Substituent Effect in the First Excited Triplet State of Monosubstituted Benzenes.

The structure of 30 monosubstituted benzenes in the first excited triplet T state was optimized with both unrestricted (U) and restricted open shell (RO) approximations combined with the ωB97XD/aug-cc-pVTZ basis method. The substituents exhibited diverse σ- and π-electron-donating and/or -withdrawing groups. Two different positions of the substituents are observed in the studied compounds in the T state: one distorted from the plane and the other coplanar with a quinoidal ring. The majority of the substituents are π-electron donating in the first group while π-electron withdrawing in the second one. Basically, U- and RO-ωB97XD approximations yield concordant results except for the B-substituents and a few of the planar groups. In the T state, the studied molecules are not aromatic, yet aromaticity estimated using the HOMA (harmonic oscillator model of aromaticity) index increases from ca. -0.2 to ca. 0.4 with substituent distortion, while in the S state, they are only slightly less aromatic than in the ground state (HOMA ≈0.8 vs ≈1.0, respectively). Unexpectedly, the sEDA(T) and pEDA(T) substituent effect descriptors do not correlate with analogous parameters for the ground and first excited singlet states. This is because in the T state, the geometry of the ring changes dramatically and the sEDA(T) and pEDA(T) descriptors do not characterize only the functional group but the entire molecule. Thus, they cannot provide useful scales for the substituents in the T states. We found that the spin density in the T states is accumulated at the C and C atoms, and with the substituent deformation angle, it nonlinearly increases at the former while decreases at the latter. It appeared that the gap between singly unoccupied molecular orbital and singly occupied molecular orbital (SUMO-SOMO) is determined by the change of the SOMO energy because the former is essentially constant. For the nonplanar structures, SOMO correlates with the torsion angle of the substituent and the ground-state pEDA(S) descriptor of the π-electron-donating substituents ranging from 0.02 to 0.2 . Finally, shapes of the SOMO-1 instead of SOMO frontier orbitals in the T state somehow resemble the highest occupied molecular orbital ones of the S and S states. For several planar systems, the shape of the U- and RO-density functional theory-calculated SOMO-1 orbitals differs substantially.