The amide I mode

Spectral evolution

A sample of the spectral evolution of the amide I mode is displayed in Figure 3.13.

Even at rather short delays after the pump pulse, a substantial bleach of the original absorption and shift of optical density to lower frequencies is visible. This is a mani-festation of what has been dubbed “harmonic energy flow”[7, 75, 81], i.e. the excitation of normal modes involving non-azulene atoms as a consequence of the internal con-version of the azulene moiety and the coupling of the amide I mode to them. Contrary to the azulene mode discussed in section 3.3.2, the signal of the amide I mode exhibits a further rise to reach its maximum at approximately 2.15 ± 0.05 ps (see section 3.3.3).

The agreement of the higher frequency flank of the bleach with the stationary is quite good (notice that the shoulders in the FTIR spectrum do not belong to the amide I absorption), indicating practically no anharmonic constants greater than zero.

Chapter 3. Intramolecular vibrational energy transport

1610 1620 1630 1640 1650 1660 1670 1680 1690 1700

−1.8

−1.6

−1.4

−1.2

−1

−0.8

−0.6

−0.4

−0.2 0 0.2 0.4 0.6

˜ ν/cm⁻¹

∆A/mOD

FTIR 0.8 ps

1.3 ps 2.5 ps 3.8 ps 5.9 ps 10.4 ps 16.8 ps 25.7 ps 37.6 ps 101.0 ps

Figure 3.13:Transient difference spectra of the amide I absorption of

O

N O O O O O O N3

H

in CH₂Cl₂after excitation at 610 nm.

An evident characteristic of the signal of the amide I mode is that its integral is non-vanishing, i.e. the absorption of the newly populated vibrational states upon exci-tation (positive signal) is less intense than that of the in turn depopulated states (neg-ative signal, “bleach”). Accordingly, the harmonic approximation of the transition dipole moment is not valid in this case and a computation of spectra according to subsection 3.3.1 cannot be supposed to agree with the experimental data without a better-than-harmonic description of the transition dipole moment.

Close inspection of spectra of similar bleach intensity before and after reaching maximum signal intensity – two pairs of which are given in Figure 3.13 – yields an-other insight. Most prominently, the maximum of the bleach – but to a lesser degree also other features – is shifted to higher frequencies by a few wavenumbers in spec-tra recorded at delays after the maximum signal as compared to those recorded before that point. As delays increase, the maximum of the bleach gradually shifts to smaller

3.3. Investigation via UV pump IR probe spectroscopy frequencies, even beyond the position it exhibited in the spectra of very short delays.

Presumably, the “later” spectra depict a situation in which IVR is largely complete; the average intramolecular energy distribution is nearly “thermal” (see subsection 3.5.2);

and energy release to the solvent is prevalent. Consequently, the red-shift of the bleach in “earlier” spectra is a signature of the non-equilibrium distributions occurring dur-ing progressdur-ing IVR and thus an indication of its ballistic nature. It can tentatively be regarded as the signature – although a weak one – of a traveling wave packet as has been suggested based on the conical intersection between the S1 and S0 states of azulene^[124].

Finally, the residual signal due to the rise of the temperature of the sample so-lution is not quite as marked as in the case of the azulene ring distortion mode (see section 3.3.2), thus indicating a somewhat smaller sensitivity of the amide I band to temperature.

Temporal evolution and comparison of substances

The evaluation used for the ring distortion vibration of the azulene moiety as described in section 3.3.2 was carried out analogously for the amide I vibration, i.e. the CO stretching vibration of the amide group. Here, however, a bi-exponential function was used to fit the data. Subtraction of an offset – the asymptotic value – and normaliza-tion to the maximum value were carried out as before. On the same grounds as stated for the azulene absorption in section 3.3.2, this can be regarded an acceptable neglect of the rising temperature of the solution. The result is displayed in Figure 3.14. As a characteristic figure, the maximum (negative) signal valuetmaxwas chosen, since this is the least ambiguous parameter and has been used by others before^{[8, 63]}.

Within error limits, all values oftmaxagree and even the kinetic traces are all fairly similar, which may at first seem contradicting the results obtained for the values of t1/2 for the azulene moieties in section 3.3.2. First, however, the differences in the signals of the azulene moieties are largest in the two molecules not containing any amide group, i.e. azulene and ^O , which obviously cannot be compared with the amide I signals here. Second, among the remaining substances, differences int1/2 are minute and occur only at fairly large delay values (& 10 ps), when IVR is practically complete (cf. section 3.3.4). Before that point the kinetic traces obtained for the azu-lene ring distortion mode are at their initial plateau value. Finally, the value oftmax

obtained for

O N H O

being somewhat larger than the remaining values might be

Chapter 3. Intramolecular vibrational energy transport

Figure 3.14:Kinetic traces of the amide I mode. Measured in CH₂Cl₂using an excitation wave-length of 610 nm.τ₁andτ₂are the time constants of the bi-exponential fit function.

3.3. Investigation via UV pump IR probe spectroscopy reminiscent of that effect, although it has to be conceded that its error limits contain the overall average of 2.15 ± 0.05 ps. Generally speaking, while the amide I mode signal depicts IVR and, more precisely, the similarity of IVR in the substances investi-gated, the signal of the azulene group discussed in section 3.3.2 is a better sensor of the succeeding VET processes.