First examples of stable carbocations are reported from several classes of thia-PAHs with four fusedrings, namely, benzo[
b]naphtho[2,1-
d]thiophene (
1) and its 3-methoxy derivative (
2), phenanthro[4,3-
b]thiophene (
3) and its 7-methoxy (
4), 10-methoxy (
5), and 9-methoxy (
6) derivatives, phenanthro[3,4-
b]thiophene (
7) and its 7-methoxy (
8) and 9-methoxy (
9) derivatives, and 3-methoxybenzo[
b]naphtha[1,2-
d]thiophene (
11). In several cases, the resulting carbocations were also studied by GIAO-DFT. Chargedelocalization modes in the resulting carbocations were probed. A series of S-alkylated oniumtetrafluoroborates, namely,
1Me+,
1Et+,
2Et+, and
7Me+ (from
1,
2, and
7),
10Me+ and
10Et+ (frombenzo[
b]naphtha[1,2-
d]thiophene
10),
12Me+ and
12Et+ (from phenanthro[3,2-
b][1]benzothiophene
12),
13Me+ (from 3-methoxyphenanthro[3,2-
b]benzothiophene
13),
14Me+ (from phenanthro[4,3-
b][1]benzothiophene
14), and
15Me+ (from 3-methoxyphenanthro[4,3-
b][1]benzothiophene
15), weresynthesized. PAH-sulfonium salts
1Me+,
1Et+,
10Me+,
10Et+,
12Me+, and
14Me+ proved to be efficientakylating agents toward model nitrogen nucleophile receptors (imidazole and azaindole). Facile transalkylation to model nucleophiles (including guanine) is also supported by favorable reaction energiescomputed by DFT. Ring opening energies in thia-PAH-epoxides from
1,
3, and
7 and charge delocalizationmodes in the resulting carbocations were also evaluated. The four-ring-fused thia-PAHs
1,
2,
3,
4,
5,
7,
8, and
11 are effectively nitrated under extremely mild conditions. Nitration regioselectivity correspondsclosely to protonation under stable ion conditions. Bromination of
4 and
6 is also reported. Comparativemutagenicity assays (Ames test) were performed on
1 versus
1NO2,
5 versus
5NO2, and
11 versus
11NO2.Compound
5NO2 was found to be a potent direct acting mutagen.
