Photochromism of Dicyanorhodanine-Pyrrole Conjugates: Substituent and Solvent Effects

Dicyanorhodanine (RCN)-pyrrole conjugates are a relatively new class of photochromic compounds capable of highly efficient reversible photoisomerization upon visible-light excitation in both solution and solid-state, and understanding the factors governing their photophysical behavior is essential for designing reliable photoswitches. In this study, a library of RCN-pyrrole chromophores is investigated to elucidate the influence of substituents with varying electronic and steric properties on their π-π* absorption characteristics in solution. Solvent polarity affects the frontier molecular orbital energies, resulting in a negative solvatochromic effect. Even so, (near)-quantitative bidirectional Z/E photoisomerization has been achieved for all the RCN-pyrroles. Intramolecular hydrogen-bonding (H-bonding) in the E isomers and intermolecular H-bonding in the Z isomers play key roles in their exceptional photoswitching efficiency and high thermal stability. Thermal isomerization could be tuned by solvents with different H-bonding capabilities, highlighting the importance of specific solute-solvent interactions. Replacing the pyrrole with an imidazole moiety reveals contrasting photoinduced side reactions inconsistent with Z/E isomerization. Complementary ground-(DFT) and excited-state (TD-DFT) calculations correlate well with the experimental absorption profiles and the photochemical outcomes. These structure-property photoisomerization analyses provide critical insights for the application of RCN-pyrrole-based photochromic systems forsmart materials design.