Photochemical synthesis and solvatochromic fluorescence behavior of imide-fused phenacenes

Chrysenes, picene, fulminene, modified with imide, bromo, and amino functionalities, were synthesized through Mallory photoreaction as the key step, and their electronic spectra were investigated. Fluorescence spectra of chrysene-diimide CHRDI and bromo-substituted phencanene-imides, BrCHRI, BrPICI, BrFULI were dependent on solvent polarity to display appreciable fluorescence color changes. The solvatofluorochromic behavior was analyzed by conventional relationships between Stokes shift and solvent polarity parameters, such as Lippert-Mataga and Bilot-Kawski equations. The results indicated that the solvatofluorochromism was derived from the intramolecular charge transfer (ICT) nature in the excited state. Theoretical studies using time-dependent density-functional theory revealed that the phenacene-imide molecules in the fluorescent state possessed ICT characters between the strongly electron-withdrawing imide moiety and moderately electron-donating phenacene cores. Amino-substituted chrysene-imide NH₂CHRI showed fluorescence band in a red region (λ_FL = 618 nm) in toluene with a very large Stokes shift (Δν∼ = 7630 cm⁻¹) suggesting that the molecule in the fluorescent state was highly polarized. The present results indicate that phenacenes would provide potential platforms for constructing future functional fluorophores through an appropriate functionalization.