FULL PAPER PPS www.rsc.org/pps Photocyclization of 2-vinyldiphenylacetylenes and behavior of the isonaphthalene intermediates Meledathu C. Sajimon and Frederick D. Lewis* Department of Chemistry, Northwestern University, Evanston, IL, 60208-3113, USA. E-mail: lewis@chem.northwestern.edu Received 11th April 2005, Accepted 10th June 2005 First published as an Advance Article on the web 28th June 2005 The conformation, electronic structure, spectroscopy, and unimolecular photoisomerization of 2-vinyldiphenylacetylene and two derivatives have been investigated. 2-Vinyldiphenylacetylene exists predominantly in a planar anti conformation. Introduction of an a-methyl substituent results in increased phenyl–vinyl dihedral angles for both syn and anti conformers, whereas a cyclic analog is constrained to a syn conformation with a large phenyl–vinyl dihedral angle. All three molecules undergo photocyclization to yield unstable cyclic allene (isonaphthalene) intermediates which undergo further reactions leading to stable products. Both the photocyclization process and behavior of the allene intermediate are dependent upon ground state conformation. The photophysical behavior of the 2-vinyl derivative, namely its short singlet lifetime and low fluorescence quantum yield, is similar to that of diphenylacetylene. It also has a low quantum yield for photocyclization. The 2-isopropenyl derivative and conformationally locked cyclic analog have relatively long singlet lifetimes and large quantum yields for fluorescence and cyclization. The difference in excited state behavior of the planar 2-vinylacetylene and its non-planar analogs is attributed to the effect of the phenyl–vinyl dihedral angle on the barriers for activated decay of the linear singlet state. However, the behavior of the 2-isopropenyl derivative does not appear to be dependent upon ground state conformation (syn vs. anti). The cyclic allene intermediates undergo sequential protonation–deprotonation in methanol solution to yield stable products. The 2-vinyl derivative yields only the fully aromatized 2-phenylnaphthalene. However, the 2-isopropenyl and cyclic derivatives yield mixtures of fully and partially aromatized products. Preferential formation of the partially aromatized products is attributed to a stereoelectronic effect on the deprotonation step. In diethyl ether solution only the fully aromatized product is formed via a free radical mechanism. Introduction The behavior of the singlet excited states of diphenylacetylene (DPA) has been the subject of continuing experimental and theoretical investigation. 1–6 Much of the interest has centered on its short-lived fluorescence, which has been attributed to an upper singlet state. To briefly summarize the current state of understanding, excitation of the linear ground state populates a linear singlet state with quinoidal character (S Q ) which undergoes a low energy barrier crossing to a longer-lived dark singlet state with stilbene-like geometry and diradical character (S t–S , Scheme 1). 1,7 The fluorescence of DPA is assigned to the short-lived S Q singlet state. This model accounts for the pronounced excitation wavelength dependence and temperature dependence of the fluorescence quantum yield and lifetime. The Scheme 1 activation parameters for this barrier crossing, as determined by Hirata et al. are E a = 2.5 kcal mol −1 and log A = 12.8 in hexane solution. 2 In comparison to the activation parameters for the extensively investigated photoisomerization of trans-stilbene (E a = 3.5 kcal mol −1 , log A = 12.6) the activation energy is somewhat smaller but the preexponentials are essentially the same. 8 DPA has been reported to undergo a variety of photo- chemical bimolecular addition reactions. 9–14 However, there is but a single report of a unimolecular photoisomerization reaction of a diphenylacetylene derivative: the conversion of 2-vinyldiphenylacetylene (1) to 2-phenylnaphthalene (1a, Scheme 2). This reaction was investigated by op den Brouw and Laarhoven over two decades ago. 15 They proposed a two stage mechanism: photocyclization to yield an unstable cyclic allene (isonaphthalene) intermediate (1i) followed by thermal isomerization to yield the observed product, which is obtained in good preparative yield in either methanol or hexane solution (Scheme 2). The regioselective incorporation of deuterium upon irradiation in methanol-O-d was offered as evidence for protonation–deprotonation, rather than the symmetry-allowed intramolecular 1,5-hydrogen migration as the mechanism for aromatization in methanol. 16,17 Scheme 2 DOI: 10.1039/b504997k This journal is © The Royal Society of Chemistry and Owner Societies 2005 Photochem. Photobiol. Sci. , 2005, 4 , 629–636 629