3. Results

Raman spectra of the two distinct polymorphs of 1,1′-binaphthyl

The Raman spectra of two purified crystals were measured at room temperature as shown in figure III-5. For a comparison sake, the Raman spectrum of the commercially obtained 1,1′-binaphthyl was also recorded (the inset of figure) clearly seen from figure III-5

98% pure sample

Figure III-5. Raman spectra of the two forms of crystalline 1,1′-binaphthyl.

The inset is the Raman spectrum of commercially obtained 1,1′-binaphthyl.

The spectra were measured with 30 mW laser power, 2.7 cm⁻¹ spectral resolution, and 0.2 sec exposure.

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that strong fluorescence is observed for the commercial crystal with the spectral pattern being almost the same as the cisoid form, whereas no appreciable fluorescence is detected for both cisoid and transoid forms after purification. In addition, the spectral pattern in the region of

>150 cm⁻¹ of the two crystal forms is consistent with the literature [67, 68]. The Raman intensities in this region appear to be similar. It should be noted that the bands in the higher-frequency region is much weaker in intensity than the bands in the low-frequency region <150 cm⁻¹, which indicates that these low-frequency vibrational motions accompany large polarizability changes than the intramolecular vibrations.

Figure III-6 shows the Raman spectra of the two polymorphs of crystalline 1,1′-binaphthyl in the −200–+200 cm⁻¹ region. The Raman bands below 200 cm⁻¹ both in Stokes and anti-Stokes sides have been recorded simultaneously. For the transoid form, the

Figure III-6. Low-frequency Raman spectra of two crystalline 1,1′-binaphthyl.

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±26 cm⁻¹ bands, which are very close to the Rayleigh scattered peak, give the strongest intensity in the whole spectrum. On the other hand, the most intense band for the cisoid form is the doublet around 105 cm^-1, which may arise from a splitting of one band into two peaked at 100 and 110 cm⁻¹. The three Raman signals in the range of 46 to 90 cm⁻¹ appearing in both crystal forms give similar peak positions. However, their band intensity in the transoid form is quite strong compared with that of the cisoid form. Such detailed information on the low-frequency Raman spectra of the crystal polymorphs of 1,1′-binaphthyl has been obtained for the first time.

It is noteworthy that there are two small peak shoulders in the transoid form near 20 cm⁻¹ and 44 cm⁻¹, which may be due to the lattice symmetry. These shoulders are so small that they can hardly be separately recorded. Thus, in order to investigate these shoulders appearing in the present data, high spectral resolution or polarized Raman measurements might be required in the future.

Raman spectral change during the rapid heating

Figure III-7 shows the low-frequency Raman spectra of a rapidly heated crystal of the transoid 1,1′-binaphthyl. Although each spectrum was recorded with as short as 0.2 sec heating. Moreover, the central band at 0 cm⁻¹ gets broad enedand the S/N ratio of the spectra become lower at 7.6 sec. It may indicate that the liquid phase of 1,1′-binaphthyl appears at this moment. If we take a closer look at the two bands at 55 cm⁻¹ and 66 cm⁻¹, they are

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getting closer to each other as the temperature increases. However, the spectral pattern remains the same for the 79 cm⁻¹ band before 7.4 sec. It is likely that these two bands are associated with quite similar vibrational motions and hence coupled to one another.

Tracing of the melting process of the cisoid form of 1,1′-binaphthyl has also been done and the result is shown in figure III-8. Normalization procedure was applied to all the spectra as mentioned before. Similarly to the previous experiment, the crystals melt at 6 sec judging from the 0 cm⁻¹ band broadening and the reduced S/N ratio. We observe that the relative intensity of the two peaks at 100 and 110 cm⁻¹ (strong doublet band) varies with increasing the temperature. It seems that the 100 cm⁻¹ band intensity decreases upon melting. It is also clear that the 75 cm⁻¹ band disappears soon while the bands at 57 and 65 cm⁻¹ remain nearly unchanged. It is in contrast with the result in the transoid form.

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Figure III-7. Low-frequency Raman spectra of the transoid form with rapid heating.

Each spectrum was measured with an exposure time of 0.2 sec, and the laser power and spectral resolution were 36 mW and 2.7 cm⁻¹, respectively.

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Figure III-8. Low-frequency Raman spectra of the cisoid form with rapid heating.

Each spectrum was measured with an exposure time of 0.2 sec, and the laser power and spectral resolution were 31 mW and 2.7 cm⁻¹, respectively.

39 shoulders for the transoid form. Since the Stokes and anti-Stokes sides of each Raman band have the same peak position and band shape, the Lorentz function used for the fitting should be symmetrical with respect to the 0 cm⁻¹ Raman shift, like the following function.

( ) ∆

+

( ) ∆

where is the Raman shift, is the peak position, is the band intensity, and ∆ is the bandwidth. This function has two peak positions at + (Stokes side) and − (anti-Stokes side). If is zero, equation III-1 reduces to one single Lorentz function located at 0 cm^-1, representing the Rayleigh scattering.

The intensities of the Stokes and anti-Stokes bands are not the same, hence an important factor must be included in equation III-1. As we mentioned in Chapter II, the anti-Stokes/Stokes intensity ratio is related to the Boltzmann distribution in the vibrational energy levels (equation II-2). Considering the Stoke and anti-Stokes intensity difference results from the thermal effect, it can be compensated by multiplying the symmetrized Lorentz function (equation III-1) and the Bose-Einstein factor ( ( )) [79] :

(III-1)