Excimer-origin CPL vs. monomer-origin magnetic CPL in photo-excited chiral binaphthyl-ester-pyrenes: critical role of ester direction

Hana Okada a, Nobuyuki Hara a, Daiki Kaji a, Motohiro Shizuma b, Michiya Fujuiki c and Yoshitane Imai *a
aDepartment of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. E-mail: y-imai@apch.kindai.ac.jp
bDepartment of Biochemistry, Osaka Research Institute of Industrial Science and Technology, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan
cGraduate School of Materials Science, Nara Institute of Science and Technology, Takayama, Ikoma, Nara 630-0192, Japan

Received 25th April 2020 , Accepted 20th May 2020

First published on 20th May 2020


Two chiral binaphthyl (BNp) derivatives bearing oppositely oriented ester linkers to two pyrene (Py) moieties [(R)/(S)-1 and (R)/(S)-2] enabled Py-origin circularly polarized luminescence (CPL), magnetic CPL (MCPL), and circular dichroism (CD). (R)-1 that exhibited (−)-sign CD showed (+)-sign Py-excimer CPL but did not exhibit MCPL. Conversely, (R)-2, with (−)-sign CD, did not show excimer-origin CPL, but exhibited clear Py-monomer MCPL.


Recently, a variety of soluble processible enantiopure luminophores, exhibiting high degrees of circularly polarized luminescence (CPL), have received significant attention because of their use in several photonic devices and applications.1–3 Generally, practical chiral luminophores are realized by tailoring chemical influences (e.g., solvents, solid matrix, chemical additives, and pH, among others).4 A knowledge and understanding of classical stereochemistry led to the notion that the (+)- or (−)-sign at the first Cotton circular dichroism (CD) band of a chiral luminophore in the ground state (GS) is controlled by the rigid framework of the enantiopure substance, which provides the same (+)- (or (−)-) sign of the CPL signal in the photoexcited state (ES).

As alternative approaches to non-classically controlled CPL signs in the ES and CD signs in the GS, we showed that, in semi-rigid C2-symmetric binaphthyl (BNp) derivatives with single rotational C–C bond between BNp rings, the CPL sign in the ES is inverted as a result of (i) the dihedral angle between the BNps,5 (ii) chemically modifying the peripheral positions of the BNp rings,6 and (iii) introducing bulky substituents at the BNps,7 irrespective of the same axial chirality of the BNp in the GS.

In recent years, an external static magnetic field (SMF) has been shown to act as a physical bias capable of inducing CD (referred to as “MCD”) and CPL (referred to as “MCPL”) from achiral substances.8 MCD and MCPL spectroscopies were developed based on the Faraday effect discovered in 1845. So-called Faraday geometry, in which north (N)-up and south (S)-up directions of the SMF relative to an incident light, is chosen to control signs in MCD and MCPL spectra. Indeed, we confirmed that three achiral pyrene derivatives in chloroform and PMMA film at room temperature reveal clear mirror-image MCPL in the Faraday geometry: the MCPL sign is controlled by the N-up and S-up directions.9 The fundamental question of how the Faraday geometry affects the MCPL sign of a CPL-functioned chiral luminophore, however, remains unanswered.

To address this question, we chose two types of chiral BNp derivatives bearing ester linkers with luminophoric bipyrene (Py) units. (R)/(S)-1 and (R)/(S)-2 were prepared from (R)/(S)-2,2′-dihydroxy-1,1′-binaphthyl (BINOL) and 1-pyrenecarboxylic acid and from (R)/(S)-2,2′-dicarboxy-1,1′-binaphthyl and 1-pyrenol, respectively, according to reported methods (Scheme 1).10 Both (R)/(S)-1 and (R)/(S)-2 are semi-rigid chiral luminophores containing seven rotational C–C and C–O bonds (Scheme 1). Note that the ester linkers between (R)/(S)-1 and (R)/(S)-2 are oppositely connected.


image file: d0cp02215b-s1.tif
Scheme 1 Preparation of axially chiral binaphthyl-ester-bipyrenes bearing oppositely oriented binaphthyl/pyrene ester linkers [(R)/(S)-1 and (R)/(S)-2].10 DPTS: 4-(N,N-dimethylamino)pyridium-4-toluenesulfonate; DIPC: diisopropylcarbodiimide.

Herein, we report that the marked directional effect of the ester linker between BNp and Py is a crucial factor that affects the emerging CPL and MCPL signals and their signs relative to the CD sign resulting from the Py moieties in chloroform and PMMA film. For example, (R)-1 showing a (+)-sign CD signal, reveals Py excimer-origin (+)-sign CPL, but did not show the excimer-origin MCPL. Conversely, (R)-2 exhibiting a (−)-sign CD signal does not provide the excimer-origin CPL, but clearly reveals mirror-symmetric MCPL of Py monomer origin. The CPL, MCPL, and CD characteristics in chloroform (1 × 10−4 M) show similar tendencies to those of the PMMA film.

To quantitatively evaluate the degree of CPL of the chiral luminophore in the ES, we evaluate gCPL (= ΔI/I = (ILIR)/[(IL + IR)/2], where IL and IR are the signal intensities for the left- and right-CPLs, respectively) upon excitation with unpolarized incident light. Similarly, the MCPL efficiency of the chiral luminophore in the ES in an SMF, gMCPL, is given by: gMCPL = (ILIR)/[(IL + IR)/2] per Tesla, where IL and IR are the intensities of the left and right MCPLs, respectively, upon excitation with unpolarized incident light. The N-to-S and the S-to-N alignments of the SMF with the incident light are referred to as “N-up” and “S-up”, respectively. The GS chirality known as Kuhn's anisotropy is defined as: gCD = (ΔεL − ΔεR) = (εLεR)/[(εL + εR)/2], where εL and εR are the left and right absorptivities, respectively.

From Fig. 1a and b, the photoluminescence (PL) and CPL spectra of (R)/(S)-1 and (R)/(S)-2 in chloroform show clear differences in Py excimer-origin emission bands around 450–650 nm. (R)-1 and (R)-2 show mirror-image (−)/(+)-sign CD bands around 340–350 nm, while the CD bands of (S)-1 and (S)-2 are mirror-images of those of (R)-1 and (R)-2, respectively.


image file: d0cp02215b-f1.tif
Fig. 1 CD (thin lines) and CPL (thick lines) spectra in upper panel and PL (thin lines) and UV-Vis (thick lines) spectra in lower panel of (a) (R)-1 (blue) and (S)-1 (red) and (b) (R)-2 (blue) and (S)-2 (red) in dilute chloroform (1.0 × 10−4 M) at 25 °C.

(R)/(S)-1 clearly exhibit a Py excimer-origin CPL at 535 nm (Fig. 1a and Table 1) arising from intramolecular Pyπ–Pyπ stacks, as the same PL band is observed in chloroform of various concentrations (Fig. S6–S8, ESI). Conversely, (R)/(S)-2 do not exhibit any CPL signals at the Py excimer PL band (Fig. 1b). The gCPL values of (R)/(S)-1 are ±4.0 × 10−2 at 535 nm, respectively, associated with their PL quantum yield (ΦF) of 0.04. Although 1 provides CPL functionality, 2 does not. Thus, the direction of the ester linkers is crucial in the ES.

Table 1 PL, CPL, and MCPL characteristics of (R)-/(S)-1 and (R)-/(S)-2 associated with the HOMO and LUMO electron densities of (R)-1 and (R)-2 determined at the B3LYP/6-31G(d,p) level11
(R)-1 (S)-1 (R)-2 (S)-2
a M and E: Py monomer and Py excimer, respectively. b N-up. c S-up. d Taken from Fig. 3.
PL (M)a, λem 412 nm 411 nm 388 nm 388 nm
PL (E)a, λem 537 nm 537 nm ∼490 nm ∼490 nm
PyE/PyMa 2.5/100 2.5/100 3.3/100 1.8/100
CPL (M)a n.d. n.d. n.d. n.d.
g CPL/10−2
CPL (E)a +4.0 −4.0 n.d. n.d.
g CPL/10−2
MCPL (M)a n.d. n.d. −0.11b −0.15b
g MCPL/10−2, T−1 +0.12c +0.17c
MCPL (E)a, +2.3b −3.8b n.d. n.d.
g MCPL/10−2, T−1 +2.0c −4.2c
PyPy stack
ED, 1st LUMOd High (Py) No (Py)
No (BNp) High (BNp)
ED, 1st HOMOd High (Py) High (Py)
No (BNp) No (BNp)
Non PyPy stack
ED, 1st LUMOd High (Py) High (Py)
No (BNp) High (BNp)
ED, 1st HOMOd High (Py) High (Py)
No (BNp) No (BNp)


To discuss the chirality of 1 and 2 in the GS, we compared their CD and UV-Vis spectra in chloroform (1.0 × 10−4 M) (Fig. 1a and b, and Table 1). (R)/(S)-1 and (R)/(S)-2 exhibit clear mirror-image CD spectral profiles due to π→π* vibronic transitions arising from the Py and BNp moieties. The absolute gCD values, |gCD|, of 1 and 2 at the first Cotton bands are 0.25 × 10−3 at 387 nm and 0.23 × 10−3 at 348 nm, respectively.

The solid-state CPL and PL characteristics of 1 and 2 in PMMA film were investigated to control the CPL signs through intermolecular interactions among multiple Py units. As shown in Fig. S10, ESI and Table 1, 1 exhibited clear Py-excimer origin CPL at 509 nm with ΦF = 0.56 (Fig. 2a) associated with |gCPL| = 1.8 × 10−3, which are similar to those observed in chloroform. It is worth noting that the significantly high ΦF value in the film is ascribable to the strong suppression of the thermal motions of Py π–π stacks in the film. However, 2 in the film does not exhibit monomer- and excimer-origin CPL signals at all (Fig. S11, ESI).


image file: d0cp02215b-f2.tif
Fig. 2 MCPL (upper) and PL (lower) spectra of (a) (R)-1 (blue: N-up; red: S-up; black: CPL) and (b) (R)-2 (blue: N-up; red: S-up; black: CPL) in chloroform (1.0 × 10−4 M) at 25 °C.

The solid-state CD and UV-visible absorption characteristics of (R)/(S)-1 and (R)/(S)-2 in PMMA film exhibit mirror-image CD spectral profiles (Fig. S10 and S11, ESI). (R)-1 adopts a similar Py geometry in the GS regardless of being in dilute chloroform or PMMA film, while (R)-1 in the ES favors the Py excimer. However, (R)-1 in the ES shows different CPL-sign behavior in solution and the solid film; the CPL sign of (R)-1 is inverted in chloroform but retained in PMMA film. It is estimated that this apparent CPL reverse is caused by differences in intra- and intermolecular Py excimer π–π* interactions. The CPL signals in solution and PMMA film originate from intramolecular and intermolecular Py–Py interactions, respectively. The CPL sign of (R)-1 in powder form is identical to that of the film (Fig. S9, ESI). The |gCD| values of 1 and 2 at their first Cotton bands are 0.43 × 10−3 at 390 nm and 0.55 × 10−3 at 362 nm, respectively.

To further determine whether or not the MCPL signals of (R)/(S)-1 and (R)/(S)-2 commonly emerge, the SMF effect in the Faraday geometry was examined. Recently, spin–orbit coupling in an SMF showed the MCPL characteristics in inorganic substances (e.g., Eu3+, Tb3+, Cu+, and Pr3+).8 The MCPL characteristics of achiral and chiral organic luminophores consisting of rigid frameworks, however, have rarely been reported. Moreover, MCPL characteristics of semi-rigid chiral organic luminophores have not yet been reported.

To elucidate the direction effect of the ester linker between (R)-1 and (R)-2, the MCPL spectra in chloroform in N-up and S-up geometries were acquired. From Fig. 2a and Table 1, (R)-1 clearly exhibits clear Py-excimer origin CPL with the same (+)-sign around 535 nm regardless of the N-up and S-up geometries and in the absence of H0; hence the Py-excimer CPL of (R)-1 is insensitive to the SMF and controlled by the (R)–(S)-BNp chirality.

On the other hand, from Fig. 2b and Table 1, despite adopting a chiral geometry in the GS associated without obvious CPL in the ES, (R)-2 exhibits clear MCPL at the Py-monomer-origin PL band at 391 nm: the gMCPL values are −0.11 × 10−2 T−1 (N-up) and +0.12 × 10−2 T−1 (S-up), respectively. Similarly, (S)-2 provides the gMCPL values of −0.15 × 10−2 T−1 (N-up) and +0.17 × 10−2 T−1 (S-up) at the Py-monomer-origin PL band (Fig. S13, ESI and Table 1). Thus, the MCPL signs of (R)-2 and (S)-2 are determined by the direction of N-up and S-up geometries, but not by (R)(S)-BNp chirality (Table 1). The SMF induces MCPL from monomeric Py moieties of 2 in the GS but not from the Py excimers.

Moreover, we examined the MCPL characteristics of 1 and 2 embedded to PMMA film. As was observed in chloroform, 1 does not exhibit any detectable MCPL at the Py excimer band (Fig. S14, ESI and Table 1), which is possibly due to a lack of efficient Zeeman splitting in the intermolecular Pyπ–Pyπ interaction in the ES. On the other hand, in PMMA film, 2 exhibits an obvious MCPL at the Py-monomer band in the Faraday geometry (Fig. S15, ESI and Table 1). The N-up and S-up conditions determine (+)- and (−)-sign MCPL signals, respectively. The gMCPL value is ±0.2 × 10−2 (T−1), leading to no obvious MCPL properties in the absence of the SMF. Intense MCPL arises from monomeric Py species in chloroform and PMMA. The MCPL sign of 2 is controlled by the direction of the SMF.

To computationally investigate the effect of ester direction in 1 and 2, we visualize significant differences in electron density (ED) at 1st HOMO and 1st LUMO for two representative Py–Py stacks as a model of Py excimer (isovalue 0.05) and non-Py–Py stacks a model of Py monomer (isovalue 0.02) of (R)-1 and (R)-2 by TD-DFT (CAM-B3LYP//6-31G(d,p))11 (Fig. 3 and Table 1).


image file: d0cp02215b-f3.tif
Fig. 3 Comparisons of electron densities at 1st HOMOs and 1st LUMOs for Pyπ–Pyπ stack (upper panels, isovalue 0.05) and non-Pyπ–Pyπ stack (lower panels, isovalue 0.02) structures of (R)-1 and (R)-2 calculated by TD-DFT (CAM-B3LYP, 6-31G(d,p) basis).11 (R)-1 and (R)-2 were optimized by DFT (B3LYP, 6-31G(d,p) basis).11

The ED at 1st HOMO of Py–Py stack (R)-1 is localized to an upper Py ring, while that at 1st LUMO is localized to a lower Py ring. This situation may account for the Py excimer origin CPL/PL by a scenario of through-space intramolecular charge-transfer between two Py rings (IntraCT). Conversely, the ED at 1st HOMO of (R)-2 is localized to a lower Py ring, while that at 1st LUMO is localized to one of two naphthyl rings in BNp. This feature is a through-space IntraCT between Py and BNp and may be related to the inefficient Py excimer PL/CPL bands.

Conversely, the ED of non-Py–Py stack (R)-1 is localized at two Py rings regardless of 1st HOMO and 1st LUMO. A minute interaction between two Py rings is responsible for the Py monomer origin PL without CPL. The ED at 1st HOMO of non-Py–Py stack (R)-2 is localized at two Py rings, while the ED at 1st LUMO is delocalized to two Py rings and BNp. The monomeric Py moieties of (R)-1 and (R)-2 in the ES may be responsible for MCPL-activity at the Py monomeric PL band and MCPL-inactivity at the Py excimer PL bands.

Actually, our recent experimental results of three achiral Py derivatives9 proved that SMF with the Faraday geometry induces Py-origin monomeric (+)-/(−)-sign MCPL, in which the sign is determined by N-up or S-up geometry. However, the SMF did not induce any detectable MCPL of Py-excimer origin PL in highly concentrated chloroform (1 × 10−2 M). Thus, the direction of the ester linkage between BNp and Py rings is the key to generating Py-excimer-origin CPL and Py-monomer-origin MCPL.

In summary, we investigated the CPL and MCPL characteristics of two semi-rigid C2-symmetrical BNp connected through differently oriented ester linkers to Py units experimentally and computationally. (R)/(S)-1 exhibited clear Py excimer-origin CPL in chloroform and PMMA film, in which the CPL sign was determined by the BNp chirality and outside environments. (R)/(S)-1 did not reveal excimer- and monomer-origin MCPL. Conversely, (R)/(S)-2, that did not reveal excimer- and monomer-origin CPL, exhibited mirror-symmetric Py-monomer-origin MCPL, whose sign was determined by the N-up/S-up directions. CAM-B3LYP/6-31G(d,p) calculations invoked that localized EDs of two Py rings at 1st HOMO and 1st LUMO are the key for the excimer-origin CPL/PL due to the scenario of through-space IntraCT between two Pys. Conversely, the through-space IntraCT between Py and BNp at 1st LUMO is connected to emerging Py-monomer origin MCPL associated with no obvious excimer-origin CPL and MCPL. Thus, the direction of the ester linkage between BNp and Py played a key role in inducing Py-origin CPL and MCPL.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by Grants-in-Aid for Scientific Research (18K05094, 19H02712, 19H04600, and 20H04678) from MEXT/Japan Society for the Promotion of Science, Research Foundation for the Electrotechnology of Chubu (R-30506), KDDI Foundation (2019-9), the Murata Science Foundation (H31-007), and the Nippon Sheet Glass Foundation for Materials Science and Engineering (H30-4).

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Footnote

Electronic supplementary information (ESI) available: Experimental methods; mass and NMR (1H and 13C) spectra; additional CPL/MCPL, PL, CD, and UV-Vis spectra. See DOI: 10.1039/d0cp02215b

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