  We report the first interstellar detection of DC$_7$N and six
$^{13}$C-bearing isotopologues of HC$_7$N toward the dark cloud TMC-1 through
observations with the Green Bank Telescope, and confirm the recent detection of
HC$_5$$^{15}$N. For the average of the $^{13}$C isotopomers, DC$_7$N, and
HC$_5$$^{15}$N, we derive column densities of 1.9(2)$\times$10$^{11}$,
2.5(9)$\times$10$^{11}$, and 1.5(4)$\times$10$^{11}$ cm$^{-2}$, respectively.
The resulting isotopic ratios are consistent with previous values derived from
similar species in the source, and we discuss the implications for the
formation chemistry of the observed cyanopolyynes. Within our uncertainties, no
significant $^{13}$C isotopomer variation is found for HC$_7$N, limiting the
significance CN could have in its production. The results further show that,
for all observed isotopes, HC$_5$N may be isotopically depleted relative to
HC$_3$N and HC$_7$N, suggesting that reactions starting from smaller
cyanopolyynes may not be efficient to form HC$_{n}$N. This leads to the
conclusion that the dominant production route may be the reaction between
hydrocarbon ions and nitrogen atoms.
