C.-H. Chen et al.
Dyes and Pigments 175 (2020) 108143
cyclohexane to high polarity dimethylformamide. For example, the red-
shift of λem for TATCN was from 531 to 610 nm where the energy dif-
ference in thess two different polarity environments was 0.3 eV, and
such shift revealed even larger energy difference for TAPCN as 0.6 eV.
Those results portrayed these three chromophores exhibiting strong ICT
process after photo-excitation. The fact that both solvent-independent
absorption spectra (Fig. S8) and solvent-dependent emission spectra
specified that the excited state of these compounds could enter a
strongly polar excited ICT state from the initial Franck-Condon state.
The electron donating and withdrawing groups linked by
a complete conjugated system allowing redistribution of
π
-bridge form
electrons.
π
In order to further understand the capability of ICT within this ter-
aryl system, the relationship between Stokes shift and solvent polariz-
ability, named Lippert-Mataga plot, was illustrated (Fig. 2d). Linear
regression fits yielded the slope, which was the key output from the
Lippert-Mataga analysis. The slopes of TATCN and CATCN are nearly
identical but smaller than that of TAPCN, indicating that TAPCN has
more significant ICT behavior because of relatively larger slope of linear
fitted results [31,32]. Structurally, the TATCN and CATCN exhibited the
Fig. 1. Absorption (solid) and emission spectra (dash) of TATCN (black),
TAPCN (red), and CATCN (blue) in cyclohexane (concentration ¼ 6.0 � 10À 6
M). (For interpretation of the references to color in this figure legend, the reader
is referred to the Web version of this article.)
same electron withdrawing group and π-bridge but different electron
donating moieties (diphenylamine and carbazole, respectively), and
both of them displayed similar feature of ICT. Thus, diphenylamine or
carbazole substituents might not majorly affect the behavior of ICT. In
by utilizing cyclic voltammetry measuring the redox potential of sub-
stances (Fig. S7), and then, the energy levels of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) could be estimated to simulate the energies of ground and
excited states. The results of electrochemical measurements were sum-
marized in Table 1. The TATCN and TAPCN displayed relatively lower
oxidation potentials (Eox ¼ 0.67 and 0.66 V, respectively) and CATCN
exhibited relatively higher Eox (1.18 V). Observably, the diphenylamine
substituent would make the molecules to be oxidized more easily than
carbazole substituent, and thus, the energy level of HOMO was affected.
Moreover, the zero-zero transition energies (E0-0) for each of species
were assessed by the absorption and emission spectra measured in the
same solvent with electrochemical experiment, and the results of HOMO
energy and E0-0 provided the estimation of LUMO energy for each of
molecules. As shown in Table 1, E0-0 of TAPCN was closed to that of
CATCN and larger than that of TATCN. The LUMO energy levels of
TATCN and CATCN were relatively more negative and that of TAPCN
was less negative. Accordingly, the adoption of diphenylamine might
increase the energy of HOMO, and the adoption of terephthalonitrile
linked with thiophene might reduce the energy of LUMO.
contrast, when the
π-bridge was benzene rather than thiophene in
TAPCN, it would benefit the occurrence of ICT. In comparison with the
results of electrochemical measurements, the property of ICT might be
interrelated with the energy level of LUMO. In other words, the ICT
process might mainly be dominated by the types of π-bridge linked with
terephthalonitrile. Here, the utilization of Lippert-Mataga analysis also
provide another information about the dipole moment change between
the excited and ground state (Δμge) [33,34]. If the molecular size was
assumed to be similar among these teraryls, the slope of linear fits would
be simply proportional to square of the Δμge. Accordingly, the Δμge of
TAPCN was larger than that of other two chromophores. Indeed, the
absorptivity of TAPCN (9.0 � 105 cmÀ 1MÀ 1) was relatively larger than
those of TATCN (7.5 � 105 cmÀ 1MÀ 1) and CATCN (7.3 � 105 cmÀ 1MÀ 1
)
at λmax. Moreover, the photoluminescence quantum yield (Φ) was
dramatically higher for TAPCN (Φ ¼ 90%) than other two correspond-
ing species (Φ ¼ 53% for TATCN and Φ ¼ 55% for CATCN, respectively)
in cyclohexane. Those results confirmed the conclusion of
Lippert-Mataga analysis.
The Φ‘s were not only decided in cycloheaxe, but also measured in
different polarity solvents, such as diethyl ether, tetrahydrofuran,
dichloromethane, and dimethyformamide. The results are shown in
Fig. 3, in which the x-axis is presented by characteristic emission
wavelength and wavenumber. Interestingly, except the Φ of TAPCN was
noticeable high at 463 nm in cyclohexane, the energy of emission light
ranged from 21000 to 18000 cmÀ 1 exhibited similar Φ. Namely, the
populations of radiative transition within these energy gaps were com-
parable. When the energy gap was below 18000 cmÀ 1, the Φ’s reduced
significantly. This phenomenon could be known as band gap law [35],
where the vibrational energy levels of ground and excited states might
overlap with each other due to small band gap and then improved the
non-radiative relaxation. In order to further understand the nature of
ICT among these three luminogens, the electronic structures of TATCN,
TAPCN, and CATCN were investigated by density functional theory
(DFT) calculations carried out based on B3LYP/6-31G(d) level (Fig. S9).
For these three chromophores, the difference of electron distribution
between HOMO and LUMO are trivial. For all of these three chromo-
phores, the electron density of LUMO localized on acceptor tereph-
thalonitrile and HOMO electron density localized on donor
diphenylamine and carbazole units. The results confirmed that signifi-
cant ICT occurred in this system.
2.3. Solvent effect and intramolecular charge transfer
It is known that a conjugated system involving electron donating and
withdrawing groups might facilitate the occurrence of intramolecular
charge transfer (ICT). This intriguing performance could be investigated
by solvent dependent emission spectra. As shown in Fig. 2a–c, all of
TATCN, TAPCN, and CATCN revealed significant bathochromic shift of
emission profiles when the solvents were changed from low polarity
Table 1
The list of Eox, HOMO, E0-0, and LUMO for TATCN, TAPCN, and CATCN.
Eoxa (V)
HOMOb (eV)
E0-0c (eV)
LUMOd (eV)
TATCN
TAPCN
CATCN
0.67
0.66
1.18
À 5.21
À 5.20
À 5.72
2.27
2.59
2.64
À 2.94
À 2.61
À 3.08
a
Oxidation potential was decided with Ag/Agþ electrode as reference.
b
HOMO energy was determined by Eox of chromophore calibrated by Eox of
ferrocene in the same electrochemical system (HOMO ¼ -(Eox of ferrocene þ
4.80)).
c
E0-0 was calculated by the overlap of absorption and fluorescence spectra, in
The strong variation of transition dipole moment in this D-π-A-π-D
which the overlapping wavelength (λ0-0) was converted into energy by 1240/λ0-
skeleton might induce a quadrupole strength resulting in nonlinear op-
tical response such as two-photon absorption. Because of extremely high
0 in unit eV.
d
LUMO ¼ HOMO þ E0-0
.
3