Elucidating the Excited State Behavior of Pyridyl Pyridinium Systems via Computational and Transient Absorption...

The tetrahedral shape-persistent molecule 1⁴⁺ , containing four identical pyridyl pyridinium units connected via a sp³ hybridized carbon atom, has been investigated in detail by means of steady-state and time resolved spectroscopy. Remarkable photophysical properties are observed, particularly in comparison with protonated and methylated analogues (1H₄ ⁸⁺ , 1Me₄ ⁸⁺ ), which exhibit substantially shorter excited state lifetimes and lower emission quantum yields. Theoretical studies have rationalized the behavior of the tetrameric molecules relative to the monomers, with DFT and TD-DFT calculations corroborating steady-state (absorption and emission) and transient absorption spectra. The behavior of the monomeric compounds (each consisting in one of the four identical subunits of the tetramers, i. e., 2⁺ , 2H²⁺ and 2Me²⁺ ) considerably differs from that of the tetramers, indicating a strong electronic interaction between the subunits in the tetrameric species, likely promoted by the homoconjugation through the connecting sp³  C atom. 2⁺ is characterized by a peculiar S₁ -S₂ excited state inversion, whereas the short-lived emitting S₁ state of 2H²⁺ and 2Me²⁺ exhibits a partial charge-transfer character, as substantiated by spectro-electrochemical studies. Among the six investigated systems, only 1⁴⁺ is a sizeable luminophore (Φ_em =0.15), which is related to the peculiar features of its singlet state.