Mechanism of Photoisomerization of 1‐Naphthyl‐2‐Phenylethylenes in Organic Glasses

Abstract

Photoisomerization reactions of cis isomers of 1-beta-naphthyl-2-phenylethylene, an o-methylated homolog and 1-alpha-naphthyl-2-phenylethylene in organic glasses at liquid nitrogen temperature were studied. Reactions were followed by changes in UV-absorption spectra of the irradiated samples. Formation of an unstable trans-photoproduct was detected only with the o-methylated homolog. The results are consistent with high regioselective Hula-twist photoisomerization at the benzylic sites of the three compounds examined. Calculated data on relative energies of the conformers of both the cis and the trans isomers are in agreement with the suspected conformational population of the starting materials and the photoproducts.

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“…Starting from E ‐isomer, simultaneous reduction of the twist angle from 180° to 1.5° and the CC distance from 5.23 Å to 3.54 Å (Table 1) leads to Z ‐isomer which, according to,57 can be named “OBF‐ Z ‐conformer.” At the same time, rotation around the C3C4 double bond keeping the C1C6 distance constant corresponds to the PES cross‐section along the β axis and perpendicularly to the r CC axis that is achieved by simultaneous rotation of the aromatic nuclei around quasi‐single bonds (angles α and γ). Simultaneous twisting of the β and α or β and γ angles leads to another conformers of Z ‐isomer which, according to,57 can be named “HT‐ Z ‐conformers” (HT1 and HT2); the optimized structures of these conformers are outside the drawing plane in Figure 3. The same HT‐conformers are also obtained from the OBF‐ Z ‐conformer by rotation of naphthalene (HT1) or benzene (HT2) rings around the corresponding quasi‐single bonds in ethylenic group (angles α and γ).…”

“…It should be noted that experimentally only reverse Z → E HT‐reactions have been observed for different diarylethylenes, E → Z photoisomerization does not occur in confined media. For example, Z ‐1SN and Z ‐2‐styrylnaphthalene undergo regioselective HT photoisomerization at the benzylic sites when irradiated in low temperature glasses 57…”

Direct evidence for hula twist and single-bond rotation photoproducts

“…1a ). Due to their fleeting character an unambiguous assignment of their molecular and electronic structure has not been possible so far, as time resolved methods 14 – 16 , 28 , 31 , 46 , 47 or isolation of these intermediates under extremely cold 17 , 19 , 20 or rigid-medium conditions 25 , 39 , 48 , 49 provide still too little direct structural information. In rigid matrices (i.e., solvent glasses) and at very low temperatures intermediates have frequently been observed with absorption or fluorescence spectroscopy but the assignment of similar spectra to a particular isomeric structure is not straight forward.…”

Photoisomerization ofcis-1-(3-Methyl-2-naphthyl)-2-phenylethene in Glassy Methylcyclohexane at 77 K

“…That conclusion was disputed because HT photoisomerization about the benzylic CH would lead to the same result . Formation of an unstable trans conformer starting from the o -tolyl analogue of c -NPE ( c -NTE) in EPA glass at 77 K was presented as confirmation of the benzylic HT pathway, Chart b . However, comparison of the absorption spectrum of the initial t -NTE photoproduct and the spectrum of the thermally stable conformer to which it relaxes with the resolved t -NPE conformer absorption spectra in solution requires reassignment of the two spectra to structures t -NTE A and t -NTE B , respectively, Chart c.…”