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We present laboratory rotational spectroscopy of five isomers of cyanoindene(2-, 4-, 5-, 6-, and 7-cyanoindene) using a cavity Fourier-transform microwavespectrometer operating between 6-40 GHz. Based on these measurements, we reportthe detection of 2-cyanoindene (1H-indene-2-carbonitrile; 2-C$_9$H$_7$CN) inGOTHAM line survey observations of the dark molecular cloud TMC-1 using theGreen Bank Telescope at centimeter wavelengths. Using a combination of MarkovChain Monte Carlo (MCMC), spectral stacking, and matched filtering techniques,we find evidence for the presence of this molecule at the 6.3$\sigma$ level.This provides the first direct observation of the ratio of a cyano-substitutedpolycyclic aromatic hydrocarbon (PAH) to its pure hydrocarbon counterpart, inthis case indene, in the same source. We discuss the possible formationchemistry of this species, including why we have only detected one of theisomers in TMC-1. We then examine the overall hydrocarbon:CN-substituted ratioacross this and other simpler species, as well as compare to those ratiospredicted by astrochemical models. We conclude that while astrochemical modelsare not yet sufficiently accurate to reproduce absolute abundances of thesespecies, they do a good job at predicting the ratios ofhydrocarbon:CN-substituted species, further solidifying -CN tagged species asexcellent proxies for their fully-symmetric counterparts.

Discovery of Interstellar 2-Cyanoindene (2-C$_9$H$_7$CN) in GOTHAM Observations of TMC-1