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

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 pi-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.

Automated summary

The study explores the impact of N-heteroaryl rings on the spectrokinetic properties of photogenerated o-quinonoid colored reactive intermediates in pyridyl- and pyrimidylethynylated benzopyrans. It reveals that the modification of photochromic behaviors and spectrokinetic properties in the presence of acid is influenced by the relative proton affinities of N-heteroaryl rings and resonance effects relayed through the ethynyl spacers. This research demonstrates the unique opportunity to modulate both photochromic and acidochromic properties of benzopyrans by leveraging N-heteroaromatic rings.