Systematic energy band gap control of pyrene based donor-acceptor-donor molecules for efficient chemosensor

Article abstract of DOI:10.1016/j.dyepig.2021.109362

We have successfully prepared a series of pyrene based donor-acceptor-donor (D-A-D) molecules, 1,6-bis[(N,N-p-(R)-diphenylamino)phenyl]pyrene (R = CN (Py-CN), F (Py-F), H (Py-H), Me (Py-Me), and OMe (Py-OMe)), containing N,N-bis(p-(R)-phenyl)aniline as an electron donor and pyrene as an electron acceptor by using the Suzuki-Miyaura cross-coupling reaction in good yield and fully characterized. By introducing various N,N-bis(p-(R)-phenyl)aniline derivatives with electron-donating or electron-withdrawing R groups, the energy band gaps of Py-R were systematically controlled and emission colors were efficiently tuned from blue to yellow (441–536 nm). Absorption spectra of all Py-R compounds showed intramolecular charge transfer (ICT) transitions in the range of 350–450 nm in the ground state. Solvent-dependent emission spectra confirmed the substituent dependence of charge transfer ability in the excited state. Then, the charge transfer tendencies by the substituents were quantitatively observed using the Lippert-Mataga equation. The HOMO and LUMO energy levels of synthesized Py-R compounds were found in the range of ?5.44 to ?5.97 eV and ?2.68 to ?2.97 eV respectively from cyclic voltammetry (CV) experimental. Theoretical studies were conducted using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to provide a basis for interpreting experimental data. Experimental and calculated values were found to be well correlated with the Hammett constants (σ_para) and confirmed that the energy band gap was systematically controlled by the substituent effect. Furthermore, pyrene based D-A-D compounds with controllable band gap were employed as selective chemosensors to detect nitro explosives, and especially Py-CN demonstrated efficient sensing ability for ortho-nitroaniline (o-NA) by fluorescence quenching strategy.

Full text of DOI:10.1016/j.dyepig.2021.109362

Dyes and Pigments 191 (2021) 109362
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Systematic energy band gap control of pyrene based donor-acceptor-donor
molecules for efficient chemosensor
So-Yeon Kim, Min-Ji Kim, Mina Ahn, Kyeong-Min Lee, Kyung-Ryang Wee*
Department of Chemistry and Institute of Natural Science, Daegu University, Gyeongsan, 38453, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
We have successfully prepared a series of pyrene based donor-acceptor-donor (D-A-D) molecules, 1,6-bis[(N,N-p-
(R)-diphenylamino)phenyl]pyrene (R = CN (Py-CN), F (Py-F), H (Py-H), Me (Py-Me), and OMe (Py-OMe)),
containing N,N-bis(p-(R)-phenyl)aniline as an electron donor and pyrene as an electron acceptor by using the
Suzuki-Miyaura cross-coupling reaction in good yield and fully characterized. By introducing various N,N-bis(p-
(R)-phenyl)aniline derivatives with electron-donating or electron-withdrawing R groups, the energy band gaps of
Py-R were systematically controlled and emission colors were efficiently tuned from blue to yellow (441–536
nm). Absorption spectra of all Py-R compounds showed intramolecular charge transfer (ICT) transitions in the
range of 350–450 nm in the ground state. Solvent-dependent emission spectra confirmed the substituent
dependence of charge transfer ability in the excited state. Then, the charge transfer tendencies by the substituents
were quantitatively observed using the Lippert-Mataga equation. The HOMO and LUMO energy levels of syn-
thesized Py-R compounds were found in the range of ꢀ 5.44 to ꢀ 5.97 eV and ꢀ 2.68 to ꢀ 2.97 eV respectively
from cyclic voltammetry (CV) experimental. Theoretical studies were conducted using density functional theory
(DFT) and time-dependent DFT (TD-DFT) calculations to provide a basis for interpreting experimental data.
Experimental and calculated values were found to be well correlated with the Hammett constants (σpara) and
confirmed that the energy band gap was systematically controlled by the substituent effect. Furthermore, pyrene
based D-A-D compounds with controllable band gap were employed as selective chemosensors to detect nitro
explosives, and especially Py-CN demonstrated efficient sensing ability for ortho-nitroaniline (o-NA) by fluo-
rescence quenching strategy.
Pyrene
Substituents
Intramolecular charge transfer
Lippert-mataga plot
Chemosensor
Stern-volmer equation
1. Introduction
[16], phenanthrene [17], and tetracene [18] have been used as emitting
materials to produce efficient and stable emission for full-color OLEDs or
Systematic electronic state control and efficient energy band gap
tuning are fundamental issues for molecular based optoelectronics or
energy conversion devices such as organic light-emitting devices
(OLEDs) [1–3], organic field-effect transistors (OFETs) [4,5],
light-harvesting arrays [6], and organic photovoltaic cells (OPVs) [7,8].
In particular, polycyclic aromatic hydrocarbon (PAH) based
donor-acceptor (D-A) systems are one of the efficient energy gap tunable
material and play an important role in emerging technologies to achieve
multi-functional optoelectronic properties, because effectively regulate
the electronic structure by building blocks (ex. donor and/or acceptor
unit) substitution at the active PAH sites [9,10]. Several research groups
are trying to develop PAH based energy band gap tunable D-A materials
that possess intramolecular charge transfer (ICT) properties [11–14].
For example, polyacene derivatives such as perylene [9,15], anthracene
organic light-emitting field-effect transistors (OLEFETs) with good
chromaticities, luminescence efficiencies, and high device stabilities.
In particular, pyrene containing polyaromatic D-A compounds have
received much attention among the various PAH based D-A materials,
because they allow effective control of absorption and emission prop-
erties via regioselective functional group substitution of active pyrene
sites at the 1, 3, 6, and 8-positions, as well as the K-regions at the 4, 5, 9,
and 10-positions [19]. In addition, pyrene based D-A compounds are
used as efficient light-emitting materials with high emission quantum
yields in solution and solid states, and their color-tunable emission
characteristics are usually originated ICT caused by donor or acceptor
units. As previously reported, emission color tuning of pyrene based
chromophores is readily controlled by functional group modification
[10,20]. The planar nature of pyrene moiety geometry leads to
* Corresponding author.
E-mail address: krwee@daegu.ac.kr (K.-R. Wee).
https://doi.org/10.1016/j.dyepig.2021.109362
Received 13 January 2021; Received in revised form 10 March 2021; Accepted 29 March 2021
Available online 4 April 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

S.-Y. Kim et al.
Dyes and Pigments 191 (2021) 109362
intermolecular
π
-
π
interactions and induces aggregation [19,21], fluo-
2. Experimental section
rescence quenching [22], red-shift characteristics [23], and broad
emission bands [10,20]. Among these properties, molecular aggregation
and fluorescence quenching have been utilized by many studies seeking
to develop fluorescence sensors [24,25].
2.1. General conditions
Based on the standard Schlenk techniques, all of the synthesis
experimental procedures were performed under a dry argon condition.
Reagents and solvents were purchased from commercial sources and
used as received without further purification, unless otherwise stated.
All reactions were monitored with thin layer chromatography (TLC)
using commercial TLC plates (Merck Co.). Silica gel column chroma-
tography was carried out on silica gel 60 G (230–400 mesh ASTM, Merck
To control energy band gap and provide effective emission color
tuning in the pyrene based D-A compounds, the strengths of donor and/
or acceptor containing the π-conjugated linker should be systematically
modulated and the amount of charge transferred from donor to acceptor
controlled. This can be achieved by (ⅰ) adjusting the configuration of D-A
systems based on considerations of their molecular symmetries and
asymmetries, for example, D-A-D and A-D-A types [10,20,26] and D-A
Co.). The synthesized compounds were characterized by 1H NMR or 13
C
and D-
π
-A types [9,27,28], respectively, or by (ⅱ) introducing an
{1H}-NMR and elemental analysis at the Core Research Center for
Natural Products and Medical Materials (CRCNM) in the Yeungnam
University. The 1H and proton-decoupled 13C spectra were recorded on a
Bruker500 spectrometer operating at 500 and 125 MHz, respectively,
and all proton and carbon chemical shifts were measured relative to
internal residual chloroform (99.5% CDCl3) from the lock solvent. The
elemental analyses (C, H, N, O) were performed using a Thermo Fisher
Scientific Flash 2000 series analyzer. The GC-MS analysis was performed
using a highly sensitive gas chromatograph/mass selective detector
spectrometer (Agilent, 7890B–5977B GC/MSD) at the Daegu Center of
Korea Basic Science Institute. The 1,6-bis(4,4,5,5-tetramethyl-1,3,2-
electron-donating or withdrawing group, which allows charge transfer
properties to be adjusted within molecules containing a donor, acceptor,
and π-linker [9,29]. Although color-tunable pyrene based D-A materials
have been reported, knowledge of wide-range color control with broad
energy band gap tuning by donor or acceptor substitutions on the
symmetrical pyrene based molecule is lacking [9,10]. Therefore, we
investigated systematic and wide energy band gap control with focus on
their optical ICT properties using symmetrical D-A-D type compounds by
connecting two-donor units grafted with various substituents on a pyr-
ene core. Furthermore, these properties of pyrene based D-A-D com-
pounds are pragmatic and useful in the fluorescence chemosensor
research area.
dioxaborolan-2-yl)pyrene
(Py-B(pin)),
4-bromo-N,N-bis(4-cyano-
phenyl)aniline (TPA-CN), 4-bromo-N,N-bis(4-fluorophenyl)aniline
(TPA-F), 4-bromo-N,N-diphenylaniline (TPA-H), 4-bromo-N,N-bis(4-
methylphenyl)aniline (TPA-Me), and 4-bromo-N,N-bis(4-methox-
yphenyl)aniline (TPA-OMe) were prepared on the basis of the previously
published method [35,36]. The UV–vis absorption spectra were recor-
ded with photodiode array spectrophotometry by Sinco Mega-2100, and
the fluorescence emission measurements were carried out using Shi-
madzu fluorometer (RF-6000) with a wavelength resolution of ~1 nm.
Fluorescence lifetimes were measured by PicoQuant FluoTime 200 that
takes advantage of the time-correlated single photon counting method.
A pulsed diode laser operated at a 20 MHz repetition rate was used as the
excitation source. The full width at half maximum (FWHM) of a laser
pulse was typically 45 ps, and the instrument response function was
~190 ps when the Hamamatzu photomultiplier tube (H5783-01) was
used. The emission quantum yields (ΦPL) were calculated using Wil-
liam’s comparative method for samples of five different concentrations
The selective recognition and sensing of aromatic compounds is an
important aspect of fluorescence recognition and is primarily driven by
the fluorescence resonance energy transfer (FRET) mechanism [30,31].
In this context, compounds exhibiting ICT are mainly used for material
sensing and selective energy transfer is controlled by emission or energy
band gap tailoring. In particular, in chemosensor research, the detection
of nitro aromatic explosive materials such as 2,4-dinitrotoluene (DNT)
and 2,4,6-trinitrotoluene (TNT) is viewed as an important issue, because
these nitro aromatic species have substantial adverse environmental
effects due to their toxicities and environmental resistances, and hence,
efficient methods of detecting these pollutants are urgently required
[32–34]. Fluorescence method, one of the methods for detecting aro-
matic compounds, is widely used because of its sensitivity and sensing
speed [30]. Wide range fluorescence emission molecules are attractive
materials for chemosensor applications based on fluorescence quench-
ing method. These molecules can be used as selective chemosensors to
detect the analytes contain nitro aromatics, which are emission
quenchers of chromophores through various quenching pathway.
In this study, we designed and synthesized a series of pyrene based D-
A-D compounds (Py-R) consisting of two triphenylamine (TPA) donor
units linked directly to pyrene at the 1,6- positions as an acceptor unit.
Donor units were placed at the active sites on the pyrene core to achieve
potent ICT effects. By introducing various TPA derivatives (N,N-bis(p-
(R)-phenyl)aniline) with electron-donating and withdrawing sub-
stituents, the energy band gaps of the D-A-D compounds were system-
atically controlled and emission colors were efficiently tuned from blue
to yellow. Furthermore, ICT character was found to depend on the
electron push-pull effects of substituents, which exhibited a gradual
increase in ICT properties as electron-donating characteristics increased.
To investigate structure-property relationships, the photophysical and
electrochemical properties of Py-R were systematically demonstrated
and compared with density functional theory (DFT) calculations.
Finally, Py-R compounds are applied as fluorescence chemosensor using
the energy band gap control characteristic, and in particular, we
observed fluorescence quenching by o-NA for detect nitro aromatic
explosive materials.
of (1–10) μM, using 9,10-diphenylanthracene (ΦPL = 0.95, ethanol) as a
reference standard [37]. A CH Instruments 701D potentiostat was used
for electrochemical measurements, and cyclic voltammetry (CV) was
performed in an electrolytic solution prepared using 100 μM electro-
active compounds and 0.1 M tetrabutylammonium perchlorate (Bu4N-
ClO4) at RT under an argon atmosphere.
A three-electrode
configuration, glassy carbon, platinum wire, and SCE were used as
working, counter, and reference electrodes, respectively.
2.2. Synthesis
2.2.1. 1,6-bis[(N,N-p-(R)-diphenylamino)phenyl]pyrene (Py-R)
A mixture of 1,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
pyrene (Py-B(pin)) (0.30 g, 0.67 mmol), 4-bromo-N,N-bis(p-(R)-
phenyl)aniline (TPA-R) (1.03 g, 2.76 mmol, R = CN (TPA-CN), F (TPA-
F), H (TPA-H), Me (TPA-Me), and OMe (TPA-OMe)), K2CO3 (0.95 g,
6.89 mmol), and tetrakis(triphenylphosphine)palladium(0) (0.0069 g, 5
mol %) in 1,4-dioxane/H2O (4:1, 30 mL) was refluxed under argon at
100 ◦C for overnight. After cooling to room temperature, deionized
water (50 mL) was added and the organic layer was separated using a
separating funnel. The water layer was washed using dichloromethane
(DCM) ( × 3) for extracted remained organic residue. After combined all
of organic solvents, the organic layer was dried over anhydrous MgSO4,
and then filtered off. The residue was purified by silica gel column
chromatography using DCM/n-hexane mixtures as eluents.
Py-CN. Brown powder (eluent DCM/n-hexane = 2:1). Yield: 71%
2

S.-Y. Kim et al.
Dyes and Pigments 191 (2021) 109362
(0.37 g). Mp 238 ◦C. 1H NMR (500 MHz, CDCl3, ppm): δ 8.19 (d, J = 7.5
Hz, 2H), 8.17 (d, J = 9.0 Hz, 2H), 8.03 (d, J = 9.0 Hz, 2H), 7.95 (d, J =
8.0 Hz, 2H), 7.60 (d, J = 8.0 Hz, 4H), 7.55 (d, J = 9.0 Hz, 8H), 7.26 (d, J
= 8.5 Hz, 4H), 7.19 (d, J = 9.0 Hz, 8H). 13C{1H} (125 MHz, CDCl3,
ppm): δ 150.2, 144.2, 141.0, 139.2, 136.6, 134.5, 134.4, 133.7, 132.4,
130.7, 127.7, 126.5, 124.8, 123.2, 118.8, 106.2. GC-MS Calculated for
concentrations for both the target analyte o-NA and Py-R compounds
using Sinco Mega-2100 and the fluorescence emission measurements
were performed by gradually increasing o-NA concentration (2.0 ×
10ꢀ 4 M ꢀ 1.8 × 10ꢀ 3 M) at a Py-R concentration of 10
μM in DCM
solvent under an argon atmosphere using Shimadzu fluorometer (RF-
6000). In order to induce fluorescence quenching by o-NA, excitation
was performed at λex = 310 nm, and their corresponding emission
wavelength was monitored from λem = 441–536 nm (from Py-CN to Py-
OMe). For o-NA sensor response was defined as I0/I - 1. Stern-Volmer
constants (Ksv) (Mꢀ 1) were calculated by Stern-Volmer equation, (I0/
I) = 1 + Ksv [Q], where [Q] is the molar concentration of o-NA. The limit
of detection (LOD) was calculated as follows: limit of detection equation
C
56H32N6 [M]+: 788.27 g/mol, Found [M]+: 788.3 g/mol. Anal.
Calculated for C56H32N6: C, 85.29; H, 4.13; N, 10.62. Found: C, 85.19; H,
4.23; N, 10.62.
Py-F. Greenish yellow powder (eluent DCM/n-hexane = 1:3). Yield:
78% (0.39 g). Mp 295 ◦C. 1H NMR (500 MHz, CDCl3, ppm): δ 8.26 (d, J
= 9.0 Hz, 2H), 8.18 (d, J = 7.5 Hz, 2H), 8.04 (d, J = 9.0 Hz, 2H), 7.98 (d,
J = 8.0 Hz, 2H), 7.50 (d, J = 9.0 Hz, 4H), 7.20–7.19 (m, 4H), 7.18–7.16
(m, 8H), 7.04 (t, J = 9.0 Hz, 8H). 13C{1H} (125 MHz, CDCl3, ppm): δ
160.0, 158.0, 147.2, 143.8, 131.4, 130.2, 128.8, 127.7, 127.3, 126.3,
125.3, 125.2, 124.4, 122.0, 116.1. GC-MS Calculated for C52H32F4N2
[M]+: 760.25 g/mol, Found [M]+: 760.3 g/mol. Anal. Calculated for
= 3σ/K. Where σ is the standard deviation and K is the slope between
fluorescence versus o-NA concentration. The linear equation obtained
from Stern-Volmer plot is y = 0.00345x-0.31462 (R = 0.992).
3. Results and discussion
C
52H32F4N2: C, 82.09; H, 4.24; N, 3.68. Found: C, 82.19; H, 4.14; N,
3.78.
3.1. Synthesis of Py-R compounds
Py-H. Greenish yellow powder (eluent DCM/n-hexane = 1:3). Yield:
75% (0.35 g). Mp 286 ◦C. 1H NMR (500 MHz, CDCl3, ppm): δ 8.38 (d, J
= 9.5 Hz, 2H), 8.33 (d, J = 8.0 Hz, 2H), 8.01 (d, J = 8.0 Hz, 2H), 7.87 (d,
J = 9.0 Hz, 2H), 7.54 (d, J = 8.5 Hz, 4H), 7.33 (t, J = 8.5 Hz, 8H), 7.28
(d, J = 9.0 Hz, 4H), 7.26–7.25 (m, 8H), 7.08 (t, J = 7.0 Hz, 4H). 13C{1H}
(125 MHz, CDCl3, ppm): δ 147.8, 147.1, 137.8, 136.4, 135.1, 131.4,
129.3, 127.9, 127.6, 127.4, 124.8, 124.6, 124.3, 123.3, 123.1. GC-MS
Calculated for C52H36N2 [M]+: 688.29 g/mol, Found [M]+: 688.5 g/
mol. Anal. Calculated for C52H36N2: C, 90.65; H, 5.24; N, 4.12. Found: C,
90.55; H, 5.34; N, 4.12.
Scheme 1 shows the synthetic procedure used to produce a series of
pyrene based D-A-D compounds, 1,6-bis[(N,N-p-(R)-diphenylamino)
phenyl]pyrene (R = CN (Py-CN), F (Py-F), H (Py-H), Me (Py-Me), and
OMe (Py-OMe)). Initially, the precursor for the acceptor unit 1,6-bis
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyrene (Py-B(pin)) and
the precursor for donor units with various electron-donating (EDGs) or
withdrawing groups (EWGs) at the para-positions of triphenylamine
(TPA), 4-bromo-N,N-bis(p-(R)-phenyl)aniline, were prepared according
as previously reported [35,36]. As shown in Scheme 1, Py-B(pin) was
synthesized by a Ni(dppp)Cl2 catalytic coupling reaction of 1,6-dibro-
mopyrene and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane. Then, the pyr-
ene based D-A-D compounds were successfully synthesized using the
modified Suzuki-Miyaura cross-coupling reaction using Py-B(pin) and 2
molar ratios of 4-bromo-N,N-bis(p-(R)-phenyl)aniline. The Py-R com-
pounds were purified by silica gel column chromatography using
n-hexane/dichloromethane (DCM) mixtures as eluents and further pu-
rified by recrystallization. The molecular structures of all compounds
were characterized by 1H and 13C{1H} NMR and elemental analysis.
Detailed synthetic procedures and characterization data are provided in
the Experimental Section and Supplementary data.
Py-Me. Greenish yellow powder (eluent DCM/n-hexane = 1:2).
Yield: 85% (0.37 g). Mp 293 ◦C. 1H NMR (500 MHz, CDCl3, ppm): δ 8.28
(d, J = 9.5 Hz, 2H), 8.17 (d, J = 8.0 Hz, 2H), 8.03 (d, J = 9.0 Hz, 2H),
7.98 (d, J = 7.5 Hz, 2H), 7.47 (d, J = 8.5 Hz, 4H), 7.19 (d, J = 9.0 Hz,
4H), 7.13 (d, J = 9.0 Hz, 16H), 2.35 (s, 12H). 13C{1H} (125 MHz, CDCl3,
ppm): δ 147.4, 145.3, 137.6, 134.1, 132.7, 131.2, 130.2, 129.9, 128.8,
127.7, 127.2, 125.4, 124.9, 124.4, 122.0, 20.8. GC-MS Calculated for
C
56H44N2 [M]+: 744.35 g/mol, Found [M]+: 744.4 g/mol. Anal.
Calculated for C56H44N2: C, 90.27; H, 5.97; N, 3.79. Found: C, 90.22; H,
5.92; N, 3.89.
Py-OMe. Bright yellow powder (eluent DCM/n-hexane = 1:1). Yield:
88% (0.48 g). Mp 228 ◦C. 1H NMR (500 MHz, CDCl3, ppm): δ 8.28 (d, J
= 9.0 Hz, 2H), 8.16 (d, J = 8.0 Hz, 2H), 8.02 (d, J = 9.5 Hz, 2H), 7.97 (d,
J = 8.0 Hz, 2H), 7.44 (d, J = 9.0 Hz, 4H), 7.19 (d, J = 9.0 Hz, 8H), 7.10
(d, J = 8.5 Hz, 4H), 6.88 (d, J = 9.0 Hz, 8H), 3.82 (s, 12H). 13C{1H} (125
MHz, CDCl3, ppm): δ 155.9, 147.9, 140.9, 137.5, 131.1, 130.1, 128.8,
127.2, 126.7, 125.4, 124.3, 120.0, 114.78, 114.75, 114.72, 55.5. GC-MS
Calculated for C56H44N2O4 [M]+: 808.33 g/mol, Found [M]+: 808.4 g/
mol. Anal. Calculated for C56H44N2O4: C, 83.16; H, 5.45; N, 3.49; O,
7.94. Found: C, 83.06; H, 5.55; N, 3.44; O, 7.89.
3.2. Photophysical properties
As shown in Fig. 1, steady-state UV–visible absorption and fluores-
cence spectra were measured both in solution and film states. Spectral
parameters are summarized in Table 1. UV–vis absorption spectra
showed structured and relatively intense absorption bands in the range
260–325 nm, which were attributed to π-π* transitions of substituted
triphenylamine (TPA) and pyrene core (Fig. S4). Broad relatively weak
absorption bands in the range 350–450 nm were attributed to intra-
molecular charge transfer (ICT) in the ground state from TPA (donor) to
pyrene (acceptor). In addition, since TPA was located at the 1,6-posi-
2.3. Density functional theory calculations
Density functional theory (DFT) calculations were performed using
the Gaussian’16 software package. Full geometry optimizations in their
ground state were performed using the B3LYP functional and the 6-31G
(d,p) basis set for all atoms. The excitation energies and oscillator
strengths for the lowest 100 singlet–singlet transitions at the optimized
geometry in the ground state were obtained in time-dependent DFT (TD-
DFT) calculations using the same basis set and functional as for the
ground state. All Isodensity plots of the frontier orbitals were visualized
by Chem3D Ultra and GaussView softwares. More detail DFT/TD-DFT
calculation results for Py-R were described in Supplementary data.
tions of the active sites of pyrene with notable π-conjugation effect, it
was confirmed that charge transfer occurs more readily in the ground
state [38,39]. We compared the absorption wavelength bands of the
Py-R compounds, to understand substituent effects. It was found that the
low-energy band is relatively more affected than the high-energy band
by the para-substituted TPA. In this context, comparisons of the ab-
sorption wavelength maxima of charge transfer bands showed absorp-
tion peak changes depended on substituents. On moving from
electron-withdrawing groups (EWGs) to electron-donating groups
(EDGs), absorption peaks were red-shifted in the following order CN < F
< H < Me < OMe. Steady-state absorption spectral comparisons be-
tween Py-CN and the other compounds revealed Py-CN had a unique
spectrum, due to the strong electron-withdrawing substituent character
of the CN group. This phenomenon is further addressed in the DFT
2.4. Chemosensor
The UV–vis absorption spectra were measured at 10
μM
3

S.-Y. Kim et al.
Dyes and Pigments 191 (2021) 109362
Scheme 1. Synthesis of 1,6-bis[(N,N-p-(R)-diphenylamino)phenyl]pyrene (Py-R).
Fig. 1. UV–vis absorption (left) and emission (right) spectra of Py-R in DCM solution at a concentration of 10 μM.
Table 1
Photophysical properties of Py-R.
λmaxabs (nm)
ker × 108
kenr × 108
τFd(ns)
λmaxPL (nm)
Stokes
Φfc
kr/knr
Shift (cmꢀ 1
)
sola
(sꢀ 1
)
(sꢀ 1
)
sola/filmb
sola/filmb
Compd
Py-CN
Py-F
353/358
379/387
379/414
384/406
400/412
441/480
471/495
483/482
497/471
536/485
5653
5154
5682
5921
6344
0.75
0.64
0.65
0.65
0.50
1.28
2.07
1.98
2.05
2.16
5.86
3.09
3.28
3.17
2.31
1.95
1.74
1.77
1.71
2.31
3.00
1.77
1.85
1.85
1.00
Py-H
Py-Me
Py-OMe
a Measured in DCM at a concentration of 10
μM at room temperature (RT).
b Measured in thin films.
c Relative to 9,10-diphenylanthracene (ФPL = 0.95, ethanol) as standards, in DCM at RT.
d Fluorescence lifetimes in DCM.
e Values of kr and knr were calculated using the equations, kr = Φf /τF and knr = (1/τF)ꢀ kr.
calculation section below. To understand the intermolecular in-
teractions in the steady-state absorption properties, the absorption
spectra in the thin films were also shown in Fig. S7, and all of the
compounds were found to exhibit a broad red-shift trend in comparison
with their corresponding absorption spectra in solution, which implies
that Py-R compounds may aggregate in a head-to-tail arrangement to
form J-aggregates [40].
emission maxima wavelengths were shifted according to the substituent
effect in the fluorescence emission spectrum. In addition, we measured
the solvent-dependent emission spectra for each Py-R compounds to
determine the origins of emissions. Fig. S9 show that all compounds
were red-shifted on increasing solvent polarity from n-hexane to
acetonitrile (ACN), which indicated the presence of intramolecular
charge transfer in the excited state. Furthermore, Py-R compounds
showed more remarkable ICT characteristics on increasing the electron-
donating power of substituents, which demonstrated charge transfer of
Py-R compounds could be finely controlled using substituent effects.
In Table 1, fluorescence quantum yields (Φf) and fluorescence life-
times (τF) were measured to explore excited state properties. Based on
As shown in Fig. 1 (right), the emission maxima of Py-R ranged from
441 to 536 nm in dichloromethane (DCM) solution and their emission
profiles ranged from blue to yellow. In the excited state, for Py-R
emission spectra, the red-shift moving from EWGs to EDGs affects the
emission color tuning. As was observed in their absorption spectra, the
4

S.-Y. Kim et al.
Dyes and Pigments 191 (2021) 109362
measured fluorescence lifetimes, all emission decay profiles were well
fitted by single exponential functions, suggesting radiative decay from a
single specific excited state in Fig. S8. Radiative and non-radiative decay
rate constants were calculated using the relationship between the Φf and
τF values. To check the tendency according to the substituents, repre-
sentatively, we compared Py-CN (an EWG) and Py-OMe (an EDG). Py-
CN had relatively high Φf and low τF values, whereas Py-OMe had low
Φf and high τF values. Py-OMe stabilized the D-A-D molecule more than
the other compounds in the excited state, because of its electron-rich
substituent character, which also resulted in a longer fluorescence life-
time. In addition, since Py-OMe had a bulky structure than the other
compounds, more reorganization energy was required to return to the
ground state structure, which affected fluorescence quantum yield.
Consequently, these properties are determined from the molecular
behavior induced by substituents in the excited state. As shown in
Table 1, there was a general increasing trend in the kr value as the
electron withdrawing power increases but not in the knr value. This
means that the kr value is controlled by the electron-donating or with-
drawing effects of substituents, and as a result, the kr value plays a key
role in the control of emission color.
with different polarities. Using eq (1), values of dipole moment changes
(Δμ) for Py-R were estimated to be 22.6, 37.8, 33.7, 46.5, and 60.3
Debye (D) for R = CN, F, H, Me, and OMe, respectively. Table S1 shows
values of the ground state dipole moments (μg) for energy minimized
structures obtained by the DFT method. Using ground state dipole mo-
ments (μg) and dipole moment changes (Δμ), the excited state dipole
moments (μe) for Py-R were calculated to be 23.2, 37.9, 33.8, 46.7, and
60.4 D, respectively. This result explained that the excited state dipole
moments of Py-R were positively correlated with charge transfer gaps in
the excited state as measured by solvent-dependent emission spectros-
copy (Fig. S9); greater electron-donating ability increased the charge
transfer gap between n-hexane and ACN. In addition, as the electron-
donating power increased, dipole moment changes (Δμ) also tended to
increase (Py-CN < Py-F ≈ Py-H < Py-Me < Py-OMe), which confirmed
that this tendency was caused by substituent effects. Since the slope of
the Lippert-Mataga plot and (Δ
μ
)2 are proportional, as shown by eq (1),
this slope was also affected by substituents. Furthermore, the slope of
the Lippert-Mataga plot quantitatively represents the ICT process in the
excited state, indicating increases in slope are associated with large in-
creases in charge transfer degree [41]. The tendency in slopes followed
the order Py-CN < Py-H < Py-F < Py-Me < Py-OMe, which confirmed
that charge transfer degree is dependent on the nature of substituents.
Therefore, Py-R compounds indicate controllable ICT property in
accordance with the substituent effect.
The intramolecular charge transfer (ICT) process was further evalu-
ated using relationships between Stokes shifts in various solvents and
the Lippert-Mataga equation. Therefore, sensitivity of the solvent po-
larity can be analyzed in terms of the difference between the dipole
moments in the ground (μg) and excited states (μe). Stokes shifts between
the maximum absorption and emission wavelengths, in wavenumbers
3.3. Electrochemical properties
(Δ
ν
), can be calculated using eq (1).
(
)
As shown in Fig. 3, the electrochemical properties of Py-R were also
investigated by cyclic voltammetry (CV), which was performed using a
three-electrode cell system; a glassy carbon electrode was used as the
working electrode, while a platinum wire and a saturated calomel
electrode (SCE) were used as the counter and reference electrodes,
respectively. All compounds showed reversible oxidation peaks, and
first oxidation onset potentials of Py-R (R = CN, F, H, Me, and OMe)
were located at 1.17, 0.84, 0.82, 0.73, and 0.64 V, respectively. Once a
unit electron is taken out from the triarylamine to generate the oxidized
ꢀ
)
2
ε ꢀ 1
n2 ꢀ 1
Δv = va ꢀ vf =
+
×
μe
ꢀ
μg 2 + constant
hca30
2
ε
+ 1 2n2 + 1
(Δ )
μ
= 2Δf ×
2 + constant
(1)
hca30
Where μe - μg (Δμ) is the difference between the dipole moments of the
excited and ground states, c is the speed of light, h is Planck’s constant,
and a0 is the radius of the Onsager cavity around the fluorophore. The
dielectric constants (ε) (ε: 1.88; n-hexane, 2.38; toluene, 7.6; tetrahy-
form, the molecules are stabilized by the π-conjugation effect. Further,
drofuran (THF), 9.1; DCM, 37.5; ACN) and the refractive indices (n) of
solvents are included in the term Δf, which is known as orientation
polarizability. Onsager radii, which were obtained using B3LYP/6-31G,
were considered to be half of the average size of 24.8, 23.5, 23.5, 25.1,
and 26.3 Å for Py-R (R = CN, F, H, Me, and OMe, respectively).
Fig. 2 shows Lippert-Mataga plots for Py-R compounds in solvents
as the electron-donating group of the substituent is stronger, electron-
rich conditions are formed, which stabilizes the molecule along with
π
-conjugation effect. These results also indicate values of the first
oxidation onset potentials are affected by the substituent.
As shown in Table 2, experimental and calculated highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital
Fig. 3. Cyclic Voltammetry (CV) performed at a scan rate of 0.1 V sꢀ 1 on Py-R
Fig. 2. Lippert-Mataga plots for Py-R compounds measured in various solvents
(1: n-hexane, 2: toluene, 3: THF, 4: DCM, 5: ACN).
compounds in DCM solution at a concentration of 100
TBAP as electrolyte.
μM containing 0.1 M
5

S.-Y. Kim et al.
Dyes and Pigments 191 (2021) 109362
3.4. Theoretical studies
Table 2
Energy band gap properties of Py-R.
HOMOa
HOMOc
(eV)
LUMOc
(eV)
Egc
opt
Fig. 5 and Fig. S11 shows HOMO and LUMO orbitals of pyrene based
D-A-D compounds (Py-R), which were obtained by Gaussian 16 and
visualized Chem3D and GaussView programs. Geometries were opti-
mized by the DFT method using the B3LYP correlation function
employing the 6-31G (d,p) level in the vacuum state. The H-1 orbitals of
Py-R were delocalized on the TPA moiety, only. Whereas, for the HOMO
orbitals, a gradual substituent-dependent changes in electron density
were observed. As the electron-withdrawing power of substituents
increased, electron density shifted strongly towards the pyrene moiety,
and conversely, as the electron-donating power of substituents
increased, it gradually expanded towards the TPA moiety. As mentioned
above, this interpretation supports the notion that HOMO energy levels
are dependent on substituent types, as shown in Fig. 4. The LUMO or-
bitals of Py-R compounds were almost delocalized on the pyrene moi-
ety, and electron densities of L+1 orbitals shifted from pyrene to TPA as
the electron-withdrawing power of substituents increased.
LUMOb
Eg
Compounds
(eV)
(eV)
(eV)
(eV)
Py-CN
Py-F
ꢀ 5.97
ꢀ 5.64
ꢀ 5.62
ꢀ 5.53
ꢀ 5.44
ꢀ 2.97
ꢀ 2.74
ꢀ 2.76
ꢀ 2.69
ꢀ 2.68
3.00
2.90
2.86
2.84
2.76
ꢀ 5.57
ꢀ 4.92
ꢀ 4.80
ꢀ 4.68
ꢀ 4.51
ꢀ 2.18
ꢀ 1.68
ꢀ 1.58
ꢀ 1.51
ꢀ 1.44
3.39
3.24
3.22
3.17
3.07
Py-H
Py-Me
Py-OMe
a EHOMO(eV) = ꢀ e(Eooxnset + 4.8).
b ELUMO(eV) = ꢀ e(EHOMO + Egopt).
c Obtained by DFT calculation.
(LUMO) energy levels of the pyrene based D-A-D series were compared.
Experimental HOMO energy levels were estimated from a first oxidation
onset potential, indicating approximately ꢀ 5.64 ± 0.33 eV. Because no
cathodic reduction processes were measured, experimental LUMO en-
ergy levels were estimated using optical band gaps calculated from the
edges of absorption spectra and HOMO energy levels. Most of LUMO
energy levels show similar values compared to HOMO energy levels,
indicating approximately ꢀ 2.76 ± 0.21 eV. Calculated HOMO energy
levels were in the range ꢀ 4.92 ± 0.65 eV. Calculated LUMO energy
levels were in the range ꢀ 1.68 ± 0.50 eV, and all compounds had higher
calculated HOMO and LUMO levels than experimental HOMO and
LUMO levels. Experimental and calculated values of the HOMO and
LUMO energy levels has been confirmed to be effectively adjusted ac-
cording to electron-donating and withdrawing characteristics of sub-
stituents in pyrene based D-A-D series. Fig. 4 shows, experimental and
calculated HOMO and LUMO energy levels of Py-R showed good linear
correlations with σp values, showing that HOMO slopes are greater than
LUMO slopes. On the basis of the correlation of HOMO/LUMO energy
levels and Hammett substituent constants (σp), we confirmed the im-
pacts of substituents on HOMO levels and band gap tuning. Optical and
calculated energy band gaps (Eg) for Py-R are summarized in Table 2.
When optical band gaps were compared on the Py-H basis, the differ-
ence of Py-F and Py-CN (0.04 and 0.14 eV, respectively) appeared ac-
cording to the electron-withdrawing ability of substituents, and the
difference of Py-Me and Py-OMe (0.02 and 0.1 eV, respectively)
appeared according to the electron-donating ability of substituents. As
the electron-withdrawing ability of the para-substituted TPA moiety
increased, both energy band gaps gradually expanded, and which
showed calculated energy gaps were larger than optical gaps. These
results demonstrate that the band gap of the pyrene based D-A-D mol-
ecules can be significantly reduced, when compared with the pure
pyrene molecule [42]. We suggest that only with effect of the sub-
stituents, emission properties of similar molecular structures are
controlled systematically by tailoring energy band gaps.
Fig. S12 and Table S3 shows the time-dependent DFT (TD-DFT) with
the same functional and basis set confirm the origin of the π-π* transi-
tions and the ICT transitions of Py-R with the spectral shift showing. TD-
DFT calculations exhibited strong two distinct absorption bands that did
not well match experimental data due to strong spectral shift of the ICT
transition. In particular, the calculated absorption spectrum of TD-DFT
allowed us to guess at the origin of the absorption spectrum of Py-CN.
Considering the transition assignment of the spectra band, the ICT band
of Py-CN was attributed of a major transition from HOMO to LUMO and
a minor transition from H-1 to L+1 or L+2. The former corresponded to
a transition from the pyrene moiety extended to phenyl containing a
nitrogen atom as an electron donor to the pyrene core as an electron
acceptor, whereas the latter corresponded to a transition from the TPA
moiety, including its substituent, as an electron donor to the diphenyl
moiety, including its substituent, as an electron acceptor. As a result,
these two distinct types of electronic transitions by π-π* transition and
ICT might explain the origin of the steady-state broad charge transfer
band (experimental value 300–400 nm). In the case of the other com-
pounds, ICT bands were attributed to major transitions from HOMO to
LUMO and minor transitions from H-1 to LUMO. Major and minor
transitions corresponded from the uniformly distributed D-A-D molecule
and intensively distributed TPA moiety as an electron donor to the
pyrene core as an electron acceptor. Through the calculation results, the
main transition that affects the absorption spectra of pyrene based D-A-D
compounds (Py-R) is due to intramolecular charge transfer and the
calculation results support the experimental data. Therefore, we propose
the energy band gap adjustment was due to the substituent-dependent
ICT effect.
Fig. 4. Plots of experimental HOMO and LUMO energy levels versus Hammett substituent constants (σpara) (left) Plots of calculated HOMO and LUMO energy levels
versus Hammett substituent constants (σpara) (right).
6

S.-Y. Kim et al.
Dyes and Pigments 191 (2021) 109362
Fig. 5. Frontier orbital distributions (HOMO-1, HOMO, LUMO, LUMO+1) of Py-R calculated by DFT with the B3LYP function and 6-31G (d,p) basis.
3.5. Chemosensor
> Py-F > Py-H > Py-Me > Py-OMe, and the overlap increased with
electron-withdrawing ability. This suggested that emission quenching
by o-NA is more advantageous as the emission region of the Py-R
compounds are blue-shifted. When the amount of o-NA was increased
(from 0 to 180 equivalents), Py-R emissions were gradually quenched
(Fig. S10), which indicated that Py-R compounds could be used to detect
nitro explosives. The observed emission quenching can be attributed to
the FRET from the photo-excited state of Py-R (electron donor) to the
ground state of o-NA (electron acceptor) [34]. To quantify the degree of
fluorescence quenching of Py-R compounds by o-NA, Stern-Volmer
constants (Ksv) were compared. The Stern-Volmer plots of Py-R (R =
CN, F, H, Me, and OMe) provided Stern-Volmer constants of 3171, 2164,
2276, 2083, and 2287 Mꢀ 1, respectively (Fig. 7). Py-CN showed highest
fluorescence quenching efficiency as shown in Figs. 6 and 7, and the
other compounds showed similar degree of quenching. In addition, the
limit of detection (LOD) of o-NA by Py-CN was determined by the limit
Efficient substituent-dependent energy band gap adjustment is ex-
pected to be applied to chemosensors. In general, higher spectral overlap
between the absorbance band of an analyte and the emission band of
luminescence material increases the probability of fluorescence reso-
nance energy transfer (FRET) and hence fluorescence quenching [43].
The quenching phenomenon is applied to the detection of various ana-
lytes [44], and in the present study, o-NA was introduced as an analyte
to detect nitro aromatic compounds used as explosives.
The sensitivities of Py-R to o-NA was estimated using Stern-Volmer
constants, Ksv, determined using eq (2).
I0
I
= 1 + Ksv[Q]
(2)
where I0 and I are fluorescence intensities in the absence and presence of
o-NA, respectively. Stern-Volmer plots were plotted as a function of o-
NA concentration, [Q]. Stern-Volmer constants, Ksv, were calculated
from slopes of Stern-Volmer plots. To observe emission quenching of Py-
R by o-NA, we carried out fluorescence emission measurements in DCM.
To prevent excessive quenching by o-NA, excitation was performed at
310 nm, at which o-NA absorption was relatively low. Py-R compounds
(R = CN, F, H, Me, and OMe) showed their emission wavelengths at 441,
471, 484, 497, and 536 nm, respectively, which confirmed spectral
overlap between the absorption of o-NA and the emission of Py-R (Fig. 6
and Fig. S10). The degree of spectral overlap followed the order Py-CN
of detection equation (3
×
standard deviation/slope
= (3 ×
0.1601)/0.00345 [ M]) and found to be 139
μ
μ
M. (Fig. 7) As shown in
Table 1 and Fig. S6, the fluorescence emission of the Py-OMe has a lower
kr/knr value (1.00) and a higher knr value (2.31) than other Py-R com-
pounds, which means that the activation of non-radiative processes has
a remarkable effect on fluorescence quenching. These results indicated
that fluorescence quenching of Py-OMe, as shown in Fig. S10, included a
relatively dominant non-radiative effect of pure Py-OMe among the
Py-R series as well as FRET for o-NA. On this basis, we found that
Fig. 6. Normalized absorption spectrum of o-NA and emission spectra of Py-R
Fig. 7. Stern-Volmer plots for fluorescence quenching of Py-R in DCM at a
in DCM at a concentration of 10
μ
M.
concentration of 10 μM when excited at 310 nm.
7

S.-Y. Kim et al.
Dyes and Pigments 191 (2021) 109362
Py-OMe has the lowest fluorescence quenching efficiency by o-NA,
despite having similar Stern-Volmer constants to Py-F, H, Me. Consid-
ering the specificity of Py-OMe, it is confirmed that effective energy
transfer has occurred in the order Py-CN > Py-F, H, Me > Py-OMe.
Thus, it was found that Stern-Volmer constants were also regulated by
the substituent effect, which confirmed that spectral overlap and
Stern-Volmer constants of Py-R compounds were positively correlated.
Representatively, Py-CN showed more effective o-NA sensing potential
than Py-OMe, as evidenced by large overlap between Py-CN emission
and o-NA absorption, which confirmed that the efficient fluorescence
emission quenching is controlled according to systematic energy band
gaps by the various substituents. Based on these results, it is predicted
that Py-OMe with a relatively longer wavelength of emission spectrum
will show remarkable sensing efficiency for analytes with longer
wavelength of absorption spectra, and such studies through energy band
gap control are under investigation.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109362.
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