Elucidating the Excited State Behavior of Pyridyl Pyridinium Systems via Computational and Transient Absorption Studies of Tetrahedral Multichromophoric Arrays and their Model Compounds

The tetrahedral shape-persistent molecule 1, containing four identical pyridyl pyridinium units connected via a sp 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 (1H, 1Me), 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, 2H and 2Me) 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 sp C atom. 2 is characterized by a peculiar S-S excited state inversion, whereas the short-lived emitting S state of 2H and 2Me exhibits a partial charge-transfer character, as substantiated by spectro-electrochemical studies. Among the six investigated systems, only 1 is a sizeable luminophore (phiem=0.15), which is related to the peculiar features of its singlet state.