Absence of Intramolecular Charge Transfer with 4‑Fluoro‑N,N‑dimethylaniline (DMA4F), Contrary to an Experimental Report Supported by Computations

With 4-fluoro-N,N-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 μe = 7.3 D confirms the identification as LE. The single exponential fluorescence decays in the nonpolar n-hexane (2.04 ns) and in the strongly polar acetonitrile (5.73 ns) are a further support. Similar results are obtained with 4-chloro-N,N-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 ΔE(S1,S2) of 8300 cm–1 in n-hexane between the two lowest singlet excited states, which is even larger than that (6300 cm–1) of N,N-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 n-hexane and MeCN at room temperature. The ICT/LE fluorescence quantum yield ratio Φ′(ICT)/Φ­(LE) reached a maximum value of ∼2, in n-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 p-substituted anilines.