Absence of Intramolecular Charge Transfer with 4‑Fluoro‑<i>N</i>,<i>N</i>‑dimethylaniline (DMA4F), Contrary to an Experimental Report Supported by Computations
2017-01-18T00:00:00Z (GMT) by
With 4-fluoro-<i>N</i>,<i>N</i>-dimethylaniline (DMA4F), only a single fluorescence from a locally excited (LE) state is observed, irrespective of solvent polarity, temperature, and excitation wavelength. The relatively small excited state dipole moment μ<sub>e</sub> = 7.3 D confirms the identification as LE. The single exponential fluorescence decays in the nonpolar <i>n</i>-hexane (2.04 ns) and in the strongly polar acetonitrile (5.73 ns) are a further support. Similar results are obtained with 4-chloro-<i>N</i>,<i>N</i>-dimethylaniline (DMA4Cl), having a chlorobenzene subgroup, a somewhat better electron acceptor than the fluorobenzene moiety in DMA4F. The absence of intramolecular charge transfer (ICT) with DMA4F is in accord with its large energy gap Δ<i>E</i>(S<sub>1</sub>,S<sub>2</sub>) of 8300 cm<sup>–1</sup> in <i>n</i>-hexane between the two lowest singlet excited states, which is even larger than that (6300 cm<sup>–1</sup>) of <i>N</i>,<i>N</i>-dimethylaniline (DMA), for which an LE → ICT reaction likewise does not occur. The results with DMA4F are in contradiction with a publication by Fujiwara et al. (Chem. Phys. Lett. 2013, 586, 70), in which the appearance of dual LE + ICT emission is reported for DMA4F in <i>n</i>-hexane and MeCN at room temperature. The ICT/LE fluorescence quantum yield ratio Φ′(ICT)/Φ(LE) reached a maximum value of ∼2, in <i>n</i>-hexane and surprisingly also in MeCN, as the excitation wavelength approaches the red-edge of the absorption spectrum. These, in our opinion, erroneous observations were supported by time-dependent density functional theory (TDDFT) calculations, which compute a perpendicularly twisted lowest ICT state (TICT) state. This is a further example of the general tendency of computations to find a TICT conformation for the lowest excited singlet state of electron donor/acceptor molecules such as <i>p</i>-substituted anilines.