NMR Structural and Kinetic Assignment of Fluoro-3H-naphthopyran Photomerocyanines¶

Abstract

The kinetic and structural behavior of a photochromic compound, 3-(2-fluorophenyl)-3-phenyl-3H-naphtho[2,1-b]pyran (F-Py), was investigated using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. Upon irradiation, the four theoretically predicted photomerocyanines appear along with a fifth form X, whose final structure has not been elucidated. This last form and two of the photomerocyanines are thermally labile, whereas the other two do not show any signs of decay. The system has been analyzed by NMR spectroscopy. This led to the structural assignment of each photomerocyanine. The kinetics of the thermal bleaching were monitored by directly and separately measuring the concentrations of each species at regular time intervals using 19F NMR spectroscopy. We therefore propose a plausible reaction mechanism. On the basis of this mechanism, the mathematical treatment and the study of the effects of temperature led to the determination of the kinetic and thermodynamic parameters (rate coefficients, enthalpy and entropy of activation) of this photochromic system. The leading role of the labile intermediate X on the formation of trans-transoid-cis (TTC) and cis-transoid-cis (CTC) photomerocyanines is pointed out.

Solvent-dependent fluorescence enhancement and piezochromism of a carbazole-substituted naphthopyran

“…As a member of typical photochromic naphthopyran derivatives, CzNP isomerizes to the ring-opened forms (open-form CzNP) via the cleavage of the C(sp 3 )-O bond of pyran ring accompanied by the formation of two isomeric merocyanine (MC) forms, transoid-cis (TC) isomer and transoid-trans (TT) isomer, with a strong absorption in visible spectrum. Thermally unstable TC returns to CF rapidly while TT which is more stable returns slowly to CF through the TC isomer [30,31]. Fig.…”

Kinetics and mechanism of photochromic transformations of a 2,3-diarylcyclopentenone

“…In particular, this method was used to study the photochromism of diarylcyclopentenones [19]. Equations (1) and (2) [23] were used to analyze the kinetic curves of photocoloration and pho tobleaching of 1 (Fig. 3).…”

“…The problem of discrepancy in the parameters of photochromic process reported in works [17] and [20] is solved. For this purpose, method of analysis of photokinetic data for photochromic pro cesses proposed in [21,23] was applied. The molar absorption coefficient of the photoinduced form 1B found in this work is sufficiently different of that reported in [17], but coincides with the data [20].…”

“…Never theless, these methods are typically laborious, espe cially when performing serial measurements. Photok inetic method [21,23] is significantly simpler than the analytical spectral procedure described in [20].…”

“…From the discussed considerations, we used for determination of quantum yields and molar absorp tion coefficient of the closed form the photokinetic method proposed in [21,23].…”