Studies in photochemistry. Part VIII. The ultraviolet, proton magnetic resonance, and mass spectra, and photocyclisation of some styrylnaphthalenes to some benzo[c]phenanthrenes and chrysenes

Abstract

Kinetic data for the first-order ring opening of the dihydro-intermediates, formed on the photocyclisation of several styrylnaphthalene analogues, are presented and some aspects of their electronic spectra are discussed. The photocyclodehydrogenation of 1,2-di-(2-naphthyl)ethylene to benzo [g,h,i]perylene proceeded via dibenzo[c,g]phenanthrene. Photocyclodehydrogenation of 2-methyl-I -(2,4,6-trimethylstyryl) naphthalene, 2-methyl-1 -styrylnaphthalene, I -( 2,4,6-trimethylstyryI)naphthalene, and 1 -( 2,6-dichlorostyryl) naphthalene yielded the expected chrysene derivatives. 1 -Bromo-2-styrylnaphthalene and 2-(2.4.6-trimethylstyryl) naphthalene photocyclised to benzo [clphenanthrene and 2,4-dimethylbenzo[c] phenanthrene respectively. No migration of substituents was observed. The effect of electron impact on these styrylnaphthalenes correlates with their photochemical behaviour. The U.V. and l H n.m.r. spectra of the styrylnaphthalenes are discussed. Iatermediates in Some Photocyclodehydrogenation Reactions.-In 1950, Parker and Spoerri isolated phenanthrene (111) on irradiation of stilbene (I).2 Since then this basic reaction has been exploited as a very useful synthetic route to polycyclic aromatic and heterocyclic ring ~ysterns.~ Unfortunately the mechanism of the that this colour disappears slowly when the irradiating source is removed or when the solution is heated.4 Only 3= A @-8 \ /photocyclodehydrogenation of cis-stilbene analogues has not received as much attention.

Photobehaviour of Z‐1,2‐di‐(3′‐methoxynaphth‐2′‐yl)ethene as model compound of biphotochromic supermolecules with Z‐ethenic bridge

“…This unusual behaviour led us to investigate in more details the photochemistry of these ethene derivatives and to study in parallel an analogous (non photochromic) compound where the ethene bridge links two methoxy-substituted naphthyl groups, the Z-di-(3'-methoxynaphth-2'-yl)ethene (DMNE). The results obtained for this model molecule can be usefully compared with those obtained for the unsubstituted Z-1,2-di-(2'-naphthyl)ethene (DNE), whose photoisomerisation and photocyclisation have been widely investigated by various groups [5,[7][8][9][10][11][12]. Therefore, briefly summarising the photobehaviour of DNE before discussing the new results will be useful.…”

“…The relatively high value (42 kJ mol −1 ) of the activation energy for photocyclisation of Z-DNE leads to a remarkable sharp decrease of its quantum yield with temperature, the formation of DHBP A being practically inhibited below −30 • C. The half-life time of the cyclised compound at room temperature is rather long in the dark and in the absence of oxidants (t 1/2 = 36.8 days) [8]. It obviously depends on the rates of thermal and photochemical ring opening and of oxidation to DBP.…”

“…In fact, Z-DNE exists in solution as an equilibrium mixture of three conformers (A, B, C, Scheme 3) due to the partially hindered rotation of the naphthyl groups around the single bonds with the ethenic carbons. The results of accurate investigations from different groups in the early seventies [7][8][9] indicated that only one of the possible conformers (A) is involved in the activated photocyclisation at room temperature (with a quantum yield φ DHBP = 0.02 and an activation energy of about 42 kJ mol −1 ), probably taking place through an unstable isomeric precursor [10] (see below). However, a flash photolysis study of Z-DNE [9] in methylcyclohexane revealed the formation of another cyclized compound with half-life of 10 −4 s at room temperature and a redshifted absorption around 530 nm.…”

“…The thermal ring opening follows first order kinetics at a rate strongly dependent on temperature (the activation energy is 97 kJ mol −1 in hexane) [8].…”

“…The latter is characterised by a relatively strong absorption in the visible region (around 420 nm) and by the presence of two reactive hydrogen atoms, which can be removed, thermally and photochemically, in the presence of oxygen or other hydrogen acceptors leading to the oxidised product, dibenzo[c,g]phenanthrene (DBP, Scheme 2) [5,[7][8][9][10][11][12]. The ring-closure takes place in a stereospecific way with respect to the possible rotational conformers.…”