From the time-resolved fluorescence measurements, we could directly observe the PT from the D and role of the bridge, solvent, and the presence of the A in the deprotonation process of the D. However, as discussed, the fluorescence measurements do not provide direct evidence for the protonation of the A during the PT process due to the overlap in the D and A emission spectra and the low quantum efficiency of the A. Therefore, we have employed ultrafast transient absorption (TA) spectroscopy in the UV-visible region to observe this process. Here, we used only MeOH as a solvent since the solubility of the samples in CHCl3 was not sufficient to perform TA measurements. At first, to assign the various measured signals to either the protonated or deprotonated states of the D or A, we measured only the UAAs in their different states (Fig. and 4b for the TA spectra at 100 ps pump-probe delay and Figure at different delays). For the D (Fig. ), upon deprotonation, we observed a shift in the negative stimulated emission (SE) band from 405 to 465 nm (consistent with the fluorescence spectra of D, Fig. ) and a change in the positive excited state absorption (ESA) band at 350 nm. For the A (Fig. ), upon protonation, we observed a red shift in the broad ESA band around 360 nm, a more intense broad SE band from 450 to nm, and a sharper ESA band at 560 nm. Overall, the spectral shapes of the TA spectra from deprotonated D and protonated A are distinctly different, although the SE peak positions are very close. Since in both D and A, we observed a strong ESA signal around 330-370 nm masking the ground state bleaching signals, we focused on the discussed SE (at 465 nm) and ESA (at 560 nm) signals to investigate the deprotonation and protonation processes of the D and A, respectively. and, respectively. At early delays, D-(EA)4 shows a similar pattern as that of the D, with a SE signal peaking around 405 nm, whereas D-(EA)4-A is a superposition of the TA spectra of D and A, both excited by the 300-nm pump pulse. However, at longer delays, only the D-(EA)4-A exhibits a broad and prominent dip, reminiscent of a negative signal, around 450-500 nm. Moreover, this is accompanied by a sharper ESA band around 560 nm, present only in the protonated A spectra, confirming the PT to the A. To extract the timescales of deprotonation of the D and protonation of the A, we performed evolution-associated spectra analysis of the individual D, A, deprotonated D, and protonated A (the fitting data at selected time delays are shown in Figures and and the fitting procedure is explained in the SI) . Due to the dominance of the A signals in the fitting While switching to the TA measurements of the (A)8 peptides (Fig. and 4h for the TA spectra at different time delays for D-(A)8 and D-(A)8-A, respectively), we can make two important observations. The first one is the lack of the ultrafast decay of the 350 nm band for D-(A)8 (Fig. ), in contrast to the results of D-(EA)4 (Fig. ). This observation serves as an indication for the lack of the D deprotonation in D-(A)8, which is in line with our fluorescence measurements. The second observation is that the TA spectra for D-(A)8-A exhibit a broad dip around 450-500 nm, which is not present in D-(A)8, and is highly similar in shape to the one of D-(EA)4-A (Fig. ). As discussed, this signal is attributed to the SE from deprotonated D and protonated A, which validates the PT from D to A through the (A)8 peptide bridge. Due to the poor signal-to-noise of the data for the (A)8 peptides (owing to their poor solubility), we could not perform the evolutionassociated spectra analysis. Nonetheless, qualitatively, we see similar time scales of the TA decay.