Figure 3.

Excitation energies of the two lowest bright ππ* states of (a) thieno[3,2-b]thiophene (S₁ and S₂) and (b) 2,2′-bithiophene (S₁ and S₃/S₄). As in Figure 2, radii of the circles are proportional to the oscillator strengths, and colors denote the orbital excitations with largest coefficients. The cc-pVQZ basis set was employed. The *EOM-CCSD and SAC–CI energies of bithiophene were computed at the cc-pVTZ level. The **CASPT2 results of bithiophene are taken from 32 and correspond to the ground state structure optimized at CASPT2 level with C_2h symmetry.