Circular Polarized Luminescence of Hydrogen-Bonded Molecular

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Circular Polarized Luminescence of Hydrogen-Bonded Molecular Assemblies of Chiral Pyrene Derivatives Hayato Anetai, Takashi Takeda, Norihisa Hoshino, Yasuyuki Araki, Takehiko Wada, Shunsuke Yamamoto, Masaya Mitsuishi, Hiromu Tsuchida, Tomoki Ogoshi, and Tomoyuki Akutagawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12747 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Circular Polarized Luminescence of Hydrogen-Bonded Molecular Assemblies of Chiral Pyrene Derivatives Hayato Anetai,† Takashi Takeda,†, ‡* Norihisa Hoshino,†, ‡ Yasuyuki Araki,‡ Takehiko Wada,‡ Shunsuke Yamamoto,‡ Masaya Mitsuishi,‡ Hiromu Tsuchida,± Tomoki Ogoshi,± and Tomoyuki Akutagawa †, ‡* †Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan, ±Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, 920-1192, Japan, and ‡Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ABSTRACT: The absorption and fluorescence spectra of chiral alklyamide-substituted pyrene derivatives (R-1 and S-1) in the solution phase were consistent with the formation of N-H•••O= hydrogen-bonded helical π-stacked one-dimensional (1D) supramolecules (R-1) n and (S-1) n in methylcyclohexane (MCH), toluene, CHCl 3 , and THF; the π-stacked structures and aggregation number (n) were governed by the concentration (c) and solvent polarity. The aggregation of (R-1) n and (S-1) n in MCH and toluene was much greater than that in THF and CHCl 3 , and excimer emission of excited state (R-1)* n and (S-1)* n supramolecules with n ≥ 2 was observed in all solvents. Interestingly, the circular polarized luminescence (CPL) spectra of (R-1)* n and (S-1)* n revealed a g lum value of 0.03 in MCH at c > 1 × 10-6 M. Circular dichroism (CD) and CPL spectra revealed the formation of hydrogen-bonded helical 1D supramolecular assemblies in the ground and excited states. The helicities of the 1D supramolecular assemblies of (R-1) n and (S-1) n in CHCl 3 were inverted as compared to those in MCH and THF. The solvent polarity and concentration were sensitive to the π-stacked structure and helical configuration of the N-H•••O= hydrogen-bonded 1D supramolecular assemblies.

INTRODUCTION Hydrogen-bonded supramolecular assemblies and their functionalities have been extensively examined in a variety of soft materials in solution, organogel, liquid crystalline, and crystalline phases.1-8 Among them, hierarchical hydrogen-bonded biological molecular assemblies such as the double helix of DNA and the secondary structure of proteins are essential for genetic information and enzyme reactivity.9 For instance, amide-type N-H•••O= hydrogen-bonding interactions within α-helix and βsheet structures play an important role in the secondary structures of proteins,1 and association-dissociation reactions can be utilized for reversible structural transformations and highly efficient enzyme reactions.9 Therefore, controlling the hydrogenbonding in supramolecular assemblies may facilitate the fabrication of novel molecular machines and multi-functional materials. A variety of hydrogen-bonded molecular assemblies have been applied in functional molecular materials.2,3 For instance, simple alkylamide (-CONHC n H 2n+1 ) substituted benzene derivatives can form one-dimensional (1D) N-H•••O= hydrogenbonded π-stacked columns, which further assemble into nanofibers, organogels, and discotic hexagonal columnar (Col h ) mesophases.10-14 Interestingly, a ferroelectric response in the polarization – electric filed (P–E) curves was observed, as a polarity inversion for the direction of intermolecular 1D hydrogen-bonding chains and ferroelectricity depends on the substitution pattern of –CONHC n H 2n+1 groups on benzene.15,16 Introduction of phenylamide (-CONHPh) groups onto pyrene πcores also forms 1D N-H•••O= hydrogen-bonded π-stacked columns, which exhibit piezochromic responses upon modulation of the π-stacking distance and dihedral angle under outer mechanical stress, resulting in drastic changes in their absorption

and fluorescence spectra.17 The introduction of intermolecular amide-type N-H•••O= hydrogen-bonding interactions into various type of π-cores may be useful in soft molecular assemblies such as nanofibers, organogels, liquid crystals, etc., which are often utilized for functional organic materials with ferroelectric and chromic behaviors. Intermolecular amide-type N-H•••O= hydrogen-bonded molecular assemblies can exist even in the solution phase.2, 14, 18 For instance, helical hydrogen-bonded 1D molecular assemblies of chiral alkylamide-substituted benzene derivatives were observed in methylcyclohexane (MCH), and temperature-dependent association and dissociation processes were reported.14, 19, 20 Although alkylamide-substituted benzene derivatives can form interesting 1D molecular assemblies, additional molecular design is necessary to incite physical responses such as fluorescence and electrical conduction. For instance, alkylamide-substituted tetraphenylethylene derivatives can form stable excimers by intermolecular hydrogen-bonding and exhibit three fluorescence modes.18 We also examined the 1D hydrogen-bonded molecular assemblies of organogels, nanofibers, and Col h phases in –CONHC 14 H 29 substituted pyrene derivative (2) by the aid of effective π-stacking interactions using an extended πcore, which also exhibited an interesting fluorescence response, ferroelectricity, and current-switching behavior according to the ferroelectric P–E hysteresis.17 The minimum concentration at which the excimer emission of 2 in CHCl 3 appeared was one thousand times higher than that of pyrene itself due to the formation of intermolecular N-H•••O= hydrogen bonds in the solution phase. The molecular aggregation of 2 results in an interesting switching phenomenon between the monomer (blue) and excimer (green) emission. The introduction of chirality into hydrogen-bonded and π-stacked molecular assemblies can generate circular polarized luminescence (CPL)19 by modifying the

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chiral molecular assembly structure. As such, the 1D hydrogenbonded π−stacked assembly of 2 was modified to a 1D helical structure by introducing chiral (R)-3,7-dimethyl-1-octylamide and/or (S)-3,7-dimethyl-1-octylamide chains (Scheme 1).

Scheme 1. Molecular structures of chiral alkylamide-substituted pyrene derivatives (R-1 and S-1) and achiral 2.

Intermolecular N-H•••O= hydrogen-bonded chiral 1D molecular assemblies in the ground and excited states were evaluated in the solution phase using concentration- and solvent-dependent absorption and emission. The formation of helical 1D molecular assemblies in the ground and excited states was examined using circular dichroism (CD) and CPL,20 respectively, which can be used to deduce qualitative information on the aggregation state of (R-1) n or (S-1) n in the ground state and that of (R-1)* n or (S-1)* n in the excited state (n ≥ 2). The association and dissociation of N-H•••O= hydrogen-bonded 1D helical molecular assemblies depend on the polarities of MCH, toluene, THF, and CHCl 3 . Interestingly, CD and CPL activities of (R1) n and (S-1) n were observed in MCH and toluene, and the intensities were one or two orders of magnitude greater than those in THF and CHCl 3 . On the contrary, the helicity of (S-1) n in CHCl 3 was inverted as compared to that in THF, toluene, and MCH. Both the solvent polarity and concentration influenced the molecular assembly structure and helical π-stacking configuration of the intermolecular N-H•••O= hydrogen-bonding interactions of chiral R-1 and S-1. EXPERIMENTAL SECTION General. Commercially available chemical reagents and solvents were used as received without further purification. Dry triethylamine was prepared by distillation from KOH. (R)-3,7dimethyl-1-octylamine and (S)-3,7-dimethyl-1-octylamine were prepared following published procedures from the corresponding citronellal isomer.21 A Bruker Advance III 400 NMR spectrometer was used to record 1H NMR spectra, and chemical shifts (δ) were expressed in ppm relative to tetramethylsilane (1H, 0.00 ppm) as an internal standard. Mass spectra and elemental analyses were recorded on a JMS-700 spectrometer and Microcoder JM10, respectively. Infrared (IR, 400-4000 cm−1) spectra were measured using KBr pellets on a Thermo Fisher Scientific Nicolet 6700 spectrophotometer with a resolution of 4 cm−1. The morphology of S-1 on a mica substrate was observed by AFM using a JEOL SPM-5200, which was fabricated by the cast method on mica and HOPG substrates from MCH (1 × 10-4 M). Preparation of S-1. A suspension or solution of pyrene1,3,6,8-tetracarboxylic acid (1.01 g, 2.32 mmol) in thionyl chloride (30 mL) was heated at reflux for 4 h.17 Following the removal of excess thionyl chloride by atmospheric distillation, the resulting crude acid chloride was dissolved in dry CH 2 Cl 2 (40 mL). (S)-3,7-dimethyl-1-octylamine 22 (1.90 g, 11.6 mmol) followed by dry triethylamine (750 µL, 5.35 mmol) were then added to the suspension, which was stirred for 18 h at room temperature. The reaction mixture was diluted with CHCl 3 and

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water, and the aqueous layer was separated and extracted using CHCl 3 . The combined organic layers were washed with brine, and dried over MgSO 4 . After removal of the organic solvent under reduced pressure, the crude product was purified by silica gel column chromatography (CHCl 3 /EtOAc = 9/1). The combined fractions containing S-1 were evaporated, and the resulting solid was purified by recrystallization from DMF, followed by precipitation from CHCl 3 /CH 3 CN to give S-1 as a yellow powder (590 mg) in a yield of 24%. 1H NMR (400 MHz, CDCl 3 ) δ = 9.16 (br s, 6H), 7.00-6.00 (br s, 4H), 3.42 (br s, 10H), 2.00-0.50 (br s, 72H); IR (KBr, cm-1): 3427 (m), 3244 (s), 3070 (w), 2955 (s), 2927 (s), 2870 (s), 1630 (s), 1572 (s), 1550 (s), 1467 (w), 1366 (w), 1317 (w), 1288 (w); HR-MS(FAB) calcd. for C 60 H 95 N 4 O 4 [(M + H)+ ] 935.7353, found 935.7346; anal. calc. for C 60 H 94 N 4 O 4 C 77.04, H 10.13, N 5.99; found C 76.78, H 10.18, N 6.02. Preparation of R-1. A similar preparation procedure to that of S-1 was applied for R-1 using (R)-3,7-dimethyl-1-octylamine.22 Yellow powder (2.67 g, yield 40%). 1H NMR (400 MHz, CDCl 3 ) δ = 9.22 (br s, 6H), 6.50 (br s, 4H), 3.44 (br s, 10H), 2.00-0.80 (br s, 74H); IR (KBr, cm-1):3427 (m), 3244 (s), 3070 (w), 2955 (s), 2927 (s), 2870 (s), 1630 (s), 1572 (s), 1550 (s), 1467 (w), 1366 (w), 1317 (w), 1288 (w); HR-MS(FAB) calcd. for C 60 H 95 N 4 O 4 [(M+H)+] 935.7353, found 935.7359; anal. calc. for C 60 H 94 N 4 O 4 C 77.04, H 10.13, N 5.99; found C 76.55, H 10.14, N 5.93. Optical measurements. UV-vis, CD, fluorescence, and CPL spectra in CHCl 3 , THF, toluene, and MCH were measured using Perkin Elmer Lambda 750A, JASCO J-820, Shimazu RF6000, and JASCO CPL-300 spectrophotometers, respectively, using a quartz cell with an optical length of 10 mm. The excitation wavelength for fluorescence and CPL spectra was fixed at 315 nm. Time-dependent fluorescence spectra from 10 to 100 ns in 10 ns intervals after excitation were measured; excitation at 355 nm was generated by a Q-switched Nd:YAG laser with 5 ns and 10 Hz. Time-resolved fluorescence spectra were detected by streaks camera C4334 (Hamamatsu Photonics KK). Theoretical calculations. The intermolecular N-H•••O= hydrogen-bonded π-dimer structure of tetra(methyl)-1,3,6,8-pyrenetetracarboxamide was optimized by density functional theory (DFT) calculations with the M05/6-31g (d, p) basis-set.23 To simplify the calculations, the chiral 3,7-dimethyl-1-octylamide groups of R-1 or S-1 were replaced with –CONHCH 3 .21 RESULTS AND DISCUSSION Ground State 1D Helical Molecular Assemblies. Hydrogenbonded 1D helical molecular assemblies of (R-1) n and (S-1) n in the ground state were evaluated by UV-vis and CD in CHCl 3 , THF, toluene, and MCH in the c-range from 5 × 10-7 to 5 × 105 M. Table 1 summarizes the UV-vis and fluorescence peak maxima together with the quantum yields of S-1 in CHCl 3 , THF, toluene, and MCH. Figure 1a shows the UV-vis spectra of S-1 in CHCl 3 (black), THF (blue), toluene (green), and MCH (red) together with achiral 2 in CHCl 3 at c = 5x10-5 M. The absorption bands of S-1 at 278, 288, and 360 nm in CHCl 3 were consistent with those of achiral 2, suggesting that the electronic structure of the pyrene π-core was not modulated by the introduction of the chiral alkyl chains (Figure S1a). The π-π* transition band of S-1 at λ max (Abs) = 360 nm was red-shifted by approximately 20 nm compared to that of pyrene itself. The


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The Journal of Physical Chemistry introduction of electron-accepting alkylamide groups decreased the energy level of the LUMO, increased the energy level of the HOMO and decreased the HOMO-LUMO gap.17 The vibronic bands of the UV-vis spectra appeared around 360 nm in THF and CHCl 3 , whereas they were suppressed in toluene and MCH. The magnitude of the molar absorption coefficient (ε) at c = 1 × 10-5 M decreased in the order of THF > CHCl 3 > MCH ~ toluene, suggesting different molecular assembly structures of (S-1) n in the various solvents. b)

a)

Figure 1. Solution phase UV-vis and IR spectra of S-1. a) UV-vis spectra in THF (blue), CHCl 3 (black), MCH (red), and toluene (green) and that of 2 in CHCl 3 (dashed line) at c = 5x10-5 M. b) IR spectra in CHCl 3 at c = 1x10-3 (blue), 5x10-3 (green), and 1x10-2 M (red).

Table 1. Solvent dependent absorption and fluorescence peak maxima and quantum yield of S-1. Solv.

c,

UV-vis, nm

Fluorescence, nm

Quant. yield

µM CHCl 3

50 1

277,287,360 278,289,355,370

398,415,494 397,414,482

0.42

THF

50 1

277,288,358,368 279,287,352,370

395,413,495 394, 413

0.31

Toluene

50 1

275,364 278,287,352,364

400, 515 400, 513

0.26

MCH

50 1

275,364 278,287,352,370

385, 515 374, 515

0.29

The formation of the N-H•••O= hydrogen-bonded 1D molecular assembly of (S-1) n in the ground state was also confirmed by the solution phase vibrational spectra in CHCl 3 (Figure 1b). An asymmetric N-H stretching band (νa N-H ) of the isolated S-1 and hydrogen-bonding assembly of (S-1) n in CHCl 3 were observed at 3450 and 3250 cm-1, respectively, at c = 1 × 10-2 M. The νa N-H band at 3261 cm-1 at c = 1 × 10-3 M was similar to that at 3250 cm1 at c = 1 × 10-2 M despite the difference in concentration, suggesting similar aggregation structures of (S-1) n at both concentrations. The intermolecular hydrogen-bonded 1D molecular assemblies of (S-1) n were stable under the measurement conditions of c > 1 × 103 M. The hydrogen-bonded 1D helical molecular assemblies of (S1) n and (R-1) n in the ground state were also evaluated using solvent-dependent (CHCl 3 , THF, and MCH) CD spectra (Figure 2). The magnitude of exciton coupling in the hydrogen-bonded, πstacked configurations can be qualitatively evaluated from the magnitude of the g-value, which is defined by g = ∆ε /ε.24, 25 Both the π-stacking distance and twisted angle of the pyrene dimer affect the g-value; relatively large g-values have been observed in effective π-stacking interactions. The magnitude of the CD signals and Cotton effects of (R-1) n and (S-1) n were confirmed in CHCl 3 , THF, and MCH at c = 5x10-5 M. Notably, a strong CD signal with a

rotary angle (θ) over 100 mdeg with g = 3.4 × 10-3 at 352 nm was observed in MCH. On the contrary, the CD signals in THF and CHCl 3 (θ ~ 5 mdeg) were approximately 20 times smaller than that in MCH. Interestingly, the CD signature of (S-1) n in CHCl 3 was inverted as compared to those in THF and MCH (Figure 2b), due to the difference in the hydrogen-bonded π-stacked configuration owing to differences in the solvation effects on the hydrogenbonded 1D helical molecular assemblies. The same solvent-dependent helicity inversion was observed for R-1 in CHCl 3 (Figure 2b). The π-stacked configuration of the N-H•••O= hydrogenbonded molecular assemblies of (R-1) n and (S-1) n depended on the solvent polarity and concentration.26

The most stable N-H•••O= hydrogen-bonded π-dimer of a pyrene derivative bearing four –CONHCH 3 groups configuration was evaluated by DFT calculations using the M05/6-31g(d, p) basis-set, resulting in a 40°-twisted π-stacking configuration with an intermolecular hydrogen-bonding N•••N distance of d NN = 2.93 Å and an average π-stacking distance of 3.70 Å (Figure 3 and Figure S2). The similar hydrogen-bonding 1D molecular assembly found in achiral 2 suggests the formation of similar hydrogen-bonding helical 1D molecular assemblies of (S-1) n and (R-1) n in the solution phase. From the 40° twisted π-stacking dimer configuration, nine hydrogen-bonded molecular assemblies of (S-1) 9 and (R-1) 9 form one helical pitch. These assembly structures are consistent with the MD simulation of pyrenetetracarboxamide.27 Excited State Helical Molecular Assemblies. The fluorescence spectra of R-1 and S-1 in CHCl 3 were similar with emission maxima around 500 nm, Stokes-shifts of 139 nm, and quantum yields of 0.42 (Figure 4 and Figure S1). The formation of similar hydrogen-bonded 1D helical molecular assemblies of (R-1)* n and (S-1)* n in the excited state to those of (R-1) n and (S-1) n in the ground state was also assumed. Solvent (CHCl 3 , THF, toluene, and MCH) and c-dependent (5 × 10-7 ~ 1 × 10-4 M) fluorescence spectra of R-1 and S-1 were measured to shed light on the molecular assemblies in the excited state. Figures 4a and 4b show the c-dependent fluorescence spectra of S-1 in CHCl 3 and MCH, respectively. The fluorescence spectra of S1 in CHCl 3 and THF were similar, suggesting similar hydrogenbonded 1D molecular assembly structures in the excited state, whereas those in MCH and toluene also indicated similar c-dependent spectral changes of (S-1) ∞ . The excimer bands in MCH and toluene showed a clear red-shift as the concentration increased, whereas those in CHCl 3 and THF were independent of the measuring concentration. The red-shift in the excimer band in MCH and toluene was saturated due to the formation of the hydrogen-bonded 1D molecular assemblies of (S-1) ∞ (Figure S5). The monomer emission energy at 400 nm and excimer emission at 500 nm were dominant under dilute and concentrated conditions, and formed an isosbestic point at 455 nm. The minimum c-value revealing the excimer emission is defined as cex min . For instance, the cex min values of S-1 in THF and CHCl 3 were cex min = 5 × 10-5 and 1 × 10-6 M, respectively, whereas the cex min values in MCH and toluene were less than 5x10-7 M. On the contrary, high-c conditions around 1 × 10-4 M in THF resulted in the monomer blue emission in the absence of the excimer emission (Figure S5). The hydrogen-bonding



a)

b)

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c)

Figure 2. Hydrogen-bonding 1D helical molecular assemblies of (R-1)n and (S-1)n in the ground state. The g-values (upper), CD spectra (middle), and absorption spectra (lower) in a) MCH, b) CHCl3, and c) THF. Dashed and solid lines are spectra of R-1 and S-1, respectively.

1D molecular assemblies of (R-1)* n and (S-1)* n in the excited state were affected by solvent polarity and concentration, similar to the case in the ground state.

excited state, which can be evaluated by life time (τ) measurements of monomer S-1* and dimer (S-1)* 2 or oligomer (S-1)* n and its relaxation process (Figure 4c and Figure S6). Achiral molecule 2 in CHCl 3 showed a c-independent monomer emission with τ = 4 nsec, whereas the excimer emission was observed after 10 nsec. a)

Figure 3. Optimized hydrogen-bonding π-dimer configuration of pyrene derivative bearing four –CONHCH 3 chains viewed along the parallel to the π-plane (left) and along the normal to the π-plane (right).

The excimer emission band of S-1 in CHCl 3 and THF redshifted upon increasing the concentration up to 1x10-4 M, which was also observed with achiral 2, although pyrene itself did not exhibit such shift.28 The introduction of alkylamide-groups onto the pyrene π-core results in effective intermolecular hydrogen-bonded 1D molecular assemblies of (R-1)* n and (S-1)* n in the excited sate, and increasing the c-value enhanced the association number n and causes a red-shift in the emission band. The c-dependent red-shift in the excimer band was not observed in MCH or toluene, suggesting that oligomeric (S-1)* n is formed in the excited state to saturate the red-shift of the emission band. Increases in the c-value of S-1 in solution gradually enhanced the association number of n from the monomer (n = 1) and dimer (n = 2) to the oligomer (n > 2) with an eventual saturation in the energy shift of the excimer band. The association number n of hydrogen-bonded 1D molecular assemblies of (S-1) n in the ground state and (S-1)* n in the excited state in THF should be lower than those in MCH and toluene. There are two kinds of emission mechanisms for the excimer: from dimer (S-1)* 2 or from oligomer (S-1)* n in the

c)

b)

d)

Figure 4. Emission behavior of S-1. Fluorescence spectra of S-1 in a) CHCl 3 and b) MCH at c-range from 5 × 10-7 to 1 × 10-4 M. c) Time-dependent fluorescence spectra of S-1 in THF from 0 to 85 nsec. d) Photograph of solution phase S-1 at c = 1 × 10-4 M in CHCl 3 , MCH, THF, and toluene (left to right) under visible (upper) and UV-light (lower).

The excimer emission band of dimer (S-1)* 2 or oligomer (S1)* n in THF can be observed at cex min = 1 × 10-4 M, in which an assuming homogeneous distribution of molecule of one S-1 molecule is approximately 1.66 × 104 nm3 with a sphere diameter of r = 15.8 nm. When the excited state S-1* molecule is randomly diffusing in THF, the Brownian motion length after 4 ns is approximately 1.24 nm,29 which is more than ten times smaller than the sphere diameter of r = 15.8 nm. Since the collision probability of isolated S-1* with randomly occupied


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The Journal of Physical Chemistry neighboring S-1 to form dimeric (S-1)* 2 is low, the excimer emission at 500 nm occurs from the hydrogen-bonding oligomer (S-1)* n and/or dimer (S-1)* 2 , rather than the monomer (S1)*. The hydrogen-bonded (S-1)* n oligomer in solution is consistent with the spectroscopic data. Although the association number n of (S-1) n in the ground state in THF is lower than those in MCH and toluene, the excimer emission was observed at c > 5 × 10-5 M. Excited State Helical Molecular Assemblies. The fluorescence spectra of R-1 and S-1 in CHCl 3 were similar with emission maxima around 500 nm, Stokes-shifts of 139 nm, and quantum yields of 0.42 (Figure 4 and Figure S1). The formation of similar hydrogen-bonded 1D helical molecular assemblies of (R-1)* n and (S-1)* n in the excited state to those of (R-1) n and (S-1) n in the ground state was also assumed. Solvent (CHCl 3 , THF, toluene, and MCH) and c-dependent (5 × 10-7 ~ 1 × 10-4 M) fluorescence spectra of R-1 and S-1 were measured to shed light on the molecular assemblies in the excited state. Figures 4a and 4b show the c-dependent fluorescence spectra of S-1 in CHCl 3 and MCH, respectively. The fluorescence spectra of S1 in CHCl 3 and THF were similar, suggesting similar hydrogenbonded 1D molecular assembly structures in the excited state, whereas those in MCH and toluene also indicated similar c-dependent spectral changes of (S-1) ∞ . The excimer bands in MCH and toluene showed a clear red-shift as the concentration increased, whereas those in CHCl 3 and THF were independent of the measuring concentration. The red-shift in the excimer band in MCH and toluene was saturated due to the formation of the hydrogen-bonded 1D molecular assemblies of (S-1) ∞ (Figure S5). The monomer emission energy at 400 nm and excimer emission at 500 nm were dominant under dilute and concentrated conditions, and formed an isosbestic point at 455 nm. The minimum c-value revealing the excimer emission is defined as cex min . For instance, the cex min values of S-1 in THF and CHCl 3 were cex min = 5 × 10-5 and 1 × 10-6 M, respectively, whereas the cex min values in MCH and toluene were less than 5x10-7 M. On the contrary, high-c conditions around 1 × 10-4 a)

1D molecular assemblies of (R-1)* n and (S-1)* n in the excited state were affected by solvent polarity and concentration, similar to the case in the ground state. The hydrogen-bonded helical 1D molecular assemblies of (R-1)* n and (S-1)* n in the excited state were evaluated by CPL. There have been several reports regarding CPL-active pyrene derivatives.30-33 For instance, the CPL of pyrene-fused RNA and helical-polymers has been examined in solution.25 However, there are no reports on the CPL of hydrogen-bonded helical supramolecular assemblies of pyrene derivatives. Figure 5 summarizes the solvent- and c-dependent CPL spectra of the hydrogen-bonded 1D molecular assemblies of (R-1)* n and (S-1)* n in the excited state. The magnitude of the CPL response can be evaluated using g lum (= 2∆I / I). Although the CPL of the monomer around 400 nm was negligible, the CPL of (R-1)* n and (S1)* n revealed an excimer band at 500 nm in CHCl 3 , THF, and MCH with c = 1 × 10-4 M, highlighting the emission process from the N-H•••O= hydrogen-bonded 1D helical π-stacked molecular assemblies of (R-1)* n and (S-1)* n . The g lum values in CHCl 3 and THF (g lum ~ 10-3) were one order of magnitude lower than that in MCH (g lum ~ 3.0 × 10-2); notably, the latter value is quite large34, 35 and is slightly larger than that of the pyrene-fused RNA polymer (g lum = 2 × 10-2).25 The hydrogenbonded 1D helical molecular assemblies of (R-1)* n and (S-1)* n can stably exist even in solution. Recently, Isobe et al. reported extremely large g lum value in the ring-type organic π-molecules.36 On the contrary, large g lum in hydrogen-bonding R-1 and S-1 was observed in quite simple molecular structure through the chiral molecular aggregation. Interestingly, the CPL signals of (R-1)* n and (S-1)* n in CHCl 3 were inverted as compared to those in THF and MCH, similar to their CD spectra. Chiral S-1 and R-1 represent the first hydrogen-bonding supramolecular system with solvent-dependent CPL inversion. The solvent dependent CD activation of organic molecules were known, 37-39 but solvent dependent CPL activation of only organic molecules were not reported. From this point of view, the large and inverted g lum values of S-1 and R-1 in MCH is the very

b)

c)

Figure 5. Hydrogen-bonded 1D helical molecular assemblies of excited state R-1 and S-1. The glum values (upper), CPL spectra (middle), and fluorescence spectra (lower) of R-1 (dashed-line) and S-1 (solid-line) in a) MCH, b) CHCl3, and c) THF.

M in THF resulted in the monomer blue emission in the absence of the excimer emission (Figure S5). The hydrogen-bonding

intersecting result. In addition, the quantum yields of R-1 and S-1 in CHCl 3 , MCH, toluene, and THF were 0.42, 0.29, 0.26,



and 0.31, respectively, and the hydrogen-bonded molecular assemblies in CHCl 3 showed relatively high quantum yields. 1D Molecular Assemblies. Although achiral molecule 2 can form a hydrogen-bonded 1D molecular assembly in an organogel and Col h mesophase, the Col h phase was not observed in chiral R-1 and S-1 until the decomposition temperature. However, organogels of R-1 and S-1 were observed at c > 5.7 × 10-2 M in MCH and toluene, where the association number n of (R1) n and (S-1) n was large enough to form mesoscale fibrous molecular assemblies. Figure 6 summarizes the organogels and xerogels of S-1. Organogels were not observed in THF and CHCl 3 , suggesting that the growth of hydrogen-bonded 1D molecular assemblies was insufficient to form bulk fibrous assemblies with adequate association numbers. 1D molecular assemblies in MCH and/or toluene were further entangled to form three-dimensional pores to capture solvents in organogels. Figures 6b and 6c show the surface morphologies of (S-1) n nanofibers on hydrophilic mica and hydrophobic HOPG substrates, respectively. A nanofiber film of S-1 was fabricated using a cast-method in MCH. A network structure of (S-1) n nanofibers with a typical height of 2.5 nm and width of 300 nm was observed on the mica surface, whereas nanofibers with a typical height of 20 nm and width of 300 nm were densely occupied on the HOPG surface. Since the surfaces of hydrogen-bonded 1D molecular assemblies of (R-1) n and (S-1) n are covered by hydrophobic alkylamide-chains, the dewetting of nanofiber surface on the mica surface resulted in the dispersion of the hydrophobic nanofibers. a)

b)

c)

MCH, toluene, CHCl 3 , and THF are ~0, 0.37, 1.04, and 1.74 Debye, respectively.40 Hydrogen-bonding supramolecular assemblies are generally stabilized by non-polar solvents in the absence of effective intermolecular hydrogen-bonding interactions, which stabilize the associations of (R-1) n and (S-1) n . The helical 1D molecular assemblies grow when solvents with low dipole moments such as MCH and toluene are utilized for the organogelation. On the contrary, polar solvents like THF and CHCl 3 effectively interact with the polar N-H•••O= hydrogenbonding sites, which disrupt the hydrogen-bonding 1D molecular assembly through solvation. Unfortunately, the reason for helicity inversion in CHCl 3 remains to be elucidated, but may be associated with the structural stabilization through intermolecular N-H•••Cl interactions.26, 34, 35 CONCLUSIONS Hydrogen-bonding chiral pyrene derivatives S-1 and R-1 bearing (R)-3,7-dimethyl-1-octylamide and (S)-3,7-dimethyl-1-octylamide chains were prepared to evaluate their intermolecular hydrogen-bonding 1D helical supramolecular assemblies in the ground and excited states in solution. Solvent- and concentration-dependent absorption, CD, fluorescence, and CPL spectra revealed differences in the association number n and π-stacking configuration of (R-1) n and (S-1) n supramolecules due to solvation effects. The intermolecular N-H•••O= hydrogen-bonded 1D helical molecular assemblies of (R-1) n and (S-1) n in the ground state were confirmed by CD, and π-stacked molecular assemblies exhibited pronounced CD signatures with relatively large exciton coupling in MCH and toluene. On the contrary, the fluorescence spectra of R-1 and S-1 in THF and CHCl 3 showed c-dependent contentious spectral changes between monomer and excimer emission, where increases in the c-value caused a red-shift in the excimer band. Especially in THF, the N-H•••O= hydrogen-bonded helical 1D molecular assembly was destabilized by the partial dissociation of the hydrogenbonded molecular assembly structures due to the effective solvation of polar THF. In MCH, relatively large association numbers n and well-defined assembly structures of (R-1)* n and (S1)* n in the excited state were observed at g lum ~ 0.03, and the helicities in CHCl 3 were inverted as compared to those in THF, toluene, and MCH based on the rotational directions of the CD and CPL spectra. Based on theoretical calculations, the 40°twisted π-stacked configuration revealed the formation of (R1) 9 and (S-1) 9 units for one pitch of the 1D helical molecular assemblies. The solvent polarity and concentration of R-1 and S-1 can effectively modify the hydrogen-bonding 1D helical molecular assemblies and CPL. Simple pyrene derivatives with g lum > 10-2 may have potential applications in 3D display devices. Helical π-stacking molecular assemblies can form fluorescent nanofibers and helical carrier transport systems.

ASSOCIATED CONTENT Figure 6. Formation of organogels and nanofibers comprised of hydrogen-bonded 1D molecular assemblies of (S-1) n . a) Organogel in MCH at c = 6.1 × 10-2 M under of visible- (left) and UV-light (right). AFM images (10 × 10 µm2) of nanofibers on b) mica and c) HOPG substrates. Each film was fabricated using a cast-method from MCH at c = 1 × 10-4 M.

The growth of N-H•••O= hydrogen-bonded helical 1D molecular assemblies of (R-1) n and (S-1) n depends on the solvent polarity and concentration. The permanent dipole moments of

Supporting Information. Preparation of R-1, solution phase UV-vis spectra, fluorescence spectra of pyrene, R-1, and S-1 in CHCl 3 , solution phase IR spectra of S-1, optimized π-stacking dimer configuration of pyrene derivative, concentration-dependent UV-vis spectra of S-1, concentration-dependent UV-vis spectra of R-1, concentration-dependent fluorescence spectra of R-1, fluorescence of S-1 in THF, and temperature-dependent fluorescence spectra of S-1. This material is available free of charge via the Internet at http://pubs.acs.org.


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The Journal of Physical Chemistry

AUTHOR INFORMATION Corresponding Author * [email protected] and [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “π-Figuration” (JP26102007), KAKENHI Kibankenkyu (B) (JP15H03791), JSPS Research Fellow (16J03265), and “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from MEXT.

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