NMR Kinetic Investigations of the Photochemical and Thermal Reactions of a Photochromic Chromene

Abstract

The photochromic behavior of 2,2-di(4-fluorophenyl)-6-methoxy-2H-1-chromene has been investigated by 19F NMR spectroscopy. Photocoloration under UV irradiation at low temperature led to the formation of three interconverting photoisomers including two merocyanines and an unprecedented allenyl-phenol isomer. Photobleaching with visible light, which was known to lead to reversion to the initial closed chromene, was shown to increase allenyl-phenol concentration. Thermal relaxation of the preirradiated system was also studied at various temperatures. In each case (UV and visible irradiations, thermal isomerization), the kinetics of each of the four species was monitored. Numerical analysis of concentration vs time profiles enabled us to unequivocally establish the global mechanism occurring in each of the experimental conditions and to interpret the specific reactivity of each photoisomer. It has been shown that, among the 12 possible isomerization processes, only some paths were active. For the first time, it has been possible to determine their corresponding thermal activation parameters and photochemical quantum yield ratios.

New benzopyranocarbazoles: synthesis and photochromic behaviour

“…The enthalpy of activation value agrees with those referred for photochromic compounds (40-120 kJ mol K1 ). 11 The negative value obtained for DS ‡ points to an expected loss of freedom in the recyclization into the original closed form but is difficult to interpret as it includes an unknown contribution from the rearrangement of the solvent (toluene).…”

“…8 According to recent studies these photoproducts can be identified, as the trans-cis (TC) and trans-trans (TT) isomers (Scheme 1) and the fast fading phase, of higher amplitude, can usually be attributed to the TC-isomer. 11 2.3. Mechanistic studies 2.3.1.…”

“…Both are responsible for the colour obtained after irradiation; however, usually, the latter isomer was found to be the most thermally stable. [10][11][12] Benzo-and naphthopyrans containing a fused carbazolo or indolo group exhibit interesting photochromic properties and have been patented. [13][14][15][16] In previous works, we described the photochromic properties of indolo-fused benzopyrans.…”

Synthesis of novel photochromic pyrans via palladium-mediated reactions

“…They originate from open merocyanine isomers and are formed via a 1,5-hydrogen shift reaction [21–22]. However, these allene intermediates can be avoided by the replacement of hydrogen in the immediate vicinity of the two aryl residues (3-position of 2 H -1-benzopyrans and 2-position of 3 H -naphtho[2,1- b ]pyrans).…”

Cleavage of photochromic compounds derived from heterocycles under electrospray tandem mass spectrometry: study of the influence of the heteroatom in fragmentation mechanisms

“…In recent years nuclear magnetic resonance (NMR) studies have revealed important details in the photochromic mechanism of this family of compounds, namely on the thermal or photochemical interconversion of the different species and the common involvement of a colourless allenyl-naphthol structure as a key intermediate. [2][3][4] All of these studies were performed in polar solvents, at low temperatures and quite high concentrations (10 À2 M) due to the relatively low instrumental sensitivity. More recently, Maurel et al showed that Surface-Enhanced Resonance Raman Scattering (SERRS) could be used for the study of the photochromic process at ambient temperature and low concentrations (10 À5 M).…”