Photochromic, Thermochromic, and Fluorescent Spirooxazines and Naphthopyrans: A Spectrokinetic and Thermodynamic Study

Abstract

The photochromic and thermochromic behavior of four commercially available Reversacol dyes are presented. The compounds studied belong to the class of spirooxazines and naphthopyrans, which are typically thermoreversible photochromic molecules. On stimulation with UV light, these compounds become colored and exhibit spectra which extend over the whole visible region. Increasing the temperature causes spontaneous coloration (thermochromism). Herein, absorption and fluorescence spectra, molar absorption coefficients of the colorless and colored forms, fluorescence and photochemical quantum yields, and kinetic parameters of thermal bleaching (rate constant, frequency factor, and activation energy) are determined in acetonitrile solution. The thermal ground-state reaction is exhaustively described in terms of thermodynamic parameters (equilibrium constant, free energy, enthalpy, and entropy). Temperature effects on photochemical and thermal colorabilities are evaluated. The results indicate that the two spirooxazines are good photochromes below room temperature, whereas they are efficient thermochromic compounds above room temperature. Naphthopyrans are better photochromes but worse thermochromic compounds than spirooxazines.

Designer molecular probes for phosphonium ionic liquids

“…Since, the BSP-MC equilibrium is almost isoentropic, the consequence of balance between two opposite contributions to ∆S 0 : a positive contribution, which is related to the increased torsional freedom in the MC isomer, and a negative contribution, which is due to solvent reorientation around the more polar MC form. 31 As expected, the experimental activation entropies …”

Metal complexes of the merocyanine form of nitrobenzospyran: Structure, optical spectra, stability

“…The equilibrium constant K 1 has been determined only for a few spiropyrans [39,40]. It can be obtained from the absorption spectrum in the visible range, where only the MC form has a detectable absorption, provided that the absorption coefficient of this isomer, e MC , is known.…”

Role of the microenvironment on the fluorescent properties of a spirooxazine

“…Photochromism and thermochromism of SO were also detected in solid powder and in a poly(methyl methacrylate) matrix (PMMA) [27]. This molecule in solution relaxes also through fluorescence, showing the presence of a dual emission that markedly depends on the solvent and excitation wavelength [26,28]. This interesting feature had been earlier identified for similar molecules containing an electron-donor and an electron-acceptor moieties linked by a single bound [29].…”

“…The compound under study exhibited fairly intense fluorescence emission with the maximum at 575 nm in MeCN solution (U F $ 3 Â 10 À2 at k exc = 386 nm) [26]. Depending on the solvent and the excitation wavelength, a dual fluorescence was detected: one emission (F A ) around 575 nm, assigned to a transition from a twisted intramolecular charge transfer (TICT) state, was favoured in polar and non-hydrogen-bonding solvents, the other (F B ) at about 480 nm was detectable in non-polar solvents [28].…”

“…photochromism and thermochromism, have been previously investigated in MeCN solution [26]. Its photocolouration is thermally reversible.…”