Circular Polarized Luminescence of Hydrogen ... - ACS Publications

Article Cite This: J. Phys. Chem. C 2018, 122, 6323−6331

pubs.acs.org/JPCC

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 § Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: The absorption and fluorescence spectra of chiral alkylamide-substituted pyrene derivatives (R-1 and S-1) in the solution phase were consistent with the formation of N−H···O hydrogenbonded helical π-stacked one-dimensional (1D) supramolecules (R-1)n and (S-1)n in methylcyclohexane (MCH), toluene, chloroform (CHCl3), and tetrahydrofuran (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 CHCl3, 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 glum value of 0.03 in MCH at c > 1 × 10−6 M. Circular dichroism 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 CHCl3 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.

■

(Colh) mesophases.10−14 Interestingly, a ferroelectric response in the polarization−electric field (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 −CONHCnH2n+1 groups on benzene.15,16 Introduction of phenylamide (−CONHPh) groups onto pyrene π-cores also forms 1D N−H···O hydrogenbonded 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 types of π-cores may be useful in soft molecular assemblies such as nanofibers, organogels, liquid crystals, and so forth, which are often utilized for functional organic materials with ferroelectric and chromic behaviors.

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 hydrogenbonded 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 β-sheet 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 hydrogen bonding in supramolecular assemblies may facilitate the fabrication of novel molecular machines and multifunctional materials. A variety of hydrogen-bonded molecular assemblies have been applied in functional molecular materials.2,3 For instance, simple alkylamide (−CONHCnH2n+1)-substituted benzene derivatives can form one-dimensional (1D) N−H···O hydrogen-bonded π-stacked columns, which further assemble into nanofibers, organogels, and discotic hexagonal columnar © 2018 American Chemical Society

Received: December 27, 2017 Revised: February 5, 2018 Published: February 27, 2018 6323

DOI: 10.1021/acs.jpcc.7b12747 J. Phys. Chem. C 2018, 122, 6323−6331

Article

The Journal of Physical Chemistry C

configuration of the intermolecular N−H···O hydrogenbonding interactions of chiral R-1 and S-1.

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.14,19−21 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 Colh phases in −CONHC14H29-substituted pyrene derivative (2) by the aid of effective π-stacking interactions using an extended π-core, 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 CHCl3 appeared was 1000 times higher than that of pyrene itself because of 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 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).

■

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.22 A Bruker Advance III 400 NMR spectrometer was used to record 1H NMR spectra, and chemical shifts (δ) were expressed in parts per million relative to tetramethylsilane (1H, 0.00 ppm) as an internal standard. Mass spectra and elemental analyses were recorded on a JMS700 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 atomic force microscopy (AFM) using JEOL SPM-5200, which was fabricated by the cast method on mica and highly oriented pyrolytic graphite (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 CH2Cl2 (40 mL). (S)-3,7-Dimethyl-1-octylamine22 (1.90 g, 11.6 mmol) followed by dry triethylamine (750 μL, 5.35 mmol) was then added to the suspension, which was stirred for 18 h at room temperature. The reaction mixture was diluted with CHCl3 and water, and the aqueous layer was separated and extracted using CHCl3. The combined organic layers were washed with brine and dried over MgSO4. After removal of the organic solvent under reduced pressure, the crude product was purified by silica gel column chromatography (CHCl3/EtOAc = 9/1). The combined fractions containing S-1 were evaporated, and the resulting solid was purified by recrystallization from dimethylformamide, followed by precipitation from CHCl3/CH3CN to give S-1 as a yellow powder (590 mg) in a yield of 24%. 1H NMR (400 MHz, CDCl3): 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 C60H95N4O4 [(M + H)+] 935.7353; found, 935.7346; Anal. Calcd. for C60H94N4O4: C, 77.04; H, 10.13; N, 5.99. Found: C, 76.78; H, 10.18; N, 6.02. Preparation of R-1. A preparation procedure similar 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, CDCl3): 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); HRMS(FAB) calcd for C60H95N4O4 [(M + H)+], 935.7353; found, 935.7359; Anal. Calcd. for C60H94N4O4: 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 CHCl3, THF, toluene, and MCH were measured using PerkinElmer Lambda 750A, JASCO J-820, Shimadzu 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

Scheme 1. Molecular Structures of Chiral AlkylamideSubstituted 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, 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, tetrahydrofuran (THF), and chloroform (CHCl3). Interestingly, CD and CPL activities of (R-1)n and (S-1)n were observed in MCH and toluene, and the intensities were 1 or 2 orders of magnitude greater than those in THF and CHCl3. On the contrary, the helicity of (S-1)n in CHCl3 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 6324


Article

The Journal of Physical Chemistry C 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 streak camera C4334 (Hamamatsu Photonics KK). Theoretical Calculations. The intermolecular N−H···O hydrogen-bonded π-dimer structure of tetra(methyl)-1,3,6,8pyrenetetracarboxamide 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-1octylamide groups of R-1 or S-1 were replaced with −CONHCH3.21

unoccupied molecular orbital (LUMO), increased the energy level of the highest occupied molecular orbital (HOMO), and decreased the HOMO−LUMO gap.17 The vibronic bands of the UV−vis spectra appeared at around 360 nm in THF and CHCl3, 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 > CHCl3 > MCH ≈ toluene, suggesting different molecular assembly structures of (S-1)n in the various solvents. 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 CHCl3 (Figure 1b). An asymmetric N−H stretching band (νaN−H) of the isolated S-1 and hydrogen-bonded assembly of (S-1)n in CHCl3 were observed at 3450 and 3250 cm−1, respectively, at c = 1 × 10−2 M. The νaN−H band at 3261 cm−1 at c = 1 × 10−3 M was similar to that at 3250 cm−1 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 × 10−3 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 (CHCl3, THF, and MCH) CD spectra (Figure 2). The magnitude of exciton coupling in the hydrogen-bonded π-stacked configurations can be qualitatively evaluated from the magnitude of the g-value, which is defined by g = Δε/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 CHCl3, THF, and MCH at c = 5 × 10−5 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 CHCl3 (θ ≈ 5 mdeg) were approximately 20 times smaller than that in MCH. Interestingly, the CD signature of (S-1)n in CHCl3 was inverted as compared to those in THF and MCH (Figure 2b) because of the difference in the hydrogen-bonded π-stacked configuration owing to differences in the solvation effects on the hydrogen-bonded 1D helical molecular assemblies. The same solvent-dependent helicity inversion was observed for R-1 in CHCl3 (Figure 2b). The π-stacked configuration of the N− H···O hydrogen-bonded molecular assemblies of (R-1)n and (S-1)n depended on the solvent polarity and concentration.26

■

RESULTS AND DISCUSSION Ground-State 1D Helical Molecular Assemblies. Hydrogen-bonded 1D helical molecular assemblies of (R-1)n and (S-1)n in the ground state were evaluated by UV−vis and CD in CHCl3, THF, toluene, and MCH in the c-range from 5 × 10−7 to 5 × 10−5 M. Table 1 summarizes the UV−vis and Table 1. Solvent-Dependent Absorption and Fluorescence Peak Maxima and Quantum Yield of S-1 solv.

c, μM

CHCl3

50 1 50 1 50 1 50 1

THF toluene MCH

UV−vis, nm 277, 278, 277, 279, 275, 278, 275, 278,

287, 289, 288, 287, 364 287, 364 287,

360 355, 370 358, 368 352, 370 352, 364 352, 370

fluorescence, nm 398, 397, 395, 394, 400, 400, 385, 374,

415, 494 414, 482 413, 495 413 515 513 515 515

quant. yield 0.42 0.31 0.26

0.29

fluorescence peak maxima together with the quantum yields of S-1 in CHCl3, THF, toluene, and MCH. Figure 1a shows the UV−vis spectra of S-1 in CHCl3 (black), THF (blue), toluene (green), and MCH (red) together with achiral 2 in CHCl3 at c = 5 × 10−5 M. The absorption bands of S-1 at 278, 288, and 360 nm in CHCl3 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 λmax (Abs) = 360 nm was red-shifted by approximately 20 nm compared to that of pyrene itself. The introduction of electron-accepting alkylamide groups decreased the energy level of the lowest

Figure 1. Solution-phase UV−vis and IR spectra of S-1. (a) UV−vis spectra in THF (blue), CHCl3 (black), MCH (red), and toluene (green) and that of 2 in CHCl3 (dashed line) at c = 5 × 10−5 M. (b) IR spectra in CHCl3 at c = 1 × 10−3 (blue), 5 × 10−3 (green), and 1 × 10−2 M (red). 6325


Article


Figure 2. Hydrogen-bonded 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, respectively. Dashed and solid lines are spectra of R-1 and S-1, respectively.

excited state. Figure 4a,b shows the c-dependent fluorescence spectra of S-1 in CHCl3 and MCH, respectively. The fluorescence spectra of S-1 in CHCl3 and THF were similar, suggesting similar hydrogen-bonded 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 CHCl3 and THF were independent of the measuring concentration. The red shift in the excimer band in MCH and toluene was saturated because of 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 ex is defined as cex min. For instance, the cmin values of S-1 in THF ex −5 and CHCl3 were cmin = 5 × 10 and 1 × 10−6 M, respectively, whereas the cex min values in MCH and toluene were less than 5 × 10−7 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-bonded 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 excimer emission band of S-1 in CHCl3 and THF redshifted upon increasing the concentration up to 1 × 10−4 M, which was also observed with achiral 2, although pyrene itself did not exhibit such a 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 S1 in solution gradually enhanced the association number of n from the monomer (n = 1) and dimer (n = 2) to the oligomer

The most stable N−H···O hydrogen-bonded π-dimer of a pyrene derivative bearing four −CONHCH3 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 dN−N = 2.93 Å and an average π-stacking distance of 3.70 Å (Figures 3 and S2). The similar hydrogen-bonded 1D

Figure 3. Optimized hydrogen-bonding π-dimer configuration of a pyrene derivative bearing four −CONHCH3 chains viewed along the parallel to the π-plane (left) and along the normal to the π-plane (right).

molecular assembly found in achiral 2 suggests the formation of similar hydrogen-bonded helical 1D molecular assemblies of (S1)n and (R-1)n in the solution phase. From the 40°-twisted πstacking 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 molecular dynamics simulation of pyrenetetracarboxamide.27 Excited-State Helical Molecular Assemblies. The fluorescence spectra of R-1 and S-1 in CHCl3 were similar with emission maxima of around 500 nm, Stokes shifts of 139 nm, and quantum yields of 0.42 (Figures 4 and S1). The formation of hydrogen-bonded 1D helical molecular assemblies of (R-1)n* and (S-1)n* in the excited state similar to those of (R1)n and (S-1)n in the ground state was also assumed. Solvent (CHCl3, THF, toluene, and MCH) and c-dependent (5 × 10−7 to 1 × 10−4 M) fluorescence spectra of R-1 and S-1 were measured to shed light on the molecular assemblies in the 6326


Article


Figure 4. Emission behavior of S-1. Fluorescence spectra of S-1 in (a) CHCl3 and (b) MCH at c-range from 5 × 10−7 to 1 × 10−4 M. (c) Timedependent 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 CHCl3, MCH, THF, and toluene (left to right) under visible (upper) and UV light (lower).

nm, and quantum yields of 0.42 (Figures 4 and S1). The formation of hydrogen-bonded 1D helical molecular assemblies of (R-1)*n and (S-1)*n in the excited state similar to those of (R1)n and (S-1)n in the ground state was also assumed. Solvent (CHCl3, THF, toluene, and MCH) and c-dependent (5 × 10−7 to 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. Figure 4a,b shows the c-dependent fluorescence spectra of S-1 in CHCl3 and MCH, respectively. The fluorescence spectra of S-1 in CHCl3 and THF were similar, suggesting similar hydrogen-bonded 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 CHCl3 and THF were independent of the measuring concentration. The red shift in the excimer band in MCH and toluene was saturated because of the formation of the hydrogen-bonded 1D molecular assemblies of (S-1)∞ (Figure S5). The monomer emission energy at 400 nm and the 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 ex is defined as cex min. For instance, the cmin values of S-1 in THF ex −5 and CHCl3 were cmin = 5 × 10 and 1 × 10−6 M, respectively, whereas the cex min values in MCH and toluene were less than 5 × 10−7 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-bonded 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.

(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)*2 in the excited state, which can be evaluated by lifetime (τ) measurements of monomer S-1* and dimer (S-1)2* or oligomer (S-1)n* and its relaxation process (Figures 4c and S6). Achiral molecule 2 in CHCl3 showed a c-independent monomer emission with τ = 4 ns, whereas the excimer emission was observed after 10 ns. The excimer emission band of dimer (S-1)2* or oligomer (S−4 M, in which an 1)n* in THF can be observed at cex min = 1 × 10 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 10 times smaller than the sphere diameter of r = 15.8 nm. Because the collision probability of isolated S-1* with randomly occupied neighboring S-1 to form dimeric (S-1)2* is low, the excimer emission at 500 nm occurs from the hydrogen-bonded oligomer (S-1)n* and/or dimer (S-1)2*, rather than the monomer (S-1)*. 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 CHCl3 were similar with emission maxima of around 500 nm, Stokes shifts of 139 6327


Article


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.

hydrogen-bonded molecular assemblies in CHCl3 showed relatively high quantum yields. 1D Molecular Assemblies. Although achiral molecule 2 can form a hydrogen-bonded 1D molecular assembly in an organogel and Colh mesophase, the Colh 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 (R-1)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 CHCl3, 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. Figure 6b,c shows 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 a width of 300 nm was observed on the mica surface, whereas nanofibers with a typical height of 20 nm and a width of 300 nm were densely occupied on the HOPG surface. Because 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 the nanofiber surface on the mica surface resulted in the dispersion of the hydrophobic nanofibers. 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 MCH, toluene, CHCl3, and THF are ∼0, 0.37, 1.04, and 1.74 Debye, respectively.40 Hydrogen-bonding supramolecular assemblies are generally stabilized by nonpolar solvents in the absence of effective intermolecular hydrogenbonding interactions, which stabilize the associations of (R-1)n and (S-1)n. The helical 1D molecular assemblies grow when

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 hydrogenbonded 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 glum (= 2ΔI/I). Although the CPL of the monomer around 400 nm was negligible, the CPL of (R-1)*n and (S-1)*n revealed an excimer band at 500 nm in CHCl3, 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 glum values in CHCl3 and THF (glum ≈ 10−3) were 1 order of magnitude lower than that in MCH (glum ≈ 3.0 × 10−2); notably, the latter value is quite large34,35 and is slightly larger than that of the pyrene-fused RNA polymer (glum = 2 × 10−2).25 The hydrogen-bonded 1D helical molecular assemblies of (R1)n* and (S-1)n* can stably exist even in solution. Recently, Isobe et al. reported extremely large glum value in the ring-type organic π-molecules.36 On the contrary, large glum in hydrogenbonding R-1 and S-1 was observed in quite a simple molecular structure through the chiral molecular aggregation. Interestingly, the CPL signals of (R-1)*n and (S-1)*n in CHCl3 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 was known,37−39 but the solvent-dependent CPL activation of only organic molecules was not reported. From this point of view, the large and inverted glum values of S-1 and R-1 in MCH provide a very intersecting result. In addition, the quantum yields of R-1 and S-1 in CHCl3, MCH, toluene, and THF were 0.42, 0.29, 0.26, and 0.31, respectively, and the 6328


Article


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 hydrogen-bonded molecular assembly structures because of the effective solvation of polar THF. In MCH, relatively large association numbers n and well-defined assembly structures of (R-1)n* and (S-1)n* in the excited state were observed at glum ≈ 0.03, and the helicities in CHCl3 were inverted as compared to those in THF, toluene, and MCH based on the rotational directions of the CD and CPL spectra. On the basis of theoretical calculations, the 40°-twisted πstacked configuration revealed the formation of (R-1)9 and (S1)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 glum > 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

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12747. Preparation of R-1; solution-phase UV−vis spectra and fluorescence spectra of pyrene, R-1, and S-1 in CHCl3; solution-phase IR spectra of S-1; optimized π-stacking dimer configuration of a pyrene derivative; concentration-dependent UV−vis spectra of S-1 and R-1; concentration-dependent fluorescence spectra of R-1; fluorescence spectra of S-1 in THF; and temperaturedependent fluorescence spectra of S-1 (PDF)

Figure 6. Formation of organogels and nanofibers comprising hydrogen-bonded 1D molecular assemblies of (S-1)n. (a) Organogel in MCH at c = 6.1 × 10−2 M under visiblelight (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.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.T.). *E-mail: [email protected] (T.A.).

solvents with low dipole moments such as MCH and toluene are utilized for the organogelation. On the contrary, polar solvents such as THF and CHCl3 effectively interact with the polar N−H···O hydrogen-bonding sites, which disrupt the hydrogen-bonded 1D molecular assembly through solvation. Unfortunately, the reason for helicity inversion in CHCl3 remains to be elucidated but may be associated with the structural stabilization through intermolecular N−H···Cl interactions.26,34,35

ORCID

Shunsuke Yamamoto: 0000-0002-6854-2477 Masaya Mitsuishi: 0000-0002-7069-9860 Tomoki Ogoshi: 0000-0002-4464-0347 Tomoyuki Akutagawa: 0000-0003-3040-1078 Notes

■

The authors declare no competing financial interest.

■

CONCLUSIONS Hydrogen-bonded 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 because of 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 πstacked 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 S1 in THF and CHCl3 showed c-dependent contentious spectral

ACKNOWLEDGMENTS 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.

■

REFERENCES

(1) An Introduction to Hydrogen Bonding; Jeffrey, G. A.; Oxford University Press, 1997. (2) Mattia, E.; Otto, S. Supramolecular systems chemistry. Nat. Nanotechnol. 2015, 10, 111−119. (3) Tayi, A. S.; Kaeser, A.; Matsumoto, M.; Aida, T.; Stupp, S. I. Supramolecular ferroelectrics. Nat. Chem. 2015, 7, 281−294. 6329


Article


(22) Koeckelberghs, G.; De Cremer, L.; Vanormelingen, W.; Dehaen, W.; Verbiest, T.; Persoons, A.; Samyn, C. Improved synthesis of Nalkyl substituted dithieno[3,2-b:2’,3’-d]pyrroles. Tetrahedron 2005, 61, 687−691. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Revision C 01; Gaussian, Inc.: Wallingford, CT, 2009. (24) Richardson, F. S.; Riehl, J. P. Circularly polarized luminescence spectroscopy. Chem. Rev. 1977, 77, 773−792. (25) San Jose, B. A.; Matsushita, S.; Akagi, K. Lyotropic chiral nematic liquid crystalline aliphatic conjugated polymers based on disubstituted polyacetylene derivatives that exhibit high dissymmetry factors in circularly polarized luminescence. J. Am. Chem. Soc. 2012, 134, 19795−19807. (26) Kulkarni, C.; Korevaar, A. P.; Bejagam, K. K.; Palmans, A. R. A.; Meijer, E. W.; George, J. S. Solvent clathrate driven dynamic stereomutation of a supramolecular polymer with molecular pockets. J. Am. Chem. Soc. 2017, 139, 13867−13875. (27) Korlepara, D. B.; Bejagam, K. K.; Balasubramanian, S. Supramolecular polymerization of N,N’,N’’,N’’’-tetra(rttradecyl)-1, 3, 6, 8-pyrenetetracarboxamide: A computational study. J. Phys. Chem. B 2017, 121, 11492−11503. (28) Förster, T. Excimers. Angew. Chem., Int. Ed. Engl. 1969, 8, 333− 343. (29) Intermolecular Forces an Introduction to Modern Methods and Results; Huyskens, P. L., Luck, W. A. P., Zeegers-Huyskens, T., Eds.; Springer-Verlag: New York, 1991; p 86. (30) Kano, K.; Matsumoto, H.; Hashimoto, S.; Sisido, M.; Imanishi, Y. A chiral pyrene excimer in γ-cyclodextrin cavity. J. Am. Chem. Soc. 1985, 107, 6117−6118. (31) Nakamura, M.; Suzuki, J.; Ota, F.; Takada, T.; Akagi, K.; Yamana, K. Helically assembled pyrene arrays on an RNA duplex that exhibit circularly polarized luminescence with excimer formation. Chem.Eur. J. 2016, 22, 9121−9124. (32) Ito, S.; Ikeda, K.; Nakanishi, S.; Imai, Y.; Asami, M. Concentration-dependent circularly polarized luminescence (CPL) of chiral N,N’-dipyrenyldiamines: Sign-inverted CPL switching between monomer and excimer regions under retention of the monomer emission for photoluminescence. Chem. Commun. 2017, 53, 6323−6326. (33) Okazaki, Y.; Goto, T.; Sakaguchi, R.; Kuwahara, Y.; Takafuji, M.; Oda, R.; Ihara, H. Facile and versatile approach for generating circularly polarized luminescence by non-chiral, low-molecular dye-onnanotemplate composite system. Chem. Lett. 2016, 45, 448−450. (34) Okazaki, M.; Mizusawa, T.; Nakabayashi, K.; Yamashita, M.; Tajima, B.; Harada, T.; Fujiki, M.; Imaia, Y. Solvent-controlled sign inversion of circularly polarized luminescent binaphthylacetic acid derivative. J. Photochem. Photobiol., A 2016, 331, 115−119. (35) Mimura, Y.; Kitamura, S.; Shizuma, M.; Kitamatsu, M.; Fujiki, M.; Imai, M. Solvent-sensitive sign inversion of excimer origin circularly polarized luminescence in bipyrenyl peptides. ChemistrySelect 2017, 2, 7759−7764. (36) Sato, S.; Yoshii, A.; Takahashi, S.; Furumi, S.; Takeuchi, M.; Isobe, H. Chiral intertwined spirals and magnetic transition dipole moments dictated by cylinder helicity. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 13097−13101. (37) Nagata, Y.; Nishikawa, T.; Suginome, M. Solvent-dependent fluorescence and circular dichroism properties of poly(quinoxaline-2,3diyl)s bearing pyrene pendants. Chem. Commun. 2012, 48, 11193− 11195. (38) Liu, M.; Zhang, L.; Wang, T. Supramolecular chirality in selfassembled systems. Chem. Rev. 2015, 115, 7304−7397. (39) Jin, Q. X.; Zhang, L.; Liu, M. H. Solvent-polarity-tuned morphology and inversion of supramolecular chirality in a selfassembled pyridylpyrazole-linked glutamide derivative: Nanofibers, nanotwists, nanotubes, and microtubes. Chem.Eur. J. 2013, 19, 9234−9241.

(4) Hirschberg, J. H. K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Helical self-assembled polymers from cooperative stacking of hydrogen-bonded pairs. Nature 2000, 407, 167−170. (5) Lin, X.; Suzuki, M.; Gushiken, M.; Yamauchi, M.; Karatsu, T.; Kizaki, T.; Tani, Y.; Nakayama, K.-i.; Suzuki, M.; Yamada, H.; Kajitani, T.; Fukushima, T.; Kikkawa, Y.; Yagai, S. High-fidelity self-assembly pathways for hydrogen-bonding molecular semiconductors. Sci. Rep. 2017, 7, 43098. (6) Yoshii, Y.; Hoshino, N.; Takeda, T.; Moritomo, H.; Kawamata, J.; Nakamura, T.; Akutagawa, T. The formation of organogels and helical nanofibers from simple organic salts. Chem.Eur. J. 2014, 20, 16279− 16285. (7) Kato, T.; Frechet, J. M. J.; Wilson, P. G.; Saito, T.; Uryu, T.; Fujishima, A.; Jin, C.; Kaneuchi, F. Hydrogen-bonded liquid crystals. Novel mesogens incorporating nonmesogenic bipyridyl compounds through complexation between hydrogen-bond donor and acceptor moieties. Chem. Mater. 1993, 5, 1094−1100. (8) Steiner, T. The hydrogen bond in the solid state. Angew. Chem., Int. Ed. 2002, 41, 48−76. (9) Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.; Walter, P. Molecular biology of the cell; Garland Science, Taylor & Francis, 2002; Vol. 4. (10) Yasuda, Y.; Iishi, E.; Inada, H.; Shirota, Y. Novel low-molecularweight organic gels: N,N’,N’’-tristearyltrimesamide/organic solvent system. Chem. Lett. 1996, 25, 575−576. (11) Timme, A.; Kress, R.; Albuquerque, R. Q.; Schmidt, H.-W. Phase behavior and mesophase structures of 1,3,5-benzene- and 1,3,5cyclohexanetricarboxamides: Towards an understanding of the losing order at the transition into the isotropic phase. Chem.Eur. J. 2012, 18, 8329−8339. (12) Shishido, Y.; Anetai, H.; Takeda, T.; Hoshino, N.; Noro, S.; Nakamura, T.; Akutagawa, T. Molecular assembly and ferroelectric response of benzenecarboxamides bearing multiple −CONHC14H29 chains. J. Phys. Chem. C 2014, 118, 21204−21214. (13) Matsunaga, Y.; Miyajima, N.; Nakayasu, Y.; Sakai, S.; Yonenaga, M. Design of novel mesomorphic compounds: N,N’,N’’-trialkyl-1,3,5benzenetricarboxamides. Bull. Chem. Soc. Jpn. 1988, 61, 207−210. (14) Stals, P. J. M.; Smulders, M. M. J.; Martín-Rapún, R.; Palmans, A. R. A.; Meijer, E. W. Asymmetrically substituted benzene-1,3,5tricarboxamides: Self-assembly and odd−even effects in the solid state and in dilute solution. Chem.Eur. J. 2009, 15, 2071−2080. (15) Fitié, C. F. C.; Roelofs, W. S. C.; Magusin, P. C. M. M.; Wübbenhorst, M.; Kemerink, M.; Sijbesma, R. P. Polar switching in trialkylbenzene-1,3,5-tricarboxamides. J. Phys. Chem. B 2012, 116, 3928−3937. (16) Bejagam, K. K.; Kulkarni, C.; George, S. J.; Balasubramanian, S. External electric field reverses helical handedness of a supramolecular columnar stack. Chem. Commun. 2015, 51, 16049−16052. (17) Anetai, H.; Wada, Y.; Takeda, T.; Hoshino, N.; Yamamoto, S.; Mitsuishi, M.; Takenobu, T.; Akutagawa, T. Fluorescent ferroelectrics of hydrogen-bonded pyrene derivatives. J. Phys. Chem. Lett. 2015, 6, 1813−1818. (18) Takeda, T.; Yamamoto, S.; Mitsuishi, M.; Akutagawa, T. Alkylamide-substituted tetraphenylethylene: three modes of fluorescence based on a hydrogen-bonded excimer. Org. Biomol. Chem. 2016, 14, 8922−8926. (19) Longhi, G.; Castiglioni, E.; Koshoubu, J.; Mazzeo, G.; Abbate, S. Circularly polarized luminescence: A review of experimental and theoretical aspects. Chirality 2016, 28, 696−707. (20) Smulders, M. M. J.; Schenning, P. A. H. J.; Meijer, E. W. Insight into the mechanisms of cooperative self-assembly: The “sergeantsand-soldiers” principle of chiral and achiral C3-symmetrical discotic triamides. J. Am. Chem. Soc. 2008, 130, 606−611. (21) García, F.; Viruela, P. M.; Matesanz, E.; Ortí, E.; Sánchez, L. Cooperative supramolecular polymerization and amplification of chirality in C3-symmetrical OPE-based trisamides. Chem.Eur. J. 2011, 17, 7755−7759. 6330


Article

The Journal of Physical Chemistry C (40) RSC handbook of chemistry and physics, 83rd ed.; Lide, D. R., Ed.; RSC Press, 2002; pp 15−18.

6331


Circular Polarized Luminescence of Hydrogen ... - ACS Publications

Recommend Documents