Photoluminescence and Crystal Structures of Chiro-Optical 1,1′-Bi-2

DOI: 10.1021/cg1004648

Photoluminescence and Crystal Structures of Chiro-Optical 1,10 -Bi-2-naphthol Crystals and Their Inclusion Compounds with Dimethyl Sulfoxide

2010, Vol. 10 3547–3554

Tu Lee* and Jen Fan Peng Department of Chemical and Materials Engineering, National Central University, 300 Jhong-Da Road, Jhong-Li City 320, Taiwan, R.O.C. Received April 7, 2010; Revised Manuscript Received June 28, 2010

ABSTRACT: Crystal structures of (R)-(þ)-1,10 -bi-2-naphthol (BINOL), (R,S )-(()-BINOL, (S )-(-)-BINOL, (R,S )-(()BINOL 3 DMSO, (R)-(þ)-BINOL 3 2DMSO, (R,S )-(()-BINOL 3 2DMSO, and (S )-(-)-BINOL 3 2DMSO (DMSO = dimethyl sulfoxide) were determined and related to their photoluminescence measurements systematically. The intensities of both excitation and emission bands of BINOL tended to rise as the average distance between BINOL molecules increased. The polarity of DMSO in the inclusion compounds with DMSO probably induced a different magnitude of the Davydov splitting and the degree of shifting of the levels of BINOL molecules. The stabilization of the excited state and the lowering of its energy led to a red-shifted emission. It was proven that photoluminescence of lumophore molecules in the solid state was able to be finetuned through supramolecular organization and the host-guest interaction by crystallization of a racemic compound (i.e., cocrystallization of R and S enantiomers), by formation of an enantiomorph, and by solvation with different stoichiometric ratios of a guest solvent. Therefore, this kind of supramolecular organization has significant design and characterization implications for organic solid-state lasers and devices.

Introduction Covalent syntheses of large molecular arrays are highly inefficient and costly. However, supramolecular organization1 of large numbers of smaller functional molecules (i.e., building blocks) by intermolecular interactions can often provide an alternative to lead to synergistic properties that are not intrinsic to the building blocks themselves.2 This strategic approach should promise to play an important role in the future research on organic solid-state lasers and devices3-6 because molecular ordering is essential in determining their performances, and making use of the intermolecular interactions wisely can sometimes simplify their fabrication and reduce their characterization cost.7 Unlike most of the flat and achiral laser dyes and organic semiconductors, our object of study was a well-known chirooptical 1,10 -bi-2-naphthol (BINOL) which is a simple conjugate building block having a C1-C10 σ-bond connecting two naphthyl groups with the presence of the hydroxyl groups in the 2 and 20 ortho-positions. The two enantiomers, labeled as S and R, are formed through the C1-C10 inter-ring (naphthol/ naphthol) rotation. Therefore, three different conformations exist for BINOL labeled as cc, ct, and tt orientations (or I1, I2, and I3, respectively)8,9 for each enantiomer and its mirror counterpart involving the rotation of the hydroxyl groups around the O-C2 and O-C20 bonds. Structures of the three isomers of BINOL and their respective enantiomers were all shown in Figure 1. In racemic (R,S )-(()-BINOL crystals, the enantiomeric (R)-(þ)-BINOL or (S )-(-)-BINOL molecules with a torsion angle, t1, of 88.9° (i.e., the rotational torsion angle between the two naphthol groups) formed a typical hydrogen bonded helical structure along the 21 screw axis with O-H 3 3 3 O of 2.21 A˚ (Figure 2). However, in (R)-(þ)-BINOL or

(S )-(-)-BINOL enantiomorphs (Figure 3), the single enantiomer molecules were stacked around a 31 screw axis to form a lefthanded helical structure through a hydrogen bond O-H 3 3 3 O of 2.22 A˚.9-11 Interestingly, the helical structures were broken12 when either the inclusion compounds of BINOL with 1,4dioxane,13,14 morpholine,13 acetone,13 tetrahydrofuran,13 dimethylsulfoxide,13 and dimethylformamide15 were produced or the inclusion complex crystals of BINOL with benzoquinone and aromatic hydrocarbons were made or the co-crystals of BINOL with 1,10-phenanthroline-5,6-dione16 and acridine16 were generated. Therefore, the aim of this paper is to make use of BINOL crystals and their inclusion compounds with dimethyl sulfoxide (DMSO), such as (R)-(þ)-BINOL, (R,S )-(()-BINOL, (S )-(-)BINOL, (R,S )-(()-BINOL 3 DMSO, (R)-(þ)-BINOL 3 2DMSO, (R,S )-(()-BINOL 3 2DMSO, and (S )-(-)-BINOL 3 2DMSO, as model systems to gain an in-depth understanding of how the fine-tuning of supramolecular structures17 by crystal engineering can sometimes affect the photoluminescent properties18-27 of the solid states. Materials and Methods

*Corresponding author. Telephone: þ886-3-422-7151 ext. 34204. Fax: þ886-3-425-2296. E-mail: [email protected].

Chemicals. (R,S )-(()-1,10 -bi-2-naphthol ((R,S )-(()-BINOL, C20H14O2, 99% purity, mp = 214-217 °C, MW = 286.32, lot 104655), (R)-(þ)-1,10 -bi-2-naphthol ((R)-(þ)-BINOL, C20H14O2, 99% purity, mp = 208-210 °C, MW = 286.32, lot 246948), and (S )-(-)-1,10 -bi-2-naphthol ((S )-(-)-BINOL, C20H14O2, 99% purity, mp = 208-210 °C, MW = 286.32, lot 246956) were purchased from Sigma-Aldrich (MO, USA). Solvents. Reversible osmosis (RO) water was clarified by a water purification system (model: Milli-RO Plus) bought from Millipore (Billerica, MA, USA). Dimethyl sulfoxide (DMSO, (CH3)2SO, 99.8% purity, bp = 189 °C, MW = 78.13, lot 66079) was received from Scharlau Chemie S.A. (Sentmenat, Spain). Experiments. Preparation of Crystalline Powders. Crystalline powders of (R)-(þ)-BINOL, (R,S )-(()-BINOL, and (S )-(-)-BINOL

r 2010 American Chemical Society

Published on Web 07/09/2010

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Figure 1. Structures of the three different conformations of BINOL and their respective enantiomers. were used as received. All other solids were crystallized from the solution of BINOL dissolved in DMSO by cooling from 60 to 25 °C in a 20-mL scintillation vial, filtered, and oven-dried at 40 °C overnight: (R,S )-(()-BINOL 3 DMSO powders were obtained from the solution of 100 mg of (R,S )-(()-BINOL dissolved in 0.33 mL of DMSO, (R)-(þ)-BINOL 3 2DMSO powders were produced from the solution of 100 mg of (R)-(þ)-BINOL dissolved in 0.66 mL of DMSO, (R,S )(()-BINOL 3 2DMSO powders were generated from the solution of 100 mg of (R,S )-(()-BINOL dissolved in 0.8 mL of DMSO, and (S )-(-)-BINOL 3 2DMSO powders were made from the solution of 100 mg of (S )-(-)-BINOL dissolved in 0.66 mL of DMSO. Preparation of Single Crystals. Single crystals of (R,S )-(()BINOL 3 2DMSO were obtained by slow evaporation of DMSO from a solution of 100 mg of (R,S )-(()-BINOL dissolved in 0.8 mL of DMSO over a period of 1-7 days at 25 °C. Single crystals of (R)-(þ)-BINOL 3 2DMSO and (S )-(-)-BINOL 3 2DMSO were produced by cooling the solution of 100 mg of (R)-(þ)-BINOL and (S )-(-)-BINOL dissolved in 0.66 mL of DMSO, respectively, from 60 to 25 °C in a 20-mL scintillation vial gradually with the water bath, filtered, and oven-dried at 40 °C overnight. Instrumentation. Optical Microscopy (OM). Crystal habits were examined and measured by an Olympus SZII Zoom Stereo microscope (Olympus, Tokyo, Japan) equipped with a Sony SSC-DC 50A digital color video camera (Sony Corporation, Tokyo, Japan). Differential Scanning Calorimetry (DSC). DSC analysis was mainly used to identify the temperature of desolvation, the temperature of solid-liquid transformation (melting), and the enthalpy of fusion. Thermal analytical data of 3-5 mg of samples in

Lee and Peng

Figure 2. View of the crystal structure of (a) (R,S )-(()-BINOL along the c-axis, (b) the 21 helical structure consisting of (R)-(þ)-BINOL molecules and its mirror image is the 21 helical structure of (S )-(-)BINOL molecules. Hydrogen bonds are shown by dotted lines. perforated aluminum sample pans (60 μL) were collected on a Perkin-Elmer DSC-7 calorimeter (Perkin-Elmer Instruments LLC, Shelton, CT, USA) with a heating rate of 10 °C/min from 50° to 230 °C under a constant nitrogen 99.990% purge. The instrument was calibrated with indium and zinc 99.999% having reference temperatures of 156.6° and 419.47 °C, respectively (Perkin-Elmer Instruments LLC, Shelton, CT, USA). Thermal Gravimetric Analysis (TGA). TGA analysis was carried out by TGA 7 (Perkin-Elmer, Norwalk, CT, USA) to monitor sample weight loss as a function of temperature. The heating rate was 10 °C/min ranging from 50° to 230 °C to minimize unwanted contamination caused by sublimation. Weight loss was usually associated with solvent evaporation close to the boiling point of a solvent as in the case of solvates or associated with sample decomposition. The open platinum pan and stirrup were washed by ethanol and burned by a spirit lamp to remove all impurities. All samples were heated under nitrogen atmosphere to avoid oxidization. About 3 mg of sample were placed on the open platinum pan suspending in a heating furnace. Transmission Fourier Transform Infrared (FTIR) Spectroscopy. Transmission FTIR spectroscopy was utilized to measure purity, to detect bond formation and to verify chemical identity. Transmission FTIR spectra were recorded on a Perkin-Elmer Spectrum One spectrometer (Perkin-Elmer Instruments LLC, Shelton, CT, USA). The KBr sample disk was scanned with a scan number of 8 from 400 to 4000 cm-1 having a resolution of 2 cm-1. Powder X-ray Diffraction (PXRD). PXRD diffractograms were collected by Bruker D8 Advance (Germany). The source of PXRD was Cu KR (1.542 A˚) and the diffractometer was operated at 40 kV

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USA). 0.05 mg of (R,S )-(()-BINOL in 2 mL of DMSO and 0.05 mg of (S )-(-)-BINOL in 2 mL of DMSO were used as baselines. Photoluminescence (PL). Solution Phase. Photoluminescence (PL) measurements were conducted on a Perkin-Elmer LS-55 photoluminescence spectrometer (Perkin-Elmer Instruments LLC, Shelton, CT, USA) equipped with a pulsed xenon lamp at 60 Hz. The excitation and emission slits were set to 10 nm during the operation. The scanning speed was 100 nm/min and the scanning range was from 200 to 700 nm. A 1% attenuator was used to reduce the intensity to correspond to the scale. The excitation wavelength was 338 nm to energize all solution samples, and the emission monitoring wavelength was 380 nm. 0.05 mg of (R,S )-(()-BINOL in 2 mL of DMSO and 0.05 mg of (S )-(-)BINOL in 2 mL of DMSO were used as baselines. 0.02, 0.04, and 0.08 mL of water were added to induce aggregation of (R,S )-(()-BINOL and (S )-(-)-BINOL molecules in DMSO. 0.02, 0.04, and 0.08 mL of DMSO were also added to another set of DMSO solutions of (R,S )(()-BINOL and (S )-(-)-BINOL molecules as a control to distinguish the aggregation effect of (R,S )-(()-BINOL and (S )-(-)-BINOL molecules in water added DMSO from the dilution effect of solvent in general. Solid Phase. Photoluminescence (PL) measurements were conducted on a Perkin-Elmer LS-55 photoluminescence spectrometer (Perkin-Elmer Instruments LLC, Shelton, CT, USA) equipped with a pulsed xenon lamp at 60 Hz. The excitation and emission slits were set to 2.5 nm during the operation. The scanning speed was 100 nm/ min and the scanning range was from 200 to 700 nm. A 1% attenuator was used to reduce the intensity to correspond to the scale. The excitation monitoring wavelength was 290 nm. About 2 mg of sample powders were used.

Results and Discussion

Figure 3. View of the crystal structure of (a) (R)-(þ)-BINOL along the b-axis and its mirror image is (S )-(-)-BINOL along the b-axis, and (b) side view of the 31 helical structure formed by the hydrogen bonding between hydroxyl groups of neighboring molecules and the mirror image for (S )-(-)-BINOL has a side view of the 32 helical structure formed by the hydrogen bonding between hydroxyl groups of neighboring molecules. Hydrogen bonds are shown by dotted lines. and 41 mA. The X-ray radiation from the source passed through a 1 mm slit. It was then incident upon the sample and reflected through a 1 mm slit, a nickel filter, and another 0.6 mm slit and finally collected by the detector. The detector type was a scintillation counter. The scanning rate was set at 0.05° 2θ/s ranging from 5° to 35°. The quantity of sample used was around 20-30 mg. Single Crystal X-ray Diffraction (SXD). Single crystal X-ray diffraction (SXD) data of samples were recorded on the Siemens SMART CCD-based Bruker X8 APEX X-ray diffractometer (Karlsruhe, Germany) equipped with Mo KR source (λ = 0.7137 A˚) operated at 3 kW. Data collection was performed by Bruker Apex2 software package and the structure was solved and refined using Bruker SHELXTL version 5.10 software package. The size of a crystal sample was from 0.1 to 1.0 mm. The crystal packing plot of SXD was drawn by Diamond 3.1 computer software (Crystal Impact GbR, Brandenberg Germany). Ultraviolet and Visible (UV/vis) Spectrophotometry. The ultraviolet absorption spectra of (R,S )-(()-BINOL and (S )-(-)-BINOL molecules in DMSO were monitored and quantified by the UV/vis absorption spectrum from 200 to 500 nm obtained using a PerkinElmer model Lambda 25 spectrophotometer (Norwalk, Connecticut,

The crystal structures of (R,S )-(()-BINOL, (R)-(þ)-BINOL (R,S )-(()-BINOL 3 DMSO, and (R,S )-(()-BINOL 3 2DMSO were reported in the literature10,11,13 except for the ones of (S )-(-)-BINOL, (R)-(þ)-BINOL 3 2DMSO, (S )-(-)-BINOL 3 2DMSO, (R)-(þ)-BINOL 3 DMSO, and (S )-(þ)-BINOL 3 DMSO. But for a full comparison, it was necessary to have the torsion angles about the C1-C10 inter-ring (naphthol/naphthol) rotation and the hydrogen bond angles and lengths of all the inclusion compounds. Therefore, we went ahead and determined the crystal structures of (S )-(-)-BINOL, (R)-(þ)-BINOL 3 2DMSO, (S )-(-)-BINOL 3 2DMSO in addition to the already known (R,S )-(()-BINOL, (R)-(þ)-BINOL, (R,S )(()-BINOL 3 2DMSO by SXD. Although we failed to grow single crystals of (R,S )-(()-BINOL 3 DMSO and single crystals and powders of (R)-(þ)-BINOL 3 DMSO and (S )-(þ)-BINOL 3 DMSO, fortunately, the literature data of (R,S )-(()BINOL 3 DMSO crystals were available.13 OM images, DSC and TGA scans, FTIR spectra, PXRD patterns, ORTEP plots, IR assignments, reflection parameters of single crystal X-ray data, selected bond lengths and bond angles of (R,S )-(()-BINOL, (R)-(þ)-BINOL, (S )-(-)-BINOL, (R,S )-(()-BINOL 3 DMSO, (R)-(þ)-BINOL 3 2DMSO, (R,S )-(()-BINOL 3 2DMSO, and (S )-(-)-BINOL 3 2DMSO are all illustrated in Figures S1-S5 and Tables S1-S4, Supporting Information, respectively. Purchased (R,S )-(()-BINOL and (R)-(þ)- or (S )-(-)-BINOL solids were orthorhombic and trigonal, in the space groups of Iba2 and P31 with Z = 8 and 3, respectively (Figures 2-3 and Table 1). (R,S )-(()-BINOL 3 DMSO powders were monoclinic in the space group P21/n with Z = 8 (Figure 4a,b and Table 1). The asymmetric unit consisted of two (R)-(þ)- or (S )-(-)BINOL host molecules (Figure 4a,b) and two DMSO guest molecules were located in the general positions. The DMSO guest molecules acted as hydrogen-bonding bridges between pairs of (R)-(þ)- or (S )-(-)-BINOL host molecules giving rise to ribbons of host-guest pairs running in the [101] direction (Figure 4a,b). The DMSO guest molecules were docked in centrosymmetric

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Table 1. Crystal Data and Data Collection and Final Refinement Parameters (R)-(þ)-BINOL CCDC No. empirical formula stoichiometry crystallization temperature (°C) formula weight temperature (K) crystal system space group a (A˚) b (A˚) c (A˚) R β γ Z volume (A˚3) density (Mg/m3) absorption coefficient (mm-1) F(000) crystal size (mm3) theta range for data collection (°) index ranges reflections collected independent reflections data/restraints/parameters goodness-of-fit final R indices [I > 2σ(I)]

R1 ωR2

CCDC no. empirical formula stoichiometry formula weight temperature (K) crystal system space group a (A˚) b (A˚) c (A˚) R β γ Z volume (A˚3) density (Mg/m3) absorption coefficient (mm-1) F(000) crystal size (mm3) theta range for data collection (°) index ranges reflections collected independent reflections data/restraints/parameters goodness-of-fit final R indices [I > 2σ(I)]

R1 ωR2

(R,S )-(()-BINOL

(S )-(-)-BINOL

761061 C20H14O2

761063 C20H14O2

761065 C20H14O2

25 286.31 296(2) trigonal P31 10.8108(4) 10.8108(4) 10.8845(4) 90° 90° 120° 3 1101.68(7) 1.295 0.083 450 0.30  0.22  0.20 2.18 to 28.30° -14 e h e 9, -9 e k e 14, -14 e l e 11

25 286.31 296(2) orthorhombic Iba2 15.7042(8) 21.6248(13) 8.6312(6) 90° 90° 90° 8 2931.2(3) 1.298 0.083 1200 0.28  0.25  0.25 1.88 to 26.35° -17 e h e 19, -15 e k e 26, -10 e l e 10 6368 2809 [R(int) = 0.0219] 2809/1/ 201 1.058 0.0376 0.0787

25 286.31 295(2) trigonal P32 10.8101(8) 10.8101(8) 10.8831(16) 90° 90° 120° 3 1101.4(2) 1.295 0.083 450 0.35  0.30  0.30 2.18 to 28.30° -14 e h e 14, -14 e k e 12, -10 e l e 14 8285 2999 [R(int) = 0.0340] 2999/1/ 201 0.933 0.0414 0.0900

6841 3424 [R(int) = 0.0163] 3424/1/ 201 1.032 1.032 0.0887

(R)-(þ)-BINOL 3 2DMSO 761062 C12H13O2 1:2 364.44 296(2) tetragonal P41212 8.7337(3) 8.7337(3) 30.2147(12) 90° 90° 90° 8 2304.70(14) 1.275 0.258 936 0.30  0.10  0.10 2.43 to 26.38° -10 e h e 7, -10 e k e 10, -37 e l e 33 11656 2363 [R(int) = 0.0282] 2363/18/154 1.274 0.1007 0.2941

gaps which exhibited disorder of the S atoms.28 (R,S )-(()BINOL 3 2DMSO, (R)-(þ)- and (S )-(-)-BINOL 3 2DMSO solids (Figure 5 and 6 and Table 1) were monoclinic, tetragonal, and tetragonal in the space groups of P21/c, P41212, and P43212 with Z = 4, 8, and 4 respectively. All molecules were in the general positions and both DMSO guests were disordered with S atoms occupying two positions. All three structures were tubular with the DMSO molecules docking in channels in the [100] and [010] directions (Figures 5a and 6a). Each hydroxyl group of BINOL acted as a hydrogen-bond donor to a DMSO molecule (Figures 5-6). All (R,S )-(()-BINOL, (R)-(þ)-BINOL, and

(R,S )-(()-BINOL 3 2DMSO 761064 C24H26O4S2 1:2 442.57 273(2) monoclinic P21/c 8.625(7) 8.999(6) 29.81(2) 90° 93.462(14)° 90° 4 2309(3) 1.273 0.257 936 0.30  0.20  0.20 1.37 to 26.49°. -10 e h e 6, -11 e k e 10, -34 e l e 37 14157 4715 [R(int) = 0.0352] 4715/4/307 1.050 0.0820 0.2251

(R,S )-(()-BINOL 3 DMSO ref 13 C22H20O3S 1:1 25/60 364.44 293(2) monoclinic P21/n 20.792(3) 8.883(1) 20.800(3) 90° 105° 90° 8 3708.8(9) 1.3053 1536 2.03 to 25.94° -13 e h e 25, -9 e k e 10, -24 e l e 25 4126 R(int) = 0.0472 6074/8/ 486 1.160 0.1197 0.2698 (S )-(-)-BINOL 3 2DMSO 761066 C24H26O4S2 1:2 442.57 296(2) tetragonal P43212 8.7358(2) 8.7358(2) 30.2334(7) 90° 90° 90° 4 2307.24(9) 1.274 0.258 936 0.25  0.20  0.18 2.43 to 26.38° -7 e h e 10, -10 e k e 10, -37 e l e 18 11126 2364 [R(int) = 0.0209] 2364/20/154 1.342 0.0979 0.3019

(S )-(-)-BINOL molecules had a cc orientation, whereas all inclusion compounds with DMSO had a tt orientation.8 The cocrystallization of both (R)-(þ)-BINOL and (S )-(-)-BINOL molecules to form a racemic compound and the inclusion of different stoichiometric amounts of DMSO in BINOL host solids could be used as a means to increase the torsion angle of a BINOL molecule from 76.6° to 107.7° and the intermolecular O1 3 3 3 O2 distances between two nearest BINOL molecules from 2.96 to 7.54 A˚ (Table 2). Although (R)-(þ)-BINOL and (S )-(-)-BINOL has a melting point of around 210 °C according to the DSC scans in

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Figure 5. View of the crystal structure of (a) (R)-(þ)-BINOL 3 2DMSO along the b-axis and the mirror image is (S )-(-)-BINOL 3 2DMSO along the b-axis, and (b) hydrogen bonding in (R)-(þ)-BINOL 3 2DMSO and the mirror image is the hydrogen bonding in (S )-(-)-BINOL 3 2DMSO. Hydrogen bonds are shown by dotted lines.

Figure 4. View of the crystal structure of (a) (R,S )-(()-BINOL 3 DMSO along the b-axis, (b) hydrogen bonding in (R,S )-(()-BINOL 3 DMSO consisting of (R)-(þ)-BINOL molecules and the mirror image is the hydrogen bonding in (R,S )-(()-BINOL 3 DMSO consisting of (S )-(-)BINOL molecules. Hydrogen bonds are shown by dotted lines.

Figure S2b,c, a complete desolvation of (R)-(þ)-BINOL 3 2DMSO and (S )-(-)-BINOL 3 2DMSO failed to regenerate (R)-(þ)-BINOL and (S )-(-)-BINOL crystals as clearly illustrated by the absence of a melting point at around 210 °C in Figure S2e,g. Probably, as the crystal lattice shrank upon the removal of DMSO, the two BINOL molecules originally 7.54 A˚ apart (i.e., O1 3 3 3 O2 distance) would need to travel a relatively long distance to become 2.96 A˚ apart (i.e., O1 3 3 3 O2 distance) which might be the cause for the final collapse of the crystalline structure (Table 2). And yet, the regeneration of (R,S )-BINOL from (R,S )-BINOL 3 DMSO was possible upon heating as shown by the DSC scan in Figure S2d with an endotherm of 218.4 °C which was very close to the melting point of 219 °C for (R,S )-(()-BINOL in Figure S2a. The relatively short distance between the two BINOL molecules originally 4.43 A˚ (i.e., O1 3 3 3 O2 distance) would only need to travel a relatively short distance to become 2.96 A˚ apart (Table 2). Therefore, the crystalline structure was likely to be transformed upon the removal of DMSO.

The absorption and photoluminescence spectroscopies of (R,S )-(()-BINOL and (S )-(-)-BINOL were first studied in the solution phase to better understand the basic spectroscopy involved. The absorption spectra of a 0.087 mM solution of (R,S )-(()-BINOL in DMSO and a 0.087 mM solution of (S )-(-)-BINOL in DMSO are shown in Figure 7. They looked almost identical and consisted of five absorption maxima. Given the less hindered intermolecular rotation that would exist in the solution phase than in the solid state about the C1-C10 bond connecting the two naphthol groups (Figure 1), it was likely that the energy states associated with each naphthol group would be decoupled from each other and observed separately in the absorption spectrum. The naphthol group on (R,S )-(()-BINOL or (S )-(-)-BINOL had a characteristic absorption spectrum18 with three distinct maxima in the range from 250 to 300 nm with a major band at 278 nm and the other minor bands at 269 and 290 nm. Moreover, there were two additional local maxima at 329 and 341 nm (Figure 7). The solution phase excitation and emission spectra of the 0.087 mM solution of (R,S )-(()-BINOL and (S )-(-)-BINOL in DMSO looked exactly the same (not shown). Therefore, only the ones for (R,S )-(()-BINOL were shown in Figure 8a. It was found that the excitation into the naphthol group consisting mainly of the π f π* transition at a wavelength of 338 nm had led to measurable fluorescence having a maximum at 380 nm (Figure 8a). As water, a bad solvent relative to (R,S )-(()-BINOL, was titrated into the

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0.087 mM solution of (R,S )-(()-BINOL in DMSO, the excitation bands and the intensity of luminescence were reduced drastically (Figure 8b-d). This phenomenon was not merely caused by the dilution effect of a solvent since it was found that when equal portions of DMSO instead of

Figure 6. View of the crystal structure of (a) (R,S )-(()-BINOL 3 2DMSO along the b-axis, (b) hydrogen bonding in (R,S )-(()-BINOL 3 2DMSO consisting of (R)-(þ)-BINOL molecules and the mirror image is the hydrogen bonding in (R,S )-(()-BINOL 3 2DMSO consisting of (S )-(-)-BINOL molecules. Hydrogen bonds are shown by dotted lines.

Lee and Peng

water were added into the 0.087 mM solution of (R,S )-(()BINOL in DMSO, no significant change upon the intensity in both excitation and emission bands was observed (Figure 9). Obviously, a concentration quenching effect27 instead of an aggregation induced emission (AIE)25 due to the aggregation of (R,S )-(()-BINOL in the presence of water had resulted. Excimers (i.e., the excited-state dimers) could have been formed when two aromatic systems were within a sufficient proximity of about 3 A˚,27 but each aromatic system was not better conjugated.25 To make the matter worse, the formation of excimers decreased the photoluminescence yields because of the greater number of nonradiative decay pathways for depopulation of the excited states.27 A similar phenomena to the concentration quenching effect in the solution phase (Figure 10a-c) was also demonstrated in the solid-state photoluminescence spectra of (R)-(þ)-BINOL, (R,S )-(()-BINOL, and (S )-(-)-BINOL crystals. The intensities of both excitation and emission bands of (R,S )-(()BINOL were lower than the ones of (R)-(þ)-BINOL and (S )-(-)-BINOL for BINOL molecules were packed more closely in a racemic compound than in an enantiomorph as evidenced by the higher density of (R,S )-(()-BINOL of 1.298 g/cm3 than the one of (R)-(þ)- or (S )-(-)-BINOL of 1.295 g/ cm3 (Table 1) and also by the shorter distance between the hydroxyl groups of two BINOL molecules (i.e., O1 3 3 3 O2) in (R,S )-(()-BINOL solids of 2.86 A˚ than the one in (R)-(þ)- or (S )-(-)-BINOL solids of 2.96 A˚ (Table 2). But as the stoichiometric amount of DMSO increased in the inclusion compounds from (R,S )-(()-BINOL 3 DMSO to (S )-(-)-BINOL 3 2DMSO, the intensity in both excitation and emission bands started to rise

Figure 7. Ultraviolet absorption spectra of a 0.087 mM solution of (R,S )-(()-BINOL in DMSO (solid line) and a 0.087 mM solution of (S )-(-)-BINOL in DMSO (dashed line).

Table 2. Hydrogen Bonding Details and Torsion Angle (t1) of BINOLa torsion angle t1/° (R)-(þ)-BINOL (R,S )-(()-BINOL (S )-(-)-BINOL (R,S )-(()-BINOL 3 DMSOa (R)-(þ)-BINOL 3 2DMSOb (R,S )-(()-BINOL 3 2DMSOc (S )-(-)-BINOL 3 2DMSOd

-76.6 -88.9 (R) 88.9 (S) 76.6 -95.5 (R) 95.4 (S) -106.4 -103.4 (R) 103.4 (S) 107.7

O-H 3 3 3 O O1-H1 3 3 3 O2-H2 O1-H1 3 3 3 O2-H2 O1-H1 3 3 3 O2-H2 O1-H1 3 3 3 O2-H2 O1-H1 3 3 3 OdS O1-H1 3 3 3 OdS O1-H1 3 3 3 OdS O1-H1 3 3 3 OdS O1-H1 3 3 3 OdS O1-H1 3 3 3 OdS

average H 3 3 3 O /A˚ 2.22 2.12 2.12 2.22 1.73 1.77 1.83 1.90 1.90 1.90

shortest O1 3 3 3 O2/A˚ 2.96 2.86 2.86 2.96 4.43 4.44 6.88 6.80 6.80 7.54

O1 3 3 3 O10 /A˚ 3.42 3.81 3.81 3.41 3.90 3.78 4.04 4.0 4.0 4.02

a Symmetry codes: a: y - 1, x, z; b: -y þ 1, -x þ 1, -z þ 1/2; c: -y, -x þ 1, -z; d: y, x, -z. 1: BINOL molecule; 10 : same BINOL molecule; 2: another BINOL molecule; R: R-configuration; S: S-configuration.

Article

Figure 8. Solution phase excitation (dashed lines) and emission (solid lines) spectra of (a) a 0.087 mM solution of (R,S )-(()-BINOL in 2 mL of DMSO, after an addition of (b) 0.2 mL of water, (c) 0.4 mL of water, and (d) 0.8 mL of water. Excitation spectra of (R,S )-(()-BINOL obtained at an emission wavelength of 380 nm, and emission spectra obtained at an excitation wavelength of 338 nm.

Figure 9. Solution phase excitation (dashed lines) and emission (solid lines) spectra of (a) a 0.087 mM solution of (R,S )-(()-BINOL in 2 mL of DMSO, after an addition of (b) 0.2 mL of DMSO, (c) 0.4 mL of DMSO, and (d) 0.8 mL of DMSO. Excitation spectra of (R,S )-(()-BINOL obtained at an emission wavelength of 380 nm, and emission spectra obtained at an excitation wavelength of 338 nm.

and the peaks became red-shifted relative to the nonsolvated BINOL solids (Figure 10d-g). Apparently, the aggregationinduced quenching effect7 in the solid state was diminished as the average distance between BINOL molecules (i.e., O1 3 3 3 O2) was lengthened from 4.43 A˚ to 7.54 A˚ (Table 2 and Figure 11). Also, the shielding of DMSO had prevented the BINOL molecules from exchanging energy. The spatial fit among BINOL host molecules and DMSO guest molecules might minimize the conformational relaxation of excited BINOL and thereby enhance the process of radiative relaxation.29 In addition, if the excited state had the larger dipole moment, an increase in solvent polarity had probably induced a different magnitude of the Davydov splitting and the degree of shifting of the levels,20,30 stabilized the excited state, lowered the energy of it, and then led to a red-shifted emission.7 No definitive trend in the intensity of the emissive peaks of (R,S )-(()-BINOL 3 2DMSO and (R)-(þ)- or (S )-(-)-BINOL 3 2DMSO was observed. Perhaps this could be due to the slight desolvation of DMSO under an ambient condition during photoluminescence measurements.

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Figure 10. Solid-state excitation (dashed line) and emission (solids line) spectra of (a) (R )-(þ)-BINOL, (b) (R,S )-(()-BINOL, (c) (S )-(-)BINOL, (d) (R,S )-(()-BINOL 3 DMSO, (e) (R)-(þ)-BINOL 3 2DMSO, (f ) (R,S )-(()-BINOL 3 2DMSO, and (g) (S )-(-)-BINOL 3 2DMSO. Excitation spectra obtained at an emission wavelength of 360 nm for (a), (b), and (c), and wavelength of 380 nm for (d), (e), (f ), and (g), and emission spectra obtained at an excitation wavelength of 290 nm.

Figure 11. The relationship between the photoluminescence intensity and the average distance between the hydroxyl groups of two BINOL molecules (average O1 3 3 3 O2 distance) in (a) (R,S )-(()BINOL, (b) (R)-(þ)-BINOL, (c) (S )-(-)-BINOL, (d) (R,S )-(()BINOL 3 DMSO, (e) (R)-(þ)-BINOL 3 2DMSO, (f ) (R,S )-(()-BINOL 3 2DMSO, and (g) (S )-(-)-BINOL 3 2DMSO.

Conclusions The average distance between chiral lumophore molecules such as BINOL in the solid state could be fine-tuned by supramolecular design through the crystallization of a racemic compound (i.e., co-crystallization of R and S enantiomers), the formation of an enantiomorph, and the production of inclusion compounds with different stoichiometric ratios of DMSO. BINOL aggregate formation in the solid state was mainly driven by a homo-intermolecular interaction such as hydrogen bonding17,31,32 but not π-π stacking in this particular case. There was a direct correlation between the aggregation-induced quenching effect on the photoluminescence emission spectra and the average distance between the two different BINOL molecules as measured by the O1 3 3 3 O2 distance between two hydroxyl groups of the

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two nearest BINOL molecules. However, this trend was not observed for the torsion angles. An increase in the solvent polarity such as DMSO was also responsible for stabilizing the excited state of BINOL molecules in the inclusion compounds with DMSO. Hence, the energy of the excited states was lowered, which led to a red-shifted emission. The underlying design and characterization implications of the supramolecular organization and the host-guest interaction to control ordering, film morphology, crystallinity, crystal orientation are extremely important for optically pumped organic solid-state lasers and other organic devices.33 Similar kinds of studies will be extended to the other inclusion compounds and co-crystals of BINOL and other lumophores. We will also carry out the polymorph search for the inclusion compounds, solvates, and co-crystals,31,34 and study their evaporation-deposition behaviors for the fabrication of solid thin films35 in the near future. Acknowledgment. This research was supported by the grants from the National Science Council of Taiwan ROC (NSC 98-2113-M-008-006). We are greatly indebted to Ms. Jui-Mei Huang and Ms. Ching-Tien Lin for their assistance with DSC, TGA, and PXRD at the Precision Instrument Center in National Central University and Ms. Pei-Lin Chen for her assistance with SXD at the Department of Chemistry in National Tsing Hua University. The authors would like to thank Prof. Anthony S. T. Chiang for his insightful comments on the TGA data. Supporting Information Available: Figures S1-S5, Tables S1-S4, and six CIF files of X-ray crystal structures of co-crystals (CCDC 761061-761066) were provided as Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org.

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