Light induced reactions in cryogenic matrices

Rui Fausto and Andrea Gómez-Zavaglia

DOI: 10.1039/9781849732826-00001

In this chapter light induced reactions in cryogenic matrices are addressed, ranging from conformational isomerizations to complex bond-breaking/ bond-forming processes. These include reactions induced by radiation in both UV-visible and IR regions, and also photochemical processes where noble gas atoms participate directly, leading to formation of covalently bound noble gas containing molecules.

1 Introduction

In the previous issues of this series (vol. 37 and 38), we provided an extensive review on the literature dealing with light induced reactions in cryogenic matrices published during the period July 2004 – December 2009. The present chapter focuses on the reports on the same subject appearing in the specialized literature during 2010.

We will concentrate in studies dealing mainly with organic compounds, since application of matrix isolation to study photoinduced processes in inorganic/organometallic systems is reviewed shortly elsewhere in this book. Section 2 considers UV-driven photochemical processes, while section 3 deals with IR-induced ground state hot vibrational chemistry. A short section (2.4) is dedicated to photochemical processes where the matrix noble gas atoms participate directly, leading to formation of covalently bound noble gas containing molecules.

The fundamentals and technical descriptions of the matrix isolation method can be found in the books by Meyer, Andrews and Moskovits, Barnes et al., Dunkin or Fausto. More specific and recent reviews on matrix isolation and its application to the study of ligh induced processes can also be consulted by those which are less familiar with the subject, including a recently published special issue of the prestigious Annual Reports on the Progress of Chemistry, the fifth report in that series dedicated to matrix isolation (after those published in 1985, 1991, 1997 and 2001). Light-induced noble gas chemistry in cryogenic matrices has been addressed by Nemukhin et al. and by Khriachtchev, Räsänen and Gerber, who also present therein their perspectives on future developments of this field of research.

Very interesting reviews by Wentrup, and Winkler and Sander, focusing on rather recent developments in aryne chemistry, with a special emphasis on the matrix isolation of benzynes, tridehydrobenzenes and related systems were published in 2010. Other particularly relevant studies reported in this year were those by Olbert-Majkut et al., where Raman spectroscopy was used to investigate the product ratios of the H2O + CO and H2 + CO2 photodecomposition channels of formic acid in different matrices, concluding that in argon matrix the dominating process is the dehydration (H2O + CO) channel, whereas the decarboxylation (H2 + CO2) channel is the most prominent one in solid xenon, and by Lapinski et al., on the infrared induced selective rotamerization of the aminohydroxy conformers of cytosine. In this latter work, the authors were able to control in an efficient way the relative populations of two aminohydroxy conformers of cytosine, differing in rotation of the OH group by ~180°, using narrowband, near-infrared laser light. For cytosine monomers isolated in a low-temperature argon matrix, laser irradiations at 7013 cm-1 and at 7034 cm-1 were found to induce effective transformations of the two conformers into each other in a reversible way. It was also demonstrated that other forms of cytosine (amino-oxo and imino-oxo) are not affected by near-IR irradiation.

2 UV-visible-induced reactions in cryomatrices

Most of the studies on the topics addressed by the present review that were reported during 2010 refer to photochemical processes induced by UV-visible light. In this section, some of the most relevant of those studies are shortly presented. Firstly, photoinduced conformational isomerization processes will be addressed. Then, the more complex bond-breaking/bond-forming reactions will be considered, including tautomerizations and other structural isomerizations, fragmentation reactions, and formation of complex and weakly bound species. The last part of this section addresses the subject of noble gas chemistry, where the matrix noble gas atoms take an active role in photochemical reactions promoting formation of covalently bound noble gas containing molecules.

2.1 Conformational isomerizations

Kalume et al. reported experimental and computational studies of the photolysis of atmospherically important 1,2-dibromoethanes of general formula 1,2-C2X4Br2 (X = H, F) in Ar matrices at 5 K. They found that a significant conformational relaxation occurs for 1,2-C2H4Br2 (observed anti/gauche ratio = 30:1) but not for 1,2-C2F4Br2 (anti/gauche = 3:1), which was explained taking into account the larger barrier to rotation about the C-C bond in the latter compound. It was suggested that the matrix pulsed deposition method, as compared with conventional continuous deposition methods, can lead to an increased conformational relaxation for systems with low barriers to internal rotation. Photolysis of 1,2-C2H4Br2 at λ = 220 nm reveals the growth of infrared bands assigned to the gauche conformer (Fig. 1), which occurs concomitantly to production of the C2H4-Br2 charge transfer and C2H3Br-HBr complexes and the C2H4Br radical. In the case of 1,2-C2F4Br2, irradiation at 220 nm was found to lead mainly to the anti and gauche conformers of the C2F4Br radical, whose vibrational and electronic spectra were characterized for the first time. The increase in yield of radical for 1,2-C2F4Br2 was attributed to the stronger C–Br bond in the fluoro-substituted radical species.

A series of studies on UV-induced conformational isomerizations in aromatic aldehydes has been reported by Fausto and co-workers. The conformational space of monomeric pyrrole-2-carbaldehyde (P2C) was investigated theoretically at the MP2 and DFT (B3LYP) levels, with the 6-311++G(d,p) basis set. The compound can assume two conformations, cis and trans, regarding the orientation of the N–C–C=O dihedral angle. The cis form was found to be the conformational ground state, being more stable than the trans conformer by ca. 15 kJ mol-1. The relative stability of the two conformers was analyzed based on the comparison of their structures and using the natural bond orbital method. In agreement with the calculations, only the signature of the cis conformer was found in the experimental IR spectra of matrix-isolated P2C monomers. Broadband UV irradiation (λ>235 nm) led to conversion of the cis into the trans form, with a photostationary equilibrium being established when the [cis]/[trans] ratio became equal to ca. 3.3:1 in both Ar and Xe matrices. The appearance of the new bands due to the trans conformer was found to be very fast, the bands being detectable already after the first minute of irradiation. However, the profile of the kinetic curves with time of irradiation is slightly different for the photochemistry in argon and xenon matrices. In argon, the photostationary state for conformational isomerization was attained after ca. 1 h of irradiation, whereas in xenon it was about twice faster. The different behavior observed in argon and xenon matrices was explained considering the presence of an additional photochemical channel, specifically the one leading to the valence isomerization of P2C into its Dewar form. This was supported by the appearance in the spectra of irradiated samples of an absorption at 1717 cm-1 that might be ascribed to the Dewar isomer. In xenon matrix, the spectroscopic results indicated that photochemical formation of the Dewar P2C valence isomer is suppressed. These observations are in agreement with the photochemistry of benzene in argon and xenon matrices. For benzene isolated in argon matrix, photochemical production of benzvalene, fulvene, and Dewar benzene valence isomers was reported, whereas no isomerization reactions were observed in xenon matrix. The authors also concluded that the observed P2C conformational isomerization most probably occurs in the S1 state or partially in the S1 potential energy surface with relaxation to the ground state through a conical intersection, since no heavy-atom effect on the conformational isomerization yield was observed in the experiments carried out in the xenon matrix.

Particularly interesting results were obtained for 4-methoxybenzaldehyde (p-anisaldehyde). This compound possesses two almost isoenergetic conformers, which are nearly equally populated in gas phase at room temperature. Once deposited in argon or xenon matrix, the population ratio of the two forms could be reversibly varied by irradiating the sample with UV light in different wavelength ranges or by varying the temperature of the matrix.

Increasing the temperature of the xenon matrix up to ca. 57 K led to conversion of the less stable O-cis p-anisaldehyde conformer into the O-trans form, shifting the O-cis/O-trans ratio to ca. 1:7. On the other hand, UV irradiation was found to lead to wavelength-specific photostationary equilibria, characterized by the O-cis/O-trans ratios of about 1:2.2, 1:1.4, 1:1.1, and 1:0.89 for λ > 328, 295, 288, and 234 nm, respectively (Fig. 2).

The observed dependence of the photostionary equilibrium on the irradiation wavelength could be explained taking into account results of time-dependent DFT calaulations. These calculations indicated that the bright states correspond to S2 and S3, with the S1 state having neglegible oscillator strengths for both conformers. Thus, the relative dominance of the O-trans form in the photostationary states produced with excitations at longer wavelengths was attributed to a higher consumption of the O-cis form, which has a lower excitation energy to the S2 state than the O-trans form. On the contrary, the prevalence of the O-cis form in the photostationary equilibria produced with shorter excitation wavelengths results from the higher oscillator strength of the transition to the S3 state in the O-trans form.

The described results also indicated that the photochemistry of p-anisaldehyde strongly depends on the matrix medium. Indeed, for the compound isolated in argon matrix only isomerization in the O-cis ->O-trans direction could be induced (using a narrowband laser source tuned at λ = 280 nm). The reverse process could be expected to be induced by irradiation at shorter wavelengths, but, contrarily to what was observed in xenon, in argon matrix irradiation at these wavelengths led to extensive production of new chemical species instead of preferential promotion of conformational isomerization.

Another studied aldehyde was 3-furaldehyde (3FA). As P2C and p-anisaldehyde, 3FA has two conformers differing in the orientation of the aldehyde group (Fig. 3). At the B3LYP/6-311++G(d,p) level of theory, the trans form was computed to be ca. 4 kJ mol-1 more stable than the cis form. In fairly good agreement with their calculated relative energies, the two conformers could be trapped in an argon matrix from the compound room temperature gas phase in proportion ~7:1. Broadband UV-irradiation (λ > 234 nm) of the matrix-isolated compound resulted in partial trans ->cis isomerization (Fig. 4), which ended at a photostationary state with the trans/cis ratio being ca. 2:1.

The obtained population ratio at the photostationary state was explained based on results of time-dependent DFT calculations. The potential energy profiles of the excited states calculated as a function of the aldehyde group rotation (Fig. 5) showed that all of S1, S2, and S3 states have a minimum at the cis conformation, which is, in those states, more stable than the trans conformation. In addition, the bright S2 state shows a third minimum (global minimum) at a nearly perpendicular conformation of the aldehyde relatively to the molecule ring (~97°). Excitation of both conformers with λ 234 nm provides the excess energy of around 5.3 eV, which is well above all the barriers on the S2 surface and can be followed by fast internal rotation in the S2 excited state to the ~97° minimum. Relaxation of S2 from the ~97° minimum can then produce either cis or trans ground state conformers by internal rotation in S0. However, since the torsional coordinate at the minimum in S2 equals ~97° is shifted in the direction of the trans conformer, thus leading to the observed more favourable relaxation to this form.

Trivella et al. studied the UV and IR photoreactivities of acetylacetone isolated at 4.3 K in four matrices (N2, Ne, Ar, Xe), using either tunable UV and IR optical parametric oscillators, or a broadband mercury lamp. They found that, contrary to what occurs in the gas phase, E-Z and conformational isomerizations were the main reactions observed: UV irradiation breaks the strong H-bond of the stable enolic form of acetylacetone (CCC; Fig 6), leading to the observation of non-chelated forms. Among the seven non-chelated stereoisomers, only two pairs of conformers (differing by a rotation of the OH moiety around the single C–O bond) were observed: CTC/ CTT and TCC/TCT. Isomerization among these two pairs of non-chelated forms (CTC <-> CTT and TCC <-> TCT) as well as back-isomerization to the chelated form were also observed under UV irradiation. Similar reactions and reaction rates were observed for the four matrices, indicating that the inter-system crossing to the T1 state involved in the isomerization processes is very fast, probably due to efficient coupling with phonons, in contrast with gas phase where inter-system crossing is rate-limiting. The differences between the gas phase and cold solid medium photodynamics of acetylacetone were discussed. The differences between the gas phase and the cold solid medium of acetylacetone 1ππ* photoreactivities can thus be explained by a different competition between cooling and reaction, the solid medium environment accelerating cooling processes. For the molecule isolated in the gas phase, C–O dissociation is the dominant reaction and consequently, dissociation is that of the chelated form and gives OH fragments. For the molecule isolated in a cold solid medium, vibrational cooling is much faster and isomerization becomes dominant.

Lapinski and co-workers reported a very interesting study on o-hydroxy-benzaldehyde (salicylaldehyde) and o-hydroxyacetophenone, where non-hydrogen-bonded isomers of the compounds were photogenerated by UV (λ > 335 nm) irradiation of the compounds isolated in argon matrix. The observed isomerizations were found to be photoreversible. Upon shorter wavelength UV irradiation, the initial forms of the compounds (with intramolecular hydrogen bonds; notated as HB in Fig. 7) were partially repopulated. The structures of the photogenerated non-hydrogen-bonded isomers of both compounds were positively identified by comparison of their IR spectra with the spectra theoretically calculated [at the DFT(B3LYP)/ 6-311++G(2d,p) level] for all possible non-hydrogen-bonded isomers of the studied compounds. The experimental IR spectra of the photoproducts generated from o-hydroxybenzaldehyde and o-hydroxyacetophenone are very well reproduced only by the theoretical spectra predicted for the isomers with both OH and formyl (or acetyl) groups rotated by 1801 (NPT3 forms), with respect to the initial, most stable hydrogen-bonded conformer. These are interesting cases where simultaneous rotations around two bonds take place during isomerization.