博 士 学 位 論 文
Creation of peptide-pyrene organic luminophore with circularly polarized luminescence (CPL) properties from
pyrenylalanine
近畿大学大学院
総合理工学研究科 物質系工学専攻
味村 優輝
博 士 学 位 論 文
Creation of peptide-pyrene organic luminophore with circularly polarized luminescence (CPL) properties from
pyrenylalanine
(
ピレニルアラニンからの円偏光発光(CPL)
特性を 有するペプチド-
ピレン有機発光体の創製)
令和3年1月6日
近畿大学大学院
総合理工学研究科 物質系工学専攻
味村 優輝
Contents
General introduction ··· 1
Chapter 1 1-1. Introduction ··· 6
1-2. Results and discussion ··· 8
1-3. Conclusions ··· 14
1-4. Experimental details ··· 15
1-5. References ··· 17
1-6. Supplementary data ··· 20
Chapter 2 2-1. Introduction ··· 27
2-2. Results and discussion ··· 28
2-3. Conclusions ··· 37
2-4. Experimental details ··· 38
2-5. References ··· 40
2-6. Supplementary data ··· 43
Chapter 3 3-1. Introduction ··· 46
3-2. Results and discussion ··· 47
3-3. Conclusions ··· 58
3-4. Experimental details ··· 59
3-5. References ··· 61
3-6. Supplementary data ··· 63
Chapter 4 4-1. Introduction ··· 67
4-2. Results and discussion ··· 69
4-3. Conclusions ··· 73
4-4. Experimental details ··· 74
4-5. References ··· 76
4-6. Supplementary data ··· 78
Chapter 5
5-1. Introduction ··· 85
5-2. Results and discussion ··· 86
5-3. Conclusions ··· 93
5-4. Experimental details ··· 94
5-5. References ··· 96
5-6. Supplementary data ··· 98
Chapter 6 6-1. Introduction ··· 114
6-2. Results and discussion ··· 115
6-3. Conclusions ··· 121
6-4. Experimental details ··· 122
6-5. References ··· 124
6-6. Supplementary data ··· 125
General conclusions ··· 139
Publication list ··· 140
Acknowledgments ··· 141
1
General introduction
In recent years, organic luminophores are being used in various fields, including optoelectronics. Among them, chiral organic luminophores exhibiting circularly polarized luminescence (CPL) have attracted considerable attention. Furthermore, extensive research is being conducted on functional organic luminophores that have high luminous efficiency and are able to easily impart specific functions by the virtue of molecular design.
Light can be decomposed into two types-light along the x-axis and light along the y- axis (Figure 1).
Figure 1. Light along the y-axis and x-axis.
A combination of these two components results in linearly polarized light, circularly polarized light, or elliptically polarized light. Linearly polarized light is obtained when the phase difference between the light along the x and y axes is the same. Circularly polarized light is obtained when the phase difference is π/2, while elliptically polarized light is obtained when the phase difference is not π/2.Natural light is linearly polarized;
that is, the right-handed circularly polarized light and left-handed circularly polarized light are equivalent (Figure 2).
Figure 2. Phase difference in linearly polarized light and circularly polarized light.
This light can be decomposed into right-handed circularly polarized light and left- handed circularly polarized light using a polarizing filter.
2
Luminescence generated from an achiral organic luminophores is linearly polarized, in which the right-handed circularly polarized luminescence and left-handed circularly polarized luminescence are equally mixed. On the other hand, luminescence generated from chiral organic luminophores is biased toward either of the two types of luminescence, and this is called CPL (Figure 3).
Figure 3. Luminescence emitted from achiral and chiral luminophores.
The anisotropic factor (g value) is often used as an index of the circular polarization intensity of a CPL-emitting organic luminophore. The g value is expressed by the following equation.
g = 2(IL-IR)/(IL+IR)
The ratios of the emission intensities of the counterclockwise and clockwise circular polarization are IL and IR, respectively. The g value ranges from 0 to 2. For linearly polarized light, g = 0 and IL: IR = 50:50. For circularly polarized light, g = 2 and IL: IR = 0:100 or 100:0. To prepare organic materials exhibiting CPL, it is necessary to develop a material with higher g value.The application of CPL materials is expected to contribute to an energy-saving society, which would use special polarized light such as backlights for energy-saving liquid crystal displays, paints for advanced security, and LED lights for controlling plant growth. However, the light emitters of the CPL light sources currently in use do not have CPL properties, and a circular polarization filter is used to convert linearly polarized light into circularly polarized light to generate right-handed and left- handed polarized light. The light intensity is significantly reduced in this method since a polarizing filter is used. Therefore, if a CPL-type luminophores is used, it may be possible to achieve high functionality without a reduction in the light intensity. However, there are only a few studies on the CPL properties of organic luminophores.
In this study, we focused on chiral peptides with high customizability. The peptides were prepared by solid-phase synthesis by allowing the reactant amino acid to bind to a solid resin and then react with the reagent on the resin. It is easier to customize the target peptide in solid-phase synthesis than in the usual synthesis method. Additionally, it is
3
easier to remove the unreacted substances and by-products after the reaction in this method. Therefore, it is possible to impart various functionalities by combining various amino acids (Figure 4). Further, the use of amino acids as a raw material is expected to yield CPL luminophores having high biocompatibility; their application to a biomarker having CPL properties is also plausible.
Figure 4. Solid-phase synthesis of peptides (Fmoc method).
The use of peptides as CPL luminophores will not only allow the fabrication of CPL- emitting organic luminophore materials but also expand their application base to biochemicals for their use as biomarkers with CPL properties.
In this study, the synthesis of novel luminophores with new functional CPL properties was attempted by designing novel peptide-pyrene organic luminophores.
In our previous study (Org. Biomol. Chem. 2015, 13, 11426-11431), a peptide-pyrene CPL material with positive (+) (D-form) and negative (-) (L-form) CPL properties was successfully prepared by introducing a pyrene unit, which is a luminescent unit, into a chiral peptide. However, in terms of chirality, the pyrene unit introduced into one peptide backbone was either the D-form or the L-form only.
The CPL sign of the pyrene excimer changes due to stacking of the pyrene units.
Therefore, by changing the combination of the chirality of the pyrene unit to be introduced, the CPL sign of the excimer can be expected to be reversed. In Chapter 1, the CPL properties of peptides with different chiralities of the pyrene units to be introduced were investigated. We succeeded in changing the CPL sign by combining the chirality of the
4
pyrene units introduced into one peptide. When positive (+) (D-form) and negative (-) (L- form) elements were introduced into one peptide, the CPL properties changed rather than being nullified.
In the studies on peptide-pyrene organic luminescent materials conducted so far, the CPL sign is inverted using both D-form and L-form of the amino acids. However, amino acids of either chirality are often expensive. Therefore, if the CPL sign can be inverted with only the chirality on one side, cheaper materials can be produced. The peptide main chain is composed of polar peptide bonds, and it is considered that the CPL sign can be inverted by changing the solvent. Therefore, in Chapter 2, among the peptides previously synthesized in our laboratory (Chemistry Select 2016, 4, 831–835), peptide 2py without methylene spacer was compared with peptide 2C4, in which four methylene spacers are present between the pyrene units. The CPL properties were investigated by changing the solvent, which is the surrounding environment of the organic luminophores. As a result, we succeeded in controlling the direction of rotation of the CPL sign by changing the solvent. The difference in the effect on the peptide backbone was found to control the direction of rotation upon changing the solvent.
In Chapter 2, we succeeded in reversing the CPL sign from the same chirality of the pyrene unit by changing the solvent. If it is possible to control the inversion of the CPL sign without changing the chirality, the range of materials can be further expanded.
Therefore, the inversion of the CPL sign due to the influence of the solvent could be controlled without changing the chirality of the pyrene unit by introducing a piperidine unit having a large steric hindrance into the peptide main chain. In Chapter 3, the peptides (2C4-pip-r, 2C4-pip-l, 2C4-pip-lr) in which the piperidine unit is introduced into the peptide main chain of 2C4 (used in Chapter 2) were treated by changing the solvent, similar to that in Chapter 2. The CPL properties were examined. As a result, in 2C4-pip- lr, CPL inversion by the solvent did not occur. Thus, solvent-dependent CPL inversion was controlled by introducing piperidine units with large steric hindrance on both sides of the peptide backbone; rotation of the peptide backbone was controlled under the influence of the solvent.
For the previously reported peptide-pyrene organic luminophores, the CPL properties were investigated only in organic solvents. Peptides are expected to be employed for biological applications. However, they are difficult to be employed for the same in organic solvents. In addition, the peptide-pyrene organic luminophores have a plurality of hydrophobic pyrene units, and it is considered that aggregates of pyrene units are formed in water. Therefore, an arginine unit, which is a water-soluble amino acid, was introduced into the peptide main chain to improve the hydrophilicity of the peptide organic
5
luminescent group. This will facilitate the study in water. In Chapters 4 and 5, anticipating the application of peptide organic luminophores to organisms, the water solubility of peptides was improved by introducing arginine units into the main chain of the peptides under aqueous conditions. The CPL properties were examined.
In Chapter 4, by changing the number of methylene spacers between the pyrene units, the excimer CPL derived from pyrene units was examined in water. The CPL sign could be successfully inverted for odd and even numbers of methylene spacers.
In Chapter 5, the change in CPL properties by adjusting the number of arginine units introduced into the peptide main chain of the water-soluble arginine peptide was examined. We succeeded in adjusting the CPL properties by adjusting the number and position of arginine units introduced on both sides of the peptide main chain. Results of Chapters 4 and 5 reveal that the peptide-pyrene organic luminophores in water balance the hydrophilicity and hydrophobicity of the peptide by adjusting the number and position of the pyrene units or arginine units.
Although numerous studies have been conducted so far, it is sometimes difficult to study the CPL properties due to aggregation of the pyrene units. This is because the flexible peptide main chain promotes the aggregation of the pyrene units depending on the structure. Therefore, we aimed to inhibit the aggregation and investigate the CPL properties by introducing 2-aminoisobutane acid (Aib), which creates steric hindrance between the pyrene units of the peptide main chain. In Chapter 6, the CPL properties of the Aib peptide with the Aib unit introduced between the pyrene units in the peptide main chain were compared with those of the Gly peptide with glycine (Gly) introduced. It was found that six pyrene units could be successfully introduced into one Aib peptide. In addition, the Aib unit succeeded in expressing the CPL properties in a chloroform solution or an ethanol (EtOH) solution. Compared with Gly peptide, aggregation was observed in Gly peptide, which is caused by the high degree of freedom of the peptide main chain.
The CPL properties could not be observed in peptides with 3 or 6 pyrene units. The Aib peptide exhibited CPL without forming aggregates. It is considered that the steric hindrance of the Aib unit inhibited the aggregation by inhibiting the interaction between the pyrene units.
6
Chapter 1
Circularly polarised luminescence of pyrenyl di- and tri- peptides with mixed D- and L-amino acid residues
1-1. Introduction
In Chapter 1, the CPL properties of peptides with different chirality combinations of pyrene units to be introduced were investigated.
In recent years, efficient circularly polarized luminescence (CPL) with high asymmetry ratio (gem), and efficient circular dichroism (CD) with high asymmetry ratio (gabs) of organic molecules, lanthanoid complexes,1,2 supramolecular,3 agglomerates,4 gels5 and film has received a lot of attention from its technical chiroptical functions.
These systems typically require enantiopure building blocks, as photoexcited and ground states chirality are determined only by one and / or multiple stereocenters within the framework.
But, non-enantiopure (called enantioimpure) systems are expected in the fields of modern stereochemistry and chiral materials science.7 As an example, even if the starting material is 0.00005% ee, almost enantiopure synthesis is possible by using the soai reaction.8 Furthermore, it is known as the majority, as evidenced by that the kiloptic amplification from subtle kiloptic signals is confirmed by the CD and CPL spectra.7 The majority-rule systems contains a (R)- and (S)-chiral source in the side chains attached to CD-silent semiflexible helical polymers to maintain rotational freedom of the main chains7a–g and supramolecular π-π stacked assemblies.7h–j These are thought to arise from a coordinated transition at the critical point in branching systems.9 These systems7,8 choose one of two helices due to mirror symmetry breaking (MSB) at double-well potentials when there is a strong conflict on either right or left. 9
Circularly polarized radiation sources are hypothesized to be the origin of homochirality on Earth. but, due to inefficient photon chirality transfer reactions, the expected enantiopurity might be very low.10 Also, sunlight which is a non-circularly polarised radiation source in the UV-visible-NIR ranges, is not the origin of homochirality.
The open question is whether oligopeptides with an enantioimpure amino acid in the backbone can construct a helical / chiral configuration in the photoexcited and / or ground
7
state. Enantioimpure oligopeptides are a good model for comparing naturally occurring
L-proteins with corresponding enantiopure synthetic polypeptides and should provide a positive answer to the origin of homochirality. We hypothesized that oligo- / poly peptides and L-proteins maintain appropriate rotational freedom due to multiple C-C, C-N and C- N single bonds along the main chain axis and C-C linkers between the main and side chains. This "appropriate freedom" refers to between a tightly restricted state and a completely unrestricted state. The appropriate rotational freedom of these single bonds should play an important role in providing MSB properties for external unpolarized light sources, and enantioimpure systems that are highly sensitive to chemical effects.
To answer this question and consider the conjecture, we focused on enantioimpure di- / tri-peptides with multiple pyrene units and the corresponding enantiopure L- and D-ones for comparison. These pyrene units act as chiral sources for photoexcited and ground- state di- / tri-peptide main chains and exhibit chirality transfer capability with or without excitation by an unpolarized light sources. These peptides exhibit main chains chirality in the photoexcited and ground states because they maintain the rotational freedom of the main chains and the methylene linker between pyrene and the main chains.11
In a previous study, we reported an interesting luminophores of enantiopure mono- and oligopeptides with multiple pyrene units, i.e. H-Sp6-L-Ala(Pyr)-Sp6-NH2 [N-L-C], H- Sp6-L-Ala(Pyr)-L-Ala(Pyr)-Sp6-NH2 [N-LL-C], and H-Sp6-L-Ala(Pyr)-L-Ala(Pyr)-L- Ala(Pyr)-Sp6-NH2 [N-LLL-C] similarly corresponding D-isomers, N-D-C, N-DD-C and N-DDD-C (Figure 1-1).11b Here, N and C exhibit the N- and C-terminus, respectively.
These enantiopure di-/tri-peptides with two or more pyrene units in chloroform (CHCl3) solution showed a pyrene excimer-origin CPL signs in the visible ranges. On the other hand, the monopeptides N-L-C and N-D-C with one pyrene showed only monomer- origin CPL signs in the UV ranges.
Here, we newly designed di-/tri-peptide containing L- and D-mixed pyrene units (Figure 1-1): H-Sp6-L-Ala(Pyr)-D-Ala(Pyr)-Sp6-NH2 [N-LD-C], H-Sp6-L-Ala(Pyr)-L- Ala(Pyr)-D-Ala(Pyr)-Sp6-NH2 [N-LLD-C], H-Sp6-L-Ala(Pyr)-D-Ala(Pyr)-D-Ala (Pyr)- Sp6-NH2 [N-LDD-C], H-Sp6-L-Ala(Pyr)-D-Ala(Pyr)-L-Ala(Pyr)-Sp6-NH2 [N-LDL-C]
and their optical antipodes, N-DL-C, N-DDL-C, N-DLL-C and N-DLD-C. The four enantioimpure peptide pairs were synthesised from the corresponding amino acids (fluorenylmethyloxy-carbonyl (Fmoc)-Sp6-OH, Fmoc-L-Ala(Pyr)-OH and Fmoc-D- Ala(Pyr)-OH) according to previously reported method.11
8
Figure 1-1. Mono-, di- and tri-peptide-pyrene luminophores.
1-2. Results and discussion
Before investigating the chiroptical properties of di-/tri-peptides, we examined the photoluminescence (PL) spectra of N-LD-C in CHCl3 solutions with different peptide sequence chirality by comparing the corresponding N-LL-C.11b The main disadvantage of PL materials is the generation of PL quenching induced by association, but N-LD-C clearly shows pyrene(s)-derived PL in dilute CHCl3 solution (1.0 × 10-4 M) (Figure 1-2 (a), black line). N-LD-C showed a monomer PL band near 370~400 nm and a weak excimer PL band around 400~550 nm. The 0–0' and 0–1' monomer PL bands of N-LD-C are detected at 380 and 396 nm, respectively, whereas the 0–0' excimer PL band of N- LD-C is detected at approximately 450 nm. The PL quantum yield (Φf) of N-LD-C is 0.09. This lower Φf value results from the rotational freedom11b of the twisted main chain.
(a) (b)
Figure 1-2. (a) CPL (top lines) and PL (bottom lines) spectra and (b) CD (top lines) and UV-Vis (bottom lines) spectra of N-LD-C (black lines), N-DL-C (grey lines), and for comparison, N-LL-C (black dotted lines)11b in CHCl3 (1.0 × 10−4 M).
0 0.5 1
350 400 450 500 550 600 -20
-10 0 10 20
I= 1/2 • (IL+ IR) 104• ΔI = IL –IR
Wavelength / nm N-LD-C N-DL-C
N-LL-C
0 0.5 1 -15
-10 -5 0 5 10 15
250 300 350 400
ε= 1/2 • (εL+ εR) / 105M-1cm-1
Δε = εL–εR/ M-1cm-1
Wavelength / nm N-LD-C
N-DL-C N-LL-C
9
Interestingly, N-LD-C is composed of L- and D-amino acids, but the excimer CPL signal (top black line) from the excimer of N-LD-C is clearly detected (Figure 1-2 (a)).
As expected, the excimer CPL spectra between N-LD-C and N-DL-C are almost mirror images. Both peptides exhibit excimer CPL signal as high as |gem| = 0.39 × 10−2 at 460 nm and 0.53 × 10−2 at 440 nm. Here, we use the dimensionless Kuhn's anisotropy factor for photoexcited state (gem)12 is defined as gem = 2(IL−IR)/(IL+IR). where IL and IR indicate the intensity ratio of the left and right CPL under excitation by unpolarized incident light, respectively. Interestingly, the CPL signal between N-LD-C and N-LL-C is the same sign (Figure 1-2 (a), top black dotted lines). However, The |gem| value of N-LD-C is about 0.78 times lower than those of N-LL-C. It is |gem| = 0.86 × 10−2 at 455 nm and 1.10 × 10−2 at 463 nm, respectively.11b To investigate the concentration dependence of pyrene excimer formation, the CPL spectra of diluted N-LD-C are measured in CHCl3 (1.0 × 10−5 M) (Figure 1-S1). No clear concentration dependence on the excimer CPL spectra is observed.
The CD and UV-Vis spectra of N-LD-C (black lines) and N-LL-C (black dotted lines) in CHCl3 are shown for comparison (Figure 1-2 (b)). To quantitatively evaluate the absolute CD intensity, we use the dimensionless Kuhn's anisotropy factor12 for the ground state is defined as gabs = Δε/ε. The subtle |gabs| value of N-LD-C is similar to that of N- LL-C. It is |gabs| = 2.3 × 10−4 at 350 nm and 0.91 × 10−4 at 349 nm, respectively.9 N-LD- C usually has several vibronic (0–0’, 0–1’, 0–2’) π–π* CD and UV-Vis bands that are characteristic of pyrene in the 290–360 nm range (Figure 1-2 (b)). As expected, the CD signals between N-LD-C and N-DL-C are almost mirror image.
These results predict that the main chain of the peptide efficiently transfers chirality to the achiral pyrene of ground state, regardless of the enantiopure and enantioimpure structure. The CD sign of the first Cotton band (λext ≈ 350 nm) of N-LL-C and N-LD-C is positive (+) in common. Interestingly, the excimer |gem| value of N-LD-C is 17-20 times higher than the corresponding |gabs| value at first Cotton CD band of N-LD-C. To investigate the origin of the CD and CPL signals of peptide-pyrene luminophores in CHCl3, N-LL-C as a model of oligopeptide is computationally optimized (Figure 1-3).
The two pyrenes in the peptide form a twisted structure with an interval of 10.22 Å.
The CD spectra calculated from the N-LL-C optimized structure show positive (+) and negative (-) CD signals at ≈360 nm and ≈340 nm, respectively (Figure 1-4). The CD and UV-Vis spectra of N-LL-C in CHCl3 at the time of the experiment are almost the same as the calculated values. The CD intensities calculated at these wavelengths originate from the exciton caplets of the π–π* transitions of each pyrene. The CD sign involves the relative spatial arrangement between the two pyrenes in this model.
10
The CD spectra (especially the CD sign) between N-LD-C and N-LL-C in CHCl3 are experimentally similar. From this result, the twisted structure between the two pyrenes of N-LD-C is assumed to be the same as that of N-LL-C, and the excimer CPL signal of N- LD-C is considered.
Figure 1-3. The theoretically optimized structure of the part extracted from N-LL-C.
Figure 1-4. Simulated CD (fwhm = 0.2 eV) (top black line) and UV-vis (fwhm = 0.3 eV) (bottom grey line) spectra of N-LL-C in a vacuum.
Next, we investigate the PL and CPL properties of the four tripeptides (N-LLD-C, N- LDD-C, N-LDL-C, and N-LLL-C) in dilute CHCl3. N-LLD-C, N-LDD-C, and N-LDL- C commonly exhibit pyrene-related monomer PL bands at maximum PL (λem) at 380 and 396 nm. (Figure 1-5 (a), (c) and (e), bottom black lines). It is clear that both N-LLD-C and N-LDD-C have Φf values of 0.08 and 0.07, respectively, and show an excimer-origin 0–0' PL band at ≈460 nm. The two enantioimpure tripeptides show λem similar to the monomer and excimer PL bands, but these Φf values are similar to that of N-LLL-C.
(Figure 1-5 (a) and (c), black dotted lines).11b
0 20000 40000 60000 80000 100000
-150 -100 -50 0 50 100 150
150 200 250 300 350 400 450 500 Absorptivity, ε (M-1cm-1) Molar ellipticity, Δε(M-1cm-1)
Wavelength / nm
11
(a) (b)
(c) (d)
(e) (f)
0 0.5 1
350 400 450 500 550 600 -20
-10 0 10 20
I= 1/2 • (IL+ IR) 104• ΔI = IL –IR
Wavelength / nm N-LLD-C
N-DDL-C
N-LLL-C
0 0.5 1 -20
-10 0 10 20
250 300 350 400
ε= 1/2 • (εL+ εR) / 105M-1cm-1
Δε = εL–εR/ M-1cm-1
Wavelength / nm N-LLD-C
N-DDL-C N-LLL-C
0 0.5 1
350 400 450 500 550 600 -20
-10 0 10 20
I= 1/2 • (IL+ IR) 104• ΔI = IL –IR
Wavelength / nm N-LDD-C
N-DLL-C N-LLL-C
0 0.5 1 -20
-10 0 10 20
250 300 350 400
ε= 1/2 • (εL+ εR) / 105M-1cm-1
Δε = εL–εR/ M-1cm-1
Wavelength / nm N-LDD-C
N-DLL-C N-LLL-C
0 0.5 1
350 400 450 500 550 600 -20
-10 0 10 20
I= 1/2 • (IL+ IR) 104• ΔI = IL –IR
Wavelength / nm N-LDL-C
N-DLD-C
N-LLL-C
0 0.5 1 -20
-10 0 10 20
250 300 350 400
ε= 1/2 • (εL+ εR) / 105M-1cm-1
Δε = εL–εR/ M-1cm-1
Wavelength / nm N-LDL-C
N-DLD-C N-LLL-C
12
Figure 1-5. (a) CPL (top lines) and PL (bottom lines) spectra and (b) CD (top lines) and UV-Vis (bottom lines) spectra of N-LLD-C (black lines) and N-DDL-C (grey lines). (c) CPL and PL spectra and (d) CD and UV-Vis spectra of N-LDD-C (black lines) and N- DLL-C (grey lines). (e) CPL and PL spectra and (f) CD and UV-Vis spectra of N-LDL- C (black lines) and N-DLD-C (grey lines) in CHCl3 solution (1.0 × 10−4 M). For comparison, spectra of N-LLL-C are shown in black dotted lines.11b
N-LLD-C (Figure 1-5 (a), black line) and N-DLL-C (Figure 1-5 (c), grey line) show excimer-origin CPL spectra similar to N-LLL-C (Figure 1-5 (a), (c) and (e), black dotted lines), but their CPL sign is greatly influenced by the L- and D-form sequences. The CPL spectra between N-LLD-C and N-DDL-C (Figure 1-5 (a)) and between N-LDD-C and N-DLL-C (Figure 1-5 (c)) are mirror images, respectively. The |gem| values of the four tripeptides are of the order of 10−2. those of N-LLD-C and N-DDL-C are 0.57 × 10−2 at 451 nm and 0.72 × 10−2 at 457 nm, respectively, and those of N-LDD-C and N-DLL-C are 0.57 × 10−2 at 478 nm and 0.62 × 10−2 at 480 nm, respectively.
However, N-LDL-C (Figure 1-5 (e), black lines) and N-DLD-C (Figure 1-5 (e), grey lines) clearly indicate the corresponding excimer PL signals, but not the excimer CPL signals. This discrepancy can be expected to occur when the ratio of left-handed to right- handed structures of these pyrene in the photoexcited state is equal.
An inversion of the excimer CPL sign is observed between N-LLD-C (Figure 1-5 (a), black lines) and N-LDD-C (Figure 1-5 (c), black lines). N-LLD-C is produced from a mixture of N-LL-C and N-LD-C, both showing a negative (-) excimer CPL sign.11b On the other hand, N-LDD-C is produced from a mixture of N-LD-C and N-DD-C, which shows the negative (-) and positive (+) excimer CPL signs, respectively, causing a conflict.11b This result can be explained by the discovery that the CPL intensity of N- LDD-C is lower than that of N-LLD-C.
The monomer CPL signal of N-LDD-C at ≈409 nm (Figure 1-5 (c), top black lines) is weak, but the corresponding PL signal is clear.11c This is completely in contrast to the results of the monomer CPL of N-LDL-C at ≈404nm. (Figure 1-5 (e), top black lines).
The |gem| values of N-LDL-C and N-DLD-C are further reduced by a third to 0.19 × 10−2 at 399 nm and 0.26 × 10−2 at 408 nm, respectively. N-LDL-C is produced from a mixture of N-LD-C and N-DL-C shows a strong conflict between the negative (-) and positive (+) excimer CPL signs, respectively.
Similarly, monomer CPL signs between N-LDD-C (Figure 1-5 (c), black lines) and N- D-C11b and between N-LDL-C (Figure 1-5 (e), black lines) and N-L-C.11b Especially, the CPL sign for N-LDD-C and N-LDL-C shows negative (-) and positive (+), respectively.
13
On the other hand, that for N-D-C and N-L-C shows positive (+) and negative (-), respectively. These disadvantages can result from chirally perturbed electronic and / or vibronic transitions between multiple pyrene units and / or between pyrene units and the peptide main chain.
Similar to N-LD-C, to investigate the concentration dependence of the monomer and excimer CPL signals, the CPL spectra of diluted N-LLD-C and diluted N-LDL-C are measured in CHCl3 (1.0 × 10-5 M) (Figure 1-S2 and 1-S3). Similarly, no clear concentration dependence on the CPL spectra was observed in these cases.
The CD and UV-Vis spectra of N-LLD-C, N-LDD-C, and N-LDL-C in CHCl3
solution were compared with that of the corresponding isomer (N-DDL-C, N-DLL-C, and N-DLD-C) to determine the ground state chirality (Figure 1-5 (b), (d) and (f), black and grey lines). The UV-Vis spectra are similar between N-LLD-C, N-LDD-C, N-LDL- C and N-LLL-C.
On the other hand, N-LLD-C, N-LDD-C, N-LDL-C and N-LLL-C show different CD bands (Figure 1-5 (b), (d) and (f), top lines). The CD signs between N-LLD-C and N- DDL-C, between N-LDD-C and N-DLL-C and between N-LDL-C and N-DLD-C is almost mirror image. This result suggests the occurrence of efficient peptide chirality transfer to multiple pyrene units in the ground state. This has already been observed with N-LD-C and N-DL-C.
The first cotton band CD signs for N-LLD-C, N-LDD-C, N-LDL-C, and N-LLL-C are clearly positive (+). However, the first Cotton CD band (λext) of N-LDL-C (361 nm) is drastically redshifted compared to that of N-LLD-C (346 nm), N-LDD-C (345 nm), and N-LLL-C (351 nm).11b The |gabs| values of N-LLD-C, N-LDD-C, and N-LDL-C at the first Cotton CD bands (345, 345, and 361 nm, respectively)that cause the CPL band,
|gabs| = ≈4.3 × 10−4, 2.3 × 10−4, and 7.5 × 10−4, respectively.
Therefore, the monomer |gabs| values of the first Cotton CD band are incleased by 9-37 times compared to the corresponding excimer |gem| values of N-LLL-C and N-DDD-C.
The CPL signs and CPL wavelengths are controlled by the number and order of the L- /
D-configurations.
For N-LDL-C, the signs for the first cotton CD band and monomer CPL band is the same as for N-L-C. On the other hand, in the case of N-LDD-C, the signs of the first Cotton CD band and the weak monomer CPL band are different. This is due to the steric effect of N-LDD-C's strong excimer formation.11b
The enantiopurity of peptide sequences is not a prerequisite for photoexcited chirality, as evidenced by a clear CPL signal, but it determines which left or right direction the photoexcited state or the ground state is preferred.
14
1-3. Conclusions
Eight ways of enantioimpure di-/tri-peptides with pyrene units (N-LD-C, N-LLD-C, N-LDD-C, N-LDL-C and their corresponding isomers) were synthesized. These peptide- pyrene luminophores expressed excimer CPL regardless of enantioimpurities. The g value was |gem| ≈ (0.19-0.72) × 10−2. However, the multiple stereocenters (D- and L-) of the peptide backbone were not the main factor. This may be due to a strong contradiction between the preferred chiroptical signs of the photoexcited and ground states. However, these results give CPL- / CD-active oligos and polypeptides more freedom to design, ignoring their enantiopurity.
15
1-4. Experimental details
General methods
CHCl3, a spectroscopic-grade (Spectrosol) solvents used for optical measurements, was purchased from Dojindo Laboratories (Kumamoto, Japan).
Synthesis of di- / tri-peptide-pyrene luminophores
We synthesised peptides that consisted of two and three pyrene units and flexible units by conventional 9-fluorenylmethyloxycarbonyl group (Fmoc)-based solid-phase peptide synthesis. The solid phase synthesis was performed on L- and D-pyrenyl alanine (L- Ala(Pyr) and D-Ala(Pyr): Fmoc-Ala(1-Pyn)-OH and Fmoc- D-Ala(1-Pyn)-OH, Watanabe Chemicals, Hiroshima, Japan) and an amino acid consisting of ethylene glycol units (Sp6:
Fmoc-NH-PEG6-COOH, Merck, Darmstadt, Germany) as monomer units, and an Fmoc- NH-SAL PEG (super acid-labile poly(ethylene glycol)) resin from Watanabe Chemicals (14 μmol scale). Sp6 was used for improving the solubility of the peptides against various solvents. Fmoc deprotection was performed with 20% piperidine in DMF for 7 min at room temperature. After six washes with DMF, each amino acid was coupled using HBTU/NMM reagents with a reaction time of 40 min per coupling at room temperature.
No capping step was performed. After the N-terminal Fmoc group was deprotected, the resin was washed with DCM and treated with 95% TFA, 2.5% water and 2.5% TIS for 1.5 hours at room temperature. The crude peptides were analysed and purified by reversed phase high-pressure liquid chromatography (RP-HPLC) on a C18 column with buffer A (0.1% TFA in water) and buffer B (acetonitrile) and monitoring at 340 nm. The purified peptides were identified using MALDI-TOF mass spectroscopy and RP-HPLC. The yields of the peptides were 62% (10.9 mg; N-LD-C), 60% (10.6 mg; N-DL-C), 46% (10.0 mg; N-LLD-C), 60% (12.9 mg; N-DDL-C), 52% (11.2 mg; N-LDD-C), 69% (14.9 mg;
N-DLL-C), 31% (6.7 mg; N-LDL-C), and 44% (9.5 mg; N-DLD-C).
16
Measurement of CPL and PL spectra
The absolute PL quantum yields in CHCl3 were obtained with an absolute PL quantum yield measurement system (Hamamatsu Photonics C9920-02, Hamamatsu, Japan) under an air atmosphere at room temperature. Dilute CHCl3 solutions of the di- / tri-peptides (1.0 × 10−4 M) were excited at 300 nm. The pass length was 10 mm. PL and CPL spectra in CHCl3 solution were measured using a JASCO CPL-300 spectrofluoropolarimeter (Hachioji-Tokyo, Japan) at room temperature with a pass length of 1 mm. The di- / tri- peptides were used at a concentration of 1.0 × 10−4 M in CHCl3. The instrument used a scattering angle of 0° from the excitation of unpolarised, monochromated incident light with a bandwidth of 10 nm at excitation at 300 nm and emission with a bandwidth of 10 nm.
Measurement of CD and UV-Vis spectra
The circular dichroism (CD) and UV-Vis spectra of the di- / tri-peptides in CHCl3
solution (1.0 × 10−4 M) were measured using a JASCO J-820 spectropolarimeter (Hachioji, Tokyo, Japan) at room temperature with a pass length of 1 mm.
Theoretical calculations
The structure of the model molecule of N-LL-C (Figure 1-3) was calculated by the hybrid density functional theory (B3LYP functional)13 with the cc-pVDZ basis set.14 In this geometric optimisation, the dihedral angles Φ (C–C–N–C) and Φ (N–C–C–N) were fixed at −57° and −47°, respectively, to force this molecule to take an α-helical conformation. The excitation energies and rotational strengths of this molecule were calculated using the time-dependent response function theory15 with the abovedescribed functional and basis set. The CD curve (Figure 1-4) was obtained by expanding each rotational strength into a Gaussian curve centred at the corresponding excitation wavelength. The Gaussian0916 program was used for these quantum chemical calculations.
17
1-5. References
1. For recent reviews. (a) J. Kumar, T. Nakashima, H. Tsumatori and T. Kawai, J. Phys.
Chem. Lett., 2014, 5, 316-321; (b) E. M. Sánchez-Carnerero, A. R. Agarrabeitia, F.
Moreno, B. L. Maroto, G. Muller, M. J. Ortiz and S. de la Moya, Chem. Eur. J., 2015, 21, 13448-13500; (c) F. Zinna and L. D. Bari, Chirality, 2015, 27, 1-13; (d) G. Longhi, E. Castiglioni, J. Koshoubu, G. Mazzeo and S. Abbate, Chirality, 2016, 28, 696-707.
2. J. L. Lunkley, D. Shirotani, K. Yamanari, S. Kaizaki and G. Müller, Inorg. Chem., 2011, 50, 12724-12732; (b) Y. Sawada, S. Furumi, A. Takai, M. Takeuchi, K. Noguchi and K. Tanaka, J. Am. Chem. Soc., 2012, 134, 4080-4083; (c) Y. Morisaki, M. Gon, T. Sasamori, N. Tokitoh and Y. Chujo, J. Am. Chem. Soc., 2014, 136, 3350-3353; (d) S. Abbate, G. Longhi, F. Lebon, E. Castiglioni, S. Superchi, L. Pisani, F. Fontana, F.
Torricelli, T. Caronna, C. Villani, R. Sabia, M. Tommasini, A. Lucotti, D. Mendola, A. Mele and D. A. Lightner, J. Phys. Chem. C, 2014, 118, 1682-1695.
3. (a) Y. Imai, K. Kawano, Y. Nakano, K. Kawaguchi, T. Harada, T. Sato, M. Fujiki, R.
Kuroda and Y. Matsubara, New J. Chem., 2008, 32, 1110-1112; (b) S. Haraguchi, M.
Numata, C. Li, Y. Nakano, M. Fujiki and S. Shinkai, Chem. Lett., 2009, 38, 254-255;
(c) N. Nishiguchi, T. Kinuta, T. Sato, Y. Nakano, H. Tokutome, N. Tajima, M. Fujiki, R. Kuroda, Y. Matsubara and Y. Imai, Chem. Asian J., 2012, 7, 360-366; (d) T. Shiraki, Y. Tsuchiya, T. Noguchi, S.-I. Tamaru, N. Suzuki, M. Taguchi, M. Fujiki and S.
Shinkai, Chem. Asian J., 2014, 9, 218-222; (e) J. Kumar, T. Nakashima and T. Kawai, J. Phys. Chem. Lett., 2015, 6, 3445-3452.
4. (a) Y. Kawagoe, M. Fujiki and Y. Nakano, New J. Chem., 2010, 34, 637-647; (b) Y.
Nakano and M. Fujiki, Macromolecules, 2011, 44, 7511-7519; (c) R. Tempelaar, A.
Stradomska, J. Knoester and F. C. Spano, J. Phys. Chem. B, 2011, 115, 10592-10603;
(d) J. Liu, H. Su, L. Meng, Y. Zhao, C. Deng, J. C. Y. Ng, P. Lu, M. Faisal, J. W. Y.
Lam, X. Huang, H. Wu, K. S. Wong and B. Z. Tang, Chem. Sci., 2012, 3, 2737-2747;
(e) M. Fujiki, A. J. Jalilah, N. Suzuki, M. Taguchi, W. Zhang, M. M. Abdellatif and K. Nomura, RSC Adv., 2012, 2, 6663-6671; (f) X. Jiang, X. Liu, Y. Jiang, Y. Quan, Y.
Cheng and C. Zhu, Macromol. Chem. Phys., 2014, 215, 358-364; (g) For a review. J.
Roose, B. Z. Tang and K. S. Wong, Small, 2016, 12,6495-6512.
5. (a) H. Jintoku, M.-T. Kao, A. Del Guerzo, Y. Yoshigashima, T. Masunaga, M.
Takafuji and H. Ihara, J. Mater. Chem. C, 2015, 3, 5970-5975; (b) D. Yang, Y. Zhao, K. Lv, X. Wang, W. Zhang, L. Zhang and M. Liu, Soft Matter, 2016, 12, 1170-1175.
6. (a) S. H. Chen, D. Katsis, A. W. Schmid, J. C. Mastrangelo, T. Tsutsui and T. N.
Blanton, Nature, 1999, 397, 506-508; (b) M. Oda, H.-G. Nothofer, U. Scherf, V.
Šunjić, D. Richter, W. Regenstein and D. Neher, Macromolecules, 2002, 35, 6792-
18
6798; (c) P. C. P. Hrudey, K. L. Westra and M. J. Brett, Adv. Mater., 2006, 18, 224- 228; (d) J.-M. Yu, T. Sakamoto, K. Watanabe, S. Furumi, N. Tamaoki, Y. Chen and T.
Nakano, Chem. Commun., 2011, 47, 3799-3801; (e) K. Watanabe, I. Osaka, S.
Yorozuya and K. Akagi, Chem. Mater., 2012, 24, 1011-1024; (f) Y. Yang, R. C. da Costa, D.-M. Smilgies, A. J. Campbell and M. J. Fuchter, Adv. Mater., 2013, 25, 2624-2628; (g) For a review: S. Furumi, Polym. J., 2013, 45, 579-593.
7. (a) M. M. Green, J.-W. Park, T. Sato, A. Teramoto, S. Lifson, R. L. B. Selinger and J.
V. Selinger, Angew. Chem. Int. Ed., 1999, 38, 3138-3154; (b) J. J. L. M. Cornelissen, A. E. Rowan, R. J. M. Nolte and N. A. J. M. Sommerdijk, Chem. Rev., 2001, 101, 4039-4070; (c) T. Nakano and Y. Okamoto, Chem. Rev., 2001, 101, 4013-4038; (d) D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes and J. S. Moore, Chem. Rev., 2001, 101, 3893-4011; (e) M. Fujiki, J. R. Koe, K. Terao, T. Sato, A. Teramoto and J.
Watanabe, Polym. J., 2003, 35, 297-344; (f) E. Yashima, K. Maeda, H. Iida, Y.
Furusho and K. Nagai, Chem. Rev., 2009, 109, 6102-6211; (g) Y.-Z. Ke, Y. Nagata, T. Yamada and M. Suginome, Angew. Chem. Int. Ed., 2015, 54, 9333-9337; (h) J.
Gestel, A. R. A. Palmans, B. Titulaer, J. A. J. M. Vekemans and E. W. Meijer, J. Am.
Chem. Soc., 2005, 127, 5490-5494; (i) J. Kumar, H. Tsumatori, J. Yuasa, T. Kawai and T. Nakashima, Angew. Chem. Int. Ed., 2015, 54, 5943-5947; (j) F. Li, Y. Li, G.
Wei, Y. Wang, S. Li and Y. Cheng, Chem. Eur. J., 2016, 22, 12910-12915.
8. (a) I. Sato, H. Urabe, S. Ishiguro, T. Shibata and K. Soai, Angew. Chem. Int. Ed., 2003, 42, 315-317; (b) A. Córdova, M. Engqvist, I. Ibrahem, J. Casa and H. Sundén, Chem. Commun., 2005, 2047-2049.
9. (a) V. Avetisov and V. Goldanskii, Proc. Natl. Acad. Sci. U. S. A., 1996, 93, 11435- 11442; (b) D. K. Kondepudi and K. Asakura, Acc. Chem. Res., 2001, 34, 946-954.
10. (a) W. A. Bonner, Origins Life Evol. Biosphere, 1995, 25, 175-190; (b) H. Nishino, A. Kosaka, G. A. Hembury, F. Aoki, K. Miyauchi, H. Shitomi, H. Onuki and Y. Inoue, J. Am. Chem. Soc., 2002, 124, 11618-11627; (c) U. J. Meierhenrich, L. Nahon, C.
Alcaraz, J. H. Bredehft, S. V. Hoffmann, B. Barbier and A. Brack, Angew. Chem. Int.
Ed., 2005, 44, 5630-5634.
11. (a) M. Kitamatsu, M. Shigeyasu, T. Okada and M. Sisido, Chem. Commun., 2004, 40, 1208-1209; (b) T. Nishikawa, N. Tajima, M. Kitamatsu, M. Fujiki and Y. Imai, Org.
Biomol. Chem., 2015, 13, 11426-11431; (c) T. Nishikawa, S. Kitamura, M. Kitamatsu, M. Fujiki and Y. Imai, ChemistrySelect, 2016, 4, 831-835.
12. E. L. Eliel, S. H. Wilen and L. N. Mander, Stereochemistry of Organic Compounds, Wiley, New York, 1994, ch. 13.
13. A. D. Becke, J. Chem. Phys., 1993, 98, 5648-5652.
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14. (a) T. H. Dunning Jr., J. Chem. Phys., 1989, 90, 1007-1023; (b) D. E. Woon and T. H.
Dunning Jr., J. Chem. Phys., 1993, 98, 1358-1371.
15. M. E. Casida, Recent Advances in Density Functional Methods, ed. D. P. Chong, World Scientific, Singapore, 1995, vol 1.
16. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M.
Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E.
Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C.
Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E.
Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K.
Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Rev. D.01, Gaussian, Inc., Wallingford CT, 2013.
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1-6. Supplementary data
Figure 1-S1. CPL (top lines) and PL (bottom lines) spectra of N-LD-C in CHCl3 solution at 1.0 × 10–4 M (black lines) and 1.0 × 10–5 M (grey lines).
Figure 1-S2. CPL (top lines) and PL (bottom lines) spectra of N-LLD-C in CHCl3
solution at 1.0 × 10–4 M (black lines) and 1.0 × 10–5 M (grey lines).
Figure 1-S3. CPL (top lines) and PL (bottom lines) spectra of N-LDL-C in CHCl3
solution at 1.0 × 10–4 M (black lines) and 1.0 × 10–5 M (grey lines).
0 0.5 1
350 400 450 500 550 600 -20
-10 0 10 20
I= 1/2 • (IL+ IR) 104• ΔI = IL –IR
Wavelength / nm
0 0.5 1
350 400 450 500 550 600 -20
-10 0 10 20
I= 1/2 • (IL+ IR) 104• ΔI = IL –IR
Wavelength / nm
0 0.5 1
350 400 450 500 550 600 -20
-10 0 10 20
I= 1/2 • (IL+ IR) 104• ΔI = IL –IR
Wavelength / nm
21
Figure 1-S4. MALDI-TOF mass spectra of N-DL-C and N-LD-C. An α-CHCA was used as a matrix. calcd. [M+H]+ = 1230.61, N-DL-C; obsd. [M+H]+ = 1231.22, N-LD-C; obsd.
[M+H]+ = 1231.06.
Figure 1-S5. MALDI-TOF mass spectra of N-DLL-C, N-LDL-C and N-DDL-C. An α- CHCA was used as a matrix. calcd. [M+Na]+ = 1523.71, N-DLL-C; obsd. [M+Na]+ = 1524.99, N-LDL-C; obsd. [M+Na]+ = 1525.44, N-DDL-C; obsd. [M+Na]+ = 1525.18.
22
Figure 1-S6. MALDI-TOF mass spectra of N-LDD-C, N-DLD-C and N-LLD-C. An α- CHCA was used as a matrix. calcd. [M+Na]+ = 1523.71, N-LDD-C; obsd. [M+Na]+ = 1525.31, N-DLD-C; obsd. [M+Na]+ = 1525.52, N-LLD-C; obsd. [M+Na]+ = 1524.49.
Figure 1-S7. RP-HPLC chart of N-DL-C. Buffer A. 0.1% TFA in water; buffer B, acetonitrile and monitoring at 340 nm with a gradient of 0-100% for 20 min.
23
Figure 1-S8. RP-HPLC chart of N-LD-C. Buffer A. 0.1% TFA in water; buffer B, acetonitrile and monitoring at 340 nm with a gradient of 0-100% for 20 min.
Figure 1-S9. RP-HPLC chart of N-DLL-C. Buffer A. 0.1% TFA in water; buffer B, acetonitrile and monitoring at 340 nm with a gradient of 0-100% for 20 min.
24
Figure 1-S10. RP-HPLC chart of N-DDL-C. Buffer A. 0.1% TFA in water; buffer B, acetonitrile and monitoring at 340 nm with a gradient of 0-100% for 20 min.
Figure 1-S11. RP-HPLC chart of N-LDL-C. Buffer A. 0.1% TFA in water; buffer B, acetonitrile and monitoring at 340 nm with a gradient of 0-100% for 20 min.
25
Figure 1-S12. RP-HPLC chart of N-LDD-C. Buffer A. 0.1% TFA in water; buffer B, acetonitrile and monitoring at 340 nm with a gradient of 0-100% for 20 min.
Figure 1-S13. RP-HPLC chart of N-LLD-C. Buffer A. 0.1% TFA in water; buffer B, acetonitrile and monitoring at 340 nm with a gradient of 0-100% for 20 min.
26
Figure 1-S14. RP-HPLC chart of N-DLD-C. Buffer A. 0.1% TFA in water; buffer B, acetonitrile and monitoring at 340 nm with a gradient of 0-100% for 20 min.
27
Chapter 2
Solvent-sensitive sign inversion of excimer origin circularly polarized luminescence in bipyrenyl peptides
2-1. Introduction
In Chapter 2, The CPL properties by changing the solvent, which is the surrounding environment of the organic luminophores, were investigated.Among them, the peptides previously synthesized in our laboratory (Chemistry Select 2016, 4, 831–835), peptide 2py without methylene spacer was compared with peptide 2C4, in which four methylene spacers are present between the pyrene units.
In recent years, chiral and metallic organic luminophores that efficiently emit CPL in the near-ultraviolet, visible, and near-infrared areas are highly anticipated for their unique optoelectronic and photonic applications.1–4 Especially, the sign and intensity of the CPL signal of chiral luminophores with unlimited rotational freedom exerts its function in a photoexcited state that is susceptible to external influences. However, since the conformation of the molecule and the complex differs greatly between the ground state and the excited state, the sign and intensity of the CPL signal (excited state) and the CD signal (ground state) differ greatly. Therefore, it is difficult to control photoexcitation chirality.3
An enantiopair of luminophores with a stereocenter in the ground state provides a mirror-image CPL signal of positive (+)- and negative (-)-signs in the excited state.
However, this approach has limitations because these pairs are not always available.
Therefore, it is a difficult problem from the conventional approach to obtain both positive (+)- and negative (-)-sign CPL signals from one enantiopure luminophore without using an enantiomer.4 Non-conventional sign inversion of the CPL sign is possible by changing conditions such as suitable solvent,4a,k host matrix,4e stirring direction at sol-gel transition temperature,4b as well as by geometrical modification of the luminophore,4c,d,g cooling and heating,4f,k decomposition and aggregation.4f,j,l
Several CPL spectra of chiral oligo- / polypeptides containing pyren units have already been reported.5 Recently, it has been found that chiral oligopeptides with multiple pyrene units emit strong excimer CPL signals.5i In addition, the CPL sign of these pyrene- peptides involved the intramolecular distance between the two pyrene units.5j However,
28
the relationship of solvent-controlled CPL sign inversion in chiral peptides has not yet been explained.
This chapter reports solvent-dependent CPL sign inversion in bipyrenyl dipeptide enantiopairs with and without a four methylene spacer to adjust the intramolecular distance between two pyrene units (Scheme 2-1): (i) H-Sp6-D-Ala(Pyr)-D-Ala(Pyr)-Sp6- NH2 [D2] and its L-isomer (L2) and (ii) H-Sp6-D-Ala(Pyr)-NH-(CH2)4-CO-D-Ala(Pyr)- Sp6-NH2 [D2C4] and its L-isomer (L2C4).
Scheme 2-1. Chiral bipyrenyl-peptide luminophores, D2, L2, D2C4, and L2C4.
2-2. Results and discussion
The two bipyrenyl peptides D2/L2 and D2C4/L2C4 were synthesized from Fmoc-Sp6- OH and Fmoc-D-Ala(Pyr)-OH (or Fmoc-L-Ala(Pyr)-OH) according to previous method.5i Since D2 showed a positive (+)-CPL band in CHCl3,5i we investigated the chiroptical properties of D2 in other seven other solvents: dichloromethane (CH2Cl2), methanol (MeOH), acetonitrile (CH3CN), DMF, N-methylpyrrolidone (NMP), N,N- dimethylacetamide (DMAc), and acetone. As shown in Figure 2-1 (bottom lines), the PL spectra of dilute D2 were similar in CH2Cl2, MeOH, CH3CN, DMF, NMP, and DMAc (bottom lines in Figures 2-1 (a)–(f), respectively, indicated by grey solid lines). The PL spectra was not shown with acetone due to the very low solubility of D2 in this solvent.
The most serious problem affecting the luminescence of the luminophore is quenching by aggregation,6 but D2 shows a clear PL in the solution derived from the pyrene unit. The PL spectra in various solvents were similar to those previously reported in CHCl3 (grey dotted line in Figure 2-1 (a)).5i D2 exhibited a sharp monomer PL band (~370 nm) and a very weak excimer PL band (~430–530 nm) (Figure 2-1), with the monomer PL band predominant in all solvents. The emission maximum of the monomer PL band (λem) was 378 nm in CH2Cl2, MeOH, CH3CN, DMF, NMP, and DMAc. The PL quantum yield (Φf) was 0.12 for CH2Cl2, 0.07 for MeOH, 0.07 for CH3CN, 0.12 for DMF, 0.09 for NMP, and 0.15 for DMAc. These low Φf values are similar to those of CHCl3 (Φf = 0.10)5i and may be due to the rotational freedom of the peptide main chain.
Conversely, as shown in Figure 2-1 (top lines, grey solid lines) the excimer CPL band of
