Intramolecular Charge Transfer with 4-Fluorofluorazene and the Flexible 4-Fluoro-N-phenylpyrrole

With 4-fluorofluorazene (FPP4F) and its flexible counterpart 4-fluoro-N-phenylpyrrole (PP4F) an intramolecular charge transfer (ICT) reaction occurs in the singlet excited state in sufficiently polar solvents. The ICT reaction begins to appear in tetrahydrofuran (E = 7.4) for FPP4F and in the more polar 1,2-dichloroethane (epsilon = 10.4) with PP4F, showing its presence by dual fluorescence from a locally excited (LE) and an ICT state. Only LE fluorescence is observed in less polar solvents such as n-hexane. The ICT reaction is more pronounced with FPP4F than for PP4F, due to the smaller energy gal) Delta E(S-1,S-2) of the former molecule, in accordance with the PICT model. The occurrence of an ICT reaction is confirmed by the ICT dipole moments mu(e)(ICT) of 12 D (FPP4F) and 10 D (PP4F), clearly larger than mu(e)(LE) of similar to 4 D for FPP4F and PP4F. Isoemissive points are found in the fluorescence spectra of FPP4F and PP4F in acetonitrile (MeCN), ethyl cyanide (EtCN), and n-propyl cyanide (PrCN) as a function of temperature, confirming the two-state (LE and ICT) reaction mechanism. From plots of the logarithm of the ICT/LE fluorescence quantum yield ratio versus the reciprocal absolute temperature in these solvents, the ICT reaction enthalpies Delta H are determined, with larger -Delta H values for FPP4F than for PP4F: 19.2 as compared with 14.9 kJ/mol in MeCN, as an example. The picosecond fluorescence decay of PP4F at -45 degrees C becomes slower with decreasing solvent polarity, 5.1 ps (MeCN), 14 ps (EtCN), and 35 ps (PrCN), from which the LE -> ICT reaction rate constant is calculated, decreasing from 19 x 10(10) to 2.1 x 10(10) s(-1) between MeCN and PrCN. The femtosecond LE excited-state absorption spectra of FPP4F and PP4F do not undergo any time development in n-hexane (no ICT reaction), but show a fast ICT reaction in MeCN at 22 degrees C, with decay times of 1.1 ps (FPP4F) and 3.3 ps (PP4F). It is concluded that FPP4F and PP4F have a planar ICT state (PICT model), indicating that a perpendicular twist of the donor and acceptor subgroups in a donor/acceptor molecule is not a requirement for fast and efficient ICT. The molecular structures of FPP4F and PP4F obtained from X-ray crystal analysis reveal that the pyrrole group of PP4F is twisted over an angle theta = 25 degrees relative to the fluorophenyl moiety in the ground state, whereas as expected r-PP4F is practically planar (theta = 2 degrees). The pyrrole-phenyl bond length of FPP4F (140.7 pm) is shorter than that for PP4F (141.8 pm).