A valence- and inner-shell electronic and photoelectron spectroscopic study of the frontier orbitals of 2,1,3-benzothiadiazole, C6H4SN2, 1,3,2,4-benzodithiadiazine, C6H4S2N2, and 1,3,5,2,4-benzotrithiadiazepine, C6H4S3N2