Electron acceptors of the fluorene series. Part 5. Intramolecular charge transfer in nitro-substituted 9-(aminomethylene)fluorenes

Strong intramolecular charge transfer (ICT) in fluorenes 2 and 3 from a donor amino group to acceptor fluorene moiety leads to exceptionally easy rotation around the C(9)(α) bond that has been registered by 1H NMR spectroscopy; single crystal X-ray analysis of 2i confirms the changes in the bond numbers. Cyclic voltammetry (CV) of compounds 2 and 3 shows two closely spaced reversible single-electron reduction waves (in the range of –0.4 V to –1.16 V, E1red½– E2red½⩽ 0.16 V) resulting in radical anions and dianions, respectively, and a third reduction wave [E3red½≈–(1.31 – 1.53) V] resulting in radical trianions. E3red½ shows very little dependence on the structure of compounds 2 and 3, whereas E1red7½ and E2red½ correlate well with σp– constants of substituents in the fluorene ring. At +0.71 V to + 1.55 V a single-electron oxidation wave yielding radical cations of the compounds 2 and 3 is observed in CV. Parameters ρcv– for reduction (E1red½ and E2red½) and oxidation (Eox½) of compounds 2 and 3 are in the region of 0.12 V to 0.20 V. The influence of the structure and solvent effects on the ICT energies have been studied by UV–VIS spectroscopy. It has been found that the ICT energies also correlate well with σp– constants in the fluorene ring; however, in contrast to CV investigations which demonstrated close ρcv– values for both series of the compounds [ρ1red–(2)= 0.195 ± 0.005 V ≈ρ1red–(3) 0.175 ± 0.005 V, in acetonitrile], the sensitivity parameter ρICT–for 2 is approximately twice that for 3 (–0.175 ± 0.008 eV and –0.085 ± 0.008 eV, respectively, in acetonitrile). A quantitative estimation of solvent effects on the ICT energies using the four-parameter Koppel-Palm equation shows that only polarity and basicity of the solvent are statistically relevant in all the cases. Spectroelectrochemical studies on the compound 2i show the disappearance of the ICT band in the visible spectra when the negative potential yielding the radical anion and/or the dianion is applied. An increased electrophotographic sensitivity of poly-N-(2,3-epoxypropyl)carbazole (PEPC) films sensitized by compound 2d has been found in the ICT region; this phenomenon can be used for elaboration of photothermoplastic films with selective regions of the photosensitivity.