Luminescent Materials and Devices, Key Laboratory of
Luminescence from Molecular Aggregates of Guangdong Province,
Center for Aggregation-Induced Emission, South China University of Technology,
Guangzhou, 510640, China
† Electronic supplementary information (ESI) available: XRD patterns of the
organic self-assemblies before and after silica covering, CD spectra of tolane-1,
emission spectra of (L, L)-T, FF and glum of materials. See DOI: 10.1039/
d1nj01327k
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property.20 When ClOꢀ ions were added into the system, the (HBTU) were purchased from GL Biochem Ltd. (Shanghai,
handedness of self-assemblies and the CPL signs were inversed. China). L-Phenylalanine methyl ester hydrochloride and
Moreover, using supramolecular self-assembly strategies, even D-phenylalanine methyl ester hydrochloride were purchased
achiral molecules can contribute to CPL amplification and from Sam Chemical Technology Co., Ltd (Shanghai, China).
inversion. Jin and co-workers prepared nanofiber-based chiral 4-Ethynylanisole, methyl 4-iodobenzoate, copper(I) iodide,
silica through a templating approach, where its chiral Information bis(triphenylphosphine)palladium dichloride (Pd 15.2%), 3-
can transfer from silica hosts to achiral luminescent guests and aminopropyltrimethoxysilane, tetraethyl orthosilicate, (R)-(+)-1-
they obtained CPL-active materials.21 Feng et al. demonstrated that phenylethanol and (S)-(ꢀ)-1-phenylethanol were purchased
the handedness of the CPL of hydrogels can be efficiently from Aladdin Chemical Co., Ltd (Shanghai, China). N,N-
inverted by achiral coumarin derivatives through noncovalent dimethylformamide (DMF), methanol (MeOH), dichloro-
interactions.22
methane (DCM), triethylamine (Et3N) and tetrahydrofuran
We have developed supramolecular templating approaches (THF) were obtained from Chinasun Specialty Products Co.,
based on supramolecular self-assembly to fabricate helical Ltd (Changshu, China). DMF, Et3N and DCM were dehydrated
organic–inorganic hybrid silicas.23 For instance, we prepared by treatment with calcium hydride for over 24 h and then
two single-handed helical tetraphenylethylene (TPE)-bridged redistilled. Trifluoroacetic acid (TFA) was purchased from
polybissilsesquioxane (TPE–silica) nanotubes with CPL Sinopharm Chemical Reagent Co., Ltd (Shanghai, China).
activities, through a supramolecular templating approach
General procedures for the synthesis of compounds
using TPE-containing bis(triethoxysilane) as the precursor,
and self-assemblies of chiral cationic amphiphilic compounds
Synthesis of tolane-1. Tolane-1 was synthesized according to
as the templates.23c In the process of preparation of hybrid the literature.25
materials, the achiral component co-assembles with the chiral
Synthesis of (L, L)-T and (D, D)-T. Boc-L-Phe-L-Phe-OMe and
templates, and the self-assemblies are trapped in silica nano- 4-[(4-methoxyphenyl)ethynyl]benzoic acid were synthesized
structures. It can be predicted that adding achiral molecules, according to the literature.26,27 The dipeptides were prepared by
such as an inorganic silica source, to chiral self-assembly a manual liquid phase synthesis procedure. All four dipeptides
systems may change the packing of the molecules. Herein, a were prepared by a manual liquid phase synthesis procedure.
pair of Phe–Phe dipeptides with a tolane group, (L, L)- and (D, D)- A solution of Boc-L-Phe-L-Phe-OMe (2.00 g, 4.70 mmol) in a mixture
T, were synthesized (Fig. 1), whose supramolecular assemblies of DCM : TFA (1 : 1) was stirred for 2 h at room temperature. A
showed CPL properties and high fluorescence efficiency. white powder (L-Phe-L-Phe-OMeꢁTFA) was obtained after removing
Single-handed helical hybrid silicas, (L, L)- and (D, D)-T-SiO2, the solvents. To a solution of 4-[(4-methoxyphenyl)ethynyl]benzoic
were fabricated through a dynamic supramolecular templating acid (3.17 mmol), L-Phe-L-Phe-OMeꢁTFA (4.70 mmol), and HBTU
approach using the assemblies of (L, L)- and (D, D)-T as (1.78 g, 4.70 mmol) in DMF, 5 ꢂ equivalent of Et3N (5.09 mL,
templates, respectively.24 They exhibited enhanced luminescence 36.5 mmol) was added under an N2 atmosphere and stirred at
efficiency and CPL activities with inversed handedness.
50 1C for 12 h. Then, the solution was washed with 5.0 wt%
NaHCO3 aqueous solution. After filtering, the resulting ester was
dried and the formed precipitate was obtained after being recrys-
tallized from methanol. Then, a solution of NaOH (219 mg,
5.48 mmol) in water (5 mL) was added slowly to a stirred suspen-
sion of the ester (1.5 g, 2.74 mmol) in MeOH (50 mL). The reaction
mixture was refluxed for 2 h. Then, it was cooled to room
temperature and poured into distilled water (400 mL). The mixture
was acidified with diluted hydrochloric acid, then filtered and
washed to neutral. The crude product was dried in vacuo at 55 1C.
The target product (L, L)-T was obtained after being recrystallized
from methanol. The synthesis of (D, D)-T was the same as (L, L)-T.
Experimental section
Materials
N-[(1,1-Dimethylethoxy)carbonyl]-L-phenylalanine (Boc-L-Phe-OH), N-
[(1,1-dimethylethoxy)carbonyl]-D-phenylalanine (Boc-D-Phe-OH), and
O-benzotriazole-N,N,N0,N0-tetramethyl-uronium-hexafluorophosphate
20
Characterization of (L, L)-T. Mp 264.8 1C. ½aꢃD = ꢀ67.2 (c =
1.0, MeOH). FT-IR (cmꢀ1): 3284 (nN–H, amide A), 2215 (nCRC),
1741 (vCQO, –COOH), 1626, 1602 (nCQO, amide I), 1532, 1514
1
(nN–H, amide II). H NMR (400 MHz, DMSO-d6, TMS) dH: 2.59–
3.13 (m, 4H; ArCH2CH), 3.76 (s, 3H; OCH3), 4.35–4.55 (m, 1H;
CONHCHCOOH), 4.60–4.80 (m, 1H; CONHCHCONH), 6.96
(d, 2H; J = 8.1 Hz, ArH), 7.06–7.35 (m, 10H; ArH), 7.76 (d, 2H;
J = 7.6 Hz, ArH), 7.48 (d, 2H; J = 8.2 Hz, ArH), 7.53 (d, 2H; J =
8.1 Hz, ArH), 8.29 (d, 1H; J = 7.4 Hz, CONHCHCOOH), 8.60
(d, 1H; J = 8.2 Hz, CONHCHCO), 12.78 (s, 1H; NHCHCOOH)
ppm. MS (MALDI-TOF) m/z [M
+
2Na]+, calculated:
Fig. 1 Molecular structures of the dipeptides and tolane-1.
591.215; found: 591.218. Elemental analysis: calculated
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(for C34H30N2O5), C 74.71, H 5.53, N 5.12%; found, C 74.43, performed on a Varian NMR 400 spectrometer (Palo Alto, USA).
H 5.83, N 4.99%. Elemental analysis was performed on a PerkinElmer series
20
Characterization of (D, D)-T. Mp 261.3 1C. ½aꢃD = +67.6 (c = 1.0, II CHNS/O analyzer 2400 (Waltham, Massachusetts, USA).
MeOH). FT-IR (cmꢀ1): 3295 (nN–H, amide A), 2216 (nCRC), Mass spectra (MS) were obtained using an UltrafleXtreme MALDI
1741 (vCQO, ꢀCOOH), 1629, 1602 (nCQO, amide I), 1534, 1514 TOF/TOF spectroscope (Bruker, USA). FT-IR spectra
1
(nN–H, amide II). H NMR (400 MHz, DMSO-d6, TMS) dH: 2.59– were recorded using a Bruker V70 spectrometer (Germany).
3.13 (m, 4H; ArCH2CH), 3.76 (s, 3H; OCH3), 4.35–4.55 (m, 1H; Wide-angle X-ray diffraction (WAXRD) patterns were recorded
CONHCHCOOH), 4.60–4.80 (m, 1H; CONHCHCONH), 7.00 (d, using an X’Pert-Pro MPD X-ray diffractometer (Almelo,
2H; J = 7.5 Hz, ArH), 7.09–7.40 (m, 10H; ArH), 7.52 (d, 2H; J = Netherlands) with Cu-Ka radiation (1.542 Å) and an Ni filter
7.6 Hz, ArH), 7.57 (d, 2H; J = 8.0 Hz, ArH), 7.80 (d, 2H; J = 7.1 Hz, at room temperature. Optical rotation was measured with a
ArH), 8.36 (d, 1H; J = 7.4 Hz, CONHCHCOOH), 8.65 (d, 1H; J = RUDOLPH Autopol IV.
7.7 Hz, CONHCHCO), 12.82 (s, 1H; NHCHCOOH) ppm. MS
(MALDI-TOF) m/z [M + 2Na]+, calculated: 591.215; found:
591.238. Elemental analysis: calculated (for C34H30N2O5), C
74.71, H 5.53, N 5.12%; found, C 74.96, H 5.64, N 4.90%.
Results and discussion
Preparation of self-assemblies and hybrid silicas
The gel-forming abilities of (L, L)- and (D, D)-T were explored.
Preparation of organic self-assemblies. 10 mg of (L, L)-T/(D, D)-T Since they were poorly soluble in water or aqueous solution of
(0.0183 mmol) was dissolved in 1.4 mL of pre-made aqueous NaOH NaOH, they could not self-assemble to form hydrogels. (L, L)-
(0.0366 mmol) solution (MeOH/H2O = 1 : 1, v/v). A white gel was and (D, D)-T can cause physical gels in an NaOH (1.04 g Lꢀ1
)
instantly formed from the reaction mixture which was kept under solution of a mixture of methanol and water (1 : 1, v/v) at 0 1C.
static conditions at 0 1C for 12 h. The field-emission scanning electron microscopy (FE-SEM)
Preparation of hybrid silicas. 10 mg of (L, L)-T/(D, D)-T images indicated that the sodium salts of (L, L)- and (D, D)-T
(0.0183 mmol) was dissolved in 1.4 mL of pre-made aqueous assembled into straight belts (Fig. 2a and b). The organic self-
NaOH (0.0366 mmol) solution (MeOH/H2O = 1 : 1, v/v). The assemblies were covered by silica through a dynamic templating
reaction mixture was kept under static conditions at 0 1C for approach using 3-aminopropyltrimethoxysilane (APTMS) as the
30 minutes. Then, APTMS (10 mL, 0.0426 mmol) was added co-structure-directing agent and tetraethoxysilane (TEOS) as the
under vigorous stirring at 0 1C. Thirty seconds later, TEOS silica source. Interestingly, the obtained hybrid silicas, (L, L)- and
(30 mL, 0.134 mmol) was added. One minute later, stirring was (D, D)-T-SiO2, were right- and left-handed coiled nanoribbons,
stopped. The reaction mixture was kept at 0 1C for 12 h.
respectively (Fig. 2c–f). Their helical pitches were about 1.0 mm.
To understand the formation of the coiled structure, FE-SEM
images of the reaction mixture were taken after different
General methods
The morphology study was performed on a Hitachi S-4800 field reaction times (Fig. 3). After the addition of APTMS and TEOS,
emission scanning electron microscope (Ibaraki prefecture, APTMS co-assembled with the gelator, and twisted nanorib-
Japan) with an acceleration voltage of 3.0 kV. Before the bons were identified at the terminals of the belts. With an
FE-SEM images were taken, platinum was sputtered on extension of the reaction time, all of the belts transformed to
the surface of the xerogel samples for 70 seconds to improve twisted nanoribbons, and coiled nanoribbons were then
the conductivity and prevent the charging effect. Transmission formed. At the same time, silica oligomers originated from
electron microscopy (TEM) images were taken on a Tecnai G220 TEOS adsorbed on the surfaces of the coiled nanoribbons
instrument operating at 200 kV. Circular dichroism (CD) and and copolymerized with the APTMS molecules. A schematic
UV-vis spectroscopy studies were performed on a JASCO J-815 illustration of the formation of right- and left-handed coiled
circular dichroism spectrometer (Tokyo, Japan) under a nitrogen nanoribbons is shown in Fig. 4. Due to this morphological
atmosphere. CD and UV-vis spectra were recorded within a transformation from straight belt to coiled ribbon, the neighbouring
wavelength range from 200 to 600 nm with a bandwidth of tolane groups were proposed to pack in a twisted structure within
1.0 nm, a 1.0 nm-interval and an average scanning speed of the coiled nanoribbons.
100 nm minꢀ1. To eliminate the effect of linear dichroism, the
To understand the organization of the tolane groups before
CD spectra of the solid samples were measured using a plate and after silica covering, the wide angle X-ray diffraction
holder which makes the samples rotate 3601 in 22.51 increments. (WAXRD) patterns of the xerogels of the sodium salts of (L, L)-
The fluorescence spectroscopy and quantum yields were and (D, D)-T and the hybrid silicas were taken (Fig. S1, ESI†).
performed on an Edinburgh Instrument FLS 980 (Edinburgh For the xerogels, many sharp peaks were identified, indicating
Instruments, UK), where the excitation wavelength was 320 nm. an ordered structure. The distance between neighbouring tolane
CPL spectra were measured on a JASCO CPL-300 (Tokyo, Japan). groups was about 0.37 nm. For the hybrid silicas, the intensity of
To eliminate the effect of linearly polarized luminescence, the the diffraction peaks was low. The molecules did not pack in a
CPL spectra of the solid samples were measured using a plate long-range order. The distance between neighbouring tolane
holder which makes the samples rotate 3601 in 22.51 increments. groups was about 0.41 nm. This molecular packing difference
1H nuclear magnetic resonance (1H NMR) analysis was can drive the difference in optical activity.
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Fig. 4 Schematic illustration of the formation of right- and left-handed
coiled nanoribbons.
absorption bands at a wavelength longer than 300 nm
originated from the tolane group. The CD signals of the
enantiomers present mirror symmetry. For (L, L)-T, a broad
and weak positive CD signal was identified at B418 nm, and a
negative one was identified at 338 nm. According to the exciton
chirality method, the sign of the signal at the longer wavelength
determines the exciton chirality.14a The negative CD signal at
320 nm originated from the electron transition within the
tolane group under a chiral surrounding condition. To confirm
the origin of the CD signals, the CD spectra of tolane-1 in (R)-
and (S)-1-phenylethanol were taken at a concentration of 10ꢀ4 M
(Fig. S2, ESI†). The results confirmed that the CD signal at
B320 nm originated from the electron transition within the
tolane group. After being covered by silica, the intensity of the
CD signals decreased sharply (Fig. 5b). For (L, L)-T-SiO2, an
obvious broad positive signal was identified at 363 nm.
Moreover, the intensity ratio of the signals at 342 and 323 nm
was increased. The neighbouring tolane groups are proposed to
pack in a twisted structure.
Fig. 2 (a–d) FE-SEM images of the xerogels of the self-assemblies of the
sodium salts and organic–inorganic hybrid silicas. (a) (L, L)-T, (b) (D, D)-T, (c)
(L, L)-T-SiO2 and (d) (D, D)-T-SiO2. (e and f) TEM images of the organic–
inorganic hybrid silicas. (e) (L, L)-T-SiO2 and (f) (D, D)-T-SiO2.
Tolane derivatives are usually highly emissive in both
solution and solid state.28 The emission spectra of (L, L)-T in
the methanol/water mixtures with different water fractions (fw) are
displayed in Fig. S3 (ESI†). (L, L)-T showed purple fluorescence
with a band at 418 nm in methanol. With an increase in the fw
values (o60%), the emission intensity decreased gradually.
Moreover, the emission maximum red-shifted to 443 nm with
To study the optical activity of the gelators before and after
silica covering, CD and UV-vis absorption spectra were
measured at a concentration of 7.14 g Lꢀ1 (Fig. 5). The UV
Fig. 3 FE-SEM images of hybrid silicas taken after different aging times. (a–f) (L, L)-T-SiO2, (g–l) (D, D)-T-SiO2. Note that ‘before’ refers to before adding
TEOS and APTMS, ‘after’ refers to after adding TEOS and APTMS.
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Fig. 6 Photographs of the (L, L)-T solution in methanol at a concentration
of 1.0 ꢂ 10ꢀ4 M and physical gel at a concentration of 7.14 g Lꢀ1 in an
NaOH (1.04 g Lꢀ1) solution of a mixture of MeOH and H2O (v/v = 1/1) under
(a) natural and (b) 365 nm UV light and those of the (L, L)-T-SiO2 powder
under (c) natural and (d) 365 nm UV light.
Fig. 5 CD and UV-vis spectra of (a) the self-assemblies of the sodium
salts and (b) the hybrid silicas.
an increase in the polarity of the solvent, that could be attributed
to the intramolecular charge transfer (ICT) between the electron
donor (–OCH3) and the electron acceptor (–COOH).28b,29 The
increased fw raised the polarity of the system and stabilized the
charge separation, and thus drove the equilibrium to the ICT
state. As the fw value increased continuously (fw 4 60%), the
emission intensity increased progressively, and the emission
maximum blue-shifted to 400 nm, showing aggregation-induced
emission (AIE) characteristics. This demonstrates that (L, L)-T
exhibited both ICT and AIE features in the methanol/water
mixtures. Due to the competition between the ICT and AIE effects,
the highest intensity was not recorded at an fw of 90%. The
fluorescence quantum yields (FF) of (L, L)- and (D, D)-T in methanol
Fig. 7 (a) Emission and (b) CPL spectra of the xerogels of the sodium salts
of the dipeptides and hybrid silicas, (c) CD and UV-vis, and (d) CPL spectra of
(L, L)- and (D, D)-T solution in methanol at a concentration of 1.0 ꢂ 10ꢀ4 M,
lex = 320 nm.
measured using an integrating sphere were 34.6% and 34.7%, covering. As expected, both the xerogels of the sodium salts of
respectively (Table S1, ESI†). the dipeptides and the hybrid silicas showed CPL activity.
Photographs of the (L, L)-T solution in methanol and physical Almost mirror images were observed in the CPL spectra
gel in an NaOH (1.04 g Lꢀ1) solution of a mixture of MeOH and (Fig. 7b). The glum values of the xerogels of the sodium salts
H2O (1 : 1, v/v) and (L, L)-T-SiO2 powder are shown in Fig. 6. of (L, L)-T and (D, D)-T were calculated to be about ꢀ2.3 ꢂ 10ꢀ3
All the samples emitted purple light at about 380 nm under and +1.1 ꢂ 10ꢀ3, respectively, whereas the glum values of (L, L)-T-
365 nm UV light. The emission spectra of the xerogels of the self- SiO2 and (D, D)-T-SiO2 were about +4.4 ꢂ 10ꢀ3 and ꢀ5.4 ꢂ 10ꢀ3
,
|
assemblies and hybrid silica powders are given in Fig. 7a. The FF respectively (Table S2, ESI†). It should be noted that the |glum
values of (L, L)-T, (D, D)-T, (L, L)-T-SiO2 and (D, D)-T-SiO2 are 53.1%, values of the hybrid silicas are higher than those of the
51.7%, 56.7% and 56.3%, respectively (Table S1, ESI†). The xerogels, and TPE–silica (ꢀ0.6 ꢂ 10ꢀ3 and +1.6 ꢂ 10ꢀ3).23c
values of the hybrid silicas are higher than those of the self- Furthermore, the CPL spectra of the xerogels and hybrid silicas
assemblies. This may be due to the fact that the inorganic matrix exhibited inversed signals. The difference in the packing
can greatly restrict the intramolecular motions of tolane units, structures of the tolane groups within the xerogels and hybrid
and protect the fluorophors from coming into contact with silicas should drive these phenomena. Within the xerogels, the
reactive oxygen species.30 It is worth noting that the FF values neighbouring tolane groups stack in a parallel or an anti-
of hybrid silicas, (L, L)-T-SiO2 and (D, D)-T-SiO2, are higher than parallel structure. Then, straight ribbons are formed. Since
those (26.2% and 21.2%) of TPE-Silicas in our previous work.23c the neighbouring tolane groups do not stack in a chiral
The intense fluorescence and molecular chirality of the manner, the handedness of the CPL signals of the xerogels
tolane-conjugated dipeptides prompted us to investigate the should be driven by the chiral surrounding condition formed
CPL properties of their self-assemblies before and after silica by the Phe-Phe dipeptide residues. Within the hybrid silica with
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a coiled morphology, the neighbouring tolane groups stack Notes and references
in a chiral manner. The handedness of the CPL signals of the
hybrid silicas should be mainly dominated by the stacking
handedness of the neighbouring tolane groups.
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For a better understanding of the origin of the CPL signals,
CD and CPL spectra of (L, L)- and (D, D)-T were measured in
methanol at a concentration of 1.0 ꢂ 10ꢀ4 M. As shown in
Fig. 7c, the CD signal at 315 nm originated from the electron
transition within the tolane group and no CD signals
were observed at longer wavelength, indicating that there
was no chiral stacking among the tolane groups. The CPL
spectra showed that (L, L)- and (D, D)-T in methanol
exhibited weak negative and positive signals, respectively
(Fig. 7d). The signs of the signals were the same as those of
their CD signals. The CD and CPL signals at long wavelength
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Conclusions
In summary, we have prepared a pair of tolane-conjugated
dipeptides, (L, L)-T and (D, D)-T, and two single-handed helical
hybrid silicas, (L, L)-T-SiO2 and (D, D)-T-SiO2, through a dynamic
supramolecular templating approach. The xerogels of the
self-assemblies of the sodium salts of (L, L)-T and (D, D)-T
showed good CPL performance with glum values of ꢀ2.3 ꢂ
10ꢀ3 and +1.1 ꢂ 10ꢀ3, and the FF values of 53.1% and 51.7%,
respectively. (L, L)-T-SiO2 and (D, D)-T-SiO2 exhibited enhanced
CPL activities with inversed handedness (+4.4 ꢂ 10ꢀ3 and
ꢀ5.4 ꢂ 10ꢀ3), and high fluorescence efficiency with FF values
of 56.7% and 56.3%, respectively. It was found that
silica covering induced intensity enhancement and handedness
inversion of the CPL signals of the supramolecular
assemblies, due to the change of the arrangement of the tolane
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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This work was supported by the National Natural Science
Foundation of China (21875152 and 22071166), the Natural
Science Foundation of the Jiangsu Higher Education Institutions
of China (20KJA430009), the Open Fund of the State Key
Laboratory of Luminescent Materials and Devices, South China
University of Technology (No. 2020-skllmd-02), the Open Fund
of Guangdong Provincial Key Laboratory of Luminescence
from Molecular Aggregates, Guangzhou 510640, China
(South China University of Technology) (2019B030301003), the
Project of Scientific and Technologic Infrastructure of Suzhou
(SZS201905), and the Priority Academic Program Development of
Jiangsu High Education Institutions (PAPD).
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