Propeller Chirality of Boron Heptaaryldipyrromethene: Unprecedented

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Propeller Chirality of Boron Heptaaryldipyrromethene. Unprecedented Supramolecular Dimerization and Chiroptical Properties Masataka Toyoda, Yoshitane Imai, and Tadashi Mori J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02492 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 5, 2016

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The Journal of Physical Chemistry Letters

Propeller Chirality of Boron Heptaaryldipyrromethene. Unprecedented Supramolecular Dimerization and Chiroptical Properties Masataka Toyoda,† Yoshitane Imai,‡ and Tadashi Mori*,† † Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan ‡ Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 34-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan * E-mail: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT. Chiral boron dipyrromethenes (BPs) enjoy high fluorescence efficiency at visible to near-IR wavelength regions with a reasonable range of dissymmetry factors. Here, we demonstrate that the (quasi)propeller chirality, similarly to hexagonal propeller in hexaarylbenzene, can be effectively induced in heptaarylated BP. In addition, supramolecular dimer was formed at low temperatures in non-polar solvent, which exhibits strong bisignate Cotton effects at BP transitions (the couplet amplitude A = 193 M-1 cm-1) in the circular dichroism (CD). Due to the bulky substituents on the propeller blades, but with void space around boron atoms, BP chromophores in the dimer are aligned in a head-to-tail manner with a small torsion (φ ≈ 15°), to avoid fluorescence quenching usually observed in H-type dimer of BPs, exhibiting strong circularly polarized luminescence (CPL) signals (glum = 2.0  10-3, Φlum = 0.45). Such supramolecular dimer formation would be viewed as an alternative approach for designing and developing novel chiroptical materials.


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TOC GRAPHICS

N B N F F aR

N

B

F F N B N

N

F F

Dimer glum = 2 x 10-3

CPL

550

600

650 700 750 Wavelength / nm

800

KEYWORDS. propeller chirality • circular dichroism • circularly polarized luminescence • boron heptaaryldipyrromethene • toroidal interaction



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Simple organic chiral luminophores, that emit circularly polarized light (CPL), have been much attracted and considerably explored recently.1 Although they are in a rather developing stage at the present time (c.f., lower luminescent dissymmetry factors, glum, the dimensionless value defined as 2 × (IL - IR) / (IL + IR), where IL and IR denote the quantum yields of the left- and right-handed CPL, respectively, upon photoexcitation by unpolarized light, of 10-2~10-5 range; much smaller than such as lanthanide complexes),2,3 possible applications have been projected in materials for 3D displays/endoscopes, information storage/communication, as well as studying of excited-state chirality and enantioselective sensors. 4 , 5 , 6 , 7 In general, organic molecules are considered more promising in controlling emission wavelength and efficiency through the rational structural modifications. Among several groups of such nominees, boron dipyrromethene (BODIPY, or simply BP)8,9 has been extensively employed as a scaffold for chiral fluorescent materials,10 as BPs are generally highly fluorescent (e.g., Φlum of parent BP has been found 0.93 in methanol).11. Rather straightforward approach for obtaining chiral BPs uses chiral substituent(s) interconnected at α- or meso-position of BP, 12 , 13 , 14 but relatively weak chiroptical properties (such as gabs in CDs) were merely recorded. Chirality was more effectively induced in BP by incorporation of chiral binaphthyl moieties at the BP-periphery, but profound loss of fluorescence efficiency was noted.15 Axially chiral BP at meso-position16 and chiral BP with asymmetric boron atom17 have been also examined, but found not encouraging in terms of the glum value. Similar approach, nevertheless, has been applied for preparation of chiral polymers, in which reasonably improved chiroptical properties were observed.18,19,20 Recently, elongation in BP π-systems via ring-fusion has been demonstrated to provide much larger glum of 6  10-4 with high quantum yield of 0.73.21 In another approach, BP moiety can be chirally distorted by direct fusion through the boron atom, while the (chir)optical features of such BPs


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may become somewhat unpredictable due to their non-planarity.22 At any rate, helical twist has been effectively evoked in O,O-connected BPs, where relatively large chiroptical properties (gabs and glum) of 10-3~10-4 have been reported with fairly large Φlum values. 23 , 24 As a different approach, the dimers of BPs are expected to provide strong bisignate circular dichroisms by exciton coupling. Indeed, such dimers of BP incorporated in chiral scaffold has been reported. 25,26 The C-C coupled BP dimers have been also described in which intrinsic axial chirality are induced and most of them actually afforded bisignate Cotton effects in their CDs.27,28,29,30 Nevertheless, less has been explored so far, especially for their CPL property. We have been recently interested in propeller chirality of hexaarylbenzenes (HABs).31 Due to the compensated conjugation effect and steric hindrance,32 radial aromatic rings (or propeller blades) in HABs are not aligned orthogonal to the central core at their optimized geometries (ideal tilt angles are ca. 59°), by which clockwise (C) or counterclockwise (CC) propeller chirality can be induced.31 Such chirality can be regarded as extended notion of axial chirality, but multiple axial chiralities do work subserviently (domino effect). Accordingly, small chiral group(s) attached at the periphery of propeller blades could induce almost complete onedirectional chirality in their propellers under the optimized conditions (i.e., at low temperature in non-polar solvents),31 while single point chirality alone could merely induce nearly negligible axial chirality, presumably due to its dipole moment.32 Another unique feature of these HABs are their hexagonal toroidal interaction,33,34,35 where the unique π-electron delocalization over the propeller blades are observed. By a careful analysis of chiroptical properties of HABs, such toroidal interaction has been successfully evaluated quantitatively. Briefly, at the temperature higher than Tc (critical temperature), the whizzing toroids become more substantial in their



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equilibrium in which toroidal interaction is maximized by nearly orthogonal alignment (in average) of floppy propeller blades against the central benzene core.31 In the present study, we examined (chir)optical properties (UV-vis/CD and FL/CPL) of heptaarylated BP (B7), in which similar propeller chirality is anticipated (Chart 1). The fully chiral heptaaryl-BP (B7), as well as achiral analog B0, were efficiently prepared by a condensation of the corresponding tetraarylcycopentadiene (to give precursors such as A7), followed by a reaction with boron trifluoride etherate in the presence of trimethylamine (detailed synthetic procedures are described in the Supporting Information, SI).36 We demonstrate that chiral B7 could also attain the (quasi)propeller chirality (as in H6), but the degree of induced chirality and its temperature dependence were quite different. In addition, we found the unprecedented supramolecular dimer formation at low temperature in non-polar solvent, which exhibited strong bisignate Cotton effects in CD (A = 190 M-1 cm-1, gabs = 2.3  10-3), as well as large dissymmetry factor in CPL (glum = 2.0  10-3) with moderate fluorescence quantum yield (Φlum = 0.45). With an aid of theoretical calculations, we also addressed the possible structure and (chir)optical properties therefrom of the monomer and the dimer of B7.


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Chart 1. Chiral and Achiral Boron Heptaaryldipyrromethene (B7 and B0) and the Relevant Hexaarylbenzene (H6) and Parent Boron Dipyrromethene (B) as Reference. Inset Represents the Propeller Chirality in Perarylated BPs

Table 1 summarizes the absorption and fluorescence spectral features of chiral BP (B7) and the corresponding achiral analog (B0), compared with those of parent boron dipyrromethene (B) in chloroform at 25 °C (Figure S14 in SI). Two low-energy bands in the UV-vis spectra at around 600 and 430 nm in heptaaryl-BPs (i.e., B0 and B7) were readily assignable to the S0-S1 and S0-S2 transitions along with long and short axes of BP chromophore. At shorter wavelength below 300 nm were found the transitions associated with aromatic blades (mainly 1Lb and 1La transitions), which are essentially absent in the spectra of B. The emission maxima in fluorescence spectra were found at around 640 nm for heptaaryl-BPs. The overall red-shifts in the UV-vis and FL spectra, larger Stokes shifts, as well as lower fluorescence quantum yields of heptaaryl-BPs



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(slightly more pronounced in B7) than those of parent B are ascribed to the extended πconjugation through the slant aromatic blades, which in turn affects the BP chromophore to be more distorted and lowers the HOMO-LUMO energy gaps. Furthermore, vibrational relaxation may become more feasible through the floppy aromatic blades.

Table 1. Photophysical Properties of Heptaarylated and Parent BPs a BP

λabs (S0-S1)

λabs (S0-S2)

λabs (1Lb)

λlum

Φlum b

B0

603

427

275

636

0.44

B7

607

432

275

640

0.32 (0.45c) 850

Bd

505

―

―

526

0.76

Stokes shift 860

790

a

Peak maxima in absorption and fluorescence spectra (in nm), fluorescence quantum yield, and Stokes shift (in cm-1) in chloroform at 25 °C. b The Φlum value of B7 was obtained by a comparison with that reported for B0 in ref. 37. Excitation wavelength was set to 440 nm. c At 120 °C. d B = parent boron dipyrromethene.11

Figure 1 shows the CD spectra of B7 in methylcyclohexane. At 25 °C, the CD signal was almost silent at longer wavelength (transitions at BP chromophore), while two negative Cotton effects of similar strengths were observed below 300 nm, the pattern of which was quite similar to those obtained with chiral HABs.31 The sign of Cotton effects is indicative of the favored induction of the (C)-propeller chirality, which is immediately confirmed by the theoretical CD calculation (Figure S15 in SI). The molar ellipticities for B7 were found -5.2 and -4.3 M-1 cm-1 (at 229 and 281 nm, respectively), which is roughly ten times smaller than those of H6, but much larger than those of corresponding aryl ether (-0.4 M-1 cm-1) and biphenyls (-0.5 ~ -1.8 M-1 cm-1) with the same chiral alkyl group(s).32 Consequently, the interaction between the blades are obviously important for observed CD in B7, but less effective than H6. The smaller interaction


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in B7 may be attributed to larger interblade distance (3.1 Å; distance between two ipso carbons of blades) than that of HABs (2.9 Å), and to the fact that the interaction in B7 is not fully circulated. Degree of interaction of open-shaped (but contiguous) aromatic arrays has been already discussed thoroughly in hexaarylbenzene derivatives.38

+20

+120

+10

+60

/ M-1cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0

-10

-60

-20 200

300


-120 200 700

300


700

Figure 1. Temperature dependence of CD spectra of B7 in methylcyclohexane at 7.7 μM. Left: from 25 (black), 0 (purple), and to -30 °C (blue). Right: from -60 (green), -90 (orange), and to 120 °C (red). The corresponding change in UV-vis spectra can be found in Figure S16 in the Supporting Information.

Generally, the strength of CD in solution is slightly enhanced at lower temperatures in many flexible molecular systems. 39 However, such effects can be considerably amplified in the propeller chirality through the domino effect.31 Moreover, the temperature dependence is quite informative about the dynamic behavior of propeller chirality. We found two different temperature domains for (chir)optical properties of HABs; i.e., (1) the (C)- and (CC)-propellers are in equilibrium at temperature lower than Tc = -50 °C, and (2) the whizzing toroid becomes substantial at higher temperature, where the average blade angles become nearly perpendicular



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by which toroidal interaction is maximized, but the CD intensity tends to be diminished. As such, we quantitatively analyzed the temperature dependence of CD for B7. Briefly, the strength of Cotton effects derived from transitions at propeller blades (below 300 nm) was once increased up to ca. -14 M-1 cm-1 (Figure 1, left) and then decreased (Figure 1, right) as continuingly lowering temperature, revealing the existence of three different temperature domains in B7. The CD signals at the BP chromophore was almost negligible at higher temperature, but the bisignate and positive Cotton effects (+/-/+) became apparent at the S0-S1 and S0-S2 transitions of BP as the temperature was decreased to -60 °C, which were further intensified by further decrease of temperature (+88/-105 and +20 M-1 cm-1 at -120 °C). The apparent excitonic coupling at the S0S1 transition in BP core at lower temperatures indicates that two (or more) BP units are located in close proximity under these conditions (vide infra). The absorption, fluorescence emission, and excitation spectra all exhibited three temperature domains (Figures S16 and S17 in SI). For instance, absorption peak at 606 nm (corresponding to S0-S1 transition in BP chromophore) gradually red-shifted to 614 nm when the temperature was decreased from 25 to -60 °C, but again blue-shifted to 608 nm as further decreased the temperature to -120 °C. Figure 2 plots several transition energies in UV-vis spectra, Cotton effects in CD spectra, as well as fluorescence behaviors against reciprocal temperature. As clearly seen, all photophysical data point out three different temperature domains with transition temperatures at around Tc = 10 and Td = -70 °C, in which different major contributors are operative in each temperature domain for (chir)optical properties of B7 (also see Figure S18 in SI).


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+10

440

-70 / °C UV

420 300

80

250 205

40

190

2

3

4 -1

5 -3

6 -1

T / 10 K

+10

120

7

0

-70 / °C

+10

192

CD 5

-70 / °C FL

189

186

2

3

4 -1

5 -3

6

7

-1

T / 10 K

183

2

3

4 -1

5 -3

6

7

-1

T / 10 K

Figure 2. Temperature dependences of the UV-vis (left), CD (middle), and fluorescence (right) spectra of B7 in methylcyclohexane. Left and middle: blue and red are for the S0-S1 and S0-S2 transitions of BP core (blue filled and open circles in CD are of positive and negative Cotton effects in their bisignate signals). Green is primarily at the 1Lb transition in peripheral aromatic blades and the signals in CD are intensified for clarity. Right: energy in excitation spectra (blue) and relative fluorescence intensity (green).

Taking the behavior of propeller chirality of HABs into account, we rationalize this unusual temperature dependence of heptaaryl-BPs as follows (Figure 3). Below Tc = 10 °C (but above Td = -70 °C), the (chir)optical properties can be approximated by a static equilibrium mixture of (C)- and (CC)-isomers (in a similar manner in HABs), as the C-CC equilibrium is relatively slow. The relative preference for more stable C over CC isomer (supported by theoretical calculations, vide infra) explains the increase of CD intensity in B7 as lowering temperature (within this temperature range). A considerably smaller shift in the UV-vis peaks (the change was slightly more evident in FL spectra, however) against the temperature is in accord with our interpretation that while the C-CC ratio is increased, the average blade angles are not much altered within this middle temperature region. Above Tc, rotation of propeller blades becomes more dynamic along



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the tilt angle surface and toroidal interaction becomes to be more effective. This contribution of whizzing toroids (T) is apparent from the blue shifts in UV-vis and FL spectra upon increased temperature, implying that the switching from J-type to H-type association between the aromatic blades. The contribution of such whizzing toroid in B7 is more evident in polar solvents as in HABs (Figure S19 and S20 in SI). The observed (chir)optical properties are significantly affected by such dynamic trajectories, particularly at higher temperatures. The higher critical temperature in B7 (Tc = 10 °C) as compared with that of H6 (Tc = -50 °C) may well be explained as the effective interaction in B7 requiring the higher energy due to the larger separation between the blades and incomplete circular toroids.

N

N B F F

T aS

N

N B F F

aR

CC 0.8 kcal/mol

N

N B F F

C 3.8 kcal/mol

N B N F F F F N B N

r d

D

Figure 3. Conceptual visualization that explains the unique (chir)optical property based on propeller chirality of B7. C and CC: heptaaryl-BP of (C)- and (CC)-propeller chirality. T: whizzing toroids that maximizes the toroidal interaction. D: supramolecular dimer, affording bisignate CD signal. Inset: optimized geometry for D at the DFT-D3(BJ)-TPSS/def2-SV(P) level.


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The observed temperature dependence of (chir)optical properties of B7 at high and middle temperature domains was quite comparable to those of HABs. However, the behavior at temperature lower than Td (= -70 °C) was totally unexpected and was not observed in any HAB derivatives examined so far. In brief, by further lowering temperature, we observed: (1) the strong bisignate Cotton effects at the transitions in BP core, suggesting two (or more) BPs are associated in a close proximity. (2) In contrast, the Cotton effects at the high-energy region (transitions at the blades) were blue shifted, suggesting the H-type association is again favored. (3) The UV-vis and FL intensity was not much affected at the BP chromophore. It is also to note that the similar bisignate Cotton effects (but much weaker in intensity) were observed for the precursor dipyrrin A7 at -90 °C (Figure S21 in SI). We propose supramolecular dimer (D), not higher order aggregate, of B7 is responsible for those observations in UV-Vis/CD/FL spectra (Figure 3) by the experiments as follows. The CD spectral intensity change plotted against the concentration of B7 at -120 °C was found well correlated with the regression line representing the dimer model (Figure S22 in SI). This also provided the Gibbs free energy change for the dimer formation at -120 °C of ΔG = -16 kJ mol-1, which is in a reasonable range for London (dispersion) interactions. In addition, the diffusion ordered NMR spectroscopic technique in methylcyclohexane-d14 at 0 °C revealed that the volume of complex was compatible with the dimer: i.e., Vcomplex/Vmonomer = 1.8 (Figure S23 in SI). Theoretical calculations on the structure of monomer and dimer of B7 provide additional insights on this peculiar behavior of (chir)optical properties for B7. The geometry optimization of (C)- and (CC)-isomers of B7 (C and CC) was performed by the dispersion-corrected density functional theory at the DFT-D3(BJ)-TPSS/def2-TZVP level. In the calculation, the most stable Tg- conformation in the chiral group(s) and alternately oriented (i.e., udududu) conformation in



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alkoxy group(s) were adopted, according to previous investigations. As showed in Table 2, tilt angle of each aromatic blade considerably differs each other depending on locations. While the aromatic blades at meso- and adjacent positions keep the similar angle to HABs, the blades at αand next (outer β) positions were found more planar due to the smaller crowding, the average tilt angle being ≈55° (compare: 59° in H6). The energetic profile of the propeller-chirality inversion of B7 is also included in Figure 3. In the most feasible domino process, one ring rotation induces small alterations in tilt angles of the adjacent rings on both sides and such a motion propagates to the whole rings, achieving the overall propeller inversion.31 The activation energy for this process was estimated by optimizing B7 conformation by fixing most crowded meso-aryl blade at 90°. Accordingly, the activation energy of 3.8 kcal mol-1 (via the domino inversion mechanism) from the (C)- to (CC)-enantiomer with the energy difference of 0.81 kcal mol-1 (0.92 kcal mol-1 at the SCS-MP2/def2-TZVPP level incorporating the COSMO solvation model for methylcyclohexane) was obtained, in accord with more feasible inversion process found in H6 (ΔE = 0.81 kcal mol-1, Ea = 2.1 kcal mol-1, Tc = -50 °C).31


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Table 2. Structural Parameters of DFT-D3 Optimized Geometries of Heptaarylated and Parent Boron Dipyrromethenes BP

Tilt angles a

Ring deviations b

(C)-B7 (C)

52.7/53.3/61.4/58.7/56.3/54.1/50.3 (55.3±3.8)

10.0/8.0/13.6 (0.112)

(CC)-B7 (CC)

54.3/53.2/58.2/57.7/56.3/52.5/48.5 (54.4±3.4)

8.9/7.9/11.9 (0.096)

B7 (T) c

54.3/53.2/58.2/88.3/56.3/52.5/48.5 (58.8±13.4) 8.9/7.9/11.9 (0.096)

B0

53.0/54.3/58.9/57.8/57.6/52.6/52.6 (55.3±2.7)

9.9/8.2/13.4 (0.111)

B0’ d

55.6/54.3/59.6/57.6/57.6/53.0/52.9 (55.8±2.6)

9.6/8.5/12.9 (0.105)

[48.4/57.0/62.6/71.5/74.0/63.8/40.0]

10.9/11.6/21.3

―

0.3/0.2/0.4 (0.005)

B a

Tilt angles of aromatic blades against the central BP plane at 3-, 2-, 1-, 8- (or meso), 7-, 6-, and 5-positions. The last values in parentheses are average. b Ring deviations (or mean-plane angles) between 5/6, 6/5, and 5/5-membered rings in BP core. The values in parentheses are the mean deviation from regression plane. c For one of the possible conformers as whizzing toroid, which was geometrically optimized with a constraint (dihedral angle at meso position was fixed at 90°). d B0’: 8-p-Anisyl-1,2,3,5,6,7-hexaphenyl boron dipyrromethene. The values in brackets are from X-ray crystal structure.37

The structure of B7 dimer were briefly discussed with an aid of theoretical calculation at the DFT-D3(BJ)-TPSS/def2-SV(P) level. We considered dimer models in which BPs are located in either head-to-tail or head-to-head manner on both side of each (C)- or (CC)-monomer. Due to the considerable complexity of the conformational variation of blades, we were not able to confirm the global minimum structure. However, the head-to-tail dimers were always optimized at local minima in quite similar energies (ΔE < 0.1 kcal mol-1), while head-to-head dimers were considerably high in energy. Such dimers were all aligned with a considerable separation (r) and displacement (d) in ca. 5.1 and 2.4 Å, respectively, in order to fill void spaces around boron atoms. In addition, the propeller blades around meso positions became more perpendicular by



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sterical demand (with tilt angels on other blades being adjusted), in accord with observed UV-vis spectral shift and decreased CD intensities at the transitions of propeller blades. Although the exciton chirality method is often applicable in predicting the CD spectra of bichromophoric systems, apparent failure was discussed recently in the BP dimers, where μm term cannot be ignored.29 Thus, we directly performed the CAM-B3LYP/def2-SVP calculations for estimation of CD spectra of this dimer. Consequently, the signs and strength of bisignate Cotton effects were reasonably well reproduced for the dimer depicted in Figure 3, inset (see Figure S25 in SI), suggesting the preferred plus-screw orientation of two BP cores (φ ≈ 15°). It is also worth mentioning that formation of dimeric BPs has been reported in confined media such as silicate glass40 or proteins.41 In these studies, the J-type dimer was found highly fluorescent, but H-type association deactivate the fluorescence process. Our dimer model, that circumvents H-type association, is in accord with its moderate fluorescent efficiency. Finally, we measured the CPL activity of B7 in methylcyclohexane (Figure 4). Strong fluorescence observed at BP chromophore at 25 °C was slightly enhanced at -120 °C (lower curves). Although nearly negligible CPL was only detected at 25 °C, this was remarkably improved at -120 °C, affording strong positive CPL with a large dissymmetric factor, glum = 2.0  10-3 (at 650 nm), ascribable to the B7 dimer (upper red curve). It is also to note that the sign of CPL was opposite for that observed for H6.


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+0.0006

+0.0003

0

-0.0003

550

600


800

Figure 4. CPL (top) and total luminescence (bottom) spectra for B7 in methylcyclohexane at 25 (black) and -120 °C (red). Excitation wavelength: 440 nm. [B7] = 25 μM.

In summary, we studied the (chir)optical properties of boron heptaaryldipyrromethene (B7) in comparison with propeller chirality of hexaarylbenzene (H6). At high temperature domain (>Tc = 10 °C), whizzing toroids play substantial role for the observed (chir)optical properties, but Tc in B7 was much higher than that of H6, in accord with the larger separation between the blades and incomplete formation of circular toroids. At lower temperature, the C-CC equilibrium becomes dominant, analogous to H6. Although weaker than H6, considerably enhanced CD (at the transitions of aromatic blades) was observed owing to the (quasi)propeller chirality also evolved in B7. Most surprisingly, unprecedented dimer was profoundly contributed at temperature lower than Td (= -70 °C) for which strong bisignate Cotton effects at the transitions of BP-chromophore became apparent (A = 193 M-1 cm-1). A theoretical calculation revealed that void spaces around



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boron atoms are crucial to arrange two heptaaryl-BP units in a head-to-tail manner. Due to the bulky substituents on the propeller blades, BP chromophores in the dimer are considerably separated (≈5.1 Å) and substantially displaced (≈2.4 Å) and tilted (≈15°) to avoid fluorescent quenching, usually observed for H-type dimer. Consequently, this supramolecular dimer afforded intense CPL activity (glum = 2.0  10-3) with a large fluorescent quantum yield (Φlum = 0.45). Such strategy (i.e., supramolecular dimer formation) may well be employed an alternative approach for developing or designing superior chiroptical materials.

Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acs.jpclett.xxxxxx. Experimental and theoretical details and extended spectral data (PDF)

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; Fax: +81-6-6879-7923 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT


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Financial supports by Grant-in-Aids for Scientific Research, Challenging Exploratory Research, and on Innovative Areas "Photosynergetics" (Grant Numbers JP15H03779, JP15K13642, JP15H01087) from JSPS, the Matching Planner Program from JST (Grant Number MP27215667549) and MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2014-2018, are greatly acknowledged.

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Propeller Chirality of Boron Heptaaryldipyrromethene: Unprecedented

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