Photogeneration of quinone methide from adamantylphenol in an ultrafast non-adiabatic dehydration reaction

The ultrafast photochemical reaction of quinone methide (QM) formation from adamantylphenol was monitored in real time using femtosecond transient absorption spectroscopy and fluorescence upconversion in solution at room temperature. Experiments were complemented by theoretical studies simulating the reaction pathway and elucidating its mechanism. Excitation with sub-20 fs UV pulses and broadband probing revealed ultrafast formation of the long-lived QM intermediate directly in the ground state, occurring with a time constant of around 100 fs. UV-vis transient absorption data covering temporal dynamics from femtoseconds to hundreds of milliseconds revealed persistence of the absorption band assigned to QM and partially overlapped with other contributions tentatively assigned to triplet excited states of the adamantyl derivative and the phenoxyl radical that are clearly distinguished by their evolution on different time scales. Our data, together with the computations, provide evidence of a non-adiabatic photodehydration reaction, which leads to the formation of QM in the ground state via a conical intersection, circumventing the generation of a transient QM excited state.


Deconvolution of rise time in transient absorption data 1.1 Results of transient absorption using THG of 120 fs at 800 nm as a pump pulse
Since the results of transient absorption measurements didn't show significant dynamics in first few picoseconds after excitation with 267 nm, 120 fs pulses we performed fitting of several kinetic profiles (at 425, 500 and 600 nm) focusing on first 10 ps. For the model function we used two exponential functions convoluted with identical Gaussian function to account for instrument response function (IRF) of our transient absorption spectrometer. Time constant of the first exponential function (t1) having negative amplitude then represented deconvoluted rise time of kinetic profile at that particular wavelength. In order to reduce number of free parameters, time constant for the exponential decay was fixed at 1 ns. Where needed, we expanded this model to account for coherent artifacts that could not be removed from the data without significantly affecting measured kinetic profiles. Hence, we included in the model function additional Gaussian in Figure S1 to account for two-photon absorption (TPA) signal around zero delay time, and two Gaussians were added to account for cross phase modulation signal (XPM) superimposed on exponential rise (Figures S7 to S10). It is known that, it the case of broadband chirped probe pulse, XPM signal splits in two, where the transient optical density will exhibit a gain (ΔOD<0) followed by a loss (ΔOD>0), i.e., a dispersion shape for the first XPM signal, while at increased delay times second XPM-induced pump-probe signal is present, which is out of phase with the first one. 1 In addition, loss and gain amplitudes in this case are not identical, and only the overall signal averages to zero. This prompted us to use two Gaussian functions with the position of the center of the peak and peak amplitude being free parameters for modelling XPM artifacts.  To evaluate slow decay of signal at longer wavelengths that we believe originates from solvated electrons we performed simple bi-exponential fitting for the entire range of time delay points (up to 1.1 ns) for kinetic profiles corresponding to 500, 550 and 600 nm.

Figure S4
General model:

Results of transient absorption using sub-20-fs UV NOPA pump pulse
The same fitting procedure was used for extracting deconvoluted rise time of second set of measurements with better IRF due to the sub-20-fs UV pump pulse driving the photochemical reaction.
General model:

Figure S7
General model:

Figure S8
General model: Finally, in order to reduce the noise in kinetic profiles we integrated the signal in 400-500 nm wavelength range. By doing so we also obtained better contrast for coherent artifacts. The XPM superimposed on the initial rise of the time trace was accounted for by two Gaussian functions of opposite sign. It is reasonable to assume that TPA, being stronger at shorter wavelengths, also contributes to this artifact additionally changing the shape and ratio between positive and negative amplitudes of XPM. For negative delay times we also observe a weaker replica of the XPM artifact visible at time zero, shifted in time due to dispersion of the first cuvette window.

Intensity dependence of transient absorption spectra
In order to clarify the origin of the signal at longer wavelengths that we attributed to solvated electrons we measured excitation intensity dependence on the transient absorption spectra for both the sample and pure solvent at fixed delay of 10 ps that show quadratic dependence in the case of pure solvent as expected for twophoton ionization. Intensity dependence for sample is almost linear suggesting that it stems from free electrons resulting from radical cation formation which is one photon process as is shown in Figure 8. with only minor contribution of two-photon ionization of solvent molecules.

Figure S11
Excitation intensity dependence on the transient absorption spectra of adamantylphenol at fixed delay of 10 ps at 425 nm (left) and 600 nm (right) that show linear dependence.

Figure S12
Excitation intensity dependence on the transient absorption spectra in pure solvent at fixed delay of 10 ps at 425 nm (left) and 600 nm (right) that show quadratic dependence as expected for two-photon ionization.

Instrument response function
Instrument response function measurement was taken with identical experimental conditions prior to FlUC measurments of adamantylphenol. Figure S18. IRF of FlUC measurements with 272 nm excitation wavelength. IRF is equal to =680 fs.

Global analysis of FlUC data
The spectro-temporal signal can be describe by three components: the first describes a rise of the signal in 0.46 ps, the second describe a decay of ca, 1/3 of the total signal. The last component is set to 2 ns (infinity in our spanned time window). The presence of the first component is necessary because the rise of the signal is much slower than the measured IRF (630 fs). If we carry out the global fit procedure with free IRF we can remove the 0.5 ps component but the IRF would be 1.2 ps (and the time zero would be accordingly shifted). It is noteworthy that the first component equalizes the sum of the other two components. This speaks for a signal, which is delayed by 0.5 ps and is not directly populated upon excitation. S17 Figure S19. Results of global analysis using fixed value of 0.68 ps for the IRF.
Since FlUC measurements could not be performed at magic angle, it is possible to ascribe 22 ps decay to rotational anisotropy decay. Actually, 22 ps is too short for such processes (typically 100-200 ps in conventional solvents) but due to the limited scan range of 50 ps we cannot distinguish between 20 ps and 100 ps. In global analysis, the effect to change the time constant would be only to change the relative amplitude of the second and third DAS component (see Figure S19). To check this explanation, we carried out an analysis where we changed the second component in order to obtain a ratio 2:1 between the second and the third component. Indeed, a pure rotational anisotropy would induce a reduction by 2/3 of the initial signal with respect to the fully randomized one. The outcome of this analysis is shown in Figure S20. Under the 2:1 condition, the second time constant changed to 90 ps, which is compatible with a rotational diffusion process Figure S20. Results of global analysis with imposed rotational anisotropy decay.

FlUC of anisole molecule
In order to verify both IRF and the presence of the 0.5 ps rise in the adamantylphenol signal we performed additional FlUC measurements using anisole as a reference system. Only the two long components were necessary for the fitting of data. Any attempts to add a rise component or to impose 1 ps IRF were rejected by the fitting. When the IRF parameter was set free the fitting procedure gives 0.79±0.08 ps.        Table S4. Electronic energies, zero-point vibrational energies, enthalpies and Gibbs energies of 3, 3', TS, and 4 in hartree computed at the PCM(water)/(TD-)ωB97XD/6-311++G(d,p) level of theory.