  We present laboratory rotational spectroscopy of five isomers of cyanoindene
(2-, 4-, 5-, 6-, and 7-cyanoindene) using a cavity Fourier-transform microwave
spectrometer operating between 6-40 GHz. Based on these measurements, we report
the detection of 2-cyanoindene (1H-indene-2-carbonitrile; 2-C$_9$H$_7$CN) in
GOTHAM line survey observations of the dark molecular cloud TMC-1 using the
Green Bank Telescope at centimeter wavelengths. Using a combination of Markov
Chain 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-substituted
polycyclic aromatic hydrocarbon (PAH) to its pure hydrocarbon counterpart, in
this case indene, in the same source. We discuss the possible formation
chemistry of this species, including why we have only detected one of the
isomers in TMC-1. We then examine the overall hydrocarbon:CN-substituted ratio
across this and other simpler species, as well as compare to those ratios
predicted by astrochemical models. We conclude that while astrochemical models
are not yet sufficiently accurate to reproduce absolute abundances of these
species, they do a good job at predicting the ratios of
hydrocarbon:CN-substituted species, further solidifying -CN tagged species as
excellent proxies for their fully-symmetric counterparts.
