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

Abstract

Investigation of photochromic and acidochromic behaviors of a set of pyridyl- and pyrimidylethynylated mono- and bis-benzopyrans reveals an intriguing influence of the N-heteroaryl ring on spectrokinetic properties of the photogenerated o-quinonoid colored reactive intermediates. While the absorption maxima of the pyridylethynylated bis-benzopyran and its photogenerated o-quinonoid colored species undergo bathochromic shifts by ca. 40 and 22 nm, respectively, in the presence of an acid (e.g., trifluoroacetic acid (TFA)), the same remain unaffected for the analogous pyrimidylethynylated bis-benzopyran and its photogenerated o-quinonoid colored species under similar conditions. Modification of the photochromic behavior of these benzopyrans and, hence, spectrokinetic properties of their photogenerated o-quinonoid species in the presence of H+ is a consequence of relative proton affinities of N-heteroaryl rings, i.e., pyridyl/pyrimidyl, and the resonance effects relayed through the ethynyl spacers in a push–pull π-delocalized-type skeleton; the mesomeric effects operate in a contrasting manner depending on the N-heteroaryl ring in the absence and in the presence of an acid. These molecular systems offer a unique opportunity to modulate both photochromic and acidochromic properties of benzopyrans and their photogenerated colored o-quinonoid intermediates by leveraging N-heteroaromatic rings.

Nonadiabatic molecular dynamics simulations for ultrafast photo-induced ring-opening and isomerization reactions of 2,2-diphenyl-2H-chromene

“…2 In order to improve the colorability, photostability and fatigue resistance performances of benzopyrans, numerous investigations have been carried out by means of modulating the kinetic equilibrium towards TC or TT isomers based on inductive, mesomeric and steric effects, heterocyclic substitution and ring annulation strategies. [6][7][8][9][10][11][12][13][14][15][16][17][18][19] Fully T-type photochromism chromenes have been synthesized with suitable substitutions toward a pyran ring that stabilizes the TC isomer, and thus prevents the ensuing formation of TT both thermally and optically [8][9][10] and guarantees promising applications in photochromic lenses. On the other hand, helicene annulation gives rise to P-type chromenes 6,7 with potential applications, such as molecular motors, high density data storage devices and electronic devices.…”

Zn(II) Complexes Based on a Schiff Base: Mechanochromism- and Solvent Molecule-Dependent Acidochromism

“…Therefore, it is important to obtain loose and easily broken molecular packing patterns to obtain efficient mechanochromic luminescence materials. Acidochromic materials exhibit excellent color or emission changes under the stimulation of acid gas and have potential applications in the detection of acid vapors. − Under the treatment of acid gas, the nitrogen atoms contained in the molecules can be mostly protonated, resulting in the change of the charge distribution and luminescence properties. When further treated with basic vapors, the deprotonation process takes place and the original luminescence state is restored, achieving reversible acidochromic properties. − …”