The first vertical excitation energies calculated at the TD-DFT/PBE0-D3(BJ)/def2-TZVP and TD-DFT/ωB97X-D/def2-TZVP levels of theory and the corresponding singlet–triplet gap for 1–4 (energies are given in eV). The oscillator strengths (f) calculated as mixed length and velocity representations are given in parenthesis.

Molecule	Irrepa	E(S1 ← S0) (f × 104)	E(T1 ← S0)	ΔEST
PBE0 functional
1	a′′	2.702 (0.200)	2.652	+0.050
2	a′′	3.000 (3.502)	2.760	+0.240
3	a	2.281 (0.125)	2.278	+0.003
4	a	2.951 (0.005)	2.736	+0.215
ωB97X-D functional
1	a′′	3.174 (0.509)	2.934	+0.240
2	a′′	3.171 (0.447)	2.928	+0.243
3	a	3.187 (0.160)	2.948	+0.239
4	a	3.173 (0.002)	2.956	+0.217

^aIrreducible representation.