Toranosuke
Tomikawa
a,
Yuichi
Kitagawa
*b,
Koki
Yoshioka
d,
Kei
Murata
c,
Tomohiro
Miyatake
d,
Yasuchika
Hasegawa
*be and
Kazuyuki
Ishii
*c
aGraduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
bFaculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
cInstitute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. E-mail: k-ishii@iis.u-tokyo.ac.jp
dDepartment of Materials Chemistry, Ryukoku University, 1-5 Yokotani, Seta Oecho, Otsu, Shiga 520-2194, Japan
eInstitute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan
First published on 25th January 2023
A switchable system between magnetic and natural circularly polarized luminescence via J-aggregation using photosynthetic antenna model compounds is demonstrated for the first time. The J-aggregation provides characteristic dispersion-type circularly polarised luminescence signals based on the degenerate J-band (gCPL = 0.03).
The magnitude of CPL can be expressed using the dissymmetry factor (gCPL).1,2 The gCPL value depends on the transition electric dipole moment (), transition magnetic dipole moment (), and angle between and . Although the combination of enhanced || and || offers a large gCPL value with bright emission, achieving this is still challenging in modern chemistry because the linear electronic motion of is fundamentally different from the rotational electronic motion of . Chiral J-type aggregation is a promising strategy for achieving large || and || values based on the exciton chirality theory.10–17 Ghosh et al. recently reported a large gCPL (= 4.6 × 10−2) with a high emission quantum yield (Φ = 43%) using helical J-aggregates composed of chiral naphthalene-diimide derivatives.10 Moreover, the flexibility of aggregation can introduce switching functions. For example, Harada et al. have achieved flexible CPL switching behaviour using pseudo cyanine J-aggregates by thermal stimulus.13 Using thiopheneboronic acid, Kameta et al. demonstrated thermal stimulus-driven CPL switching via morphological transformation from H- to J-aggregates (gCPL ≈ 0 → gCPL = 3.1 × 10−3).14 Thus, flexible CPL switching systems with a large gCPL value can open up new possibilities for photonic applications.
Magnetic CPL (MCPL) is the CPL phenomenon under a magnetic field. MCPL is observed in both chiral and achiral compounds,18–20 and the magnitude of MCPL for organic compounds is related to the orbital angular momentum of the aromatic π-electron system.21,22 Specifically, the multi-function switching based on both natural CPL and MCPL is expected to provide new insights for the development of magneto-photonic materials based on polarised light.23–25
Herein, we provide a novel system, in which MCPL and natural CPL are switched upon J-aggregation. To demonstrate our MCPL/CPL switching concept, we employed chiral Zn-chlorin (zinc 3-devinyl-3-methoxymethyl pyropheophorbide-a monoester: ZnChl, Fig. 1a) with a large aromatic π-electron system because of the following reasons.26 (1) The interaction of Zn(II) with a methoxy-substituent gives the formation of J-aggregates (Fig. 1b), which correspond to a model compound of light-harvesting antennas in green photosynthetic bacteria, i.e., chlorosomes.27 (2) The transition to the lowest excited state in the chlorin unit, called the Qy band, has a large transition probability,28 resulting in strong light absorption and emission properties as well as good chiroptical properties in the Qy band via J-aggregation.29 (3) The large orbital angular momentum derived from the large aromatic π-electron system of the chlorin unit imparts effective magneto-optical properties to the Qy band, which is weakened by the J-aggregation.29 Based on these facts, the J-aggregation of ZnChl should be an appropriate system for realizing the switch between MCPL and natural CPL (Fig. 1c).
Fig. 1 (a) Molecular structure of ZnChl and (b) schematic image of the chiral J-aggregates of ZnChls.31–38 (c) Magnetic and natural circularly polarised luminescence switching via J-aggregation (Red arrow: transition electric dipole moment). |
ZnChl was prepared according to the previous method.26 Here, the Zn ion was employed as the central metal ion instead of a Mg ion, since Zn chlorins are more stable than Mg chlorins.30 The J-aggregates of ZnChls were prepared by diluting the methanol solution of ZnChl with 99-fold volume of water (ESI†).31–38 The electronic absorption, emission, MCPL, and CPL spectra of the ZnChl monomer are shown in Fig. 2. The absorption bands at 581, 611, and 658 nm were assigned to the Qx(0,0), Qy(1,0), and Qy(0,0) bands, respectively (Fig. 2a).29 Strong emission bands were observed at 665 and 720 nm, corresponding to their Qy bands (Fig. 2b). The emission quantum yield and lifetime were 0.067 and 4.7 ns, respectively. The Qy emission bands showed a distinguishable, positive MCPL signal (Fig. 2c, gCPL = 0.003) in contrast to the negligible CPL signal (Fig. 2c) derived from the poor chiroptical property of the asymmetric centre at positions 17 and 18 (Fig. 1a).
In our previous study,29 the electronic transition bands were calculated using the time-dependent density functional theory (TD-DFT, B3LYP/6-31G(d)) method. In the calculation, we employed the structure of a ZnChl derivative having an ethyl group at position 17. The spectroscopic properties of the Q bands can be reasonably explained by Gouterman's model based on four orbitals (b1, b2, c2, and c1) and four electronic configurations ((b1c2), (b2c2), (b1c1), and (b2c1)), as shown in Fig. 3a.39,40 Because the energy difference between the (b2c2) and (b1c1) configurations is relatively large, the Qy transition mainly originates from the single electronic (b2c2) configuration (Table S1 in ref. 29). On the other hand, the (b1c2) configuration is heavily admixed with the (b2c1) configuration, resulting in weak Qx bands. Thus, with reference to the magnetic circular dichroism intensity, a strong Faraday B-type MCPL signal is observed for the Qy emission band because of the magnetic moment between the Qx and Qy bands; this magnetic moment originates from the large orbital angular momentum of the aromatic π-electron system (i.e., 〈b2|lz|b1〉 and 〈c2|lz|c1〉).22
Fig. 3 (a) Molecular orbitals of ZnChl.29 (b) Energy diagram of the ZnChl monomer and their J-aggregates for explaining the ineffective MCPL due to J-aggregation. The molecular orbitals (isosurface value = 0.02) were calculated by the DFT method (B3LYP/6-31G(d)). |
In the electronic absorption spectrum of the chiral J-aggregates of ZnChls, the Qy band remarkably shifted to the red-side (729 nm) owing to the exciton interaction between the ZnChl units, whereas the shift of the Qx band was very small. The narrow J-band (FWHM of 280 cm−1) reflects the formation of well-ordered J-aggregates. Consequently, the J-aggregation promotes the energy separation between the Qy and Qx bands (monomer, 2120 cm−1; J-aggregates, 3510 cm−1). In the emission spectrum, a sharp signal was observed at 732 nm with a small Stokes shift (ΔE = 60 cm-1), as shown in Fig. 2b. The emission quantum yield was 0.004.41 In the emissive J-band region, no distinguishable MCPL signal was observed, although the ZnChl monomer exhibited MCPL (Fig. 2c). This is reasonably explained by the increase in the energy gap between the Qx and Qy bands via J-aggregation, because the MCPL B term, which is proportional to Im((〈Qy|m|Qx〉/(E(Qx) − E(Qy)))〈S0|μ|Qy〉〈Qx|μ|S0〉), is in inverse proportion to the energy gap, E(Qx) − E(Qy) (〈Qy|m|Qx〉: the magnetic moment between the Qx and Qy bands, 〈S0|μ|Qi〉: the electric dipole moment of the Qi band, E(Qi): the Qi band energy). On the other hand, an opposite trend was observed for CPL. Whereas the ZnChl monomer exhibited negligible CPL signal, the chiral J-aggregates of ZnChls exhibited intense positive/negative CPL signals in the J-band region (Fig. 2d, gCPL = 0.03, positive peak: 723.5 nm, negative peak: 739 nm). The CPL spectrum is similar to the dispersion-type CD spectral pattern in the J-band region (Fig. S1, positive peak: 728.5 nm, negative peak: 735 nm, ESI†), which indicates the coexistence of the lowest excited singlet state and the nearby excited singlet state in the J-band region. The energy splitting (120 cm−1) between these excited states is comparable to the thermal energy (∼200 cm−1), and therefore, the dispersion-type CPL spectral pattern was observed as a sum of the negative CPL from the lowest excited singlet state and the positive CPL from the thermally activated, nearby excited singlet state. Thus, using photosynthetic antenna model compounds, we successfully demonstrate the first system in which the MCPL/CPL behaviour is switched upon J-aggregation (Fig. 2c and d).
Finally, we attempted to theoretically clarify the difference in gCPL between the monomer and J-aggregates. gCPL is given by
(1) |
(2) |
(3) |
(4) |
Here, J and J are the transition electric dipole moment and transition magnetic dipole moment of the J-aggregates, respectively, and the label (p) denotes the p-th ZnChl unit in the J-aggregates (Fig. 4a). According to the exciton chirality method,12 the J is expressed using the following equation:
(5) |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc04841h |
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