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Figure 4.18 shows the initial dynamics of MGL in methanol probed at different wave- lengths together with the fitting curves. The Table 4.3 presents the results for the fitting parameters obtained by assuming multi-exponential decays and a Gaussian cross correlation (for details see Appendix A).

-1 0 1 2 3 4 0 1 2 λ_probe= 610 nm λ_probe= 460 nm λ_pump= 270 nm λ_probe= 560 nm λ_probe= 700 nm ∆ OD ∗ 100 ∆t (ps)

Figure 4.18 Time resolved data (circles) of MGL in methanol and corresponding fits (black lines) probed at 460 nm (blue), 560 nm (green), 610 nm (red) and 700 nm (pink). Traces at 460, 560 and 610 nm are shifted along x and y axes for better visibility.

Table 4.3

Fitted time constants with corresponding amplitudes in brackets

λprobe τ1 (amplitude) τ2 (amplitude)

460 nm 0.15 ps (-0.58) --

560 nm 0.15-0.4 ps (0.28) 3-4 ps (-0.55)

610 nm 0.15 ps (0.09) 2-4 ps (-0.45)

700 nm 0.4 ps (-0.25) 2-4 ps (0.46)

In the wavelength region of the DMA radical cation absorption band (460 nm) the con- tribution (build up) of the ultrafast component of 0.15 ps is the strongest one indicating that the CT process is as fast as in aprotic solvents. In the middle wavelength region (λprobe = 560 nm and 610 nm) there is first a small contribution to the signal as a decrease of absorption with a time constant of ~0.15 ps. This region corresponds to the Sn← S1 absorp-

tion band of DMA (λFWHM = 550, 650 nm) [121], which is locally excited chromophore of MGL, as discussed above (see Section 4.2 and 4.3.3). The ESA band of DMA appears in- stantaneously within our time resolution. Subsequently absorption signal decrease in this re- gion with a time constant of 0.15 ps, with which the absorption signal appears in a region of DMA radical cations. This is an additional evidence for the intramolecular CT process, which eliminates the assumption of direct CT transition. It should be noted that this fast de- cay corresponds to a time constant of low amplitude. A higher contribution to the transient absorption in this region (λprobe = 560 nm and 610 nm) comes from an absorption increase on a time scale of 2 - 4 ps, which corresponds to the appearance of the MG cation absorption band [113] as discussed above (see Section 4.6.2). This time constant indicates its close rela- tionship with diffusive solvation time of 3.2 – 3.3 ps [27, 35] or a dielectric relaxation time of 3.1 ps [112] reported for methanol (see Table 4.1). This subsequent process of solvation appears as a respond of the polar solvent molecules to a change of a charge distribution which results from initial CT process observed with a time constant of 0.15 ps. The dielec- tric solvent respond corresponds to the orientation of the hydroxyl group in methanol by rotation around its C-OH bond, which seems to be responsible for the opening of the lactone ring. As discussed above (see Figure 4.16), the chemical reaction of lactone ring opening is followed by the formation of MG cation band, which is detected with a time constant of 2 - 4 ps.

In the long wavelength region (700 nm) a decrease of the absorption signal is observed with the same time constant of 2-4 ps, what can reflect depopulation of the CT state, which corresponds to the broad band of DMA radical cation as discussed above (see Figure 4.7).

The long time behavior, shown in Figure 4.19, shows a double exponential decay at all probed wavelengths. The time constant of τ3 = 20 ps is stronger in the wavelength region of the DMA radical cations while the slower one τ4 = (300-500 ps) is stronger in the region of dissolved MG cations (amplitudes of fitted constants are shown in the Table 4.4).

Two different long time decays (~50 ps in the region of CT state and ~400 ps in the region of MG cations) imply that no population transfer between the CT state and MG+ state is possible after 4 ps, so two independent return path ways to the ground state occur. The transient spectra (see Figure 4.17) shows also a faster decay of the DMA radical cation absorption band and a slower one of the MG cation absorption band.

-100 0 100 200 300 400 0 1 2 3 λ_probe λ_pump= 270 nm 700 nm 610 nm 560 nm 460 nm ∆ OD ∗ 100 ∆t (ps)

Figure 4.19 Long time behavior of MGL in methanol and corresponding fits (black lines) probed at 460 nm (blue), 560 nm (green), 610 nm (red) and 700 nm (pink).Traces probed at 460, 560 and 610 nm are shifted along x and y axes for better visibilty.

Table 4.4

Fitted time constants with corresponding amplitudes in brackets

λprobe τ3 (amplitude) τ4 (amplitude)

460 nm 20 ps (3.03) 317 ps (0.13)

560 nm 20-40 ps (1.34) 400 ps (0.27)

610 nm 20 ps (0.51) 500 ps (0.55)

700 nm 15-30 ps (0.17) 405 ps (0.11)

The 20 ps time constant is observed over a whole observed spectrum range with different amplitudes reflecting a shape of CT state, which occurs also in MGL dissolved in acetoni- trile (see Figure 4.7). This 20 ps decay indicates a new additional relaxation process of the CT state in protic environment, which could be related to another dielectric relaxation time of methanol reported for alcohols [112, 126]. This intermediate time constant in the region from 20-50 ps originates from the rotation of the free monomeric molecule and is also observed as one time constant of multi-exponential fit describing polar solvation dynamics

[27]. It is possible that this kind of molecular reorientation of protic solvents causes faster recombination of the radical ion pair of the CT state.

The longer time constant (τ4) reflects the recombination of MG cation to the ground state. A small contribution of τ4-time constant in the wavelength region outside the MG cation absorption band (λprobe = 460 nm and 700 nm) indicates that corresponding absorption band is broader than shown with cw-spectrum (see Figure 4.17(b)).