Blue-orange emitting carbazole based donor-acceptor derivatives: Synthesis and studies of modulating acceptor unit on opto-electrochemical and theoretical properties

Article abstract of DOI:10.1016/j.jphotochem.2021.113457

In order to demonstrate an effect of varying acceptor unit on optoelectronic properties of Donor–Acceptor (D–A) assembly, we herein designed and synthesized C–N coupled, carbazole based dyes 1–8 by employing Buchwald–Hartwig coupling amination reaction and fully characterized by spectroscopic methods. Presence of intramolecular charge transfer (ICT) transitions (λ_max = 403–467 nm) in absorption spectra of all dyes reminiscent D–A assembly in it and emit in blue–yellow region (λ_emm = 467–565 nm) in solution state with marked positive solvatochromism. The photophysical studies of some of the dyes in different THF/water mixtures revealed aggregation-induced emission (AIE) feature with the nanoparticles formation, as confirmed by dynamic light scattering (DLS) technique. The HOMO (?5.42 to ?5.74 eV) and LUMO (?3.03 to ?3.50 eV) energy level of these molecules measured by cyclic voltammetry suggest electron transporting property in it. Further, DFT/TDDFT calculation of dyes, indicate quite comparable theoretical and experimental photophysical and electrochemical data. Moreover, calculated singlet (S₁) and triplet (T₁) excitation energy levels by DFT/B3LYP/6–311 G+(d,p) shows small ΔE_ST value for some dyes and hence called as TADF emitters. Thus obtained all optoelectrochemical properties of dyes propose its potential use in optoelectronic devices and in biosensing.

Full text of DOI:10.1016/j.jphotochem.2021.113457

Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Blue-orange emitting carbazole based donor-acceptor derivatives: Synthesis
and studies of modulating acceptor unit on opto-electrochemical and
theoretical properties
*
Pooja S. Singh, Purav M. Badani, Rajesh M. Kamble
Department of Chemistry, University of Mumbai, Santacruz (E), Mumbai 400 098, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
In order to demonstrate an effect of varying acceptor unit on optoelectronic properties of Donor–Acceptor (D–A)
assembly, we herein designed and synthesized C–N coupled, carbazole based dyes 1–8 by employing Buch-
wald–Hartwig coupling amination reaction and fully characterized by spectroscopic methods. Presence of
intramolecular charge transfer (ICT) transitions (λmax = 403–467 nm) in absorption spectra of all dyes remi-
niscent D–A assembly in it and emit in blue–yellow region (λemm = 467–565 nm) in solution state with marked
positive solvatochromism. The photophysical studies of some of the dyes in different THF/water mixtures
revealed aggregation-induced emission (AIE) feature with the nanoparticles formation, as confirmed by dynamic
light scattering (DLS) technique. The HOMO (ꢀ 5.42 to ꢀ 5.74 eV) and LUMO (ꢀ 3.03 to ꢀ 3.50 eV) energy level of
these molecules measured by cyclic voltammetry suggest electron transporting property in it. Further, DFT/
TDDFT calculation of dyes, indicate quite comparable theoretical and experimental photophysical and electro-
Intramolecular charge transfer (ICT) transitions
Donor–acceptor (D–A) architecture
Aggregation-induced emission (AIE)
Electron-transporting materials
chemical data. Moreover, calculated singlet (S
1
) and triplet (T
1
) excitation energy levels by DFT/B3LYP/6–311
G+(d,p) shows small ΔEST value for some dyes and hence called as TADF emitters. Thus obtained all opto-
electrochemical properties of dyes propose its potential use in optoelectronic devices and in biosensing.
1
. Introduction
development of many functional D–A materials [17–23]. It possess rigid,
planar and aromatic molecular structure, amorphous nature, extended
In the field of modern opto-electronic devices, low molecular weight
organic -conjugated materials arouse tremendous attention of re-
π
-conjugation, reasonable modification, good chemical stability, an
π
intrinsic electron-donating property, very high photoconductivity,
excellent quantum yield and good charge (hole)-transporting ability
[17–23]. Moreover, hole transporting ability of Cz can be modulated
into electron transporting/n-type materials by amalgamation with
various strong electron accepting segments [24–26]. As a result it is used
to achieve the current increased demand for electron transporting (n-
type) materials for the development of high-efficiency and environ-
mentally stable organic electronic devices than widely available p–type/
hole transporting materials possessing good stability under ambient
conditions [27].
searchers from the academic and industrial laboratories in the several
past years [1–6]. Small organic molecule bears well-defined molecular
structure, reproducible synthetic pathways, easy purification methods
and versatile functional properties, which can be tuned by small struc-
tural alteration [1–6]. Thus, they are widely used in organic light
emitting diodes (OLEDs) [7,8], solar cells (OPVs) [9], organic field effect
transistors (OFETs) [10], non-linear optics (NLOs) [11], thermally
activated delayed fluorescent (TADF) emitters [12] and in molecular
bio/chemosensors [13].
Further, compounds having suitable electronic donor (D) and
acceptor (A) units particularly that display effective intramolecular
charge transfer (ICT) property have been widely demonstrated as effi-
cient functional materials in electronic devices [14–16]. For the past
several years 9H-carbazole (Cz), a widely available small heterocyclic
Additionally among various donor used in D–A systems, Cz possess
high triplet energy and small difference between singlet and triplet en-
ergy levels (ΔEST) characteristics which are beneficial for their use as a
host materials for triplet emitters, thermally activated delayed fluores-
cence (TADF) emitters and hybridized local and charge transfer (HLCT)
emitters respectively [21,23,28–34]. Furthermore, D–A system formed
aromatic compound adopted as
a promising donor block for
*
Corresponding author.
E-mail address: kamblerm@chem.mu.ac.in (R.M. Kamble).
https://doi.org/10.1016/j.jphotochem.2021.113457
Received 20 May 2021; Received in revised form 11 July 2021; Accepted 15 July 2021
Available online 22 July 2021
1
010-6030/© 2021 Elsevier B.V. All rights reserved.

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
by insertion of a Cz group in a molecule show high thermal and
morphological stabilities [17–26,28–34]. Recent past study on Cz de-
rivatives also suggest good aggregation-induced emission (AIE) or
aggregation-induced emission enhancement (AIEE) characteristics due
to locking conformation or conformational restricted rotation of phenyl
ring in it which in turn show worthy solid state application in organic
electronics mainly in OLED’s [35–43]. By owing such a fabulous char-
acteristics its derivatives are successfully applied in organic electronics
insight of its photophysical including AIE, electrochemical and thermal
characteristics with modulation of various acceptor units used in it.
Additionally, all these properties are correlated with theoretical DFT
and TDDFT study. Its singlet and triplet energy levels were also inves-
tigated theoretically for their possible application as TADF emitters.
Structures of dyes 1–8 are shown in Fig. 1.
2. Experimental
[
17–27,28–37] and as receptors in fluorescent sensors [44,45].
Thus, by considering the role of Cz donor block in D–A type mole-
2.1. Materials and methods
cules and its importance in optoelectronics, herein we designed and
synthesized a series of Cz derivatives 1–8 (Scheme 1) where Cz coupled
with quinoxaline, phenazine, indolo-quinoxaline and pyrido-pyrazino-
indole acceptor cores using Buchwald–Hartwig C–N coupling amina-
tion reaction [46,47]. Further the dyes were studied to get the detailed
All materials were purchased from commercial sources as Sigma
Aldrich and Alfa Aesar, used without any further purification. The
organic solvents (HPLC and spectroscopic grade) dried using standard
procedures and handled in moisture free atmosphere. SD Fine silica gel
Scheme 1. Synthesis of carbazole based D–A derivatives 1–8.
2

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
Fig. 1. Molecular structures of carbazole based D–A architecture 1–8.
(
60–120 mesh), were used for column chromatography with n-hexane
of the resultant solutions were then performed immediately.
and chloroform as an eluent. Thin layer chromatography (TLC) used to
monitor the progress of reactions and purity of the compounds were
visualized with UV light (365 nm) and in an iodine chamber. The
2.3. Synthesis and characterization
1
13
melting point was measured by open capillary method. H and C NMR
Precursor aryl halide molecules for the synthesis of target com-
spectra of 5–8 were recorded on a Bruker 300 Ultrashield spectrometer
pounds 1–4, 6 and 8 were prepared by simple condensation reaction as
1
13
(
300 and 75 MHz, respectively) in CDCl
3
with TMS as an internal
specified in our previous report and the characterization ( H, C NMR,
IR, Maldi-TOF and Elemental analysis) data, melting point of same are
found in literature [48–53]. While the synthesis, characterization data
and spectra of precursor aryl halide for 5 and 7 are given in Supporting
information (SI) (Figs. S9, S14, S19). The target molecules 1–8 were
prepared by Buchwald–Hartwig C-N coupling amination reaction
[46,47] as illustrated in Scheme 1. The detailed synthetic procedure and
characterization data of compounds 1–8 are given below.
1
3
standard. While C the solid state NMR spectra of 1–4 were recorded on
a Bruker (Germany) Avance III HD (125 MHz and 4 mm probe) due to
poor solubility compounds in all organic solvents. Fourier transform
infrared (FT–IR) spectra were recorded using solid powder sample on a
Perkin Elmer Frontier 91579. Mass spectrometric measurements were
recorded using MALDI–TOF (Bruker) and elemental analysis was per-
formed on an EA Euro–elemental analysis instrument. UV–Visible
-
5 mol -1
spectra recorded in 10
MADZU UV–2401PC at room temperature. Solid films (2 mg mL
sample in chloroform) prepared on quartz glass plate using spin coater
Holmarc HO–TH–05) at 1000 rpm for 2 min. The emission spectra were
measured on SHIMADZU RF–5301PC fluorescence spectrophotometer.
L
solutions and in solid film on a SHI-
ꢀ 1
2.3.1. 2,3-bis(4-(9H-carbazol-9-yl)phenyl)benzo[g]quinoxaline (1)
In a three-necked round bottom flask equipped with a reflux
condenser, nitrogen inlet and outlet ports, 2,3-bis(4-bromophenyl)
benzo[g]quinoxaline (200 mg, 0.4 mmol) an aryl halide precursor and
9H-carbazole amine (135 mg, 0.8 mmol) were dissolved in anhydrous
toluene (30 ml) under nitrogen atmosphere. The palladium catalyst
(
-
5 mol -1
AIE UV–visible spectra and emission spectra of 10
solutions with different percentages of water fraction were studied on a
similar UV–Vis and fluorescence instrument. Hydrodynamic particle
sizes were obtained by DLS analysis using Zetasizer(7.12)
L
2
THF/H O
2 3
[Pd (dba) ] (20 mg), 2-dicyclohexylphoshpino-20,60-dimethylbiphenyl
(SPhos) (20 mg) as co-ligand and sodium-t-butoxide (80 mg) a base were
added to the reaction mixture. The reaction mixture was thoroughly
stirred under nitrogen atmosphere while the temperature was slowly
–
MAL1180779. TGA were performed on a Metler–Toledo instrument
under nitrogen atmosphere. Cyclic voltammetry studies were carried out
at inert atmosphere on a computer-controlled PalmSens3 potentiostat or
galvanostat instrument. Typically, a three-electrode cell equipped with a
glassy carbon working electrode, Ag/AgCl (non-aqueous) reference
electrode and platinum (Pt) wire counter electrode was employed. The
◦
raised to 110 C. The reaction mixture was stirred at this temperature for
5–12 h and then cooled to room temperature and extracted with chlo-
roform (3 × 50 ml) followed by water washing (3 × 50 ml). All the
2 4
organic layers were combined and dried over anhydrous Na SO and
1
measurements were carried out with a scan rate of 100 mVs– at room
evaporated to afford the crude product, which was further purified by
silica gel column chromatography (eluent:- n-hexane:chloroform:
temperature in anhydrous DCM with tetrabutylammonium hexa-
fluorophosphate (TBHP) solution (0.1 M) as supporting electrolyte. The
potential of the Ag/AgCl reference electrode was calibrated using the
ferrocene/ferrocenium redox couple with the known oxidation potential
of + 4.8 eV. Theoretical study (DFT/TDDFT and TADF) was performed
using Gaussian DFT/B3LYP/6–311 G+(d,p) basis set as implemented in
the Gaussian09 program.
15:85) to obtain green solid.
◦
Green solid, yield 0.126 g (47%), mp 245 C, FTIR (solid,
ν
max
/
cm 1): 3066, 2907 (Ar. = C–H str.), 1598, 1516, 1448 (Ar.–C = C str.),
ꢀ
1
1339 (C–N str.), 1229, 725 (Ar.–C–H bend), H NMR could not be ob-
tained because of very poor solubility, 13C NMR (solid, 125 MHz, 25 C)
◦
δ(ppm): 148.93, 148.12, 147.36, 146.81, 146.25, 144.29, 143.75,
1
40.91, 139.04, 135.12, 132.90, 132.38, 129.36, 125.28, 122.40,
22.01, 119.38, 114.94, 109.63, 106.90, 104.89, MALDI-TOF: mass
1
2
.2. Preparation of nanoparticles
+
+
calcd. for C48
H
30
N
4
[M ]: 662.78; found [M ]: 662.98, elemental anal.
ꢀ
Aliquots of the stock THF solution (10 4 M) were transferred to 5 ml
4
calcd. for C48H30N : C, 86.98; H, 4.56; N, 8.45%; found: C, 86.84; H,
4
.66; N, 8.50%.
volumetric flasks. After appropriate addition of THF, water was added
ꢀ 5
drop wise under vigorous stirring to furnish 10 M solutions with
different water contents (0–90 vol%). The photophysical measurements
3

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
2
7
.3.1.1. 2,3-bis(4-(9H-carbazol-9-yl)phenyl)naphtho[2,3–f]quinoxaline-
,12-dione (2). Similar procedure adopted as for 1 using, 2,3-bis(4-
184.07 (ꢀ C = O), 149.04, 147.59, 146.88, 141.10, 140.62, 136.08,
131.75, 126.52, 123.63, 122.17, 120.98, 120.55, 120.29, 109.99,
+
bromophenyl)-naphtho[2,3–f]quinoxaline-7,12-dione (200 mg, 0.35
mmol) an aryl halide precursor, 9H-carbazole amine (117 mg, 0.7
108.05, MALDI-TOF: mass calcd. for C52
H
28
4
N O
2
[M ]: 740.80; found
+
[M ]: 740.82, elemental anal. calcd. for C52
H
28
N
4
O
2
: C, 84.31; H, 3.81;
mmol), palladium catalyst [Pd
2
(dba)
3
] (25 mg), 2-dicyclohexylphosh-
N, 7.56%; found: C, 84.37; H, 3.73; N, 7.60 %.
pino-20,60-dimethylbiphenyl (SPhos) (25 mg) as co-ligand and
sodium-t-butoxide (90 mg) a base were added to the reaction mixture.
Eluent:- n-hexane:chloroform: 10:90, orange solid, yield 0.158 g
2.3.1.5. 2-(9H-carbazol-9-yl)-5-ethyl-5H-benzo[g]indolo[2,3–b]quinoxa-
line (6). Similar procedure adopted as for 1 starting from, 2-bromo-5-
ethyl-5H-Benz[g]indolo[2,3–b]quinoxaline (200 mg, 0.6 mmol) an aryl
halide precursor, 9H-carbazole amine (100 mg, 0.6 mmol), palladium
catalyst [Pd (dba) ] (15 mg), 2-dicyclohexylphoshpino-20,60-dimethyl-
◦
ꢀ 1
(
61%), mp > 250 C, FTIR (solid,
ν
max/cm ): 3057, 2935 (Ar. = C–H
str.), 1667 (ꢀ C = O), 1595, 1448 (Ar.–C = C str.), 1339 (C–N str.), 1216,
1
7
25 (Ar.–C–H bend), H NMR could not be obtained because of very
2
3
13
◦
poor solubility, C NMR (solid, 125 MHz, 25 C) δ(ppm): 180.35 (ꢀ C =
O), 179.69 (ꢀ C = O), 150.16, 149.11, 147.72, 146.38, 133.17, 132.28,
biphenyl (SPhos) (15 mg) as co-ligand and sodium-t-butoxide (60 mg) a
base were added to the reaction mixture.
1
1
28.20, 126.33, 124.94, 119.76, 117.87, 116.64, 116.02, 106.27,
Eluent:- n-hexane:chloroform :: 45:55, orange solid, yield 0.156 g
+
ꢀ 1
◦
04.21, MALDI-TOF: mass calcd. for C52
H
30
N
4
O
2
[M ]: 742.82; found
(64%), mp 230 C, FTIR (solid,
ν
max/cm ): 3055, 2975, (Ar. = C–H
+
[
M ]: 742.93, elemental anal. calcd. for C52
H
30
N
4
O
2
: C, 84.08; H, 4.07;
str.), 2866 (Aliph.–C–H str.), 1598, 1448 (Ar.–C = C str.), 1336 (C–N
1
◦
N, 7.54%; found: C, 83.98; H, 4.01; N, 7.72%.
str.), 1121, 875, 738 (Ar.–C–H bend), H NMR (CDCl , 300 MHz, 25 C)
3
δ (ppm): 8.84 (1H, s, ArH), 8.63–8.56 (2H, m, ArH), 8.16–8.07 (3H, m,
2
.3.1.2. 3,6-di(9H-carbazol-9-yl)tribenzo[a,c,i]phenazine
(3). Similar
ArH), 7.78–7.25 (12H, m, ArH (1H extra due to residual CHCl
3
in
procedure adopted as for 1 starting from, 3,6-dibromotribenzo[a,c,i]
phenazine (200 mg, 0.4 mmol) an aryl halide precursor, 9H-carbazole
CDCl
3
)), 4.49 (2H, q, N–CH
7.2 Hz); C NMR (CDCl
2
, J = 7.2, 7.2 Hz), 1.52 (3H, t, ꢀ CH
3
, J = 7.2,
1
3
◦
3
, 75 MHz, 25 C) δ (ppm): 145.74, 143.86,
amine (133 mg, 0.8 mmol), palladium catalyst [Pd
2
(dba)
3
] (20 mg), 2-
141.58, 137.86, 137.32, 134.23, 133.64, 131.53, 131.43, 128.47,
127.89, 127.76, 126.95, 126.47, 125.91, 125.26, 124.79, 121.41,
dicyclohexylphoshpino-20,60-dimethylbiphenyl (SPhos) (20 mg) as co-
ligand and sodium-t-butoxide (80 mg) a base were added to the reaction
mixture.
120.36, 113.72, 110.66, 109.72, 36.25 (N–CH
2
), 13.44 (ꢀ CH
3
)),
+
+
MALDI-TOF: mass calcd. for C32
H
22
N
4
[M ]: 462.54; found [M ]:
Eluent:- n-hexane:chloroform :: 20:80, orange solid, yield 0.124 g
462.65, elemental anal. calcd. for C32
22 4
H N : C, 83.09; H, 4.79; N,
◦
ꢀ 1
(
46%), mp > 250 C, FTIR (solid,
ν
max/cm ): 3044, 2921, 2825(Ar. =
12.11%; found: C, 83.12; H, 4.70; N, 12.18%.
C–H str.), 1598, 1448 (Ar.–C = C str.), 1338 (C–N str.), 1216, 725 (Ar.–
1
C–H bend), H NMR could not be obtained because of very poor solu-
2.3.1.6. 2-(9H-carbazol-9-yl)-6-methyl-6H-indolo[2,3–b]quinoxaline
(7). Similar procedure adopted as for 1 starting from, 2-bromo-6-
methyl-6H-indolo[2,3–b]quinoxaline (200 mg, 0.64 mmol) an aryl
halide precursor, 9H-carbazole amine (107 mg, 0.64 mmol), palladium
catalyst [Pd (dba) ] (15 mg), 2-dicyclohexylphoshpino-20,60-dimethyl-
bility, 13C NMR (solid, 125 MHz, 25 C) δ (ppm): 133.37, 128.85,
◦
1
27.20, 119.17, 116.32, 103.02. MALDI-TOF: mass calcd. for C48
H
28
N
4
+
+
[
M ]: 660.76; found [M ]: 660.91, elemental anal. calcd. for C48
H
28
N
4
:
C, 87.25; H, 4.27; N, 8.48%; found: C, 87.16; H, 4.32; N, 8.53%.
2
3
biphenyl (SPhos) (15 mg) as co-ligand and sodium-t-butoxide (65 mg) a
base were added to the reaction mixture.
2
.3.1.3. 3,6,11-tri(9H-carbazol-9-yl)dibenzo[a,c]phenazine (4). Similar
procedure adopted as for 1 starting from, 3,6,11-tribromodibenzo[a,c]
phenazine (200 mg, 0.4 mmol) an aryl halide precursor, 9H-carbazole
Eluent:- n-hexane:chloroform :: 60:40, yellow green solid, yield
◦
ꢀ 1
0.132 g (52%), mp 210 C, FTIR (solid,
ν
max/cm ): 3057, 2921 (Ar. =
amine (198 mg, 1.2 mmol), palladium catalyst [Pd
2
(dba)
3
] (25 mg), 2-
C–H str.), 2839 (Aliph.–C–H str.), 1598, 1448 (Ar.–C = C str.), 1332
1
◦
3
, 300 MHz, 25 C)
dicyclohexylphoshpino-20,60-dimethylbiphenyl (SPhos) (25 mg) as co-
ligand and sodium-t-butoxide (110 mg) a base were added to the reac-
tion mixture.
(C–N str.), 1118, 752 (Ar.–C–H bend), H NMR (CDCl
δ (ppm): 8.52–8.44 (2H, m, ArH), 8.33–8.30 (1H, m, ArH), 8.18–8.15
(2H, m, ArH), 7.98–7.94 (1H, m, ArH), 7.71–7.56 (3H, m, ArH),
Eluent:- n-hexane:chloroform :: 5:95, orange solid, yield 0.134 g
7.47–7.29 (7H, m, ArH, (1H extra due to residual CHCl
3 3
in CDCl )), 3.98
◦
ꢀ 1
13
◦
(
45%), mp > 250 C, FTIR (solid,
ν
max/cm ): 3071, 2921 (Ar. = C–H
(3H, s, N–CH
3
), C NMR (CDCl
3
, 75 MHz, 25 C) δ (ppm): 145.16,
str.), 1609, 1462 (Ar.–C = C str.), 1339 (C–N str.), 1216, 738 (Ar.–C–H
140.86, 135.19, 131.39, 129.11, 127.86, 126.46, 126.07, 123.58,
1
bend), H NMR could not be obtained because of very poor solubility,
122.88, 121.18, 120.33, 120.24, 109.95, 109.86, 109.30, 29.69
1
3
◦
+
C NMR (solid, 125 MHz, 25 C) δ (ppm): 139.75, 138.57, 135.07,
34.18, 133.94, 126.02, 121.91, 119.09, 115.95, 107.90, 103.83,
(N–CH
3
), MALDI-TOF: mass calcd. for C27
H
18
N
4
[M ]: 398.46; found
: C, 81.39; H, 4.55; N,
+
1
1
[M ]: 398.96, elemental anal. calcd. for C27
H
18
N
4
+
03.39, 102.71, MALDI-TOF: mass calcd. for C56
H
33
N
5
[M ]: 775.89;
14.06%; found: C, 81.46; H, 4.45; N, 14.10%.
+
found [M ]: 775.47, elemental anal. calcd. for C56
H
33
5
N : C, 86.69; H,
′
′
4
.29; N, 9.03%; found: C, 86.61; H, 4.40; N, 8.98%.
2.3.1.7. 3-(9H-carbazol-9-yl)-10-methyl-10H-pyrido[3 ,2 :5,6]pyrazino
2,3–b]indole (8). Similar procedure adopted as for 1 starting from, 3-
[
′
′
2
8
.3.1.4. 2,17-di(9H-carbazol-9-yl)dibenzo[a,c]naphtho[2,3–h]phenazine-
,13-dione (5). Similar procedure adopted as for 1 starting from, 2,17-
bromo-10-methyl-10H-pyrido[3 ,2 :5,6]pyrazino[2,3–b]indole
mg, 0.64 mmol) an aryl halide precursor, 9H-carbazole amine (107 mg,
0.64 mmol), palladium catalyst [Pd (dba) ] (15 mg), 2-dicyclohexyl-
(200
dibromodibenzo[a,c]naphtho[2,3–h]phenazine-8,13-dione (200 mg,
.35 mmol) an aryl halide precursor, 9H-carbazole amine (117 mg, 0.7
mmol), palladium catalyst [Pd (dba) ] (20 mg), 2-dicyclohexylphosh-
2
3
0
phoshpino-20,60-dimethylbiphenyl (SPhos) (15 mg) as co-ligand and
sodium-t-butoxide (60 mg) a base were added to the reaction mixture.
Eluent:- n-hexane:chloroform :: 35:65, Yellow solid, yield 0.142 g
2
3
pino-20,60-dimethylbiphenyl (SPhos) (20 mg) as co-ligand and
sodium-t-butoxide (85 mg) a base were added to the reaction mixture.
Eluent:- n-hexane:chloroform :: 25:75, red solid, yield 0.112 g (43%),
◦
ꢀ 1
(56%), mp 218 C, FTIR (solid,
ν
max/cm ): 3044, 2907 (Ar. = C–H str.),
2839 (Aliph.–C–H str.), 1598, 1475 (Ar.–C = C str.), 1393 (C–N str.),
◦
ꢀ 1
1
◦
3
, 300 MHz, 25 C)
mp 242 C, FTIR (solid,
ν
max/cm ): 3057, 2907, 2839 (Ar. = C–H str.),
1254, 1134, 738, 466 (Ar.–C–H bend), H NMR (CDCl
1
669 (ꢀ C = O), 1570, 1448 (Ar.–C = C str.), 1336 (C–N str.), 1257, 708
δ(ppm): 9.41 (1H, s, ArH), 8.88–8.51 (2H, m, ArH), 8.21–8.18 (2H, m,
1
◦
(
Ar.–C–H bend), H NMR (CDCl
3
, 300 MHz, 25 C) δ (ppm): 9.78 (1H, d,
ArH), 7.81–7.26 (10H, m, ArH (1H extra due to residual CHCl
3
in
1
3
◦
ArH, J = 9.0 Hz), 9.57 (1H, d, ArH J = 9.0 Hz), 9.05–8.93 (m, 2H, ArH),
CDCl
3
)), 4.10 (3H, s, N–CH
3 3
), C NMR (CDCl
, 75 MHz, 25 C) δ (ppm):
8
.36–8.02 (18H, m, ArH), 7.73–7.56 (6H, m, ArH), 7.25 (1H due to
152.70, 151.48, 150.85, 148.27, 141.64, 140.34, 132.43, 128.01,
126.08, 123.86, 123.30, 122.30, 121.02, 120.09, 109.68, 101.76, 27.93
1
3
◦
residual CHCl
3
in CDCl
3
), C NMR (CDCl
3
, 75 MHz, 25 C) δ (ppm):
4

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
+
(
N–CH
3
), MALDI-TOF: mass calcd. for C26
H
17
N
5
[M ]: 399.45; found
Table 1
+
[
M ]: 399.78, elemental anal. calcd. for C26
H
17
N
5
: C, 78.18; H, 4.29; N,
Absorption data of 1–8.
1
7.53%; found: C, 78.26; H, 4.18; N, 17.57 %.
a
ꢀ 1
ꢀ 1
b
Dye
λ
abs , nm (log
ε
max
M
cm
)
λabs, nm
Toluene
THF
CHL
DMSO
Solid film
3
. Results and discussion
1
293 (5.10),
290 (5.13),
341 (4.58),
403 (4.34)
286 (5.21),
341 (4.67),
403 (4.41)
293 (5.09),
341 (4.50),
404 (4.38)
306,
350,
411
3
4
41 (4.45),
05 (4.40)
3
.1. Synthetic procedure
2
3
286 (5.21),
289 (5.14),
341 (4.64),
420 (4.20)
291 (4.98),
341 (4.65),
453 (4.14)
291 (5.02),
340 (4.63),
438 (4.16)
295,
344,
463
The synthesis of the target compounds 1–8 is illustrated in Scheme 1.
3
4
41 (4.67),
45 (4.19)
Precursor aryl halide molecules for the synthesis of compounds 1–8
were prepared by simple condensation reaction. The molecules 1–8
were prepared by Buchwald–Hartwig coupling amination reaction
293 (4.98),
302 (4.98),
322 (4.94),
406 (4.25),
427 (4.42),
454 (4.35)
293 (4.96),
322 (5.02),
406 (4.25),
429 (4.44),
459 (4.35)
283 (4.90),
322 (4.89),
408 (4.13),
431 (4.30),
457 (4.30)
304,
367,
449
3
4
23 (5.03),
06 (4.26),
[
46,47] and found in 43–64% yield as green–red solids after purification
using column chromatographic techniques (eluent: n-hexane: chloro-
form). The identity and purity of target compounds 1–4 were confirmed
by solid state 13C NMR (SI Figs. S1ꢀ S4) due to poor solubility of it in
common organic solvents, MALDI-TOF mass spectrometry (SI Fig. S10,
S11), FT-IR (SI Fig. S15, S16) and elemental analysis. While, molecules
430 (4.45),
51 (4.34)
4
4
286 (5.11),
339 (4.51),
293 (5.18),
339 (4.50),
385 (4.33),
405 (4.50),
452 (4.37)
291 (4.97),
339 (4.50),
384 (4.33),
405 (4.50),
451 (4.37)
293 (4.91),
341 (4.36),
386 (4.25),
407 (4.39),
448 (4.35)
293,
322,
341,
470
3
85 (4.33),
406 (4.50),
53 (4.38)
1
13
5
–8 were confirmed by H and C NMR spectroscopy using CDCl
3
4
solvent (SI Figs. S5–S8), MALDI-TOF mass spectrometry (SI Fig. S12,
S13), FT-IR (SI Fig. S17, S18), and elemental analysis. The melting
points of 1–8 were determined by open capillary method. The detailed
synthetic procedure and characterization data of compounds 1–8 are
given in the experimental section (2.3).
5
6
286 (5.11),
269 (5.02),
341 (4.46),
435 (4.34),
490 (3.83)
275 (5.06),
341 (4.46),
438 (4.41),
492 (3.91)
270 (5.08),
341 (4.46),
435 (4.41),
493 (3.88)
298,
351,
458,
520
3
4
41 (4.46),
38 (4.40),
490 (3.97)
290 (5.27),
289 (5.20),
302 (5.15),
385 (4.33),
468 (3.69),
500 (3.58)
299 (5.02),
311 (4.95),
388 (4.38),
466 (3.69),
504 (3.58)
298 (5.05),
311 (4.96),
389 (4.38),
467 (3.89),
504 (3.57)
301
311,
397,
496,
528
3
12 (5.07),
3
.2. Photophysical properties
385 (4.34),
4
4
65 (3.79),
98 (3.67)
In order to compute the light-absorbing capability of the dyes, ab-
sorption and emission spectra were taken in 10-5 M of various solvents
viz. toluene, chloroform, tetrahydrofuran (THF), DMSO having different
7
287 (5.22),
286 (5.02),
341 (4.36),
376 (4.17),
413 (4.12)
284 (5.14),
343 (4.41),
382 (4.15),
414 (4.15)
287 (5.00),
342 (4.39),
378 (4.14),
415 (4.13)
298,
345,
389,
421
3
3
4
42 (4.40),
80 (4.14),
13 (4.16)
T
Reichardt polarity indices (E (30)) [54,55] and in solid film.
The absorption spectra of the dyes in toluene and chloroform are
displayed in Fig. 2 (for other solvents and solid film see SI Fig. S20, S21)
and the pertinent data collected in Table 1. The emission spectra of 1–8
in chloroform (CHL) and solid film are displayed in Fig. 3 (for other
solvents see SI Fig. S22) and the relevant data are given in Table 2.
Molecules 1–8 exhibits multiple absorption bands originating either
8
287 (5.22),
283 (4.99),
341 (4.42),
429 (4.10)
282 (5.03),
340 (4.44),
429 (4.14)
287 (4.98),
342 (4.42),
428 (4.12)
297,
344,
446
3
4
41 (4.44),
29 (4.18)
a
-5
b
–1
ꢀ 1
Recorded in10 M solution,
ε
is Extinction coefficient (L mol cm ) (in
parentheses).
from different n-
and varied in accordance with the strength of acceptor segment or extent
of -conjugation in it (Table 1, Fig. 2, SI Fig. S20, S21). Strong absorp-
tion peaks at lower wavelength ~ 290 nm (log
belongs to the localized * transitions of the Cz moiety
17–21,56,57]. The weak absorption peaks at 302–389 nm (log
π*,
π-
π* and charge–transfer transitions in D–A system
core present in dyes 1–8 (Table 1, Fig. 2, SI Fig. S20, S21)
17–21,38–43,56,57].
[
π
ꢀ
1
ꢀ 1
)
Notably efficiency of ICT interaction in 1–8 was found improved
ε
max = ~5.00 M cm
with the enhancement of the role of the electron acceptor and -conju-
π
π
– π
gation pathway in it, resulting in a bathochromic absorption shift. As in
1 and 2 (both having quinoxaline as an acceptor segment), later one
shows red shifted ICT transition (Δλ = ~17–55 nm) than former one,
[
ε
max
=
ꢀ 1
ꢀ 1
5
.00–4.14 M cm ) either corresponds to n–
π* or
π
–
π
* transitions
originating from the orbitals delocalized over the entire conjugation
pathway [17–21,56,57]. Further weak absorption peaks at longer
wavelength (above 400 nm) observed due to the intramolecular charge
transfer (ICT) transition arises from the Cz donor to the various acceptor
reminiscent strong
π
-accepting property due to presence of highly
extended conjugated chromophoric segment (C = O) with quinoxaline
acceptor core in 2 (Table 1). Strong - accepting property in 2, also
π
Fig. 2. UV–Vis absorption spectra of compounds 1–8 in toluene (a) and chloroform (b).
5

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
Fig. 3. Emission spectra of compounds 1–8 in chloroform (a) and solid film (b) (excitation wavelength = 390 nm).
Table 2
Emission data, optical band gap, Stoke’s shift and quantum yield of 1–8.
a
opt
b
-1
c
Dye
λemi nm, (Eg , eV)
λ
emi, nm
Stokes shift, cm , (ϕ
F
(%))
,
Tol.
THF
CHL
DMSO
Solid Film
Tol.
THF
CHL
DMSO
1
2
3
4
5
6
7
8
467 (2.85)
–
538 (2.78)
494 (2.66)
–
512, 547 (2.56)
468 (2.89)
481 (2.82)
508 (2.83)
–
542 (2.72)
533 (2.62)
–
525, 556 (2.54)
476 (2.82)
490 (2.74)
515 (2.77)
–
542 (2.61)
532 (2.61)
–
526, 558 (2.51)
498 (2.78)
512 (2.76)
558 (2.65)
–
544 (2.58)
547 (2.61)
–
528, 565 (2.52)
501 (2.77)
524 (2.65)
510
–
593
549
–
552, 606
510
539
3278 (7.40)
–
3585 (3.14)
1907 (18.88)
–
1974 (82.22)
2845 (2.57)
2520 (20.74)
5128 (25.90)
–
3624 (5.00)
3362 (10.55)
–
5396 (25.92)
–
3665 (3.88)
3375 (13.84)
–
7016 (12.96)
–
3692 (3.72)
4039 (1.69)
–
2473 (56.78)
4136 (31.48)
4280 (7.40)
2319 (61.11)
3204 (37.03)
2901 (33.14)
(
2447 (91.27)
4074 (35.01)
3778 (38.88)
)
opt
1
240.8
c
a
-5
b
Recorded in 10 M solution, Optical band gap (in parentheses) estimated using emission and excitation spectra Eg
=
eV. Quantum yield (in pa-
λoptedge
rentheses) with reference to quinine sulphate (ϕ = 0.54 in 0.5 M H
2 4
SO ).
suggested by theoretically computed higher electron affinity (EA) value
emission maxima of dyes follow similar trends in accordance with the
acceptor strength in it. Thus 1, 3 and 4 show a bathochromic shift in the
order as 1 < 4 < 3 (Table 2). While λemm of 6–8 varied as, 7 < 8 < 6 in
accordance with the variation in acceptor strength and donating
strength of Cz based on position of it on core molecules (Table 2).
in it (2.43 eV) than 1 (1.69 eV) (Table 4). In comparison of 1–3, 3 (EA =
1
.96 eV) possess higher degree of
π-conjugation than 1 (EA = 1. 69 eV)
but < 2 (EA = 2.43 eV) consequently their ICT maxima follows the
order: 1 (405 nm) < 3 (430 nm) < 2 (445 nm) (parenthesis: λ value in
toluene) (Tables 1 and 4). Similarly, among dye 3–5 devouring phena-
Dye 2 and 5 having sterically crowded large
π
frameworks which
zine as a strong
π-conjugated acceptor segment exhibit varied ICT λmax
force the molecule to twist out of the plane (SI Fig. S37 and S39). Such
highly pretwisted D–A system generally show dominant electron trans-
fer (ET) character which severely quenched the fluorescence through
twisted intramolecular charge transfer transition (TICT) arises in mol-
ecules having D–A unit joined by single bond [58,59]. Twisted D–A
assembly in 2 and 5 was also confirmed by highly charge separated
HOMO-LUMO orbitals than other molecules (Fig. 10). In case of 6
observed dual emission in all solvents (Fig. 3(a) and SI Fig. S22) indicate
mixed emission from a high energy band through relaxation of the
locally excited (LE) state and from a lower energy band due to emission
from the TICT state [58,59].
with variation in their acceptor strength as 4 (406 nm) < 3 (430 nm) < 5
(
438 nm) (Table 1).
To sum up ICT λmax in 1–5 follows the order as 1 (405 nm) < 4 (406
nm) < 3 (430 nm) < 5 (438 nm) < 2 (445 nm) (Table 1). Likewise on
comparing indole based D–A dyes 6–8 their ICT λmax varied as: 7 (413
nm, EA = 1.17 eV) < 8 (429 nm, EA = 1.39 eV) < 6 (465 nm, EA = 1.51
eV), in accordance with their
π
-accepting strength or extent of conju-
gation in it (Tables 1 and 4). Further in addition with the acceptor
strength, the substitution of Cz on indole (electron donor) side of core
moiety in 6 also effectively increase the electron donating strength of a
molecule than on quinoxaline/pyrido-pyrazine (electron acceptor) side
in 7 and 8, consequently molecule 6 shows bathochromic shift than 7
and 8. Moreover, dye 3–6 shows shoulder absorption peak with λICT at
around 406, 451 in 3, 453 in 4, 490 in 5 and 498 in 6, reminiscent highly
conjugated or unresolved vibrational-electronic (vibrionic) transition
Moreover, emission spectra of 1–8 in different polarity of solvents
(E
T
(30)) from nonpolar (toluene, E
T
(30) = 33.9) to polar (DMSO, E
T
(30) = 45.0) indicate the occurrence of notable positive solvatochrom-
ism i.e. red shift in emission wavelength as shown in Fig. 4(a and b)
[54,55]. This notable solvatochromic property in D–A based dyes 1–8,
attribute solvation of the excited state of the ICT and stabilization of
enhanced dipole moments in the polarized excited state of the ICT-
fluorophores by a highly polar solvent leads to decrease in the energy
gap of the excited state and causes a red shift in the emission [54,55,58].
Such kind of solvatochromism are generally observed in fluorescence
property rather than the corresponding absorption characteristics
(Table 1 and 2) [54,55,58]. Moreover the emission profiles of dyes are
broadened with increasing solvent polarity from toluene to DMSO
(Fig. 3(a), SI Fig. S22), suggesting the stabilization of CT excited state by
(
Table 1, Fig. 2, SI Fig. S20) [58]. Besides the dyes 1, 3, 4–8 showed
slight variations (<7 nm) in the CT absorption band on changing the
solvent polarity indicate molecules are exposed to the same local envi-
ronment in the ground and excited states (Table 1, Fig. 2 and SI Fig. S20)
[
58]. While on changing the solvent polarity dye 2 shows considerable
variation in CT absorption band suggests solvent-induced change of the
electronic ground-state structure [55,58].
Further, exciting at 390 nm, molecules show emission in the range of
4
67–565 nm in solvent state (Table 2, Fig. 3 and SI Fig. S22). As like λICT
,
6

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
Fig. 4. Emission images of 1–8 in toluene, THF, chloroform, DMSO, solid film and powder under 365 nm UV illumination (a), Plot of emission wavelength vs polarity
index E (30) (b), Plot of stokes shift vs polarity index E (30) (c), Plot of stokes shift versus orientation polarizability (d).
T
T
polar solvents [54,55,58]. Additionally, the solvent sensitivity of the
fluorophores were further examined by Lippert-Mattaga plot and
to (i) rigid
π
-conjugated structure which minimize radiation less deac-
tivation due to rotation or vibration of side groups or (ii) strongly con-
jugated electron donor and acceptor charge transfer transitions, often
Reichardt empirical solvent polarity parameter (E
T
(30)) versus Stoke’s
shift correlations (Fig. 4(c) and (d)). The Lippert-Mattaga plot of dyes
F
display bright fluorescence [58]. Further the ϕ in 6 found high in
(
(
Fig. 4(d)) displayed linear variation with good correlation coefficient
nonpolar solvent and decreases on addition of polar additives, could be
due to decreased non-radiative decay rate in nonpolar media [58].
While compound 3 with a conjugated system, exhibits low quantum
yield than other dyes could be due to (i) complex molecular structure of
R = > 0.8) over the entire solvent range (toluene to DMSO) indicate
presence of charge-transfer excited state from the donor Cz to the
acceptor unit [17,55,60]. The positive solvatochromic behaviour and
presence of specific dipole–dipole interactions or intermolecular sol-
ute–solvent interactions of dyes were further attributed from the in-
the dye (ii) high
theoretical section) increases the
significant drop in fluorescence emission intensity and ϕ
F
π
–orbital overlapping than other dyes (as explained in
stacking interaction and causes a
[58,62].
π–π
crease in the Stoke’s shift of molecules with increase in solvent E
T
(30)
values (Fig. 4(c)) [54,55,61]. In addition, the higher Stoke’s shift of dyes
Furthermore, absorption property in solid film (Table 1 and SI
Fig. S21) of 1 was found slightly red shifted (3–5 nm) and of 2–8 were
remarkably red shifted with broadening of peaks than solution state
(Table 1, Fig. 2 and SI Fig. S20). As intermolecular aggregation theory
suggests, more or less red-shifting in the solid state absorption spectrum
originates from the intermolecular charge transfer between aggregate
molecules. This intermolecular interaction includes the long-range
coulombic coupling which is weak through space and its perturbation
ꢀ 1
(
1907–7016 cm ) (Table 2), suggest a change in the ground and excited
state geometry and correspond to ICT excited state [54,55]. Large
Stoke’s shift also ensure that there is no reabsorption of the emitted
photons and hence desirable for the application of it in optoelectronics.
opt
The optical band gap (Eg ) of dyes were determined from optical edge
opt
(
λ
) obtained from the emission and excitation spectra are in the range
edge
of 2.51–2.89 eV (Table 2). The fluorescence quantum yield of 1–8 are in
ꢀ 5
1
over the energy level of S is slight, hence the red-shift in absorption is
the range of 2.57–91.27%, calculated by using 10 M solution of qui-
nine sulfate as a standard (ϕ = 0.54 in 0.5 M H SO ) (Table 2). Besides,
molecule 1 shows larger variation in ϕ with solvent polarity than other
dyes and found high ϕ in polar media than in nonpolar solvent. This
thereby quite small. Thus, more or less red shifted and broader CT band
in 1–8 reminiscent occurrence of intermolecular aggregation in the solid
state [38–43,63]. Further the solid state ICT processes and emission
characteristics in D–A molecules 1–8 are dependent on the degree of
rotational freedom between the D and A segment [59,64]. The opti-
mized geometry of 1–8 (Fig. 11 and SI Fig. S37–S42) shows dihedral
F
2
4
F
F
observed feature could be due to restricted rotation of the amino group
which decreases the non-radiative decay rate (knr) in polar media [58].
As the excitation coefficient and radiative decay rates are often not very
sensitive to solvent polarity, thus variation in quantum yield on
changing the solvent polarity is basically based on decrease or increase
in knr or due to a conformational change in the fluorophore [58]. While
◦
angle between D (carbazole) and A unit, around ± 50–128 . Which
clearly indicate that the D–A bonds are in little bit twisted due to the
lesser degree of rotational freedom and larger intramolecular steric
hindrance caused by rigid Cz donor, thereby limiting the complete
planarization of molecule. Hence based on the extent of planarization or
F
molecule 6, shows a very high ϕ among other compounds could be due
7

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
twisting of molecule induced by aggregate formation all dyes (except 2
and 5) exhibit more or less intense emission in the solid state (Fig. 3(b))
In nonpolar THF solvent all dye shows intense emission as a result of
intramolecular charge transfer (ICT) from the Cz donor to acceptor core
(SI Fig. S24). While addition of small amount of water (10%) into THF
solution of 1, emission is red shifted (537 nm) and catastrophically
weakened (Fig. 5(a), SI Fig. S24(a)) because of (i) increase in solvent
polarity and (ii) changes in conformation and formation of partially
charge separated twisted fluorophores, leads to the transformation of CT
state to TICT state which often associated with a non-radiative
quenching or decrease in emission process usually termed as a ‘dark
state’ [58,68,69]. By achieving the dark state further addition of water
percentage (20–60%) emission get quenched (Fig. 5(a), SI Fig. S24(a)).
[
36,65]. Formation of intermolecular aggregation followed by more
complex charge separation and recombination dynamics by electron
transfer process in D–A system attached by single bond could be the
reason for solid state fluorescence quenching in the anthraquinone
based dye 2 and 5 [66].
Further, it is assumed that the solid state emission exhibited by D–A
pairs 1, 3, 4, 6, 7 and 8 may exhibit AIE properties which would be
advantageous for their application in organic electronics. Thus, to
evaluate and understand molecular aggregation characteristics, we
studied the AIE effect in these molecules, as depicted in the following
section.
However, after addition of 70–90% f
w
emission intensity gets improved
designates aggregation/clustering of molecules due to hydrophobic
environment created by insolubility of dye in water (Fig. 5(a), SI Fig. S24
(
a)) [38,39,68,69]. Such improved emission at 70–90% f
w
show blue
3
.3. AIE and size-dependent emissions of the nanoparticles
shifted maxima than previous quenched media (60% f ), because of
w
elimination of the solvatochromic and TICT effect caused by formation
of aggregates in high polar (water) solvent media [38,68,69]. Hence
local environment becomes less polar due to decrease in dipole–dipole
interaction among dye molecules than that between polar solvent mol-
ecules and fluorescent aggregate molecules which leads to regain of CT
transition from TICT state [38,68,69]. Strikingly, the emission wave-
To evaluate the AIE characteristics of the selected dyes, a series of
THF/water mixtures (0–90%) of the dyes without altering the final
ꢀ 5
concentration, (10 M) were prepared in accordance with the reported
procedure for formation organic nanoparticles [67] and observed them
under visible and 365 nm UV illumination. The corresponding observed
fluorescence changes in THF/water mixtures with varying percentage of
length in aggregate state (70–90% f
emission maxima i.e., 510 nm (Table 2 and SI Fig. S24(a)) which further
gives the evidence for the formation of nanoparticles at high f in THF/
O mixtures by elimination of the solvatochromic and TICT effect
37–39]. Additionally, aggregate formation substantially hampers the
w
) matches well with the solid film
water fraction (f
left) exhibit emission enhancement by formation of nanoparticles in
homogenous suspension at high f designate possibility of AIE phe-
nomenon in it. While in case of 3, 4 and 6 decreased emission at high f
w
) (0–90%) of dye 1, 7 and 8 are shown in Fig. 5(a–c)
(
w
w
H
2
w
[
in a mixture suggest probability of aggregation-caused quenching (ACQ)
effect in it (SI Fig. S26(a–c)) [68,69]. Furthermore, aggregation based
emission quenching and enhancement characteristics of the chosen
molecules in solution of different THF/water mixture were further
evaluated using UV–Vis and fluorescence spectroscopy (SI Fig. S23,
S24).
intramolecular motions, hence activating the restricted intramolecular
motion (RIM) process which often tends to inhibit the dark TICT process
and blocks the non-radiative channel and boosting the emission effi-
ciently in the aggregate state [68,69]. Hence, AIE effect dominates in
w
this case. Moreover, as at high f , it is hard to control the size, shape and
Fig. 5. Emission images showing AIE effects with varying percentage of water fraction in THF–H
percentage (%) of water fraction in THF/water mixtures (right) of 1 (a), 7 (b) and 8 (c).
2
O mixtures (10 M) (left) and Plots of emission intensity versus
μ
8

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
number of nanoparticles formation. Consequently an irregular pattern in
emission intensity was observed at 70–90% f
w
(Fig. 5(a), SI Fig. S24(a))
reminiscent different physical constraints and numbers of nanoparticles
formed in different THF/water mixtures [69,70].
A similar effect was also observed in the cases of 7 and 8 while
enhancement in emission with the formation of nanoparticles observed
w
at 80 and 90% f and suggesting AIE activity in it (Fig. 5(b and c), SI
Fig. S24(e and f)). Besides, dye 8 shows dual emission in THF solvent
signifying presence of mixed LE and TICT state (SI Fig. S24(f)) [58,59].
While on addition of water in THF solvent (upto 70%) due to confor-
mation changes in molecule and stabilization of TICT state in polar
media, emission get decreased and broadened [58,59,68,69]. After-
wards, formation of aggregate at 80 and 90% f
characteristics as in pure THF, with slightly improved emission than
emission in 70% f (SI Fig. S24(f)). Moreover, though a hydrophobic
w
regain its spectral
w
environment is created inside the aggregate solution of 3, 4 and 6 (SI
Fig. S24(b–d), S25 and S26) alleviated TICT effect but emission is not
recovered because of the ordinary ACQ effect which indicates that not
every TICT dye is AIE active. As AIE activity is dependent on the extents
of conformational flexibility and rotational motion of molecule, thus in
an AIE active TICT system, there must be multiple rotors to maximize
the emission quenching effect in solution and omnidirectional structural
rigidification to restrict the intramolecular motions in aggregates of
molecule [68,69]. From the ground state optimized geometry (Fig. 11
and SI Fig. S37–S42) it is clearly evident that molecule 1 (Fig. 11) has
larger flexible rotors than other selected dye for AIE study, as all the free
aromatic rings i.e. B1, B2, B3, B4 and B5 are rotatable in some degree
between each other (SI Table S2), which precludes co-facial alignment of
the molecules and restrict the intramolecular motions in aggregate state
efficiently [36,68]. While in comparison of 3, 4 shows lesser degree of
rotation due to restricted rotation between B2-B5 and B4–B5 (dihedral
Fig. 6. Plot of relative quantum yield of 1, 3, 4, 6, 7 and 8 versus solvent
composition of the THF/Water mixture calculated with reference to quinine
sulphate (ϕ = 0.54 in 0.5 M H SO ).
2
4
(
80, 90% f
w
) reminiscent homogeneity in the homogeneous suspensions
O mixtures (Fig. 7, SI Fig. S27, S31 and S32) and were
of the THF–H
2
quite stable for a long period of time without coalescence. However, the
hydrodynamic average size of the nanoparticles formed in suspension of
1
at 70% f
tained by DLS (SI Fig. S27). While for 7 and 8 at 80% f
namic average size is 165 and 392 nm respectively and at 90% f
w
was 119 nm, at 80%, 124 nm and at 90%, 127 nm as ob-
the hydrody-
, 158
w
w
and 522 nm respectively (SI Fig. S31, S32), which suggest different
conformation, random packing and different arrangement of the nano-
◦
angle ~ 0 ) due to bonding between C17 and C18 atom which makes the
w 2
particles formed in the high f of THF/H O mixture. Hence, molecule 1,
molecule structurally most rigid, planar and more
π
-conjugate (SI
-conju-
7
and 8 showed nanoparticle size-dependent emission behaviour and
Table S4 and S5) [69]. Besides, indole based dye 6 has more
π
AIE activity (Fig. 5(a–c) and SI Fig. S27, S31 and S32).
gate system due to additional phenyl ring in plane than 7 and 8 and thus
show red shifted absorption band than 7 and 8 as explained in photo-
Besides, PDI value for 3, 4 and 6 (SI Fig. S28–S30) at high water
percentage in THF solvent, was found relatively high indicate that the
sample has a very broad or multiple particle size distribution due to fast
agglomeration of molecules in that media [72]. Also, broad particle size
physcical section (3.2). Increased extent of
π
-conjugation in 3, 4 and 6
by melding more and more aromatic rings together leads to strong
π
– π
stacking interactions which prompt the formation of excimers as a
detrimental species leading to the observed ACQ effect (SI Fig. S24(b–d),
S25) [69]. Henceforth among all selected molecules, dye 1 have robust
AIE or AIEE (as emission intensity in aggregate state is found higher than
in THF solution) activity due to presence of maximum number of rotors
w 2
population at high f of THF/H O mixture further evident the ACQ ef-
fect in it.
3.4. Electrochemical properties
and lesser
accordance with the number of flexible rotors and decreasing extent of
conjugation follows the order as 1 > 7 > 8 (Fig. 5(a–c)). Further the
UV–Visible spectra (SI Fig. S23) of a series of THF/water mixture of the
dyes showed presence of scattered light at high f , with levelling off tail
π
-conjugation. To sum up, AIE activity in 1, 7 and 8 in
π
As, the electronic energy levels of D–A materials directly influence
the charge injection and transport in the molecular layer of optoelec-
tronic devices, it become vital to understand their redox prospective.
Thus to ascertain the redox propensities of 1–8, cyclic voltammetry (CV)
measurements were carried out in dichloromethane solvent and its
corresponding redox potentials are listed in Table 3. Cyclic voltammo-
gram of blank, 1 and 7 are shown in Fig. 8 (for other see SI Fig. S33).
Moreover, no alteration in CV waves with repeated scan were observed,
suggest electrochemical stability of molecules under oxidative and
reductive conditions.
-
w
characteristics at longer wavelengths commonly called as ‘‘Mie-scat-
tering’’, which signifies formation of nanoparticles in suspension by
hydrophobic environment [71].
To have a quantitative picture, the emission percentage quantum
yields (ϕ
mixtures, using quinine sulphate (ϕ = 0.54 in 0.5 M H
reference (Fig. 6). The ϕ value after addition of water upto 60% f
THF solution of 1 and 70% f in solution of 7 and 8 decreases smoothly,
whereas it was found increased swiftly afterwards. Such striking change
in emission ϕ with variation in percentage of water in THF solution of 1,
and 8 further gives evidence of formation of nanoscopic aggregates
and confirmed the AIE activity in it (Fig. 6). While other molecules 3, 4
and 6 shows decrease in ϕ after the formation of nanoscopic aggregates
F
) of selected dyes were estimated for different THF/H
SO ) as the
in
2
O
2
4
F
w
Nevertheless, observed quasi-reversible waves around 1.44–1.72 V
at anodic sweep of 1–5 attribute towards oxidation of Cz donor unit
(Table 3, SI Fig. S33). While cyclic voltammogram of 6–8 at anodic
sweep display two quasi reversible oxidation peak corresponding to loss
of two electrons, where lower potential (1.27–1.49 V) contribute
oxidation from Cz segment and higher potential (1.65–1.80 V) due to
oxidation of indole unit (Table 3, SI Fig. S33).
w
F
7
F
and propose ACQ phenomenon in it (Fig. 6).
On the other hand at cathodic sweep observed two reversible wave
for molecule 1 (Fig. 8) and 3 (SI Fig. S33) corresponds to the two elec-
tron reduction of benzo[g]quinoxaline and tribenzo[a,c,i]phenazine
core segment respectively. While reduction of dibenzo[a,c]phenazine
core in 4 (SI Fig. S33) shows one quasi-reversible wave at cathodic
Further, the homogeneity and variation in size of nanoparticles
formed in different high percentage of f
evaluated by dynamic light scattering (DLS) experiment. Low poly-
dispersity indices (PDI) obtained by DLS data of 1 (70–90% f ), 7 and 8
w
in THF/water suspension were
w
9

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
Fig. 7. DLS plot of the nanoparticles formed in 70% f
w
of 1 (a), 80% f
w
of 7 (b) and 80% f
w
2
of 8 (c) in THF–H O homogeneous suspension.
material are associated with the corresponding HOMO-LUMO energy
levels [27]. Subsequently, HOMO and LUMO energy levels of com-
pounds 1–8 were calculated by comparing the first oxidation and
reduction potentials of the dyes with oxidation potential of ferrocene
measured under same condition and are found–5.42 to–5.74 and–3.03
to–3.50 eV, respectively (Table 3). Obtained HOMO and LUMO energy
levels of dyes were found varying with their corresponding D–A inter-
action by altering acceptor core, number and position of substituting
carbazole donor unit. Calculated electrochemical band gap are in the
range of 2.09–2.53 eV (Table 3) which are comparable with the theo-
retically computed band gap (Fig. 10). Besides, obtained lower LUMO
energy levels (less than–3.0 eV) of 1–8 are comparable with previously
reported most widely used electron-transporting molecules suggest po-
tential use of it as n-type materials in opto-electronic devices [73–78].
Table 3
Electrochemical data of 1–8.
Compd
peak
oxi
a
peak
red
b
HOMO
c
LUMO
d
EC
e
E
g
E
E
1
2
3
4
5
6
7
8
1.51
1.52
ꢀ 1.03, ꢀ 1.49
ꢀ 1.07, ꢀ 1.90
ꢀ 0.94, ꢀ 1.18
ꢀ 1.26
ꢀ 5.72
ꢀ 5.74
ꢀ 5.72
ꢀ 5.56
ꢀ 5.59
ꢀ 5.62
ꢀ 5.47
ꢀ 5.42
ꢀ 3.41
ꢀ 3.37
ꢀ 3.50
ꢀ 3.03
ꢀ 3.36
ꢀ 3.26
ꢀ 3.26
ꢀ 3.33
2.31
2.37
2.22
2.53
2.23
2.36
2.21
2.09
1.53, 1.72
1.44, 1.58
1.44, 1.62
1.49, 1.80
1.36, 1.65
1.27, 1.73
ꢀ 0.91, ꢀ 1.51
ꢀ 0.98, ꢀ 1.35, ꢀ 1.83
ꢀ 1.01, ꢀ 1.60
ꢀ 0.93, ꢀ 1.14, ꢀ 1.28
a
peak
oxi
b
peak
red
E
oxidation peak potential (V).
E
reduction peak potential (V).
c
peak
+
HOMO energy level calculated from EHOMO = ꢀ [Eoxi – Eredox (Fc/Fc ) + 4.8]
d
peak
+
eV. LUMO energy level calculated from ELUMO = –[Ered – Eredox (Fc/Fc ) + 4.8]
eV. E calculated from Eg = [HOMOꢀ LUMO] eV.
e
EC
EC
g
3
.5. Thermal properties
Thermogravimetric analysis (TGA) was carried out at 10 C min
sweep of cyclic voltammogram. In compound 2 and 5 (SI Fig. S33),
obtained two quasi-reversible wave corresponds to two electron reduc-
tion of two carbonyl group in naphtho[2,3–f]quinoxaline-7,12-dione
and naphtho[2,3–h]phenazine-8,13-dione core respectively. Moreover,
molecule 6 (SI Fig. S33) show three quasi-reversible peaks for reduction
of benzo[g]indolo[2,3–b]quinoxaline core, while 7 (Fig. 8) show two
quasi-reversible waves at–1.01, ꢀ 1.60 V, correspond to the formation of
mono and diradical anions of the 6H-indolo[2,3–b]quinoxaline unit. On
the other hand, three quasi-reversible reduction waves observed in cy-
clic voltammogram of 8 (Fig. S33) correspond to three electron reduc-
◦
ꢀ 1
under nitrogen atmosphere at normal pressure to check the thermal
stability of molecules 1–8 and melting points determined by open
capillary method. TGA plot of 1–8 displayed in Fig. 9, which illustrate
the thermal decomposition temperatures corresponding to 5% and 10%
◦
◦
weight loss are in the range of 189–319 C and 220–404 C, respectively.
Hence reveals good thermal stability with no weight loss at low tem-
perature. The order of thermal stability in dyes is 4 (319/346) > 5 (316/
380) > 6 (275/292) > 7 (256/297) > 1 (247/404) > 8 (236/263) > 2
(209/314) > 3 (189/220) (Parentheses: Decomposition temperature at
5/10% weight loss derived by TGA.)
′
′
tion of pyrido[3 ,2 :5,6]pyrazino[2,3–b]indole core.
In optoelectronic devices, the interfacial charge transport kinetics of
+
Fig. 8. Cyclic voltammogram (full scan) of blank, 1 and 7 (inset: Half scan of 1 and 7 w.r.t Fc/Fc ).
1
0

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
the optimized geometries of 1 and 3 are presented in Fig. 11 (for others
see SI Figs. S37ꢀ S42). The UV/Vis absorption spectra of the optimized
geometries of the 1–8 are calculated in gas phase as well as in solvents
viz. toluene, chloroform, THF and DMSO using TDDFT calculation by
similar basis sets and given in SI Fig. S36. Pertaining wavelengths and
oscillator strength (f) data given in Table 4. Computed Ionization po-
tentials (IP), electron affinities (EA), and dipole moment (µg) of all dyes
is given in Table 4. The vertical electronic transitions in gas phase,
Cartesian coordinates and Mulliken charges of 1–8 given in SI Table S1
and S10¡S17 respectively.
Computed geometrical parameters of molecules suggest negligible
variation in bond length (~1.42 Å) between Cz and A segment by
changing the core unit (Fig. 11, SI Fig. S37–S42). The observed
maximum bond length variation is between 0.001 and 0.007 Å.
Conversely, corresponding dihedral angle between Cz donor and A
◦
segment were found in the range of ± 50–128 (Fig. 11, SI Fig. S37ꢀ S42)
◦
which are slightly higher than reported Cz based TADF emitters (~45 ),
suggesting twisted molecular conformation of D (Cz) and A segment in
all dye [80,81]. Further significantly altered dihedral angle was
observed between Cz and varied acceptor segments in 1–8 suggests
difference in twisting pattern of a D–A assembly (Fig. 11, SI
Fig. S37–S42). Consequently HOMO and LUMO orbitals wave function
distributions on the D and A segments get varied which leads to devia-
tion in ΔEST value in 1–8 (Fig. 10).
Fig. 9. Thermogravimetric plot of 1–8.
.6. Theoretical studies
3
The changes in the structure–property relationships at molecular-
level with varying acceptor unit in 1–8 are investigated by density
Generally, the key strategy of separating the wave function distri-
butions of HOMO and LUMO orbitals on the D and A segments respec-
functional theory (DFT) and time-dependent density-functional
(
TDDFT) calculations using the B3LYP/6-311G+(d,p) basis set as
tively, assist extremely small ΔE ordinarily required for conversion of
ST
implemented in the Gaussian09 program [79]. The theoretically
computed ground-state Frontier molecular orbitals (FMO) of 1–8 are
shown in Fig. 10 (HOMO–1 and LUMO + 1 frontier orbitals are given in
Fig. S34, S35). In order to analyse the TADF characteristics of 1–8 with
modulation in acceptor core unit attached to Cz donor segment, the
optimized geometrical parameters (bond length and dihedral angle) of
molecules at ground state (SI Table S2–S9) and its vertical excitation
energies of singlet (ES1) and triplet (ET1) states (Fig. 10 (below)) were
calculated by using similar basis sets. The variation of dihedral angles in
non-radiative triplet (T1) excitons to radiative singlet (S1) excitons
through efficient reverse intersystem crossing (RISC) process [82–85].
Such kind of strategy for locating smaller ΔE gap in D–A system
ST
theoretically based on the principle that S1 and T1 states of a dye
correspond to HOMO to LUMO transition and have substantial charge-
transfer (CT) excitation character [82–85]. Based on this principle
various Cz based twisted D–A systems having small ΔE value have
ST
been reported as a TADF emitters [22,23,28–34,86,87].
◦
However, in some reports observed moderate dihedral angles ~ 45
Fig. 10. Ground-state Frontier molecular orbitals (FMO) contour plots of the HOMO and LUMO levels (above) and calculated excitation energies of singlet (ES1) and
triplet (ET1) States (below) of 1–8 based on B3LYP/6–311 G+(d,p) level of theory.
1
1

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
Fig. 11. The variation of dihedral angles in the B3LYP/6-311G+(d,p) optimized geometries of 1 and 3.
Table 4
Ionization potentials (IP), electron affinities (EA), wavelengths, oscillator strengths (f) and vertical electronic transition, dipole moment (µ
B3LYP/6–311 G+(d,p) Level.
g
) of 1–8 in gas phase at the
Dye
I
p
E
a
Toluene
THF
CHL
DMSO
Gas
µ
g
(
eV)
(eV)
1.69
2.43
1.96
1.84
2.55
λ
max
f
λ
(nm)
max
f
λ
(nm)
max
f
λ
max
(nm)
f
λ
max (nm)
f
(
nm)
1
2
3
4
5
6.66
6.7
493.89
0.2798
0.0156
0.0426
0.1381
0.0475
0.0031
0.0046
0.3843
0.0606
0.5144
0.0798
0.0058
0.1223
490.64
480.12
447.64
681.4
0.2756
0.0134
0.0424
0.1439
0.0386
0.0035
0.0035
0.3742
0.0586
0.5042
0.0802
0.0064
0.1106
491.57
480.74
447.76
680.97
639.49
536.32
544.71
528.67
489.63
516.23
489.75
474.94
688.96
0.2792
0.0139
0.043
489.85
479.58
447.81
683.11
642.63
533.75
544.71
527.03
489.96
513.55
486.76
472.06
689.02
0.2808
0.0123
0.044
496.22
0.1801
2.51
0.69
2.12
0.8
4
4
82.45
47.8
486.29
0.0199
443.09
0.02
680.25
0.1433
0.0413
0.0034
0.0037
0.3803
0.0598
0.5101
0.0803
0.0063
0.1145
0.1497
0.0351
0.0038
0.003
676.49 632.89
545.72
0.0847 0.0493
0.0015
6
5
37.37
39.11
640.55
535.25
544.46
527.68
489.61
514.88
488.46
473.69
688.64
6.7
545.27
543.94
530.11
484.99
518.44
497.84
482.08
687.28
0.0101
0.2525
0.0362
0.381
5
4
30.91
89.49
0.3795
0.0594
0.5094
0.081
6.56
6.76
519.11
4
4
92.87
77.92
0.07
0.0067
0.1021
0.0024
0.0737
689.66
0.46
0
.1410
6
5
66.46
87.45
0.1356
0.0002
0.0255
0.0129
0.0015
0.1263
0.0372
0.0002
0.1471
0.0171
0.0001
670.71
582.84
532.93
466.81
413.33
464.18
404.96
384.27
487.92
419.64
408.77
0.0002
669.39
584.15
532.57
467.48
414.48
465.2
0.1408
0.0002
0.0249
0.0116
0.0016
0.1248
0.0394
0.0002
0.1447
0.0161
0.0001
673.57
580.9
0.1519
0.0002
0.0243
0.0107
0.0019
0.1232
0.0437
0.0003
0.143
657.26
594.96
528.86
469.6
0.1128
0.0002
0.0135
0.01
6
7
8
6.62
6.8
1.51
1.17
1.39
531.81
0.0241
0.011
534.1
3.12
4.1
4
4
69.08
17.72
466.1
0.0017
0.1224
0.04
411.73
463.2
427.14
471.46
399.4
0.0006
0.0826
0.0003
0.0192
0.0992
0.0001
0.0106
467.79
4
3
01.83
89.61
404.1
406.46
382.06
487.68
422.46
407.81
0.0002
0.142
385.75
488.44
417.99
409.42
394.42
488.84
415.95
403.54
6.95
489.61
3.65
4
4
13.95
11.18
0.0156
0.0001
0.0158
0.0001
between Cz and A segments in D–A architecture sometimes leads to
ineffective twisting for efficient charge separation, as a consequence
higher ΔESTs was obtained which hinder the RISC process for TADF
characteristics [80,81]. Besides, efficiency of TADF characteristics in D-
molecular frame of dye 1 and 2, accepting property is greater in later one
as suggested by higher EA value in 2 (2.43 eV) than 1 (1.69 eV) (Table 4)
due to two electron withdrawing carbonyl group which form highly
twisted D–A–A type conformation in it and causes sufficient steric hin-
drance and separation between D–A segment. Accordingly, HOMOs of 2
are located on only one of the donor Cz segment (B1) and partially
extended to second Cz unit (B3) and phenyl rings (B2, B4) attached to
naphtho[2,3–f]quinoxaline-7,12-dione acceptor moiety, whereas its
LUMOs are mainly distributed over the entire acceptor moieties
(Fig. 10). While in case of 1, HOMOs are fully located on both the Cz
donor segment i.e. B1 and B3 rings and corresponding LUMOs are
confined over the entire acceptor core (Fig. 10). Thus greater spatial
separation of the HOMO and LUMO distributions in 2, resulting in
A-D, D-
π
-A, D-
π
-A-
π
–D skeleton were found more than simple D–A
-accepting or donating group increases
molecules because additional
π
the distance between D and A segment and help to achieve small ΔEST by
enhancing the twisting/steric hindrance/dihedral angle between them
[
88].
In case of 1–8 aromatic rings B1-B2/B3-B4/B2-B5/B4-B5/B6-B7
rotate to some degree between each other, which precludes co-facial
alignment/non-planar configuration of the molecules but retain conju-
gation at the same time (Fig. 11, SI Fig. S37–S42). Among twisted
1
2

P.S. Singh et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113457
smaller ΔEST (0.10) value and enhances TADF behaviour, while obvious
overlap of orbitals in former leads to larger ΔEST (0.82) which suppress
the TADF performance in it (Fig. 10). As compared to 1, HOMO is fully
localized on all over the molecule and LUMO on acceptor segment leads
to completely overlapped HOMO–LUMO orbitals in 3 results in slightly
higher ΔEST (0.84) value than 1 (Fig. 10). However dye 4 having D–A–D
architecture with one extra Cz ring (B7) than 3 which enhances the steric
hindrance/twisting between D and A segment effects less overlapping
and smaller ΔEST (0.39) as its HOMO lies on the two Cz donor (B1 and B7
ring), phenyl ring (B2 and B6) attached to phenazine acceptor and
partially extended to the third Cz (B3) ring, while LUMO located on
phenazine acceptor (Fig. 10). As like dye 2, smaller ΔEST (0.12) value
found in 5 due to highly separated HOMO-LUMO orbitals (Fig. 10).
Further indole based molecule 6 having D–D–A structure exhibit HOMO
distribution on all over the molecule and LUMO purely on acceptor
segment leads to high ΔEST (0.75) value (Fig. 10). While D–A–D struc-
tured indole based dye 7 and 8, show HOMO localized on Cz segment
and partially extended on the acceptor unit whereas LUMO confined on
acceptor unit only. As a result efficient charge separation was observed
in 7 and 8 yielding smaller ΔEST (0.43 and 0.35) value than 6 (Fig. 10).
Thus as like literature reports [88] herein too, D-A-A (2 and 5) D-A-D (4,
characteristics were found significantly influenced by the nature of
-acceptor strength/electron affinity (EA) of molecules. Dye 1, 7 and 8
having higher number of flexible rotors and lesser -conjugation than 3,
4 and 6 shows strong solid state emission and AIE dominates emission by
nanoparticles formation in homogenous THF/H O suspension. Nano-
π
π
2
particles formation in homogenous suspension of these dyes was
confirmed by DLS analysis. Measured HOMO and LUMO energy levels
by CV are in the range of–5.42 to–5.74 eV and–3.03 to–3.50 eV
respectively which are in correlation with the theoretical energy levels
estimated by DFT/B3LYP/6-311G+(d,p) basis set and found comparable
with reported n-type materials. The unique electronic and geometric
features obtained by TDDFT calculations further verify (i) the presence
of the D–A architecture thus ICT in the molecules and (ii) the fact of
dependency of photophysical characteristics on
π
-acceptor strength.
The theoretically computed small ΔEST value in D–A–A (2 and 5) and
D–A–D (4, 7 and 8) structured dyes shows potential TADF characteris-
tics in it. Thus, experimental and theoretical results along with its good
thermal stability designate potential use of these molecules in various
optoelectronic applications.
Declaration of Competing Interest
7
and 8) structured dyes are found suitable to be used as TADF emitter
than D-A (1, 3) or D-D-A (6) assembled dyes.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Further the HOMO and LUMO energy levels of 1–8 obtained by
theoretical calculations are in the range of–5.56 to–5.80 eV and–2.73
to–3.70 eV respectively with energy band gaps from 2.10 to 3.01 eV
(
Fig. 10). The experimental and theoretical computed HOMO-LUMO
energies and band gap of 1–8 considerably matches with each other
Table 2–4). IP and EA were estimated by the differences in the total
Acknowledgements
(
The authors are greatly thankful to Micro–Analytical Laboratory,
Department of Chemistry, University of Mumbai for providing Instru-
mental facilities, Department of chemistry, IIT Bombay for solid state
energies of the neutral and cationic/anionic species, respectively.
Computed IP/HOMO value remains almost constant in all dyes however
EA/LUMO altered with alteration of acceptor strength in 1–8 assigning
to its electron-accepting nature (Table 4). The ground state dipole mo-
ments of all the derivatives in the gaseous phase were found to be small
implies that the dipole vectors cancel one another out due to their
opposite directions (Table 4).
1
3
C NMR, TIFR–Mumbai for providing MALDI-TOF.
Appendix A. Supplementary data
1
Experimental data of precursor aryl halide for dye 5 and 7, H NMR,
C NMR, FTIR and MALDI–TOF spectra of 1–8; UV-vis absorption
The simulated electronic transitions of 1–8 in various solvent and in
gas phase exhibit multiple transitions (Table 4) among which λmax with
high oscillating strength correspond to HOMO → LUMO transitions
except for 3 and 5 in which transition arises from HOMO–1 → LUMO.
Such HOMO–LUMO transitions in molecules 1–8 are directly related to
their substantial intramolecular charge transfer (CT) excitation from
donor Cz to various A unit, hence clearly indicate the presence of D–A
architecture in it [89]. As like experimental ICT λmax (Table 1),
computed high oscillating λmax was found varied with the modulation in
acceptor strength/electron affinity in 1–8 (Table 4). Consequently, like
experimental absorption data variation in computed λmax in 1–5 follows
the order: 1 (493 nm) < 4 (519 nm) < 3 (530 nm) < 5 (666 nm) < 2
13
spectra of 2–6 and 8 in different solvents and solid film; emission spectra
in Toluene, THF, DMSO; AIE spectrum and DLS plot; cyclic voltammo-
gram of 1–8, DFT/TDDFT calculation data along with molecular orbital
diagram, optimized structure and computed geometrical parameters are
available in SI. Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jphotochem.2021.113457.
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(
680 nm) while in 6–8, order of λmax found as 7 (467 nm) < 8 (489 nm)
6 (531 nm) (parentheses: λmax in toluene) (Table 4). However, mol-
[
[
[
[
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