Photochromism of dihydroindolizines part VII: multiaddressable photophysical properties of new photochromic dihydroindolizines bearing substituted benzo[i]phenanthridine as a fluorescing moiety

Sixteen benzoi]phenanthridine derivatives 8a‐p were prepared via photocyclization of cis‐trans substituted 4‐styrylquinolines in low‐to‐moderate yields. The chemical structure of the photocyclized benzoi]phenanthridine derivatives was unambiguously elucidated by means of both spectral and analytical tools. The photochromic (PC) dihydroindolizines (DHIs) 8a‐p based on benzoi]phenanthridines were prepared in 19–57% yields via nucleophilic addition of benzoi]phenanthridines 4a‐p to spirocyclopropenes 5 . The 1D, 2D, NOESY NMR spectra, mass spectrometry, and elemental analysis were used for characterization of the chemical structures of the newly synthesized DHIs 8a‐p . Developing and tuning of the photophysical properties of the synthesized compounds by substituents in the base part have been achieved. The absorption maxima (λ_max) and the half‐lives (t_1/2) of the colored zwitterionic forms 7a‐p were detected in all cases by flash‐photolysis measurements due to the fast 1,5‐electrocyclization back to the DHI system. Irradiation of DHI 8a‐p in'CH₂Cl₂ solution with polychromatic light leads to the formation of green to green–blue colored betaines 7a‐p after cooling with liquid nitrogen. The kinetics of the fast bleaching process of betaines 7a‐p to DHIs 8a‐p , studied by flash photolysis as well as low temperature FT‐UV/VIS, were found to take place in the millisecond range (432–2675 ms) in dichloromethane solution and fitted well a first‐order thermal back reaction. The fluorescence spectra as well as the fluorescence quantum yield were studied. Noticeable bathochromic and hypsochromic shifts in the emission spectra by changing the substituents in the base part were monitored. Interestingly, the photo‐fatigue resistance of some studied betaines 7a‐p showed a higher t₃₀‐value than the standard one (dicyanopyridazine DHI). Large solvatochromic effects on the absorption maxima (λ_max) as well as a substantial increase in the half‐lives (t_1/2) with solvent polarity of betaines 7a‐p were also observed. The multiaddressable PC properties of DHIs 8a‐p will help these compounds to find applications. Copyright © 2007 John Wiley & Sons, Ltd.