Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Trans- and cis-stilbene isolated in cryogenic argon and xenon matrices
Graphical abstract
Introduction
Stilbene exists as two possible stereoisomers about the double bond, trans (TS) and cis (CS) (Scheme 1). Being the simplest 1,2-diarylethylene, stilbene has been extensively used as a model system for investigation of the photochemical dynamics concerning the cis → trans isomerization about a CC bond. This type of photoisomerization is of fundamental importance in many areas of chemistry, physics, biochemistry and materials science, and along the years there has been a continuing interest in this topic [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28].
CS is well-known to present a non-planar structure [29], [30], [31], [32], due to the stress imposed by the presence of the two large phenyl substituents in the some side of the double bond. On the other hand, the structure of TS has been a source of controversy, because the two phenyl groups can rotate with a very low energy cost [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. Electron diffraction and photoelectron spectroscopy studies on gaseous TS [33], [34] suggested the molecule to be non-planar with phenyl groups rotated by ∼30° around the C–phenyl bonds, in line with some theoretical results [43], [44]. However, the most recent theoretical data favor the idea that TS has, as its minimum energy structure, a strictly planar form [40], [41]. This is in agreement with experimental results for the compound in molecular beams [45] (in solid state TS molecules were also found to exhibit a planar configuration [36], [46], [47], [48], [49], while in solution and liquid phase appear to be non-planar [50], [51], [52]). The better available estimations of the relative energies of the planar and ∼30° distorted geometries of TS, undertaken at CCSD/cc-pVDZ and Møller–Plesset levels of theory in conjunction with Dunning’s correlation consistent polarized valence basis sets [40], [41], yielded values from 2 to ∼0.4 kJ mol−1, favoring the planar form. Moreover, it was also recently shown that zero-point-vibrational corrections are comparable to the energy increase between the planar and non-planar forms, so that this effect (“vibrational quasi-planarity” [40]) can be used to explain the controversy on the planar/non-planar conformational preferences of TS. A general consensus exists regarding the low energy – large amplitude nature of the C–phenyl torsional vibrations in TS [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49].
As mentioned above, the cis → trans photoisomerism in stilbene has been an area of intense study [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. The photoisomerization has been explained as proceeding with the intermediacy of a non-planar excited state minimum and relaxation to the ground state through a conical intersection. A recent matrix isolation (argon, krypton and N2 matrices) study of the cis → trans photoisomerism in stilbene [53], successfully allowed observation of a non-planar form of the photoproduced TS.
In the present study, one intended to investigate in further detail the structure and vibrational spectra of matrix-isolated trans- and cis-stilbenes, and characterize better structrally and spectroscopically the non-planar TS form resulting from cis → trans photoisomerization of stilbene in a cryogenic matrix. The studies were undertaken for the compounds isolated in argon matrices and, for the first time, also in xenon matrices.
Section snippets
Matrix isolation FTIR and photochemical experiments
Trans-stilbene (TS; trans-1,2-diphenylethylene) and cis-stilbene (CS; cis-1,2-diphenylethylene) were purchased from Sigma–Aldrich, purity ⩾96%. To prepare the cryogenic matrices, the vapors of the studied compounds were deposited, together with a large excess of argon (N60) or xenon (N45) (both supplied by Air Liquide), onto a cryogenic CsI window, which was used as optical substrate. During deposition of the matrices, the temperature of the substrate was kept at 15 K. The temperature was
IR spectra of TS and CS monomers isolated in argon and xenon matrices
Infrared spectra of matrix-isolated TS and CS are shown in Fig. 1, Fig. 2, where they are compared with the corresponding DFT(B3LYP)/6-311++G(d,p) calculated spectra. The detailed assignment of these spectra and the results of the normal mode analyses are given in Table 1, Table 2. The definitions of the symmetry coordinates used in the normal coordinate analyses are provided in Table S1 (Supporting Information).
As it can be noticed in Fig. 1, Fig. 2, the theoretical calculations reproduce very
4. Conclusions
Monomers of cis- (CS) and trans-stilbene (TS) (planar and non-planar structures) isomers were isolated in cryogenic inert gas matrices (Ar and Xe). The identification/characterization of these species was done by IR spectroscopy and extensive theoretical calculations undertaken at the DFT(B3LYP)/6-311++G(d,p) level of approximation. Full assignment of the infrared spectra of the stilbenes in argon and xenon matrices was undertaken.
In situ broadband UV irradiation of the matrix-isolated CS led
Acknowledgements
These studies were performed under project PTDC/QUI-QUI/111879/2009 (also funded by COMPETE-QREN-EU). The authors would like also to thank Prof. A.A.C. Pais and Tania Firmino for their helpful discussions regarding the chemometrics analysis described in this article. O.U. acknowledges YÖK (Higher Education Council of Turkey) for full financial support to his postdoctoral research.
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