Polycyclic aromatic hydrocarbons (PAHs) are among the most widespread compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent electric dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. Here we report the detection of 2-cyanopyrene and 4-cyanopyrene, isomers of the recently detected 1-cyanopyrene. We find that these isomers are present in an abundance ratio of ~2:1:2, which mirrors the number of equivalent sites available for CN addition. We conclude that there is evidence that the cyanopyrene isomers formed by direct CN addition to pyrene under kinetic control in hydrogen-rich gas at 10 K and discuss constraints on the H/CN ratio for PAHs in the Taurus molecular cloud (TMC-1). Our detections of the cyanopyrene isomers suggest that small PAHs like pyrene must be either formed in or transported to the cold interstellar medium, challenging assumptions about the origin and fate of PAHs in space.

多环芳烃 （PAH） 是宇宙中分布最广泛的化合物之一，占所有星际碳的 ~25%。由于大多数未取代的 PAH 不具有永久电偶极矩，因此射电天文学看不到它们。限制它们的丰度依赖于对极性化学替代物的检测，例如芳香族腈。在这里，我们报告了 2-氰基芘和 4-氰基芘的检测，它们是最近检测到的 1-氰基芘的异构体。我们发现这些异构体以 ~2：1：2 的丰度比存在，这反映了可用于 CN 添加的等效位点的数量。我们得出结论，有证据表明，在 10 K 的富氢气体中，在动力学控制下，通过直接将 CN 添加到芘中形成的氰基芘异构体，并讨论了金牛座分子云 （TMC-1） 中 PAH 的 H/CN 比率的限制。我们对氰基芘异构体的检测表明，像芘这样的小 PAH 必须在寒冷的星际介质中形成或运输到寒冷的星际介质中，这挑战了关于 PAH 在太空中起源和命运的假设。