Synthesis and Structure–Photophysical Property Relationship of CF2CF2-Containing Tetracyclic Isoquinoline Derivative

Fluorescent isoquinoline derivatives with structurally unique frameworks remain largely unexplored. In this study, a series of CF2CF2-containing tetracyclic isoquinoline derivatives bearing methoxy substituents at different positions was synthesized, and their photophysical properties were systematically investigated. Comparison with the corresponding diester analog revealed that replacement of the CH2C(CO2Et)2 moiety with a CF2CF2 unit induced a bathochromic shift in the emission spectrum while having only a minor influence on the absorption properties. The position of the methoxy substituent markedly affected both the absorption and emission characteristics, with the 6-methoxy derivative exhibiting the largest Stokes shift owing to hypsochromically shifted absorption and bathochromically shifted emission. Solvatochromic studies demonstrated pronounced positive solvatochromism in the emission spectra, and protonation with trifluoroacetic acid resulted in further bathochromic emission shifts, suggesting the formation of a more polar excited state with charge-transfer character. Time-dependent density functional theory calculations, including natural transition orbital analysis and excited-state geometry optimization, revealed enhanced intramolecular charge-transfer character for the 6-methoxy derivative and suggested that the CF2CF2 unit contributes to stabilization of the excited state. These experimental and theoretical results provide new insight into the structure–photophysical property relationships of fluorinated tetracyclic isoquinolines and provide useful guidelines for the design of CF2CF2-containing fluorophores with tunable emission and acid-responsive properties.