You have a complex function, so you should define what exactly you want to plot of this function. Some alternatives are the followings (note that the imaginary number in Matlab is 1j):
t = 0:0.1:10;
P= (2/3) * exp( 1j*(4*pi*t + 3*pi/4) );
plot(t,real(P)), hold on
plot(t,imag(P))
plot(t,abs(P))
legend({'Real','Imag','Absolute'})
