Diverse models of intramolecular
charge transfer (ICT) have been
proposed for interpreting the origin of the charge-transfer (CT) state
in donor–acceptor (D–A) dyes. However, a large variety
of fused-heterocyclic dyes containing a pseudo-aromatic ring in the
rigid structure have shown to be incompatible with them. To approximate
a solution within the ICT concept, we reported a novel ICT model called
partially aromatized intramolecular charge transfer (PAICT). PAICT
involves the generation of a CT state from an ICT that occurred within
a pre-excited D–A fused-heterocyclic structure possessing a
pseudo-aromatic or unstable aromatic ring as the acceptor moiety.
The model was proposed from the multiple-emissive mesomeric D–A N
1
-aryl-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one, whose excited
mesomeric states, which are defined by the aromatic and pseudo-aromatic
forms of the pyrindin-4(1H)-one ring, led to a common
partial aromatized CT state upon excitation via PAICT. The latter
was supported through theoretical calculations on the excited mesomeric
states, one-dimensional (1D) and two-dimensional (2D) excitation–emission
measurements in different solvents, and the detection of three excited
states by lifetime measurements upon 370 nm excitation. The existence
of mesomerism was supposed from: (i) two overlapping bands at 370–390
(or 400–420 nm) in UV–vis spectra, (ii) the direct interaction
between the pyridinic nitrogen of one molecule and the carbonylic
oxygen of the other found in the solid state and, (iii) the detection
of three excited states by lifetime measurements. The PAICT opens
new perspectives for interpreting the charge-transfer phenomenon in
fused-heterocyclic dyes, in particular, those containing a pseudo-aromatic
or unstable aromatic ring as an acceptor moiety.