Competition between Photoisomerization and Photocyclization of the Cis Isomers of n-Styrylnaphthalenes and -Phenanthrenes

Abstract

The isomerization and cyclization photoreactions of the cis (Z) isomers of n-styrylnaphthalenes (n = 1 and 2), n-styrylphenanthrenes (n = 1, 2, 3, 4, and 9), and two related compounds, 3-styrylchrysene and 3-styrylbenzo[c]phenanthrene, were investigated by spectrophotometric and chromatographic techniques. The quantum yields of the two photoreactions were measured in aerated and deaerated nonpolar solvent at room temperature and compared with those reported in the literature for some of the molecules investigated and other related compounds. The combined use of high-performance liquid chromatographic and spectrophotometric techniques made easier the separation of the components of the photoreaction mixtures thus simplifying the study of the isomerization/cyclization competition and the measurement of the UV-vis absorption spectra and the thermal decay kinetics of the dihydrophenanthrene-type intermediates. The conformational equilibria in the ground state and the positional isomerism (n values) notably affect the prevalence of one or the other competitive photoreaction. Oxygen also plays an important role: practically it does not affect the quantum yield of the Z --> E process but has a drastic effect on the formation of the final oxidation product which can proceed even in the presence of traces of air. In three cases, non-negligible formation of a side colorless product was detected. It was assigned to an isomer of the normal photocyclization intermediate (4a,4b-dihydrophenanthrene-type), formed by shift of one or both hydrogen atom(s) to other positions of the rings. Parallel ab initio calculations of the potential energy of the possible conformers helped to explain the structure effects on the competitive photoreactive relaxation pathways. Some correlations between the computed reagent/product energy difference in the ground state and the photocyclization yield and thermal stability of the intermediates were also verified.

Osteogenic response to BMP‐2 of hMSCs grown on apatite‐coated scaffolds

“…Due to steric hindrance between hydrogen atoms at double bond and in neighbouring benzene ring of the naphthalene nucleus, E ‐1SN exists in solution predominantly as s‐cis ‐conformer (rotamer) 26. Scheme shows open‐ and closed‐ring forms of 1SN and interconversions between them.…”

“…We compared the structures of all diarylethylenes which undergo one‐photon E ‐isomer‐to‐DHP photocyclization and found that they all comprise framework of 1‐styrylnaphthalene (1SN). This compound itself is known to undergo E → Z photoisomerization with quantum yield φ = 0.10,25 and photoisomerization of Z ‐isomer (φ = 0.11) competes with photocyclization (φ = 0.15) 26. Quantum‐chemical calculations (CNDO/S) have shown that the first excited singlet state of 1SN has ethylenic character 27…”

Photoisomerization and Photocyclization of 5-Styryloxazole

“…As tested in a previous work, 27 the combined use of HPLC and spectrophotometric techniques was very useful for the separation of the components of the photoreaction mixture thus allowing the evaluation of the isomerization/cyclization competition and the measurement of the spectra and decay kinetics of the DHP-type intermediate. Our experiments in CH were carried out in different steps.…”

Deciphering the working mechanism of aggregation-induced emission of tetraphenylethylene derivatives by ultrafast spectroscopy

“…However, the working mechanism behind the AIE phenomenon remains a subject of much debate and intense elucidation efforts. 13 – 27 Among several hypotheses concerning the mechanism of the AIE effect, the restriction of intramolecular rotation (RIR) 15 – 19 and restriction of intramolecular vibration (RIV) 20 – 22 subsumed under the overarching concept of restriction of intramolecular motion (RIM) 2 , 21 are the most frequently cited ones, with photocyclization 23 – 25 as well as E – Z isomerization 18 , 26 , 27 being invoked in a number of cases. Although the discourse nowadays appears to lend much support to the general mechanism of RIM due to an extensive body of experimental evidence substantiated in part by computational studies, a clear, consistent, and deep fundamental understanding of different elemental relaxation processes associated with the basic molecular motions in non-radiative excited-state dynamics even in the simplest archetypal AIE-active compounds such as tetraphenylethylene (TPE) is yet to emerge.…”