Transient Absorption Measurements in THF

Transient absorption (TA) spectra recorded in THF are presented in Figure 7.4, and are very similar independently of the number of linked NDI units. They are characterized by a negative band around 430 nm that can be assigned to the ground-state bleach (GSB) of the S2 ←S0 transition of the ZnP unit, structured negative bands with maxima around 560 and 605 nm due to the GSB of the S1 ← S0 transition of NDI, and positive features in a region from 450 to 550 nm and above 640 nm that could be ascribed to the excited-state absorption of NDI or/and ZnP unit (Chapter 6). The overall dynamics is much faster then that found with the individual ZnP constituent,^{42, 131} and points to a new decay pathway of the excited state.

However, the TA spectra do not present any feature that could be assigned to a charge-separated state (CSS). This might point to a faster charge recombination (CR) than CS. Therefore, the CSS population does not accumulate enough to be clearly visible in the TA spectra. Nevertheless, one difference is observed in the region between 430 and 470 nm, namely the appearance of a band between 415 and 450 nm that shifts to the blue with time, and is a typical sign of a vibrational hot ground state (HGS) of ZnP (Chapter 6). It becomes more pronounced after a few picoseconds and with increasing number of attached NDIs. Additionally, a weak initial increase of TA signals above 600 nm is observed in the early dynamics. It might be associated with the presence of the ZnP radical cation, or NDI radical anion (Chapter 6), that is overlapping with the ESA of the arrays in that region.

The temporal evolution of the TA data was analyzed by using global target analysis assuming a 𝐴^!!𝐵^!!𝐶 ^!! 𝐷^!!𝐺𝑆 scheme. In the case of the arrays with one and two NDIs, τ1 (A-B step) was kept fixed to the ones obtained from up-conversion measurements. This approach yielded the species-associated difference absorption spectra (SADS) shown in Figure 7.5 (panels A and B). The step A-B is attributed to CS, and the SADS of B is very similar to that observed with the ZnP-HNDI dyad in Chapter 6 and which can be ascribed to CSS. The step B-C yields the same time constant for both arrays, and can be attributed to CR. The much faster time constant for the CR process obtained from target analysis is in good agreement with

7.4 Transient Absorption

our assumption of faster CR than CS, and explains the absence of strong features related to the CSS. In the case of the arrays with three and four NDIs, the time constant, τ2, for CR is fixed to the value obtained from the analysis of the first two arrays Figure 7.5 (panels C and D).

Figure 7.4 Transient absorption spectra recorded with the ZnP-NDI arrays in THF at selected time delays upon 400 nm excitation together with the negative of steady-state absorption spectra (black dotted-line).

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Figure 7.5 Species-associated difference spectra obtained from target analysis assuming an A → B → C → D → GS scheme of the transient absorption data measured with the ZnP-NDI arrays in THF upon 400 nm excitation.

In all cases, the resulting SADS are very similar independently of the number of attached NDI moieties (Figure 7.5). As it has been previously

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7.4 Transient Absorption

explained, the HGS becomes more pronounced with increasing the number of the attached NDIs, and this is also confirmed by global target analysis.

This can be explained by an acceleration of CS as the amount of NDIs gets larger, namely increasing a number of the acceptors speeds up CS but does not affect CR. However, as it can be seen from the data analysis (Figure 7.5), the CS time constant does not really accelerate with the number of the attached NDIs, and in some cases it is rather the inverse, namely the CS time constants found for ZnP-2NDI and ZnP-3NDI are approximately the same, while the CS process with the maximum number of attached acceptor groups, ZnP-4NDI, is found to be the slowest one. It can be explained by the fact that the core ZnP loses its donating properties with increasing number of the attached NDIs that are substituting on the amino groups of the anilines.

The same measurements have been performed upon 555 nm excitation, and are presented in Figure 7.6. Excitation at this wavelength leads to the population of the Q state of ZnP. The resulting TA spectra and their temporal evolution are essentially the same as those measured upon 400 nm excitation, and therefore are attributed to the same processes (Figure 7.7).

Consequently, one can conclude that CS should take place from the ZnP unit in the Q state. It means that IC in the investigated arrays is ultrafast, and occurs on a faster timescale then it found with the individual ZnP constituent.¹³¹ Such acceleration can be caused by its substitution leading to structural distortions and has already been observed with ZnP containing systems.41, 99, 134

Figure 7.6 Transient absorption spectra recorded with the ZnP-NDI arrays in THF at selected time delays upon 555 nm excitation.

-40 -20 0 20

ZnP-2NDI in THF 0.4 ps 10 ps 2 ps 60 ps

4 ps A

-40 -20 0 20

∆A x 103

ZnP-3NDI in THF 0.4 ps 10 ps 2 ps 60 ps

4 ps B

-60 -40 -20 0 20

700 650 600 550 500 450 400

wavelength / nm ZnP-4NDI in THF

0.4 ps 10 ps 2 ps 60 ps

4 ps C

7.4 Transient Absorption

Figure 7.7 Species-associated difference spectra obtained from target analysis assuming an A → B → C → D → GS scheme of the transient absorption data measured with the ZnP-NDI arrays in THF upon 555 nm excitation.