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Absence of Intramolecular Charge Transfer with 4‑Fluoro‑N,N‑dimethylaniline (DMA4F), Contrary to an Experimental Report Supported by Computations
journal contribution
posted on 2017-01-18, 00:00 authored by Klaas A. Zachariasse, Attila Demeter, Sergey I. DruzhininWith
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.