7. Let $z$ be a complex number, and $|z|=1$. When $\left|1+z+3 z^{2}+z^{3}+z^{4}\right|$ attains its minimum value, then at this time the complex number $z=$ $\qquad$ , or $\qquad$ .
7. $z=-\frac{1}{4} \pm \frac{\sqrt{15}}{4} \mathrm{i}$