S Aupetit

S Sobanska

Coussan

P Rousselot-Pailley

J Mascetti

A Pizzo

¶ C Aupetit

S Coussan

UV Photoreaction Pathways of Acetylacetaldehyde Trapped in Cryogenic Matrices

UV photoreaction pathways of acetylacetaldehyde trapped in cryogenic matrices

Introduction

Acetylacetaldehyde [START_REF] Rousselot-Pailley | UV Photochemistry of Acetylacetaldehyde Trapped in Cryogenic Matrices[END_REF][START_REF] Chiavassa | Vibrational Studies of Methyl Derivatives of Malonaldehyde: Determination of a Reliable Force Field for βdicarbonyl Compounds[END_REF][START_REF] Bothner-By | Conformational Preferences in Malondialdehyde and Acetylacetaldehyde Enols Investigated by Nuclear Magnetic Resonance[END_REF][START_REF] George | Nuclear Magnetic Resonance Spectra of Acetylacetaldehyde and Malondialdehyde[END_REF][START_REF] Nowroozi | Hydrogen Bonding in Acetylacetaldehyde: Theoretical Insights from the Theory of Atoms in Molecules[END_REF] (hereafter referred as AAD) is one of the most simple molecule, together with malonaldehyde [START_REF] Trivella | UV and IR Photochemistries of Malonaldehyde Trapped in Cryogenic Matrices[END_REF] (hereafter referred as MA) and acetylacetone ? ? (hereafter referred as AA), which exibits an intramolecular proton transfer between its two oxygen atoms.

From our previous works, supported by selective and broad band UV and IR irradiations, together with theoretical calculations, we know that after deposition, only the chelated enol form (hereafter also referred as chelated) is present in cryogenic matrices and H is located on one O atom (Figure 1). We have also put in evidence the ability of nonchelated forms (forms producted by UV irradiations. Figure 2) to isomerize among themselves (in MA [START_REF] Trivella | UV and IR Photochemistries of Malonaldehyde Trapped in Cryogenic Matrices[END_REF] and AA ? cases, we also induced IR nonchelated nonchelated isomerizations). One of the main issue concerning this suite of simple intramolecularly H-bonded molecules, is to understand their respective electronic relaxation pathways. If in the case of MA [START_REF] Trivella | UV and IR Photochemistries of Malonaldehyde Trapped in Cryogenic Matrices[END_REF] it seems that this relaxation occurs purely through singlet states, as supported by the theoretical study of Coe et al, [START_REF] Coe | On the Extent and Connectivity of Conical Intersection Seams and the Effects of Three-State Intersections[END_REF] who put in evidence a triple conical intersection between S 0 , S 1 and S 2 states, triplet states seem to be involved in that of AA. ? Upadhyaya et al [START_REF] Upadhyaya | Photodissociation Dynamics of Enolic-Acetylacetone at 266, 248 and 193 nm: Mechanism and Nascent State Product Distribution of OH[END_REF] suggested that in gas phase, upon a 266 or 248 nm irradiation, AA is initially prepared in the 1 (ππ * ) state, then OH fragment is formed in the lowest 3 (ππ * ) state. Upon 193 nm irradiation, AA reaches Rydberg state which crosses over to the nearest σ * repulsive state along C-OH bond and dissociates to give OH • . Phillips and co-workers [START_REF] Chen | Theoretical Studies of the Photochemical Dynamics of Acetylacetone: Isomerization, Dissociation, and Dehydration Reactions[END_REF] have carried out a comprehensive theoretical study of the photochemistry of AA. One of their main conclusion is that after having promoted S 2 ← S 0 transition, AA's electronic relaxation proceeds, first through a S 2 /S 1 vibronic interaction, then through a conical S 1 /T 1 /T 2 intersection. They add that C-O bond breaking is the predominant phenomenon in the 3 (ππ * ) state, and that keto-enolic tautomerization is very unlikely in gas phase. This theoretical work is strongly supported by that experimental ultrafast dynamics of Mestdagh et al . [START_REF] Poisson | Ultrafast Dynamics of Acetylacetone (2,4-Pentanedione) in the S 2 State[END_REF] They give a transfer time between S 2 and S 1 states of 1.4 ± 0.2 ps, after a departure from Frank-Condon region in 70± 10 ps. Therefore the question was whether or not the relaxation of AAD also involves triplet states? In cryogenic matrices two types of effects can reveal a triplet state transition: i) the "heavy atom effect" which would favour a transition through the triplet state by increasing the atomic or molecular weight of the matrix, which increases the spin-orbit coupling, and thus promotes Inter System Crossing (ISC) between singlet and triplet states; ii) the "O 2 quenching" effect which could quench triplet but also singlet states. [START_REF] Abdel-Shafi | Mechanism of the Excited Singlet and Triplet States Quenching by Molecular Oxygen in Acetonitrile[END_REF][START_REF] Schweitzer | Mechanism of Photosensitized Generation of Singlet Oxygen during Oxygen Quenching ofTriplet States and the General Dependence of the Rate Constants and Efficiencies of O 2 ( 1 Σ + g ), O 2 ( 1 ∆ g ), and O 2 ( 3 Σg ) Formation on Sensitizer Triplet State Energy and Oxidation Potential[END_REF][START_REF] Gijzeman | Oxygen Quenching of Aromatic Triplet States in Solution[END_REF][START_REF] Patterson | Oxygen Quenching of Singlet and Triplet States[END_REF] O 2 can also catalyse ISC of the partner molecule, keeping its triplet structure, [START_REF] Gijzeman | Oxygen Quenching of Aromatic Triplet States in Solution[END_REF][START_REF] Patterson | Oxygen Quenching of Singlet and Triplet States[END_REF] this phenomenon remaining minority. The complex forms between O 2 and, here, AAD, is called collision complex, [START_REF] Abdel-Shafi | Mechanism of the Excited Singlet and Triplet States Quenching by Molecular Oxygen in Acetonitrile[END_REF] illustrating the necessity of a close vicinity between O 2 and AAD. This close vicinity is necessary to achieve energy transfer between AAD and O 2 . The fate of this energy, once transferred from AAD to O 2 , depends on the nature of the collision complex formed, with or without charge transfer. Moreover, a part of the excitation energy could remain on AAD in the form of vibrational energy, leading eventually to a further reactivity. The energy gap between the triplet level of oxygen and the singlet or triplet levels of AAD also determines the efficiency of this quenching. In our case, as we are working in the cryogenic condensed phase, the viscosity of the crystal in the quenching efficiency is not relevant. Abdel-Shafi and Worall be due to a competition between quenching and the creation of radical ions. One of the main conclusions of this work will therefore be to understand whether it is the presence of a methyl (AAD) or the coupling of the two 17 (AA), compared to MA, that causes an electronic relaxation by triplet state of the AAD.

Experimental Matrix Experiments

Acetylacetaldehyde synthesis has already been described in details in our first publication. )). This is why we re-publish it. From UV theoretical results, three groups of UV absorptions can be conveniently discriminated as follows:

1. CCC CO and CCC OH with allowed transitions at 234 and 241 nm, respectively.

2. XXX CO nonchelated species with allowed transitions ranging between 209 and 221 nm.

3. XXX OH nonchelated species with allowed transitions ranging between 215 and 232 nm.

It is also very convenient to consider not the absolute values of the UV transitions, but rather their relative deviations, as shown in Figure 3. Following this reasoning, three groups of isomers can be redefined:

1. CCC OH which serves as the origin.

2. CCC CO , CTT OH , CTC OH and TCC OH which have deviations between -7 and -14 nm.

3. The remaining isomers with deviations between -20 and -32 nm.

It is very interesting to note that the open isomers closest to CCC OH are all XXX OH in nature.

As detailed in the experimental results section below, we had to consider the presence of mixed AAD-water hetero-complexes, presumably dimers. We therefore performed calculations, on those hetero-complexes, in two steps: simulated annealing calculations (semiempirical), which gives a first quick view of the semi-empirical fundamental potential surface, followed by DFT calculations on the semi-empirical minima, in order to obtain the DFT minima together with their harmonic frequency sets. This procedure has already been decribed in some of our past works.

Experimental Results and Discussion

UV Spectra

UV spectra of AAD trapped in argon and nitrogen are shown in Figures 4 and5, respectively.

What should be noted at the outset, is that it seems that there are not only chelated forms present after deposition. This fact is due to the presence of water traces in the samples as attested by the observation of the vibrational spectra we recorded just after deposition. As can be seen from calculations on AAD-water complexes (Figures ?? and As reported in our first work on AAD, 1 even if these UV spectra were recorded in liquid phase and at room temperature, we should only observe two maxima separated by ≈ 7 nm after deposition. In argon (Figure 4), immediately after deposition, bands at 272.5 and 266.5 nm, attributed to CCC OH and CCC CO , respectively, can be observed, but at the same time, bands at -14.5, -24.5 and -35 nm, relative to CCC OH . In nitrogen (Figure 5), one observes after deposition two bands at 269.0 and 258.5 nm, respectively assigned to CCC OH and CCC CO , and two others shifted by -21.5 and -32.5 nm with respect to CCC OH . From the first five seconds of irradiation, the other bands in the argon and nitrogen spectra are observed at -41.5, -49 and -50.5 nm, and -43.5 and -46.5 nm, respectively. Based on the vibrational assignments made in the previous paper, [START_REF] Rousselot-Pailley | UV Photochemistry of Acetylacetaldehyde Trapped in Cryogenic Matrices[END_REF] and by comparing the shifts in the UV absorption bands, in argon and nitrogen, with the theoretical results, we can reasonably assume that all the nonchelated isomers are formed and observed except for CCT. However, can we make a reliable electronic attribution of the UV spectra? Without selective UV irradiation, it will be hard to clearly assign all the UV bands. However, interesting information can be retrieved from the structure of these spectra. If we look closely at the theoretical results, as explained above, the nonchelated forms should present two distinct groups: one, closest to the chelated forms, comprises three XXX OH forms, should present three bands on 5 nm, and shifted to the blue between -9 and -14 nm, and a second, which presents a gap of about 6 nm with the first, should present 5 or 6 bands (at the resolution of the spectrometer, the bands of TCC OH , TTT OH and TTC OH 1 nm apart could overlap) on 12 nm and shifted to the blue between -20 and -32 nm. Under these conditions, an approximate attribution of the UV spectra in argon and nitrogen can be as follows: the bands between -35 and -50 nm in argon can be attributed to species of the second group (furthest from XXX OH ), like those observed at -46.5 and -43.5 nm in nitrogen. For the other bands, if the two observed at -14.5 and -21.5 nm, in argon and nitrogen respectively, lead us to believe that they belong to the first group, as they are in proximity to the bands of the chelated forms, the situation of those at -24.5 and -32.5 nm, in argon and nitrogen, is less clear. In this respect, it is worth noting the good correlation between the band profiles recorded in these two matrices.

This leads us to suggest that the latter two bands are due to the same isomer, whatever the matrix. The larger shifts in the nitrogen matrix are due to dipole-quadrupole (-OH...N 2 )

interactions, as already noted in our AA study. ? This weak interaction is sufficient to shift some energy levels. It may be tempting to attribute the latter two bands also to isomers of the first group, but we cannot at this stage be completely certain.

Electronic Relaxation Pathways

As already mentioned, one of the aims of this work was to identify the electronic relaxation mechanisms of chelated and nonchelated forms, following the effects of UV broad band irradiations, by recording the vibrational spectra of the chelated and nonchelated species.

We recall that MA 6 relaxes along a purely singlet path while AA ? relaxes along a triplet path. Two phenomena can be exploited to reveal a triplet state transition: i) a heavy atom effect, supposed to favour an ISC (Inter System Crossing), and consequently accelerate the disappearance of the parent species or of the photo-produced, nonchelated species, which themselves photoreact; ii) quenching of triplet states by oxygen. For these two phenomena to take place, it is necessary that the "heavy atom" or the oxygen be in the vicinity of the molecule whose passage through the triplet state is to be revealed. In the case of a cryogenic matrix, the rare gas atoms, from Ne to Xe, surround the doping molecules, while the oxygen must interact in a permanent way with the molecule of interest, in the injected gas phase, and this complex must survive the landing on the deposition surface. Therefore, possibly, not seeing any effect in the context of oxygen doping may come from the non-trapping in proximity of the molecule of interest and that of oxygen. During the kinetics we focus on the disappearance of the parent chelated species, and the growth of the nonchelated photoproduct species. The overall effects on the vibrational spectra of these irradiations are shown in Figures 6 and7, and the kinetics, in the case of Ne, are shown in Figure 8.

We will not go into detail about the heavy atom effects that we have already detailed in our first work. [START_REF] Trivella | UV and IR Photochemistries of Malonaldehyde Trapped in Cryogenic Matrices[END_REF] We did not observe any convincing effect of accelerated disappearance of the parent species on the kinetic constants. We renewed this work and again confirmed that we do not observe any heavy atom effect. Therefore, we focused on the oxygen quenching effects. Basically oxygen can acts in three ways : i) oxygen can quench singlet states; ii) oxygen can quench triplet states; iii) oxygen can catalyse intersystem crossing. Let's consider an intersystem catalysis: in this case we should have seen an effect on kinetic constants, as we should have seen a heavy atom effect. The fact is that we saw anything conlusive. If we now consider a singlet state quenching, it appears that this kind of unusual quenching is highly sensitive to diffusion. [START_REF] Patterson | Oxygen Quenching of Singlet and Triplet States[END_REF] As our experiments take place in cryogenic matrix at low temperature (4 K), diffusion is an extremely minor phenomenon, which we can therefore almost exclude, and with, this singlet state quenching. Let us consider the last scenario, oxygen quenches triplet states. If we first consider the vibrational spectra in Figures 6 and7, the dashed lines at 2143 and 1820-1720 cm -1 represent the location of CO for the first one, while the last two delimit the area in which we should observe fragments. It is clear that without oxygen, whatever the matrix, very few fragments are observed, as well as a rather small amount of CO. One can immediately conclude that without oxygen, in matrix, fragmentation is a minority relaxation pathway. Here, already by chemical intuition, if fragmentation is observed, considering the structure of chelated and nonchelated molecules (Figure 2), one suspects that in order to provoke a fragmentation of these structures it will be necessary to "break" the conjugation generated by the chelated structure, but also that, less important, of the nonchelated forms. To do this, a passage through a triplet state will be necessary. It can therefore be suggested that the electronic relaxation path does indeed pass through a triplet state. But who fragments? the chelated forms, as in the gas phase, ? or the nonchelated forms? As can be seen in Figure 8, the fragmentation takes place as soon as nonchelated forms are predominant. The first conclusion is that, it is not the chelated forms which fragment, otherwise we should have seen fragmentation since the first minutes of irradiation, but uniquely nonchelated ones. Those latter forms are "easier" to fragment because their conjugated system is less delocalized than those of chelated ones. As a result we think that nonchelated forms, because of the broad band radiation of the mercury lamp, absorb in their turn UV photons which lead them to a passage by dissociated or pre-dissociated triplet state, which will be "quenched" by triplet oxygen. An additional argument in favor of this scenario is that the triplet oxygen, when it quenches the triplet state of the nonchelated forms, will be promoted into two singlet excited states 1 ∆ g and 1 Σ + g . While the former is a long-lived state, the latter is an extremely reactive state, which will either relax to the 3 Σ - g form, or react efficiently with radical species in its close environment. The fact that we observe not only fragments in the 1820-1720 cm -1 region, but also generation of CO and CO 2 (Figures 6 and7), correlated with nonchelated forms consumption, perfectly illustrates the existence of radical oxygen in the reaction medium and thus the quenching of the triplet states of the nonchelated forms. The presence of oxygen, T 0 , in its fundamental state (even if it can be much rarer S 2 ), thus makes it possible to reveal that indeed a fragmentation process comes into play during the electronic relaxation, not of the chelated parent forms, but of that of the photoproduced nonchelated forms. Indeed oxygen is known to be highly reactive in the presence of free radicals. Once again, the passage through a triplet state is necessary to break the conjugate system and obtain fragments. We can therefore affirm that in the case of nonchelated species, there is indeed a passage through a triplet state. Does this mean that the relaxation of chelated forms does not pass through a triplet state? That is to say that the CCC to nC isomerisation would take place thanks to a passage through a singlet state (through a S 2 /S 1 /S 0 conical intersection), as in the case of MA 20 ? There is no evidence for this, but perhaps it is due to the fact that the matrix prevents fragmentation in the case of chelated forms, even when passing through a triplet state, and that the channel largely favoured is that of isomerisation. Another experimental fact needs comment: in the Figure 6 the diffusion of fragments, what will lead to their partial recombination and thus to isomerisation. This does not compromise the proposed mechanism, but illustrates that, depending on the matrices, competition between the relaxation processes does not lead to the same fragment/isomer ratii.

One last point remains to be considered: are Π * ← Π or Π * ← n transitions at the origin of these matrix fragmentations, because in the case of AA, in molecular beam, it is Π * ← Π transition that is directly at the origin of the fragmentation of the chelated form. [START_REF] Yoon | Photodissociation Dynamics of Acetylacetone: The OH Product State Distribution[END_REF] To answer this question we carried out irradiations using interference filters. The effects of these irradiations on the vibrational spectra are shown in Figure 9. It is clear that whether irradiating mainly the chelated form of the CCC CO type (266 nm, Π * ← Π) or the nonchelated forms of the XXX CO type (222 nm, Π * ← Π), no fragmentation is observed; in the first case there is isomerisation of the parent form, whereas in the second case there is mainly intra-chelated forms isomerisation. The conclusion is that these fragmentations are therefore due to transitions of the Π * ← n type, in the case of the nonchelated forms. One of these bands is observed at 443 nm in Ar (Figure ??).

Conclusion

In this work, we investigated the broadband UV photochemistry of the hybrid form of malonaldehyde and acetylacetone, acetylacetaldehyde (Figure 2), trapped in four different matrices, Ne, Ar, N 2 and Xe. This species presents two distinct forms at 4 K, whether for the chelated form, which is the only one present after deposition, or the nonchelated, photoproduced forms, because the proton is localised. One of the main questions we wanted to answer was whether the electronic relaxation path of acetylacetaldehyde was a triplet path, like acetylacetone, or singlet like malonaldehyde. And consequently to know if this difference in behaviour was due to the coupling between the two methyls. ? To detect a triplet state, two effects were explored, heavy atom effects and quenching of triplet states by oxygen.

These two types of experiments did not give salient results, notably on the value of the kinetic constants, but on the other hand, oxygen doping revealed a transition to a triplet state in the case of UV photochemistry of nonchelated species. In the presence of oxygen, which in its T 0 form is a particularly reactive with free radicals, fragmentation of the nonchelated species was observed, with subtle differences in the fragment to isomer ratio depending on the matrix.

This fragmentation process is particularly visible in the Ne and Xe matrices, which are respectively the softest (Ne), and the most spacious (Xe), allowing for non-recombinatorial diffusion of these fragments. The last two of the conclusions reached are that, unlike in the gas phase, it is Π * ← n type transitions that are at the origin of these fragmentations, and it is therefore not the coupling between the two methyls (in AA case compared to MA) that causes this electronic relaxation through the triplet state. Consequently, the end of the work on this trilogy can be summarised as follows (Figure 10): malonaldehyde exhibits electronic relaxation through singlet states, whereas acetylacetone and acetylacetaldehyde relax through triplet states. In the gas phase, acetylacetone fragments by excitation of Π * ← Π transitions of the chelated form, while in cryogenic matrix, the latter molecule as well as acetylacetaldehyde fragment by excitation of Π * ← n transitions of the nonchelated forms. under broadband UV irradiation (average power 100 W, no water filter, the beam was not focused). The original spectrum is noted t=0, the final spectrum t=32min35s. The irradiation times are: 5s, 10s, 15s, 25s, 35s, 65s, 2min5s, 3min5s, 13min5s, 32min35s. The relative shifts (a) are calculated with respect to the band at 272.5 nm. The wavelength (b) values have been given by deconvolution fits presented in Figure S1. Figure 5: Evolution of the UV spectra of AAD trapped in nitrogen matrix (AAD/N 2 = 2/1000) under broadband UV irradiation (average power 100 W, no water filter, the beam was not focused). The original spectrum is noted t=0, the final spectrum t=32min35s. The irradiation times are: 5s, 10s, 15s, 25s, 35s, 65s, 2min5s, 3min5s, 13min5s, 32min35s. The relative shifts (a) are calculated with respect to the band at 272.5 nm. The wavelength (b) values have been given by deconvolution fits presented in Figure S1. Integration intervals for CCC: 1476-1452 cm -1 ; for nC: 1718.6-1712.9 cm -1 ; for CO: 2166-2118 cm -1 ; for CO 2 : 2358-2362 cm -1 ; for fragments: 1780-1720 cm -1 . Dashed line is located at 49 min. Values have not been normalized. Filter@266 nm CO Figure 9: Effects of interferential filters on O 2 doped Ne matrice in ν C=C/C=O region. Average UV lamp power: 160 W (no beam focussing, no water filter). Dashed lines are centered at 2343 (CO 2 ), 2143 (CO), 1820 and 1720 cm -1 . The last two delimit the region in which the AAD fragments are to be observed. Bottom spectra: AAD/O 2 /Ne = 1/1/1000. Black spectra: spectra after deposition; green spectra: after 4 min irradiation without filter, to get nonchelated forms, red spectra: spectra after 505 min UV broad band irradiation using a 50% transmission filter centered at 222 nm. Top spectra: AAD/O 2 /Ne = 1/1/1000. Black spectra: spectra after deposition; red spectra: spectra after 509 min UV broad band irradiation using a 50% transmission filter centered at 266 nm.

TCC OH/CO TCT OH/CO R R 2 R 2 R 1 R 1 R 2 R 1 R 2 R 1 R 2 R 1 R 2 R 1 R 2 R 1 R 2 R 1 = H or CH 3 R 2 = H or CH 3 R 1 R 2

18 ?

 18 Some of those forms are shown in Figures ?? and ?? together with their relative energies with respect to the two most stable CCC CO -H 2 O and CCC OH -H 2 O forms. The main lesson of these two-step calculations is that the presence of a water molecule in the vicinity of the chelated form of AAD is sufficient to induce the opening of the pseudo-ring and consequently to observe nonchelated forms as soon as they are deposited (as discussed below in the experimental results section).In order to identify the experimental maxima, displayed in Figures4 and 5, we have matched the UV spectra with a deconvolution laboratory-made program.(Figure ??).

  ??), the presence of water induces the existence of nonchelated isomers. If we take into account the relative shift of these bands with respect to CCC OH , they are mainly of a XXX OH nature, althougth the theoretical results show that the three nonchelated forms XXX CO -water are slightly more stable than those XXX OH -water (Figures ?? and ??). If we proceed by analogy by matching the nonchelated forms XXX CO -water and XXX OH -water, the energy differences are systematically in favour of the XXX CO -water forms: CTC CO is more stable than TCT OH by 2.55 kcal mol -1 , TTC CO by 3.07 kcal mol -1 compared to TTT OH and TCT CO by 1.4 kcal mol -1 compared to CTT OH .

  it seems that this fragmentation occurs mainly in the Ne and Xe matrices. It is clear that fragmentation takes place in Ar and N 2 , but unlike Ne and Xe, a non-negligible amount of nonchelated species remains when photostationary equilibrium is reached. This observation is accompanied by the fact that much less CO and CO 2 are generated. This fact illustrates perfectly the role played by the matrix and its influence on the relaxation processes. The fact that the observation of almost complete consumption of the nonchelated species associated with fragmentation occurs in Ne and Xe, is certainly due to the fact that Ne is the softest matrix (crystal lattice parameter: d NN = 3.16 Å; lattice stiffness constant: κ = 3.60 10 3 × mdyn Å-1 ) and Xe has the largest cages (crystal lattice parameter: d NN = 4.34 Å; lattice stiffness constant: κ = 10.80 10 3 × mdyn Å-1 ). Indeed, if the matrix allows a better diffusion of fragments because of the size of its cages or because of a low stiffness constant, it is easy to understand that after dissociation the fragments do not recombine. The argon and nitrogen matrices, which have similar characteristics ( Ar: crystal lattice parameter: d NN = 3.75 Å; lattice stiffness constant: κ = 8.20 10 3 × mdyn Å-1 ; N 2 : crystal lattice parameter: d NN = 3.99 Å; lattice stiffness constant: κ = 5.40 10 3 × mdyn Å-1 ), will not ease

Figure 4 :

 4 Figure4: Evolution of the UV spectra of AAD trapped in argon matrix (AAD/Ar = 2/1000) under broadband UV irradiation (average power 100 W, no water filter, the beam was not focused). The original spectrum is noted t=0, the final spectrum t=32min35s. The irradiation times are: 5s, 10s, 15s, 25s, 35s, 65s, 2min5s, 3min5s, 13min5s, 32min35s. The relative shifts (a) are calculated with respect to the band at 272.5 nm. The wavelength (b) values have been given by deconvolution fits presented in FigureS1.

Figure 6 :Figure 7 :

 67 Figure 6: Effects of O 2 doping observed in the four matrices in ν C=C/C=O region. Black spectra: spectra after deposition; red spectra: spectra after UV broad band irradiation. Average UV lamp power: 160 W (no beam focussing, no water filter), excepted for AAD/Ar matrice for which a 300 W power has been used. Dashed lines are centered at 2143 (CO), 1820 and 1720 cm -1 . The last two delimit the region in which the AAD fragments are to be observed. MG: Matrical gas. Ar: AAD/Ar = 5/1000, irradiation time: 500 min; AAD/O 2 /Ar = 2/10/1000, irradiation time: 509 min. N 2 : AAD/N 2 ≈ 2/1000, irradiation time: 509 min; AAD/O 2 /N 2 = 2/10/1000, irradiation time: 509 min. Ne: AAD/Ne = 3/1000, irradiation time: 509 min; AAD/O 2 /Ne = 2/2/1000, irradiation time: 629 min. Xe: AAD/Xe = 1/1000, irradiation time: 509 min; AAD/O 2 /Xe = 3/3/1000, irradiation time: 469 min. * traces of water. 20

Figure 8 :

 8 Figure8: Kinetics in Ne with (right column) and without oxygen (left column). These kinetics were recorded at 4K. These kinetics were recorded without the use of a water filter and without focusing the beam, at an average power of 160 W. CCC: chelated forms; nC: non-chelated forms CTT OH /CTC OH /CTC CO . 1 (a): CO 2 values have been divided by 10. Integration intervals for CCC: 1476-1452 cm -1 ; for nC: 1718.6-1712.9 cm -1 ; for CO: 2166-2118 cm -1 ; for CO 2 : 2358-2362 cm -1 ; for fragments: 1780-1720 cm -1 . Dashed line is located at 49 min. Values have not been normalized.

Figure 10 :

 10 Figure 10: UV relaxation pathways: history of a trilogy.

Table 1 :

 1 Theoretical UV transitions calculated for the two first excited electronic states of each AAD isomers at the ωB97XD/6-311++G(2d,2p) level of theory (allowed transitions are written in bold characters). This Table has already been published in ©Coussan et al, J. Phys. Chem. A, 2020, 124, 4916-4928, but CCC OH/CO , TTT OH π * ← π and π * ← n and TCT CO π * ← n transition values were erroneous (they have been calculated with a different method: B3LYP/6-311++G[2d,2p)). Erroneous values are given in parenthesis for information.

	Isomers	Transition	Wavelength (eV) Wavelength (nm) Oscillator strengths
	CCC CO S 1 ← S 0 /π * ← n	4.3377	285(300)	0.0007(0.0006)
		S 2 ← S 0 /π * ← π	5.3017	234(241)	0.2510(0.2353)
	CCC OH S 1 ← S 0 /π * ← n	4.2269	283(308)	0.0010(0.0009)
		S 2 ← S 0 /π * ← π	5.1339	241(248)	0.2866(0.2717)
	CCT CO S 1 ← S 0 /π * ← n	3.8909	319	0.0003
		S 2 ← S 0 /π * ← π	5.6969	218	0.3390
	CCT OH S 1 ← S 0 /π * ← n	3.7335	332	0.0006
		S 2 ← S 0 /π * ← π	5.4595	227	0.3829
	CTC CO S 1 ← S 0 /π * ← n	4.1344	300	0.0001
		S 2 ← S 0 /π * ← π	5.6561	219	0.3315
	CTC OH S 1 ← S 0 /π * ← n	3.9257	316	0.0006
		S 2 ← S 0 /π * ← π	5.3902	230	0.3528
	CTT CO S 1 ← S 0 /π * ← n	4.1178	301	0.0001
		S 2 ← S 0 /π * ← π	5.5999	221	0.3293
	CTT OH S 1 ← S 0 /π * ← n	3.9075	317	0.0005
		S 2 ← S 0 /π * ← π	5.3392	232	0.3551
	TCC CO S 1 ← S 0 /π * ← n	4.1554	298	0.0002
		S 2 ← S 0 /π * ← π	5.7360	216	0.3489
	TCC OH S 1 ← S 0 /π * ← n	4.0061	309	0.0005
		S 2 ← S 0 /π * ← π	5.4518	227	0.4013
	TCT CO S 1 ← S 0 /π * ← n	4.0700	305(310)	0.0002(0.0002)
		S 2 ← S 0 /π * ← π	5.8784	211	0.3756
	TCT OH S 1 ← S 0 /π * ← n	3.8900	319	0.0004
		S 2 ← S 0 /π * ← π	5.6012	221	0.4589
	TTC CO S 1 ← S 0 /π * ← n	4.1763	297	0.0002
		S 2 ← S 0 /π * ← π	5.9195	209	0.4142
	TTC OH S 1 ← S 0 /π * ← n	4.0964	303	0.0003
		S 2 ← S 0 /π * ← π	5.7732	215	0.4823
	TTT CO S 1 ← S 0 /π * ← n	4.1365	300	0.0002
		S 2 ← S 0 /π * ← π	5.8749	211	0.4191
	TTT OH S 1 ← S 0 /π * ← n	4.0577	305(326)	0.0003(0.0002)
		S 2 ← S 0 /π * ← π	5.7295	216(231)	0.4848(0.1514)

H

  Figure 1: Keto-Enol equilibrium of malonaldehyde, acetylacetone and acetylacetaldehyde (R1 = H, R2 = CH 3 , and vice-versa). ©Coussan et al, J. Phys. Chem. A, 2020,

	124, 4916-1928

CCC OH/CO CCT OH/CO TTC OH/CO TTT OH/CO CTC OH/CO CTT OH/CO

  Figure 3: Theoretical UV absorption shifts with respect to CCC OH . CCT OH/CO are not represented because they are not observed. 1

		3.8
																AAD/Ar
		3.6		t = 32 min 35 s
				TCT CO		TCC CO		TCT OH
		3.4											CTC OH		CCC CO	CCC OH
				TTT CO		TTT OH		CTT CO	TCC OH		CTT OH
	Absorbance	3 3.2	TTC CO		TTC OH		CTC CO
				t = 0
		2.8
			-32	-30	-26	-25		-22	-20		-14		-11		-9	-7
		2.6
						a	-50.5	-49 -41.5	-35	-24.5	-14.5	-6	0
		2.4
						b	222	223.5 231	237.5	248	258	266.5 272.5
	-35		190	-30 200	210	-25	220	-20 230	-15 250 Wavelength shifts (nm) 240 Wavelength (nm)	260	-10	270	280	-5	290	0 300
	Figure 2: Picture of all AAD isomers. Nonchelated forms are obtained by UV irradiation
	of CCC, only form present after deposition. C and T stand for Cis and Trans characters
	relative to C-C, C=C and C-O bonds. OH and CO underscripts stand for the "side" methyl
	group is attached, or C=O or O-H one. For relative energies obtained at the B3LYP/6-311G++(2d,2p) level of theory, see ©Coussan et al, J. Phys. Chem. A, 2020, 124, 4916-1928 .

Acknowledgements

The authors would like to thank Mrs Frédérique Berger, glassblower, and Mr Grégory Giacometti and Mr Alexandre Poyé, for their technical assistance. It was Alexandre Poyé who developed the UV spectra band deconvolution programme.

Supplementary Material

This section contains two figures, labelled S1 and S2, diplaying the four calculated lowest energy AAD-water hetero dimers, one figure, labelled S3, displaying an example of UV spectrum devonvolution, and a last figure, labelled S4, displaying the full UV spectrum of AAD trapped in argon matrix.