Contrasting the excited state properties of different conformers of trans- and cis-2,2′-bipyridine oligomers in the gas phase

Abstract

In this article, we present conformation-dependent photophysical and excited state properties of trans- and cis- BPY oligomers. Oligomers up to tetramers for three conformers, namely, o-, m-, and p-, are constructed and optimized at the B3LYP-D3/def2-SVPD level. The photophysical and excited state properties are interpreted in terms of UV and CD spectra at the RI-ADC(2)/def2-TZVPD level. The UV spectra of oligomers of the m-conformer show high-intensity and red-shifted UV bands compared to o- and p-oligomers. The CD spectra of p-oligomers show intense CD bands compared to o- and p-oligomers in the case of trans-structures. In contrast, oligomers of each conformer of cis-structures show high-intensity CD bands. The excited states of (BPY)₂ and (BPY)₄ are also characterized by analysis of one-electron transition density matrix considering three descriptors: ω_CT, d_exc, and PR_NTO. The ω_CT values of dimers are in the range of 0.06–0.32, which indicates the excited states are mainly LE states, whereas, for (BPY)₄, the ω_CT values range from 0.17 to 0.53, indicating the possibility of partial CT in the excited states. These observations are also explained using the NTOs and e–h correlation plots.