O=S(=O)(c1ccccc1)n1cc(C[P+](c2ccccc2)(c2ccccc2)c2ccccc2)c2ccccc21.[Br-]
Molecules with a polar surface area of greater than 140 angstroms squared (Å2) tend to be poor at permeating cell membranes. What's the psa of this molecule?