Solid‐State Photochromism of Chromenes: Enhanced Photocoloration and Observation of Unstable Colored Species at Low Temperatures

Abstract

Solid-state photochromism of benzopyrans and naphthopyrans (chromenes) was investigated in the temperature range between 300 and 80 K. Variable-temperature diffuse reflectance spectroscopy of microcrystalline powders showed that the extent of photocoloration was greatly enhanced at low temperatures. All the chromenes examined exhibited solid-state photochromism at low temperatures, even when they showed little or no photocoloration at room temperature. The solid-state photochromic properties of the chromenes were quite similar to those reported for analogous photochromic compounds of spiropyrans and spirooxazines, which indicates that these classes of compounds are generally photochromic even in the solid state. Photobleaching reactions of the colored merocyanine forms proceeded at low temperatures through the formation of a colorless intermediate, instead of directly resuming the original closed form. In addition to two stable planar merocyanine forms, which are usually observed in the photochromic reactions in solution, photoreactions at low temperatures allowed us to observe unstable colored species, which were tentatively assigned as nonplanar cisoid forms, and were stabilized in the solid state at low temperatures.

PyridiniumN-Phenolate Betaine Dyes

“…This type of thermochromism does not involve any changes in the molecular structure of the dye, in contrast to the thermochromic effects resulting from a shift in the chemical equilibrium associated with a change from a colored to a noncolored species due to changes in the temperature. − Thus, the thermochromism observed with the betaines has been called negative thermosolvatochromism, being due to an increase in the differential stabilization of the zwitterionic electronic ground state of the dye relative to its excited state with decreasing temperature. ,, The explanation for this is that both specific and nonspecific intermolecular interactions involving the molecules of dye and solvent are strengthened with a decrease in temperature, , leading to higher E T (30) values. In other words, a decrease in temperature of a solution improves the solvation capability of the solvent, demonstrating that solvent polarity is temperature-dependent .…”

Contrasting Photochromic and Acidochromic Behaviors of Pyridyl- and Pyrimidylethynylated Mono- and Bis-Benzopyrans

“…Mechanistic details of the photochromic properties of benzopyrans have been extensively investigated. ,− Needless to mention that the TC and TT isomeric forms of the o -quinonoid reactive intermediates shown in Figure are mainly responsible for the observed coloration attendant upon photoirradiation of benzopyrans. The TT isomer is generated by a two-photon absorption of the benzopyran through initial formation of the TC isomer such that the former is likely to be formed only minimally when the precursor benzopyran is subjected to photoirradiation for a shorter duration.…”

“…173 K) due to the extremely fast thermal reversion of the photogenerated o -quinonoid intermediate at room temperature . Thus, modulation of the spectrokinetic properties of o -quinonoid reactive intermediates and the associated photochromic phenomenon of diarylbenzopyrans have been a subject of extensive investigations. − …”

Synthesis and photochromic behaviour of a series of benzopyrans bearing an N-phenyl-carbazole moiety: photochromism control by the steric effect

“…9,10 This behaviour has also been observed for the reference molecule having this skeleton, 3,3diphenyl-3H-naphtho [2,1-b]pyran (herein named RN, Scheme 1), that was extensively reported in the literature, especially over the last three decades. 5,7,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The transformation of a benzo-or a naphthopyran into a photochromic system possessing only a T-Type or a P-Type behaviour might undoubtedly be of interest from both points of view of basic research and practical applications. This could be achieved by choosing a suitable solvent, the temperature range or introducing modifications of the molecular structure.…”