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 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.本研究针对一系列吡啶基乙炔基化（pyridylethynylated）与嘧啶基乙炔基化（pyrimidylethynylated）的单苯并吡喃（mono-benzopyrans）和双苯并吡喃（bis-benzopyrans）的光致变色（photochromic）与酸致变色（acidochromic）行为展开系统探究，结果揭示了氮杂芳环（N-heteroaryl ring）对光生邻醌型有色活性中间体（photogenerated o-quinonoid colored reactive intermediates）的光谱动力学性质（spectrokinetic properties）存在显著调控作用。在三氟乙酸（TFA）存在的条件下，吡啶基乙炔基化双苯并吡喃及其光生邻醌型有色物种的吸收峰最大值（absorption maxima）分别发生约40 nm与22 nm的红移（bathochromic shifts）；而在相似实验条件下，结构类似的嘧啶基乙炔基化双苯并吡喃及其光生邻醌型有色物种的吸收光谱则未受明显影响。这类苯并吡喃在质子（H+）存在时，其光致变色行为发生改变，对应光生邻醌型物种的光谱动力学性质也随之变化，这一现象源于两类氮杂芳环（吡啶基/嘧啶基）的相对质子亲和势（proton affinities），以及通过乙炔基间隔基（ethynyl spacers）在推拉型π离域骨架（push–pull π-delocalized-type skeleton）中传递的共振效应（resonance effects）；在有无酸的两种情境下，中介效应（mesomeric effects）的作用模式会因氮杂芳环的不同而截然相反。这些分子体系为通过调控氮杂芳环结构，实现苯并吡喃及其光生有色邻醌型中间体的光致变色与酸致变色性能精准调控提供了独特的研究路径。