F. Bejarano‡
a,
I. Alcon‡b,
N. Crivillersa,
M. Mas-Torrent
*a,
S. T. Bromley
bc,
J. Veciana
a and
C. Rovira
*a
aInstitut de Ciència de Materials de Barcelona (ICMAB-CSIC), Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Campus de la UAB, 08193 Bellaterra, Spain. E-mail: cun@icmab.es; mmas@icmab.es
bDepartament de Ciència de Materials I Química Física, Institut de Química Teórica i Computacional, Universitat de Barcelona (IQTCUB), E-08028 Barcelona, Spain
cInstiuició Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain
First published on 7th March 2017
The E–Z isomerisation of two perchlorotriphenylmethane derivatives containing stilbene units has been investigated, both thermally and photochemically. The irreversibility of the E → Z isomerisation in both compounds is experimentally demonstrated and supported by density functional calculations.
Compounds 1 and 2 were obtained through a Wittig and a Horner–Wadsworth–Emmons reaction, respectively. In the case of 1, the reaction was carried out between the PTM phosphonium bromide salt and the 4-(acetylthio)benzaldehyde (see details on the synthesis in ref. 11). Molecule 2 was prepared through the coupling of PTM phosphonate and 4-bromobenzaldehyde (synthesis adapted from ref. 12). It is known that the 1H-NMR chemical shift of the hydrogen atoms of CC groups, as well as their coupling constant J, allows for the identification of the two possible isomers that can be obtained from these synthetic routes. For the compounds presented here, the J value of the E isomers is around 17 Hz, while for the Z isomers it is around 12 Hz. It was determined that the E isomer was the major component of the reaction mixtures, with a ratio of ca. 92% and 99% for E-1 and E-2, respectively (determined by 1H-NMR signal integration).
In order to follow and quantify the E/Z isomerisation processes for compounds 1 and 2, High Performance Liquid Chromatography (HPLC) was employed using reversed-phase conditions, at room temperature and under an isocratic regime. Mixtures of acetonitrile/tetrahydrofuran (9:
1) and acetonitrile/chloroform (8
:
2) were found as optimal conditions for an adequate separation of the corresponding isomers for 1 and 2, respectively.
After reaching a good separation of the two isomers, we proceeded to study the photo-isomerisation conversion from the E to the Z form of 1 and 2. The samples were irradiated with a Hg(Xe) lamp 300 W placed at around 30 cm from the sample. In the case of 1, a 0.07 mM solution in a THF/ACN (1:
1) mixture was used for this experiment. To follow the evolution of the isomerisation process, several aliquots were taken during the irradiation time and analysed by HPLC. Fig. 1 shows the corresponding chromatograms and the retention times. The relative peak areas (in %) are depicted for each analysed aliquot in Table 1. See ESI† for the monitoring performed for 2.
Time (min) | % Area (Z-1) | % Area (E-1) | (Z/E) Peak areas ratio |
---|---|---|---|
0 | 7.5 | 92.5 | 0.08 |
10 | 23.0 | 77.0 | 0.30 |
46 | 67.7 | 31.3 | 2.16 |
102 | 75.5 | 24.5 | 3.08 |
115 | 87.9 | 12.1 | 7.26 |
145 | 88.8 | 11.2 | 7.93 |
As expected, the Z/E ratio increases in both cases during the irradiation time until the photostationary state is reached. The 1H-NMR (see ESI†) of the sample after irradiation confirmed that the peak at a retention time of 12 minutes corresponds to the expected Z-1 compound. The same analysis was performed for compound 2.
The characterization by ultraviolet-visible (UV-vis) spectroscopy shows that there is an overlap of the absorption spectra of both isomers (see ESI†). In addition, the Z isomer absorbs white light more poorly than the E isomer. For these two reasons the back Z → E phoisomerization could not be carried out.
Since the most thermodynamically favoured isomer for stilbenoid compounds is usually the E isomer, we proceeded with the study of the thermally induced conversion of the Z form back to the E. To do that, the photostationary state mixture obtained by irradiation was refluxed in different solvents: chloroform, tetrahydrofuran, cyclohexane, toluene and o-dichlorobenzene for, at least 3 hours. These solvents do not only allow access to different heating temperatures, but also they provide different polarities of the media, as well as interactions between the solvent and the PTM derivatives through their ability to form π–π and Cl–Cl interactions. Again, the solution evolution was followed by HPLC. In this case, the thermal back conversion was not observed for either compound 1 or 2. This result is in agreement with previous studies which determined an activation barrier for the Z → E isomerisation for stilbenoid compounds of the order of 1.9 eV (43.8 kcal mol−1), which makes the thermal back reaction unlikely.13
We attempted to carry out the PTM Z → E isomerisation process using catalytic amounts of iodine. This is an established methodology to convert isomeric mixtures to the E derivative.14 The obtained photostationary state mixture of 2 was dissolved in toluene (0.022 M) with iodine (0.03 eq.) and the solution was refluxed for 5 hours. The evolution of the mixture was followed by HPLC and no back isomerisation was observed by HPLC monitoring. Then, 0.03 more equivalents of I2 were added and the solution was refluxed 19 additional hours. After such a long period of time at high temperature many products of decomposition appeared (see Fig. S5 in ESI†) but no isomer E formation was observed. Therefore, this suggests that back isomerisation, in the best case, is less favoured than several decomposition pathways.
Density functional theory (DFT) calculations were carried out to better understand the irreversible isomerisation process Z → E observed for both PTM derivatives 1 and 2. It is known that, unlike azobenzene derivatives, in ethylene derivatives photo-activated twisting of the CC bond is the main mechanism for E–Z isomerisation15 as represented with the red arrow on the stilbene structure in Fig. 2a. To obtain the ground state energy profile associated with such a twisting process in compounds 1 and 2, we systematically varied the corresponding C
C dihedral angle and performed constrained optimisations of the molecular structure for each conformation. In all cases, the E conformer (φ = 0°, 360°) is slightly more stable than the Z conformer (φ = 180°), with the perpendicular conformation (i.e. φ = 90°, 270°) being the most energetically unstable by 1.9 eV (i.e. the E ↔ Z energetic barrier height, see ESI†). These results are fully in accordance with previous studies.13,14 Time dependent DFT (TD-DFT) calculations were then used to obtain approximate absorption spectra for E and Z conformers for both derivatives 1 and 2. For the E conformers we found the most intense peak at 340 and 345 nm (for 1 and 2, respectively). In the corresponding Z conformers, these peaks slightly shift towards higher energies (315 and 317 nm for 1 and 2, respectively) and decreases in intensity by ∼60%. These calculations coincide with our experimental UV-vis spectroscopy measurements (see ESI Fig. S1†), where the peak at ∼310 nm for the E conformer decreases in intensity upon photo-generating the Z conformer and shifts towards higher energy wavelengths. This UV-vis band variation has been observed for other stilbene derivatives, where such optical absorptions were associated with the photo-activation of the C
C moiety.16
The higher absorbance of the E conformer might explain the high production of the Z conformer upon light absorption. However, the thermal irreversibility of the photo-isomerisation of Z towards the E isomer (even under I2 catalyzed conditions) suggests that the different UV-vis absorption characteristics of the two conformers may not fully explain the experimental findings.16 In the CC twisting process assessed by constrained DFT optimizations (see ESI†) we found that, for both 1 and 2, the aryl ring groups bonded to the ethylene unit had to be twisted to specifically chosen angles (black arrows on stilbene structure in Fig. 2a) in order to prevent steric interactions and hence allow the rotation around the C
C bond axis (red arrow on stilbene structure in Fig. 2a). The key role of such aryl ring twist angles for the E ↔ Z photo-isomerisation has been demonstrated both theoretically17 and experimentally,18 and proven to be more relevant for the Z → E process19 than for the E → Z one.20 Based on this, we performed ab initio molecular dynamics (AIMD) simulations at 450 K (see computational details below) to compare the thermal fluctuations of such aryl ring twist angles in compounds 1 and 2 and stilbene (where the E ↔ Z isomerisation is completely reversible) in each conformation.
Extracting the twist angle of the central ethylene unit (red arrow in Fig. 2a) we could find that, for all cases, the CC bond hardly rotates (see ESI†). This is in full accordance with its associated high twisting energy and corroborates the need for light irradiation to induce the E ↔ Z transition, even at high temperatures (see ESI†). In contrast, by following the variation in the twist angles of the aryl rings with respect to the ethylene unit (black arrows in Fig. 2a), it is possible to observe significant differences between the E and Z conformer profiles (Fig. 2b and c).
For both PTM derivatives and stilbene in the E conformation such aryl ring twist angles fluctuate significantly over time (Fig. 2b) which is consistent with the little steric interaction between the two aryl ring groups bonded to the CC unit. For stilbene in the Z conformation the two aryl rings tend to keep rotating in opposite directions leading to a gradual increase in the magnitude of the associated twist angle (see blue lines in Fig. 2c). Although in this case the two rings are close to each other, by counter rotating in this manner they are able to avoid significant steric hindrance. In the Z conformers of derivatives 1 and 2, however, the aryl ring twist angles quickly converge to fixed values of about 60°. The lack of rotation in these cases comes from two different factors. From one side, the bulky PTM group possesses relatively high inertia to move which impedes the rotation of that bulky group attached to the ethylene unit. Also, the close proximity of the ortho-chlorine atoms within the PTM unit with the opposite aryl ring (see the Z-1 structure in Fig. 2c) gives rise to large steric constraints which prevent the free rotation of the brominated aryl ring. Hence, overall, the rotational freedom of both groups bonded to the ethylene unit in the Z conformation of 1 and 2 is significantly suppressed.
As noted above, when calculating the twisting of derivatives 1 and 2 about the CC bond specific twist angles of the aryl ring groups are needed to avoid significant steric interactions and allow the isomerisation to occur. In the E conformer such specific aryl ring twist angles should be easily accessible due to the freely rotating behaviour in such conformation for 1, 2 and stilbene (see Fig. 2b). However, in the Z conformation, aryl rings in compounds 1 and 2 do not rotate due to steric interactions between the two aryl ring groups and the larger inertia of the PTM unit. Such an effect does not occur for stilbene (blue curve in Fig. 2c), where the E ↔ Z isomerisation is completely reversible.16 These results suggest that the complete and irreversible formation of Z isomers under light irradiation (Fig. 1) for PTM derivatives 1 and 2 could be at least partially explained by the lack of free rotation of their aryl rings in the Z conformation, causing a kinetic blockage. This suggestion may also help explain why even when using a chemical catalyst (I2) the Z → E process does not seem to occur and also the uniqueness of these observations as compared to less hindered stilbene derivatives reported in previous studies.16 Therefore, as represented schematically in Fig. 3a, in stilbene the E and Z conformations stay in an equilibrium under conditions of light irradiation and high temperature, due to the high conformational flexibility of the molecule. Contrary, as sketched in Fig. 3b, in PTM derivatives 1 and 2 the E conformation is kinetically blocked, even when the ethylene unit is electronically excited with light, which impedes the Z → E process and, consequently, shifts the isomerisation equilibrium towards the less stable Z isomer.
In summary, we have studied the photo-induced isomerisation of E-p-vinylphenylene functionalised perchlorotriphenylmethane derivatives to the Z isomer. The isomerisation process has been monitored by HPLC which reveal an almost complete isomerisation. The higher absorption ability of the E isomer as compared to the Z isomer at the same wavelengths prevents the back Z → E isomerisation. No thermal back isomerisation takes place although the E isomer is the most thermodynamically favoured. Theoretical calculations point to the steric hindrance within the Z conformers leading to a possible kinetic blocking of the Z → E transition as the effect contributing to the thermal irreversibility of the E → Z process.
Synthesis of 2 (adapted from ref. 12): potassium tert-butoxide (95 mg, 0.87 mmol) was added to a solution of diethyl 4-[bis(2,3,4,5,6-pentachlorophenyl)methyl]-2,3,5,6-tetrachlorobenzyl phosphonate (500 mg, 0.57 mmol) in dry tetrahydrofuran (10 mL) at −90 °C, in a N2/acetone bath. The resulting yellow-orange ylide solution was stirred for 30 minutes. Then, p-bromobenzaldehyde (188 mg, 1.04 mmol) dissolved in 4 mL of dry THF was added drop-wise. The solution was allowed to stir for 24 hours without changing the N2/acetone bath, which reached room temperature along the reaction. The reaction was monitored by thin-layer chromatography (silica-gel). Once the reaction finished, one drop of concentrated HCl was added to the resulting dark purple solution, which turned on a yellow suspension. The reaction mixture was extracted with water (10 mL) and dichloromethane (10 mL). The phases were separated and the aqueous phase was washed with further dichloromethane (2 × 10 mL). The combined organic layers were dried over anhydrous sodium sulfate, filtered and concentrated under vacuum. Almost pure E-2 isomer (97%) (473 mg, 92%) was obtained through purification by chromatography (silica gel with hexane as eluent).
The Z-2 isomer was obtained after irradiating a 3.7 mM solution in THF of the as synthesized 2 for 300 min. Then the solvent was evaporated to characterize the solid.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra28618f |
‡ Equally contributed. |
This journal is © The Royal Society of Chemistry 2017 |