Chop won't work here. I'll try to explain why.
0.445723 I is of type
Complex. Internally, both of its real and imaginary parts are stored. This is reflected by it
Thus internally it cannot be simplified any further. This is the simplest possible form.
Formatting the expression on screen is a different matter. By default Mathematica always shows both the imaginary and real part of a floating point complex number, even if the real part is
0.. I suspect this choice has to do with the fact that
0. is considered an inexact zero. It is comparable to how the symbolic expression
0. x is not automatically simplified to an exact
0. An inexact zero can be the result of roundoff errors in a numerical calculation and may be inaccurate (i.e. doing the same calculation with higher precision may in principle yield a very small but nonzero result). Thus Mathematica always displays inexact zeros.
This situation differs from something like
1.2 + 0. I.
Chop does work here: it will convert this expression of type
Complex to one of type
Real. This is not possible for
0. + 1.2 I because Mathematica has no separate type to represent a purely imaginary number with an exact zero real part and an inexact imaginary part.
To sum up: the two expressions you ask about are in fact exactly the same thing internally. When you input them, both forms will be represented as the same internal data structure. When Mathematica prints it out, it chooses to use the first printed form.
You could ask about how to cause it to print the very same expression without the
0. (and not about how to simplify its internal structure: that cannot be done). I'll leave this to other answers, like the one by @george2079.