Triplet state measurements

In order to understand the key photophysical properties of the lanthanoid complexes, it is essential to determine where the triplet state of the ligand lies. The energies of the triplet states of 6a, 4 and 25 were therefore determined from the emission spectra of the compounds in a frozen methanol matrix at 77 K and compared with the ones at 77 K in the presence of Gd³⁺. The results for 4 and 25 are reported in Figure 4.13, in which both excitation and emission spectra are displayed. The emission spectrum of calixarene 4 in frozen methanol presents a broad structureless band spanning the 350 nm to 500 nm spectral region. After addition of an excess of Gd³⁺ the emission profile is exactly the same. The measured lifetime of this band was found to be 65 μs, thus the band was attributed to emission from the triplet state of the calixarene. This finding is in agreement with the fluorescence spectrum of 4 at room temperature, that shows the emission from the singlet state centered at 300 nm. The energy value of the triplet state was calculated from the 0-0 transition, indicated by the asterisk in Figure 4.13, and was found to be 25316 cm^-1, very similar to the values reported in the literature for calixarenes.^28,61,62

Wavelength/nm Molar absorptivity/M-1 cm-1

4 25 6a 6a + Gd³⁺

Figure 4.13. Normalized excitation and emission spectra of 4 (λem = 400 nm; λexc = 280) and 25 (λem = 412;

λexc = 350 nm) in methanol at 77 K. The # symbol indicates the 0-0 transition for the singlet excited state;

the * symbol indicates the 0-0 transition for the triplet excited state.

The emission spectrum at 77 K of 25 (Figure 4.13, right) displays three different bands, centered around 400 nm, 500 nm and 600 nm. The main band, spanning the visible spectral region from 390 nm to 450 nm, was attributed to emission from the singlet of the antenna. After the addition of Gd³⁺, in fact, this band remains unchanged. On the contrary, the band centered at 500 nm is slightly increased in intensity and was thus attributed to emission from the triplet state, which is enhanced due to the improvement in the efficiency of the ISC as a consequence of the presence of the heavy Gd³⁺ ion. The measured lifetimes confirmed the assignment, yielding a value of 9 μs for the triplet state; lifetime of the singlet state instead could not be measured.

The band centered at 600 nm was attributed to the formation of an aggregated state.^63,64 Emission spectra registered at different concentrations (Figure 4.14), in fact, showed increased intensity of the band at 600 nm when raising the concentration, whereas the one at 500 nm decreased greatly, probably quenched by aggregation.

Wavelength/nm

Normalized intensity/a.u.

Wavelength/nm

Normalized intensity/a.u.

4 4 + Gd³⁺

25 25 + Gd³⁺

exc= 280 nm _exc= 350 nm

*

*

#

Figure 4.14. Emission spectra (λexc = 350 nm, MeOH, 77 K) of 25 + Gd³⁺ at different concentrations.

The singlet and triplet state energies of 25 were estimated from the lowest wavelength of each emission band, as shown by the asterisk and the hash mark in Figure 4.13, and found to be 26247 cm^-1 and 20704 cm^-1.

Emission spectra of ligand 6a (Figure 4.15) at 77 K were recorded using two different excitation wavelengths, 280 nm for the excitation of the phenyl rings of the calixarene, and 350 nm to selectively excite the antenna chromophore. The emission profiles are basically given by the sum of the emission of the calixarene and the antenna alone.

Figure 4.15. Normalized excitation (λ_em = 425 nm) and emission spectra (λ_exc = 280, 350 nm) of 6a in methanol at 77 K. The # symbol indicates the 0-0 transition for the singlet excited state; the * symbol

indicates the 0-0 transition for the triplet excited state.

Wavelength/nm

Normalized intensity/a.u.

10^-3 M 10^-4 M 10^-6 M

Wavelength/nm

Normalized intensity/a.u.

6a 6a + Gd³⁺

6a 6a + Gd³⁺

exc= 280 nm

exc= 350 nm

# *

*

By exciting at 280 nm, in fact, the emission profile of the ligand is again given by a broad structureless band spanning the 400 nm to 600 nm spectral region, which is not changed by the addition of Gd³⁺. The only difference lies in the small red-shift of the band that yields a triplet state energy of 23364 cm^-1. Lifetime value at 400 nm is consistent with the lifetime of calixarene 4 triplet state and was found to be 67 μs.

Excitation at 350 nm, on the other hand, results in a structured band at 400 nm assigned to the singlet of the antenna, and in the aggregation band at 600 nm. The addition of Gd³⁺ promotes the band at 480 nm, which was therefore assigned to the triplet state of the antenna, whose energy is 20878 cm^-1. Lifetime of the triplet state was found to be 500 μs. Emission from ligand 6a was measured at different concentrations (10^-3 M, 10^-4 M, 10^-5 M, 10^-6 M) and again the intensity of the aggregation band was found to increase when raising the concentration. Triplet state energy of the aggregated form was estimated to lie around 18000 cm^-1.

Emission spectra of 4, 25 and 6a at 77 K were measured also in DCM and the values of the triplet states energies were comparable to those calculated in methanol.

After determining the triplet state energies of 4, 25 and 6a, calix[4]arene/Ln³⁺

complexes were prepared by adding stoichiometric amounts of the nitrate salts of Tb³⁺, Eu³⁺ and Yb³⁺ to 10^-4 M methanol solutions of 4 and 6a. A graphical representation of the triplet states energies, together with the accepting lanthanoid states, is reported in Figure 4.16.

Figure 4.16. Electronic states energies of ligand 6a and Tb³⁺, Eu³⁺ and Yb³⁺ ions. Filled circle denotes the emissive states of the lanthanoid with the corresponding energies reported in black.