Synthesis, photophysical, electrochemical and thermal investigation of Triarylamines based on 9H-Xanthen-9-one: Yellow-green fluorescent materials

Article abstract of DOI:10.1007/s12039-015-0973-0

Triarylamines containing 9 H-Xanthen -9-one core and aromatic units such as phenyl, naphthyl and p -methoxyphenyl were synthesized by employing palladium catalyzed C -N bond forming amination reaction in good yields. The photophysical studies revealed that the absorption and emission spectra are influenced by the nature of the peripheral amines. The photoemission spectra can be readily tuned in the range 483-532 nm (solution) and 525-576 nm (film) displaying green or yellow emission (film) depending on the nature of the amine segment with optical band gaps in the range 2.52-2.75 eV (film). The ionization potential and electron affinity were found to be in the range 5.332-5.686 eV and 2.705-2.776 eV, respectively. Thermal studies revealed that the synthesized compounds have good thermal stability with 5% and 10% weight loss at temperature ranging from 260-330^°C and 340-370^°C, respectively.

Full text of DOI:10.1007/s12039-015-0973-0

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0973-0
Synthesis, photophysical, electrochemical and thermal investigation
of Triarylamines based on 9H-Xanthen-9-one: Yellow–green fluorescent
materials
BHARAT K SHARMA, AZAM M SHAIKH and RAJESH M KAMBLE∗
Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai 400 098, India
e-mail: kamblerm@chem.mu.ac.in
MS received 14 April 2015; revised 7 September 2015; accepted 18 September 2015
Abstract. Triarylamines containing 9H−Xanthen−9−one core and aromatic units such as phenyl, naphthyl
and p−methoxyphenyl were synthesized by employing palladium catalyzed C−N bond forming amination
reaction in good yields. The photophysical studies revealed that the absorption and emission spectra are influ-
enced by the nature of the peripheral amines. The photoemission spectra can be readily tuned in the range
483–532 nm (solution) and 525–576 nm (film) displaying green or yellow emission (film) depending on the
nature of the amine segment with optical band gaps in the range 2.52–2.75 eV (film). The ionization potential
and electron affinity were found to be in the range 5.332–5.686 eV and 2.705–2.776 eV, respectively. Thermal
studies revealed that the synthesized compounds have good thermal stability with 5% and 10% weight loss at
temperature ranging from 260–330◦C and 340–370◦C, respectively.
Keywords. Triarylamines; 9H−Xanthen−9−one; Palladium−catalyzed aryl amination; Yellow−Green
florescent materials.
1. Introduction
the D or A moiety, or from intramolecular charge trans-
fer (ICT) excited states12 or even from intermolecu-
lar excimers or exciplexes.13 Thus, a judicious choice
of the D/A units could allow the simultaneous control
of the HOMO/LUMO levels and the emission color
of the D−A molecule. EL colors spanning the vis-
ible region, including blue,14 green,15 yellow,16 and
red17 have been achieved from OLEDs based on D−A
molecules.
Further, the incorporation of polyaromatic groups
in the triarylamine core extends the conjugation in
the molecular structure and has been found to be
highly beneficial by effectively enhancing the opti-
cal properties such as absorption, emission as well
as thermal stability.10,18 Additionally, the presence
of bulky groups around the trigonal amine nitrogen
often hinders the aggregation of molecules in the
solid state. The colour of the emission in such com-
pounds was tuned by the electronic nature of the aro-
matic group. For instance, the anthracene,7 fluorene,8
pyrene9 and perylene-based19 triarylamines usually dis-
play blue emission, while the triarylamines derived
from carbazole10 and fluoranthene20 emit either green
or yellow photons.
Organic semiconductors have received considerable
attention during the last few decades because of funda-
mental curiosity about their electro-optical properties
and technological applications. Organic π–conjugated
molecules have been explored for their application
as organic light emitting diodes (OLEDs),1 organic
field effect transistors (OFETs)2 and dye-sensitized
solar cells.3 Since the breakthrough work by Tang
and VanSlyke4 in 1987, OLEDs appear to be the
display devices in future. Donor−acceptor (D−A)
molecules have recently attracted considerable atten-
tion as electroluminescent multifunctional materials
for OLEDs.5 Molecules containing electron−donating
amino groups have been found to exhibit interest-
ing charge−transporting properties in electrolumines-
cent devices.6 Triarylamines-based rigid polyaromatic
chromophores such as anthracene,7 fluorene,8 pyrene,9
carbazole,10 dibenzothiophene,11 etc., have been uti-
lized in electronic devices with dual functions, i.e., hole
transporting and fluorescent characteristics especially
in OLEDs.
The electroluminescence (EL) from such D−A
molecules can originate from locally excited states of
However, triarylamines derived from electron–
deficient units such as quinoxaline/pyrazine,21 and
oxadiazole22 exhibit balanced charge–transporting
∗For correspondence

Bharat K Sharma
properties which are important for the performance of 2.1b Redox and Spectroscopic properties: Cyclic
electronic devices particularly in OLEDs. Similarly, voltammetry studies were carried out on a computer
9H−Xanthen−9−one (Xanthone), a heteroaromatic controlled AUTOLAB PGSTATE 30 electrochemical
molecule contains carbonyl group due to which it analyzer equipped with USB electrochemical interface
has high electron affinity and also possesses planar using GPES software version 4.9.005. Typically, a three
structure due to which the molecular overlap and inter- electrode cell equipped with a glassy carbon work-
molecular electron migration occurs easily with high ing electrode, Ag/AgCl (non-aqueous) reference elec-
efficiency in solid state.23
trode and platinum (Pt) wire as counter electrode was
Keeping the above mentioned donor−acceptor archi- employed. The measurements were carried at room
tecture with the benefits of functional triarylamines in temperature in anhydrous acetonitrile with tetrabutyl
mind, we report the synthesis of novel donor−acceptor ammonium hexafluorophosphate solution (0.1 M) as
compounds featuring xanthone as core unit (acceptor) supporting electrolyte at a scan rate of 100 mVs−1. The
and various diarylamines (donors) derived from phenyl, potential of Ag/AgCl reference electrode was calibrated
naphthyl and p−methoxy−phenyl groups by employ- by using ferrocene/ferrocenium redox couple whose
ing palladium−catalyzed C−N bond forming amina- known oxidation potential is + 4.8 eV.
tion reaction and their optical, electrochemical and
thermal properties.
The HOMO energy values were estimated from the
peak potentials (Eox) of the oxidation event according
to the equation (1).24
EHOMO = −[Eopxeak − Eox(Fc/Fc+) + 4.8] eV
(1)
2. Experimental
Where Eox (Fc/Fc+) is the peak potential of Fc/Fc+
2.1 Materials and Methods
peak
ox
couple against the Ag/AgCl electrode and E
is
2.1a Chemicals and Instruments: All the starting
materials and reagents were purchased from commer-
cial source (Sigma Aldrich and Alfa Aesar) and were
used without any further treatment and purifications
unless otherwise noted. The organic solvents were of
HPLC and spectroscopic grade and were dried and
freshly distilled using the standard procedures and han-
dled in moisture free atmosphere.
oxidation peak potential for the derivatives.
UV−Visible spectra were recorded for 10−5 M
toluene solutions in a 1 cm path length quartz cuvette
and for thermally deposited thin solid films, on SHI-
MADZU UV–2401PC at room temperature. The exci-
tation and emission spectra were carried out on a Perkin
Elmer LS 55 Fluorescence spectrophotometer. The flu-
orescence quantum yields (φF ) were calculated relative
to Alq3 in benzene (φF = 0.16)25 as a standard for green
emitting materials.
Investigation and confirmation of chemical structures
of all the synthesized materials were done using FT–
IR, 1H NMR, 13C NMR and high resolution mass spec-
1
troscopy (HRMS). H NMR spectra and 13C NMR
spectra were recorded using CDCl3 on a Bruker
300 Ultrashield spectrometer with Tetramethylsilane
2.2 Synthesis of materials (2–4)
(TMS) as internal reference at a working frequency The detailed synthetic route adopted for the synthesis
of 300 MHz and 75 MHz, respectively. Fourier trans- of triarylamines based on xanthone 2–4 is depicted in
form infrared (FT-IR) spectra were recorded on a Perkin scheme 1 and the molecular structures of the synthe-
Elmer Frontier 91579. High resolution Mass spectro- sized derivatives are illustrated in chart 1. The start-
metric measurements were recorded on Tof Micro- ing bromo derivative, 2,7−Dibromoxanthen−9−one
mass YA−105 HR−MS system. Melting points of the (1) required for the present study was synthe-
products were determined by open capillary method. sized according to the reported procedure26 and the
The thermogravimetric analysis (TGA) was performed bromo precursor was conveniently converted to the
using Perkin Elmer Pyris Diamond TG/DTA under triarylaminated derivatives 2–4 by treating it with
nitrogen atmosphere.
the corresponding diarylamine using palladium cat-
Column chromatography was carried out using alyzed Buchwald−Hartwig amination reaction27 utili-
SD−Fine silica gel (60–120 mesh), eluting with zing palladium−bis(dibenzylideneacetone) [Pd(dba)2]
n−hexane and chloroform. The progress of reaction as a catalyst, 2−dicyclohexylphoshpino−2ꢁ,6ꢁ−dimet-
and the purity of the compound were checked by thin hylbiphenyl (SPhos) as a ligand and sodium tert–
layer chromatography (TLC) on silica gel coated glass butoxide as a base, under nitrogen atmosphere for 12–
plates, in which the spots were visualized with UV light 24 h at 100◦C, giving derivatives 2–4 with yield more
(λ 254–365 nm) and in iodine chamber.
than 50%.

Triarylamines based on 9H−Xanthene−9−one
Scheme 1. Synthesis of 2–4.
The C−N bond formation can be verified by the utilizes the following steps to get the desired product
presence of peak at 1235–1303 cm−1 in FT−IR Spec- 2,7−Dibromoxanthen−9−one (1). Bromine (10 mL)
tra, corresponding to the stretching frequency of C−N was added dropwise over 20 min to a solution of xan-
bond (figure S1, see Supporting Information). Fur- thone (5 g; 25.5 mmol) in acetic acid (45 mL). The solu-
ther, the weak C−H stretching was observed for all the tion was stirred continuously and heated at reflux tem-
molecules which may be due to the pseudosymmetry perature for 20 h, after which half of the solvent was
in these molecules. Substitution of xanthone with ary- removed by vacuum distillation. The reaction mixture
lamine affords the corresponding triarylamine deriva- was allowed to cool and then poured over crushed ice to
tives 2–4 as yellow to orange coloured solid soluble in obtain the off−white solid. The resulting solid was fil-
common organic solvents including dichloromethane, tered and collected and washed with sodium bicarbon-
toluene, chloroform, acetonitrile etc. However, they ate to neutralize the acetic acid and further treated with
are sparingly soluble in alcohols and insoluble in 20% aqueous solution of sodium bisulfate to remove
water.
excess bromine. The obtained solid was then dried
under vacuum and purified using column chromatogra-
phy (eluent: n−hexane) to afford (6.7 g, 52.4%) pure
white crystalline solid desired product (1) M.p.: 211◦C;
Lit. M.p.: 211–212◦C.28
2.2a 2,7–Dibromoxanthen–9–one (1): Bromination
of xanthone was done by the reported method which
Chart 1. Structure of synthesized Compounds 2–4.

Bharat K Sharma
2.2b 2,7−Bis(N−phenylanilino)xanthen−9−one (2): 144.67, 148.18, 151.48, 176.43. Anal. For C45H31N2O2,
A mixture of 2,7−Dibromoxanthone (1) (354 mg, Calcd.(%): C-85.55, H-4.94, N-4.43; Found (%):
1 mmol) and diphenylamine (530 mg, 2 mmol) was C-85.58, H-4.88, N-4.47.
dissolved in anhydrous toluene (20 mL). To this
solution, palladium bis(dibenzylideneacetone) [Pd
2.2d Synthesis of 2,7−Bis(4,4ꢁ −methoxydipheny-
lamine)xanthen−9−one (4): A mixture of 2,7−Di-
bromoxanthone (354 mg, 1 mmol) and 4,4’methoxy-
diphenyl amine (530 mg, 2 mmol) was dissolved in
anhydrous toluene (20 mL). To this solution, palla-
diumbis (dibenzylideneacetone) [Pd(dba)2] (30 mg,
6×10−5 mol), 2–dicyclohexylphoshpino−2ꢁ, 6ꢁ−dimet-
hylbiphenyl (SPhos) (45 mg, 11×10−5 mol) and sodium
tert–butoxide (300 mg, 3.1 mmol) were added. The
reaction mixture was continuously stirred under nitro-
gen atmosphere at 100◦C for 16 h. Reaction mixture
was then cooled to room temperature and extracted
with chloroform. The solid thus obtained was then puri-
fied by column chromatography (eluent: n−hexane:
chloroform ratio as 80:20) to obtain a bright orange
solid. Yield: 338 mg, 52.16 %; M.p.: 195◦C. IR (KBr)
ν/cm−1 1235 (C–N str), 1656 (C=O str), 1035 (–
O– str); HR–MS m/z calcd for [M+] C41H35N2O6
(dba)2] (30 mg, 5.21 × 10−5 mol), 2−dicyclohexyl-
phoshpino−2ꢁ, 6ꢁ−dimethylbiphenyl (SPhos) (45 mg,
11 × 10−5 mol) and sodium tert–butoxide (300 mg,
3.1 mmol) were added. The reaction mixture was
continuously stirred under nitrogen atmosphere at
100◦C for 12 h. Reaction mixture was then cooled
to room temperature and extracted with chloroform.
The solid thus obtained was then purified by column
chromatography (eluent: n−hexane: chloroform ratio
as 80:20) to obtain a bright yellow solid. Yield: 63.4%;
M.p.: 173◦C; IR (KBr) ν/cm−1 1264 (C–N str), 1654
(C=O str), 1073 (–O– str); HR–MS m/z calcd for
[M+] C37H27N2O2 531.2073; found m/z: 531.2071;
1H NMR (300 MHz, CDCl3, δ, ppm): 6.998–7.093
(m, 12H, ArH) 7.219–7.394 (m, 8H, ArH), 7.465 (d,
2H, J1 = 2.7 Hz), 7.495 (d, 2H, J1 = 2.7 Hz) 7.96
(d, 2H, J = 2.7 Hz); 13C NMR (75MHz, CDCl3, δ,
ppm): 119.00, 120.07, 122.16, 123.17, 124.06, 129.45,
131.66, 144.11, 147.48, 152, 176.27; Anal. Calcd. (%)
For C37H27N2O2: C-83.59, H-5.11, N-5.27; Found (%):
C-83.64, H-5.15, N-5.18.
1
651.2495; found m/z: 651.2474; H NMR (300 MHz,
CDCl3, δ, ppm): 3.768 (s, 12H, −OCH3), 6.810 (d,
8H, J = 9.0 Hz, ArH), 7.024 (d, 8H, J = 8.7 Hz,
ArH), 7.245 (d, 2H, J = 2.7 Hz, ArH,), 7.326 (d, 2H,
J = 2.7 Hz ArH), 7.356 (d, 2H, J = 2.7 Hz, ArH,);
13C NMR (75 MHz, CDCl3, δ, ppm): 55.466, 114.47,
114.89, 116.11, 118.558, 121.858, 124.16, 126.12,
128.86, 140.78, 144.96, 150.95, 155.96, 176.41; Anal.
For C41H35N2O6 Calcd. (%): C-75.55, H-5.41, N-4.29;
Found (%): C-75.59, H-5.44, N-4.23.
2.2c Synthesis of 2,7−Bis(N−naphthylanilino)xanthen
−9−one (3): A mixture of 2,7−Dibromoxanthone
(354 mg, 1 mmol) and N−naphthylphenylamine (530 mg,
2 mmol) was dissolved in anhydrous toluene (20 mL).
To this solution, palladium bis(dibenzylideneacetone)
[Pd(dba)2] (30 mg, 5.21 × 10−5 mol), 2−dicyclohexyl-
phoshpino−2ꢁ, 6ꢁ−dimethylbiphenyl (SPhos) (45 mg,
11 × 10−5 mol) and sodium tert–butoxide (300 mg,
3.1 mmol) were added. The reaction mixture was con-
tinuously stirred under nitrogen atmosphere at 100◦C
3. Results and Discussion
3.1 Photophysical properties
for 24 h. Reaction mixture was then cooled to room In order to realize the functioning of organic materi-
temperature and extracted with chloroform. The solid als in device applications, studying their photophysi-
thus obtained was then purified by column chromatog- cal properties is important. The photophysical proper-
raphy (eluent: n−hexane: chloroform ratio as 80:20) ties of the synthesized molecules are greatly influenced
to obtain a bright yellow solid. Yield: 340 mg, 54.0%; by the nature of peripheral amines. The photophysical
M.p.: 265◦C; IR (KBr) ν/cm−1 1303 (C– N str), 1648 studies were performed on the 10−5 M solution of 2–4
(C=O str), 1143 (–O– str); HR–MS m/z calcd for in various solvents as well as thin solid film using UV–
[M+] C45H31N2O2 631.2386; found m/z: 631.2402. Vis absorption and fluorescence spectroscopy. The per-
1H NMR (300 MHz, CDCl3, δ, ppm): 6.91–7.016 (m, tinent data are summarized in table 1 and table S1 (see
10H, ArH,), 7.145–7.399 (m, 14H, ArH) 7.760 (d, 2H, Supporting Information) and the spectra are displayed
J = 8.1 Hz), 7.863 (d 2H, J = 9.3 Hz, ArH,) 7.903 in figures 1 and 2.
(d, 2H, J = 8.4 Hz, ArH); 13C NMR (75 MHz, CDCl3,
The synthesized molecules show similar UV−Vis
δ, ppm) 117.35, 118.85, 121.59, 121.94, 122.10, absorption spectra in solution and thin solid film
123.97, 126.20, 126.35, 126.50, 126.80, 127.16, (figure 1). However, parent molecule xanthone shows
128.51, 129.29, 129.62, 130.96, 135.38, 143.06, two absorption peaks at 285 and 338 nm arising due to

Triarylamines based on 9H−Xanthene−9−one
Table 1. Photophysical data of the synthesized compounds 2–4.
Toluene
Thin Film
Stokes
Stokes
λmaxa, /nm
λem
,
shiftd, Egopt e
λem
,
shiftd, Egopt e
c
c
f
Compounds
(εmax/M−1cm−1 × 103)b
/nm
/cm−1
4716
4421
4732
/eV
2.71
2.75
2.56
φF
λmaxa,/nm
/nm
/cm−1
5237
5165
5894
/eV
2.52
2.75
2.56
2
3
4
306 (99.93), 327 (107.47), 493
400 (10.42)
0.14
307, 328, 413
527
286 (55.80), 318 (68.39),
352 (28.06), 398 (10.03)
299 (74.29), 325 (70.88),
425 (7.47)
483
0.22 265, 318, 357, 413 525
0.22 301, 328, 430 576
532
aAbsortion spectra recorded in toluene 10−5 mol L−1 and for thin solid film form.
bExtinction coefficient (/M−1 cm−1 × 103) measured in toluene.
cEmission spectra recorded in toluene 10−5 mol L−1 and for thin solid film form.
dStokes shift calculated from charge transfer absorption and emission maxima observed in solution and thin solid film form.
eOptical bandgap estimated using emission and excitation spectra in toluene and thin solid film form.
Egopt = (1240/λopt) eV. λopt /nm values are 457 (2), 450 (3), 484 (4)
fQuantum yield relative to Alq3 (φF = 0.16 in benzene).25
π –π∗ and n–π∗ transition. Upon substituting the elec- absorption for all the derivatives as thin films are found
tron donating amine groups at the 2nd and 7th position of to be nearly same order relative to the solution spectra.
xanthone, the absorption maxima are found to be blue
The compounds 2 and 3 emit green light with λmax
shifted in all the derivatives. The absorption spectra of at 493 and 483 nm (figure 2a) upon excitation at their
the compounds 2–4 (figure 1) exhibit a well-resolved corresponding absorption maxima in toluene with a
absorption profile in the range of 286–425 nm. Among bathochromic shift of 34 nm and 42 nm in thin solid
these transitions, the higher energy absorption between film, respectively (figure 2b), while the compound 4
286–352 nm is probably originating from the π–π∗ and shows green emission with λmax at 532 nm in toluene
n–π∗ transition located within the arylamine units and (figure 2a) and a significantly (44 nm) red−shifted yel-
the xanthone core while the lower energy band between low emission at λmax 576 nm as thin film due to the
398–425 nm is attributed to the charge transfer transi- auxochromic effect of electron−donating p−methoxy
tion between the electron–donating diphenylamine and groups at phenyl rings of amine (figure 2b). The emis-
the electron–accepting 9H−xanthen−9−one core.
sion peaks of the dyes 2–4 in the toluene are in the
In addition, the absorption spectrum of the deriva- range 483–532 nm with a bathochromic shift as the
tive 3 is found with a shoulder peak at 352 nm which thin film ranging from 525–576 nm which may be due
may be due the π–π∗ transition overlapping with lower to the intermolecular aggregations29 in the solid state
energy charge transfer transition (figure 1). The λmax in (table 1).
Figure 1. UV−Vis absorption spectra of compounds 2–4 and xanthone in toluene solution (a), and thin solid film (b).

Bharat K Sharma
Figure 2. Emission spectra of the compounds 2–4 and xanthone in toluene solution (a), and thin solid film (b).
Further, the absorption and emission behavior of the Molar extinction coefficients (ε) of 2-4 are high in solu-
synthesized derivatives were recorded in two other sol- tion which indicate that the long wavelength band in the
vents, chloroform and acetonitrile (figure S5) and it was absorption spectra is a charge transfer band through the
observed that the absorption spectra for all the deriva- π–π∗ transition.
tives was of the same order while the emission spectra
The Stokes shifts of molecules 2–4 were high,
were found to be red shifted by 36–39 nm for compound 4421–4732 cm−1 in toluene and 5165–5894 cm−1 in
2 in chloroform and acetonitrile and 43 nm for com- thin films ensuring that there is no re−absorption of the
pound 3 in chloroform 30 nm blue shift for compound 3 emitted radiation. The fluorescence quantum yield of
in acetonitrile was observed as compared with the emis- 2–4 were calculated using tris(8−hydroxyquinolinato)
sion of the dyes in toluene. The emission intensity of the aluminium(III) (Alq3) (φF = 0.16 in benzene)25 as
derivative 4 was quenched in polar solvents like chloro- standard. The quantum yields of the derivatives are
form and acetonitrile (table S1). The shifts only in the in the range 0.14–0.22 in toluene (table 1) (for chlo-
emission spectra with change in polarity of the solvent roform and acetonitrile see table S1 in Supporting
indicate that the ground state geometry of the deriva- Information). Material performance of the synthesized
tives is not affected by the polarity of solvent how- molecules 2–4 for electroluminescent application is
ever excited state geometry of the compounds is greatly evaluated by Commission International de I’Eclairage
affected by the nature and polarity of the solvent.
(CIE) coordinates. The (CIE) coordinates of com-
Optical band gap (Egopt ) for the derivatives in toluene pound 2–4 are determined by using photolumines-
and thin film were calculated using emission and cence spectra in thin solid film. The obtained CIE
excitation spectra (figure S7) and are in the range are located in yellow−green region (see Supporting
2.56–2.75 eV and 2.52–2.75 eV respectively (table 1). Information).
Table 2. Electrochemical and thermal data for compounds 2–4.
peak a
ox
peak b
red
Compound
E
(V)
E
(V)
HOMOc
LUMOd
E cgv
Td (◦C)f
Tmax (◦C)g
Tm (◦C)h
2
3
4
1.218
1.129
0.864 (1.315)
−1.658
−1.677
−1.612
−5.686
−5.597
−5.332
−2.724
−2.705
−2.776
2.962
2.892
2.556
280 (340)
330 (360)
260 (370)
410
450
563
173
265
195
aEopxeak oxidation peak potential Vs Fc/Fc+.
bErpeedak reduction peak potential Vs Fc/Fc+.
peak
ox
cHOMO energy level calculated from E HOMO = −[E
− Eox (Fc/Fc+) + 4.8 ] eV.
dLUMO energy calculated from E LUMO = −[Epeak − Ered (Fc/Fc+) + 4.8 ] eV.
eEgcv calculated from Egcv = (HOMO−LUMO)reeVd .
fTd Decomposition temperature at 5% and 10% (in parentheses) and weight loss derived by TGA.
gTmax Derivative weight loss
hTm Melting point.

Triarylamines based on 9H−Xanthene−9−one
substituents attached to the amine nitrogen. An addi-
tional reversible oxidation couple at low positive poten-
tial at 0.864 V is also observed for compound 4 which
probably may arise from the oxidation of p−methoxy
segment.30
On cathodic sweep, parent xanthone molecule
exhibits two steps of reductions (figure S8 in SI).
The first quasi-reversible peak occurs at −1.78 V and
another irreversible reduction peak at −2.27 V due to
the double reduction of carbonyl group forming dian-
ion. 2–4 show only one irreversible reduction peak
between −1.612 to −1.677 V at lower potential com-
pared to xanthone due to the presence of terminal elec-
tron donor diarylamine groups thus suggesting forma-
tion of radical anion.
Figure 3. The cyclic voltammograms of compounds
2–4 measured in acetonitrile solution against ferroce-
nium/ferrocene, at 100 mV/sec.
The HOMO energy levels of the synthesized
materials 2–4 are in range of −5.332 to −5.686 eV
which are similar to or slightly higher than that
of most widely used hole transporting material
3.2 Electrochemical properties
4,4ꢁ−bis(1−naphthylphenylamino)biphenyl
(NPB)
The charge transport capability of a molecular material
can be identified from its propensity to stabilize both
its cation and anion. Electrochemical measurements can
be used to estimate the capability of the dye to pro-
duce and stabilize the cation and anion. We studied 2–4
by cyclic voltammetric measurements using ferrocene
as an internal standard to calibrate the redox potentials.
The observed parameters are reported in table 2. The
cyclic voltammograms are displayed in figure 3.
(HOMO = −5.20 eV and LUMO = −2.4 eV) and thus
might be beneficial for the hole−injection and trans-
port properties.31 The LUMO energy levels are in the
range of −2.705 to −2.776 eV. The energy band gaps
determined from the cyclic voltammetry measurements
are in range of 2.556–2.962 eV and are comparable
with the optically calculated band gaps.
On anodic sweeps, no oxidation wave was observed
for the parent xanthone core (see figure S8 in SI). How-
3.3 Thermal properties
ever, irreversible wave was presented by all of the dyes The thermal stability of the synthesized molecules
due the oxidation processes at relatively low oxida- was determined by TGA and DTA in the tempera-
tion potentials at 1.129–1.315 V. It is attributable to ture range of 30–1000◦C at a constant heating rate of
the oxidation of the peripheral amines30 at the 2nd and 10◦C min−1 under nitrogen atmosphere. The observed
7th position of xanthone core. The oxidation poten- TGA parameters are reported in table 2 and displayed
tials increased in the following order 4<3<2. This in figure 4. TGA plots of compounds 2–4 indicate that
trend is attributed to the electronic nature of the aryl the derivatives have high thermal stability with the
Figure 4. TGA thermograms (a) and derivative weight loss (b) of compounds 2–4 under nitrogen atmosphere at normal
pressure. Heating rate, 10◦C/min.

Bharat K Sharma
decomposition temperature corresponding 5% and 10% their CIE Coordinates as thin solid film (table S1), Emis-
weight loss in the range of 260–330◦C and 340–370◦C sion and Excitation Spectra (figure S7), complete scan
respectively (figure 4(a)) and are comparable with Cyclic Voltammograms (figure S8), Derivative weight
the previously reported hole−transporting materials loss and DTA plots (figure S9), CIE coordinates (figure
such as 9,10−bis(diphenylamino) anthracene (PPA), S10) and Photograph of compounds in solution form
9,10−bis(1−napthylphenylamino)anthracene (NPA), (figure S11) are given in the Supporting Information.
1,4−bis(phenyl−m−tolylamino) biphenyl (TPD), 4, Supplementary Information is available at www.ias.ac.
4ꢁ−bis(1−naphthylphenylamino)biphenyl (NPB) with in/chemsci.
stability in range 185–382◦C.7c,31a The order of thermal
stability among the diarylamine derivatives is 4<2<3
(table 2).
Acknowledgments
We thank the Micro-Analytical Laboratory, Depart-
ment of Chemistry, University of Mumbai for providing
Instrumentation facility. We also thank the Depart-
ment of Chemistry IIT-Mumbai for providing HR-MS
Spectra.
The derivative weight loss plot figure 4(b) shows a
narrow derivative peak with high decomposition tem-
perature with Tmax in the range of 410–563◦C (table 2).
The DTA plots of synthesized compounds 2–4 (see
Supporting Information) show sharp endothermic peaks
which are attributed to the corresponding melting points
of compounds and exothermic peaks were observed
above 350◦C indicating that no crystallization takes
place below 350◦C. Melting points of the derivatives
2–4 are in the range of 173–265◦C.
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