Evolving Fluorophores into Circularly Polarized Luminophores with a

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Evolving Fluorophores into Circularly Polarized Luminophores with a Chiral Naphthalene Tetramer: Proposal of Excimer Chirality Rule for CPL Kazuto Takaishi, Kazuhiro Iwachido, Ryosuke Takehana, Masanobu Uchiyama, and Tadashi Ema J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02582 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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Journal of the American Chemical Society

Evolving Fluorophores into Circularly Polarized Luminophores with a Chiral Naphthalene Tetramer: Proposal of Excimer Chirality Rule for CPL Kazuto Takaishi,*,†,‡ Kazuhiro Iwachido,† Ryosuke Takehana,† Masanobu Uchiyama,‡,§ and Tadashi Ema*,† †

Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Okayama 700-8530, Japan ‡ Cluster for Pioneering Research (CPR), Advanced Elements Chemistry Laboratory, RIKEN, Hirosawa, Wako, Saitama 3510198, Japan § Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Supporting Information Placeholder

Supporting Information Placeholder ABSTRACT: A versatile method for converting various fluorescent polycyclic aromatic hydrocarbons into circularly polarized luminescence (CPL) excimer dyes with high glum and FL values is reported. This method involves the functionalization of a chiral quaternaphthyl with six fluorophores via ester linkages in the last step of the synthesis. The usefulness of this approach was demonstrated for 1-, 2-, and 4-pyrenyl, 2- and 3-perylenyl, and 2-anthryl dyes. Most of them are the first or rare examples of CPL dyes. In the ground state, the fluorophores are tightly arranged by cumulative steric and electronic effects. In the excited state, the fluorophores form a twist excimer that maintains the ground-state conformations. The local chiral excimer directly affected the CPL properties. The systematic study on the signs of the CPLs allowed us to find a rule called the excimer chirality rule: right- and left-handed excimers exhibit (+)- and (–)-CPL, respectively.

Circularly polarized luminescence (CPL) organic dyes, such as helicenes, binaphthyls, and cyclophanes, have been actively investigated owing to the great potential in chiroptical materials science.1,2 In designing CPL dyes, researchers focus on altering known fluorescence (FL) dyes to the corresponding CPL dyes. Polycyclic aromatic hydrocarbons (PAHs) are ideal targets because they often show bright excimer FL with a large Stokes shift, which is suitable for creating CPL dyes. Actually, several CPL-active 1pyrenyl dyes, representative PAHs, have been reported.3,4 However, perylenyl5 and anthryl dyes6 with clear CPL have not been extensively studied, mainly due to the short lifetimes of their excited states.5a,7 A versatile method for creating CPL dyes is therefore necessary for the further development. Although the fundamental theory of CPL has been reported,8 there are neither reliable theoretical basis nor systematic study on excimer CPL. Hence, there is no empirical rule for the prediction and/or explanation of excimer CPL properties including the CPL signs.9 We have developed several CPL-active PAH dyes.4,10 Among them, hexa-1-pyrenyl appended axially chiral (R,R,R)-quaternaphthyl 1a exhibited intense CPL from intramolecular pyrene excim-

ers.4 The glum value was very high, reaching +0.037, where the dissymmetry factor glum is defined as 2(IL – IR)/(IL + IR). Such an intense CPL could not be achieved in binaphthyl analogues, while an

Figure 1. (a) Chemical structures of (R,R,R)-1a–f and (b) photographs of solutions of 1a–f. Conditions: 6.6  10–6 M, rt, CH2Cl2 solutions for 1a–c and 1f and THF solutions for 1d and 1e.

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octapyrenyl appended quaternaphthyl displayed the same properties as 1a. Therefore, 1a was found to be the smallest unit of prominent CPL dyes. We postulated that the quaternaphthyl skeleton is a useful framework for appending various PAHs to create excellent CPL dyes. We decided to systematically study the scope and limitations of this quaternaphthyl-based strategy by using 1a, 2- and 4pyrenyl adducts 1b and 1c, 2- and 3-perylenyl adducts 1d and 1e, and 2-anthryl adduct 1f (Figure 1). Interestingly, all of the linked PAHs displayed intense and colorful excimer CPL. The CPL-active 2- and 4-pyrenyl, 2-perylenyl, and 2-anthryl dyes represent the first examples. The strategy developed here is a versatile method for evolving achiral FL dyes into CPL dyes. Furthermore, this systematic study of 1 allowed us to find a rule named the excimer chirality rule: the signs of CPL, + or –, can be explained by the direction of the twist of excimers, right-handed or left-handed chirality.

Scheme 1. Synthesis of (R,R,R)-1a–f

Compounds (R,R,R)-1a–f were synthesized via the condensation of a common intermediate, (R,R,R)-quaternaphthyl-hexaol 3, with PAH-carboxylic acids 4a–f immediately after dealkylation of (R,R,R)-2 in moderate yields (Schemes 1 and S1). Hexaol 3 was slightly unstable in air. (R,R,R)-2 was prepared from (R)-BINOL in three steps without optical resolution.4 Enantiomers (S,S,S)-1a–f were also synthesized from (S)-BINOL. The enantiomer pairs were readily prepared in quantity, which is the advantage of this method. Although solutions of 1a–f under natural light were colorless or yellow, the solutions under UV light fluoresced in yellow-green, green, light-blue, deep-blue, or orange while maintaining a high FL of 0.03–0.67 (Figure 1). UV-Vis spectra, electronic circular dichroism (ECD), FL, and CPL spectra are shown in Figure 2. The quaternaphthyl skeleton and the PAHs absorbed light of below and above 320 nm wavelength, respectively.11 The spectral shapes comprised the sum of the quaternaphthyl and PAH spectra, indicating no appreciable homodimeric interactions. The quaternaphthyl skeleton significantly affected the ECD spectra at shorter wavelengths, and signals derived from the linked PAHs were detected above 320 nm in all 1a–f solutions. These results indicate that the PAHs are located in asymmetric environments. Among them, only PAHs in 1e displayed a clear negative split ECD. The exciton coupling theory12 indicates that the perylenes of 1e are arranged in a counter-clockwise manner. The other compounds showed no apparent split CD probably because of multiple and complex exciton couplings between six PAH chromophores in 1. For 1f, anthracene coupling overlapped with naphthalene coupling.13 All compounds 1a–f displayed excimer FL with little or no monomer FL. 1a, 1b, 1e, and 1f displayed em values typical of the excimers of each PAH.3–6,13,14 Although the excimer FL of 4pyrenyl and 2-perylenyl compounds has not been reported, the em values of 1c and 1d were greatly red-shifted as compared with the em of the monomer FL (em,max was 76 nm for 1c and 42 nm for 1d, Figure S1). The adjacent R2–R3 and R4–R5 pairs formed excimers efficiently after rapid energy transfer between R1–R6 (as

Figure 2. ECD, UV-Vis, CPL, and FL spectra of (a) 1a–c in CH2Cl2, (b) 1d and 1e in THF, and (c) 1f in CH2Cl2. Conditions: ex = 355 nm for 1a and 1c, 338 nm for 1b, 380 nm for 1d and 1e, 340 nm for 1f, 20 °C, light path length = 10 mm, [1a–e] = 1.0  10–6 M, [1f] = 1.3  10–6 M.

shown in Figure 1a, each PAH was sequentially numbered R1–R6 from one end). In general, perylenes and anthracenes seldom form excimers due to the extremely short lifetimes of the excited states.6b,7 In contrast, the PAH pairs in 1 are proximal in the ground state and ready to form excimers in the excited state. To our delight, all solutions of 1a–f were CPL-active, and the (R,R,R)- and (S,S,S)-forms displayed clear mirror images, indicating that the linearly polarized emission was not significant. It should be noted that 1b, 1c, 1d, and 1e represent the first examples of CPL-active 2-pyrenyl, 4-pyrenyl, 2-perylenyl, and 2-anthryl dyes, respectively, to the best of our knowledge. The |glum| values were very high: 2.8  10–3 for 1b, 0.014 for 1c, 3.3  10–3 for 1d, 0.013 for 1e, and 1.0  10–3 for 1f. The |glum| of (R,S,R)-1a, a diastereomer of (R,R,R)-1a, was 1.8  10–3, which is 20 times smaller than that of (R,R,R)-1a (Figure S2). Therefore, the quaternaphthyl skeleton with (R,R,R) or (S,S,S)-axial chirality is necessary for achieving intense CPL. Interestingly, the CPL signs differed among compounds 1: (R,R,R)-1a, 1c, 1d, and 1f exhibited (+)-CPL whereas (R,R,R)-1b and 1e exhibited (–)-CPL. These results suggested that these excimers adopt completely different conformations, even on the same chiral quaternaphthyl scaffold. The signs of the CPL agreed with those of the ECD at the longest signals, weakly suggesting that the ground and excited states have similar conformations.


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Journal of the American Chemical Society 9.4 kcal/mol, Figures S6 and S7). The proximal PAH pairs are oriented clockwise for 1a, 1c, 1d, and 1f and counter-clockwise for 1b and 1e. Importantly, these twist directions agree with the signs of the CPL: clockwise and counter-clockwise twists of the adjacent PAH pairs provide (+)-CPL and (–)-CPL, respectively.

Figure 4. (a) Experimental VCD and IR spectra of (R,R,R)-1a and (S,S,S)-1a (1,1,2,2-tetrachloroethane, 0.02 M, 20 °C, light path length = 0.15 cm, BaF2). (b) Calculated VCD and IR spectra of 6a extracted from the DFT-optimized (R,R,R)-1a at the CAM-B3LYP/6-31G(d,p) level. Half-width at half height = 4 cm–1. The calculated wavenumbers are not corrected.

Figure 3. Top views of the DFT-optimized ground-state structures of (R,R,R)-1a–f (PAHs are shown alternately in blue and orange) at the CAM-B3LYP/6-31G(d,p) level.

We next obtained DFT-optimized ground-state structures of 1a– f to clarify the relationship between their conformations and CPL properties (see Figure 3 for top views and Figures S4 and S5 for side views).15 DFT calculations were carried out using the Coulomb-attenuating method (CAM)-B3LYP/6-31G(d,p) level, which is suitable for polycyclic aromatic compounds.16 The four naphthalene rings in 1 are arranged with a shifting angle of almost 90° each to form a left-handed helix. Prior to analyzing the conformations of the PAHs, the orientations of the transition dipole moments of the lowest energy transition were theoretically predicted using the corresponding methyl esters 5a–f. The ester group affected the alignment of the transition moment to some degree (Figure S3). The optimized structures of 1a–f displayed three common properties: (1) R1 and R2, R3 and R4, and R5 and R6 are directed away from each other via steric repulsion; (2) adjacent carbonyl groups are oriented in opposite directions due to steric and dipole repulsions; and (3) two adjacent PAH pairs that are supposed to form excimers (R2–R3 and R4–R5) are oriented with unidirectional twisting. Although R1 and R6 are not components of excimers, their presence is essential for reinforcing the twist conformations. The PAHs are conformationally constrained via the cumulative steric effects, that is, the sergeants-and-soldiers effects4,17; when some PAHs were directionally inverted, the conformers became quite unstable (E = 1.6–

Vibrational circular dichroism (VCD) can be used to determine the absolute configurations of dicarbonyl compounds by comparing their experimental signals with theoretical signals as reported by Taniguchi and Monde.18 Information about the solution-state structure of 1 was obtained by analyzing the VCD spectra of (R,R,R)-1a, which confirmed that those of (R,R,R)- and (S,S,S)-1a were mirror images of each other (Figure 4a). A strong and sharp experimental VCD signal at 1730 cm–1 (C=O stretching) for (R,R,R)-1a was observed as a positive split signal. The observed VCD signal agreed well with the theoretical VCD signal predicted for diester 6a with chirally twisted pyrenes, which is a local

Figure 5. TD DFT-optimized excited-state structures of 6d and 6e at the CAM-B3LYP/6-31G(d,p) level. The structures extracted from the optimized ground-state structures of (R,R,R)-1d and (R,R,R)-1e were used as initial structures.



structure containing R2 and R3 in DFT-optimized (R,R,R)-1a (Figure 4b). This agreement supports the optimized structures of 1, especially the orientations of the adjacent carbonyl groups shown in Figure 3. Finally, the excited-state structures were examined to address the origin of the CPL using time-dependent (TD)-DFT calculations (Figures 5 and S8). In diester 6d with two 2-perylene rings, which is a partial structure of (R,R,R)-1d, the two perylene rings formed a stacked dimer (two rings at a distance of 3.7 Å), which is an excimer with right-handed chirality (Figure 5a). By contrast, in diester 6e with two 3-perylene rings, two perylene rings formed an excimer with left-handed chirality and a two-ring distance of 3.7 Å (Figure 5b). The twist directions of the perylene excimers are the same as those in the ground state. The right-handed excimer emitted (+)-CPL, while the left-handed excimer emitted (–)-CPL. This rule can also be applied to 1a–1c and 1f (Table 1, Figure S8). That is, the chiral twisting of the intramolecular excimers is the origin of the CPL, and the CPL sign originates from the twist direction. We call this relationship the “excimer chirality rule”. It should be noted that CPL spectra provided useful information about the chiral conformation of the PAH moieties in contrast to ECD spectra giving only poor information (Table 1 and Figure 2). The CPL spectra give us useful information resulting from only the local chirality of the adjacent PAH pairs that can form tight excimers. CPL based on the excimer chirality rule may be a powerful tool for the determination of the conformation/configuration of a specific chiral moiety in the excited state of simple or complex molecular systems, which is impossible with the CD exciton chirality method12,18 for the groundstate conformation/configuration.

Table 1. Correlation between CPL or ECD and Twist Conformation of Proximal PAH Pairs in (R,R,R)-1a–f compd 1a 1b 1c 1d 1e 1f

twist of PAHsa right-handed left-handed right-handed right-handed left-handed right-handed

sign of CPL + – + + – +

sign of split ECD nd nd nd nd – nd

a

Twist patterns of proximal PAH pairs in DFT-optimized 1 in the ground and excited states were the same.

In conclusion, we have developed a versatile method for the creation of PAH excimer CPL dyes using chiral (R,R,R)- or (S,S,S)quaternaphthyl skeletons. This method involves the simple functionalization of the quaternaphthyl with six PAHs via ester linkages in the last stage of the synthesis. The qurternaphthyl scaffold strategy was applied to various PAHs with different regioisomerism and lifetimes. This CPL arises from local chiral PAH excimers that are preorganized in the ground state. In the present systematic study with intensive DFT calculations, we found a rule called the excimer chirality rule: the right-handed and left-handed twist excimers emit the (+)- and (–)-CPL, respectively. This method and the rule will contribute greatly to the further development and application of CPL dyes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Synthesis, spectra, and computational details (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (KT) *E-mail: [email protected] (TE)

ORCID Kazuto Takaishi: 0000-0003-4979-7375 Masanobu Uchiyama: 0000-0001-6385-5944 Tadashi Ema: 0000-0002-2160-6840

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant No. JP17K05786, the Okayama Foundation for Science and Technology, the Wesco Scientific Promotion Foundation, and Tobe Maki Scholarship Foundation. We thank the JASCO Corp. for VCD measurements, and Otsuka Electronics Co., Ltd for measurements of the absolute fluorescence quantum yields. Allotment of computational resources from HOKUSAI GreatWave (RIKEN) is gratefully acknowledged.

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Evolving Fluorophores into Circularly Polarized Luminophores with a

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